Guerrilla guide to CNC and resin casting

Home manufacturing tutorial for robot builders, model makers, and other hobbyists
Copyright (C) 2013, 2014, 2015 by Michal Zalewski (lcamtuf@coredump.cx)

My latest book, Practical Doomsday, is now out. Please check it out!

1. Preface

1.1. The purpose of this guide

I'm a computer geek by day, and a hobbyist robot builder by night. I quickly learned that the craft of robotics - just like many other precision hobbies - requires either remarkably deep pockets, or a combination of outstanding manual skills and easy access to a well-equipped machine shop. Urban dwelling hobbyists who are not blessed with any of these graces tend to give up, resort to expensive and simplistic premade kits, or fall back to junkyard-quality engineering.

Curiously, the simplest tasks often prove to be the most challenging ones: many of us can program a microcontroller without breaking a sweat, but making a simple actuated joint, a gearwheel, or a cover in a desired shape, can be beyond the realm of imagination. For the past decade, we are being promised a revolution in desktop manufacturing - but unbeknownst to many, a simple, affordable, and home-workshop-friendly solution is already well within reach. The only problem with computer numerical control is that the workflows and materials suitable for small scale, hobby engineering are almost completely undocumented, and quite difficult to discover on your own.

Several years ago, I took a huge leap of faith, decided to buy a small CNC mill (Roland MDX-15), set up a resin casting workshop, and invested months of intermittent trial, error, and triumph to understand and befriend both technologies - and document them so that others don't have to go through all the pain. It was well worth it, to be sure: I can now routinely crank out remarkably cool and precise designs in no time, and with only minimal cost:

The approach also works for others; this hybrid legged robot by Marc Hamende (video) is one of my recent favorites.

If this sounds interesting, and if you are willing to spend around $2,000 to set up a CNC workshop, simply read on. Conversely, if you care only about resin casting or other non-CNC parts - you can skip the first chapter and go directly to one of the later bits:

Many of these sections will be applicable to other types of projects and other manufacturing processes - including hand-made and 3D-printed designs.

Oh, to get it out of the way: note that this page is pure, unadulterated labor of love; there are no affiliate links, paid product placements, or ads anywhere in the guide.

1.2. A bit on CNC machining, and how it compares to 3D printing

In basic terms, computer numerical control milling is a "subtractive" method for processing raw material (workpiece or stock), usually with a drill-like rotating cutter (end mill), through a set of computer-driven movements of the cutting head. You can think of it as a robot equipped with a Dremel tool.

Some of the most basic CNC mills resemble a drill press on steroids. A growing number of them is designed specifically for home and office uses, and have more sophisticated looks - but still, in comparison with technologies such as 3D printing, which produces 3D shapes by additive deposition, CNC machining seems pretty savage.

Alas, for now, entry-level additive FDM technologies remain fairly useless for low cost, high quality hobbyist work; this is slowly improving with the advent of affordable SLA ("resin") printers, but the new technology is still finicky, messy, and slow. Today, CNC has a clear upper hand on at least four fronts:

On the flip side, the fundamental advantage that 3D printers have over CNC mills is that they can often produce basic internal geometries inside enclosed parts - whereas milling can only process the areas for which there is a sufficient cutter clearance. It is not trivial to do that on low-cost 3D printers, though - and the constraint of CNC mills is seldom a big deal: almost everything can be split into halves, rotated, or so. CNC machining is the staple of industrial manufacturing. Keep in mind that almost every mass-produced item - from iPads to soda bottles - copes with this reality in a graceful way.

1.3. The virtues of resin casting

Resin casting is a nifty process that involves creating a mold, and then pouring a liquid, two-component polymer resin into the mold cavity. Once the resin sets, the finished part is removed - and the mold can be used again. This method of replicating parts is popular artists of all trades, but is seldom employed in amateur CNC or 3D printing work.

Despite sounding needlessly complicated, the overhead of this process is actually pretty modest - and it offers some interesting advantages over directly machining (or printing) the desired shape:

Best of all, when done correctly, resin casting is essentially pixel-perfect, providing copies with amazing dimensional accuracy and picking details so fine that you can only see them under a microscope.

If you want to do any real engineering work in plastics, it is a very good idea to rely on resin casting for a good chunk of your projects - and consequently, this guide is (in part) geared toward this goal.

1.4. But how much will this really cost me?

As with any hobby, sky is the limit! That said, if you want to get good results on a budget, the breakdown of expenses is roughly this:

For the processes advocated in this guide, the ongoing maintenance costs for a CNC mill are negligible: after several thousand hours of machining, you may have to replace several bearings or spindle motor; cutters should last for hundreds of hours, too. The savings add up quickly, especially if you are already paying for specialty or made-to-order parts.

2. Setting up a CNC mill

Okay, still interested? Let's dive in, then. The first "proper" section of this guide deals with shopping for a mill and understanding its operating characteristics; picking the appropriate cutting tools; and monitoring the performance of your setup to achieve perfect results every time.

Again, if you're using another manufacturing process and are interested strictly in the CAD tutorial or the resin casting bits, feel free to skip ahead.

2.1. Picking the right machine

Some purists make a distinction between what they call "CNC routers", suited chiefly for working in wood and plastics, and "true" CNC mills, designed for machining steel. This guide will not dwell on the distinction; almost any CNC machine can handle almost any material, albeit not always as quickly as a beefier mill. Just as importantly, the purists' focus on metal is a distraction. Sure, industrial CNC mills spend the bulk of their time processing aluminum and steel, but that's mostly because you need durable and temperature-resistant patterns to make injection-molded plastic parts. In hobby workshops, metal molds aren't cost-effective, and other materials reign supreme.

With this in mind: gneral purpose, benchtop-sized CNC mills start at around $600 and go up to $20,000 or so. There are numerous manufacturers of benchtop CNC mills around the world; some of the best-known brands include Roland DG, Sherline, Taig, and Tormach, Syil. But be sure to search around; there are quite a few other companies that cater to local markets - say, Probotix, Deepgroove1, LittleMachineShop, Romaxx, Microkinetics, MAXNC, Microproto, Light Machines, Minitech, Flashcut, Smithy, ShopBot, Torchmate, CNC Masters, ACT, Charlyrobot, EasyCut, or Laguna Tools. Some people had luck with ultra-low cost mills made in China, too.

Of course, it is also possible to build your own machine from scratch. Doing so is not necessarily economically sound, because there is a significant price tag attached to high-precision linear motion systems, machine spindles, and powerful servos or stepper motors; on top of that, you will probably have to iterate through several designs, and the project will consume several months of your time. Still, if you are so inclined, there are low-cost plans and kits available on the Internet.

When shopping for a pre-made system, there are several key characteristics to pay attention to; let's have a look at them, and use them as an excuse to discuss some of the inner workings of CNC milling jobs.

2.1.1. Number of axes

This is perhaps the most fundamental quality of any CNC mill. In the most basic design, the cutting head can move in three directions - X, Y, and Z - and the tool itself always points down, aligned with the Z axis. In this setup, the machine can only machine shapes that can be represented using a two-dimensional "depth map" projected onto the workpiece: the cutter may descend lower for some X-Y coordinates, and move up for others, but it will not enter the workpiece from any other side. This video is a pretty good illustration of the process:

In this machining mode, the machinable geometries are outlined here:

Note: CAM applications are designed to fail safely in regard to part geometry; that is, if a cavity on the part can't be reached without going through an area that isn't supposed to be cut, the problematic region won't be machined at all. The gray regions in the two workpieces on the right correspond to the material that will be left in place.

The limitations of three-axis machining may seem severe, but seldom truly are. Every section of an industrial injection mold or a metal forming die typically needs to be a depth projection anyway, so that the processed material could be pulled out of it easily. Even in direct machining, it is common to simply flip the workpiece with the aid of registration pins. This video illustrates the manual rotation process fairly well.

That said, there are some shapes that truly benefit from automated, multi-directional machining; this includes exotic types of gears (helical, herringbone, and worm geometries), screws / bolts, and certain categories of jewelry (say, rings). For these uses, some CNC mills come with additional rotary "axes": the so-called A axis corresponds to rotating the workpiece around the X axis (see video); B axis stands for rotation around Y; and C axis is the rotation around Z. The four-axis AXYZ setup is the most common one.

The premium for fourth axis starts at around $100 for manual indexers (a precision rotary chuck that holds the workpiece, but where the angle needs to be dialed in manually); and from $500-$1,000 for computer controlled units.

What to buy: 99% of your moldmaking work will not appreciably benefit from a fourth axis, so three axes are perfectly fine. You may want to get a mill where fourth axis is an option, though, especially if you are also planning to do artistic work.

2.1.2. Mechanical movement ranges

Greater X-Y-Z tool movement ranges translate to the ability to make larger parts in a single pass. It's important to pick a mill that won't get in the way of your imagination - but to make this call, you need to calibrate your expectations sensibly.

As an extreme example, let's consider building a man-sized biped robot. You don't need a man-sized mill for that job - for at least three reasons:

What to buy: do your own math. In my experience, about 15 x 10 cm in the X-Y plane is a good starting point, and about 30 x 20 cm will accommodate almost any medium-size robotic job. In the Z axis, you will probably not need more than 4 cm or so; and going over 8 cm is usually pointless. Whatever you do, do not confuse movement ranges with table dimensions, though.

2.1.3. Use of a specialized spindle

Spindle - the part that connects the motor and the rotating tool - has a profound impact on the accuracy of any CNC mill. Its role is to ensure that the rotation of the tool is highly concentric and vibration-free, and that it stays this way under load. If the whole rig is not perfectly centered, you may end up with a situation such as this:

The total amount of back-and-forth wobble - in other words, the difference between the intended and effective diameter of the tool - is known as total indicated runout, or TIR. High TIR will not only affect the dimensional accuracy of machined parts, but will also ruin surface finish, and prematurely wear the tool. In fact, the effect is pretty dramatic: in some materials, eccentricity of 0.01 mm can reduce tool life by 50%.

Proper CNC spindles are usually long, round or rectangular blocks of metal with precision ball bearings mounted on both ends (and often pre-tensioned with a spring). Inside, there is a heavy-duty rotating shaft that couples the motor belt drive system to the tool holder. With quality spindles, TIR usually can be kept below 2 µm.

Some of the low-end manufacturers don't bother with a proper spindle, however; the most common example of this are CNC mills that use repurposed manual rotary tools. These cases are a bit of a gamble: some of them may have still somewhat bearable TIR in the vicinity of 0.01 mm - but some will be as bad as 0.10 mm, which makes them completely useless for precision work. Runout aside, you also can't be sure if the tool is perfectly aligned with the Z axis or not; if it isn't, that opens a yet another can of worms.

Note: to put all these numbers in perspective, 0.10 mm is roughly the diameter of a human hair; level differences of this magnitude can be easily felt when sliding your finger across a hard surface. Notches down to about 0.05 mm can be easily seen on smooth but matte finishes - and on glossy surfaces, the threshold may be closer to 0.01 mm or so.

What to buy: try to avoid CNC mills without real spindles; if you need to get one, ask the manufacturer about TIR. If they are not sure, it's an obvious red flag: the parameter can be trivially measured with a $50 tool, and is one of the most rudimentary things to examine when designing a mill. Note that there are aftermarket spindles that can be fitted into certain mills, though!

2.1.4. Movement precision

There are many factors that contribute to the real-world precision of a CNC mill, but one of the most important aspects is repeat accuracy: the ability to return to the same position over and over again. Along with spindle characteristics, this quality has a tremendous impact on surface finish, and on the dimensional accuracy of small parts.

Repeat accuracy is affected chiefly by two things:

Unfortunately, there is no widely accepted standard for testing repeat accuracy; many manufacturers don't bother to advertise it, and others test it with varying levels of honesty. In fact, the good guys will give you a figure that represents the worst-case, momentary deviation following a rapid long-distance movement - but that's not really representative of most types of fine work with sub-millimeter tools.

Now, don't despair: the good news is that most of the commercially available mills are actually pretty good in this department, especially when moving slower and doing precision cuts in easily machinable stock. You can expect many entry-level mills to conduct themselves within 0.02 mm or better during normal work; more expensive units with ball screws and servo motors will probably stay around 2-5 µm.

Accuracy aside, mechanical resolution is the other important piece of the puzzle. Stepper or servo motors in a CNC mill can assume only a certain number of positions per turn, and that translates to a specific minimum distance by which the table or the cutting head can be moved around. Insufficient mechanical resolution means that the mill will have difficulty smoothly approximating certain curves, and may end up producing unattractive finish.

What to buy: The basic rule is that you should not expect a plywood-based contraption with acme screws to reach 1 µm repeat accuracy. If the manufacturer advertises an improbable value, ask them to explain. If they advertise a suspiciously high figure (over 0.1 mm or so), be wary, too. As for the mechanical resolution: look for 5 µm or better.

2.1.5. Machining speeds

Time is money. When it comes to CNC machining, the time needed to complete a job is to a surprising extent dependent on your skill and the capabilities of your software - but with a skilled operator and good toolpath decisions, the final part of the equation is always the performance of the mill itself.

To understand how the mill's performance is tied to the numbers you see in the datasheet, it is helpful to look at the geometry of a typical end mill. Upon closer inspection, the tool closely resembles a drill: it consists of a round shaft with several blades (flutes) wrapped around it in a spiral fashion. As opposed to a drill, however, these flutes have a sharp, exposed edge running along their entire length; this is because the bulk of their work is meant to be done by moving sideways. This is how it looks from the top:

Even in the most easily machinable material imaginable, the cutter is able to scoop away only a certain amount of swarf per turn - just enough to fit under the flute. If you exceed that capacity, you will end up dragging a clogged, non-cutting tool across the workpiece - which ends with one or the other eventually giving up.

For every material and cutter geometry, there is an optimal ratio of linear speed and cutter RPM that leads to efficient, high-quality machining. This is often expressed as feed per tooth. In plastics and similar materials, the ideal values are:

In practical terms, it's healthy to aim for mills where the ratio between maximum movement speed (mm/min) and maximum RPM hovers around 0.4 to 0.8 for optimum performance during rough cutting. At the same time, there is also some value in shopping for the highest maximum RPM you can get - as it lets you move faster during the precision finishing steps.

Of course, there are some gotchas:

What to buy: at least 6,000 RPM is nice; and if the aforementioned speed ratio is favorable, there are no real downsides to going up to 20,000 RPM. Maximum movement speed, in mm/min, should be ideally at least 6-10 times the movement range, so that it doesn't take more than several seconds to traverse the table.

2.1.6. Tool sensor support

Spare for some pathological situations, the mill is intrinsically aware of the position of its spindle at any given time; but the actual cutting action takes place beneath the spindle - at a distance dictated by how far the tool sticks out from its holder.

And here lies the problem: most toolholding systems do not allow you to precisely preset tool extension length, or to maintain it when you replace the cutter. If you switch the tool in the middle of a machining process, and don't compensate for the difference, the results will be off; in fact, the tool may unexpectedly hit an uncut area and break.

There are several manual tricks that can be used to work around this issue. One of them is to place a thin strip of paper or foil in a fixed reference location, and then slowly lower the tool until the strip gets caught between the cutter and whatever happens to be underneath. By comparing the Z position of the spindle at that point with the reading obtained for the previous tool, the appropriate offset can be calculated and communicated to the machine. But of course, this technique is somewhat inconvenient, and accurate only to perhaps 0.05 mm.

A better approach is to incorporate a tool height sensor into the mill. The sensor can be just a flat block of precisely machined metal; the mill automatically lowers the tool onto the sensor until contact is made - which, in the simplest design, is detected by noticing the flow of current between the probe and spindle body. The accuracy of this approach is often better than 0.01 mm. More complex (and costly) solutions involving optical sensors can also be used.

What to buy: try to find a mill that has a built-in sensor, or can be equipped with one. Failing this, you can always rig a manual tester that uses the same operating principle, and simply illuminates a LED.

2.1.7. Tool mounting method

The spindle must be terminated with some sort of a tool holding device. This can be a direct system, where the tool slides into the spindle assembly with no intermediate components; a collet-based approach, where the spindle accepts a small clamping device that actually grips the cutter and is tightened with a nut; or a model where the spindle has a large tapered bore that accepts standalone toolholders used by automatic tool changers.

Leaving automatic tool changers and exotic cutters aside, the simplest way to clamp an endmill is to use a fixed-diameter opening, be it using a set screw or thermal expansion (shrink-fit). The solution can be very accurate, but is also quite cumbersome when it comes to tool changes. For example, on a Roland MDX-15 mill, you have to replace the entire spindle assembly to switch between two different diameters of the shank.

On the opposite end of the spectrum, you have jaw chucks, similar to the solution seen in power drills. They can grip a wide range of tool diameters, but tend to suffer from poor eccentricity, and TIR is seldom better than 0.2 mm. For precision work and rigid CNC machines, that's a bad deal.

A reasonable middle ground is a system that accommodates fixed-diameter collets fastened with a nut; the spindle has a single taper to accommodate a family of collets (e.g., ER16), and every collet is precision-ground to hold a specific diameter of a tool. Because collet changes are quick and the collets themselves are inexpensive, this is a good balance between accuracy and ease of use. The ER system is a particularly popular and dependable choice.

What to buy: If you can get ER or a similar collet-based system, go for it. Otherwise, just make sure that the toolholding system is versatile enough to accommodate common shank sizes (3, 4, and 6 mm for metric cutters; 1/8" and 1/4" for imperial system tools), and will be sufficiently precise for your needs.

2.1.8. Availability of CAM software

Manually programming your CNC machine is about as much fun as building a steamboat out of toothpicks. For a higher-level approach, you need to turn to CAM software: it automatically analyzes the provided geometry (created with any 3D modelling application) and converts it to a set of paths that need to be retraced by the tool to approximate the desired shape. Once these toolpaths are ready, the software then breaks them down into a sequence of painfully basic instructions that actually make sense to the controller embedded in the mill; say, "set speed to 12,000 RPM" or "move cutter to X = 10.245, Y = 5.000, Z = -2.000".

The toolpath generation stage is largely hardware-agnostic; but the program generation one isn't. It's good to shop for a machine that speaks a common and well-documented language - or, lacking this, is popular enough to be supported by some of the best-known CAM apps. Keep in mind that even if the manufacturer bundles the mill with some starter software, you don't want to be left out in the cold if the application one day refuses to work with your new PC - or if it simply turns out to be of poor quality.

The most common quasi-standard language used by almost all CNC mills is called G-code (aka "NC"). Calling it a real standard may be a stretch: there are very significant variations in how the syntax is implemented by the manufacturers. Still, having support for G-code spells rudimentary compatibility, or at least easy integration, with almost any CAM application on the market. For other languages, this is not always given.

What to buy: check if the mill is supported by common third-party packages (Deskproto, VisualMILL, madCAM, MeshCAM, Mayka, etc); if it's not, and if it speaks something else than a clearly documented variant of G-code, be wary.

2.1.9. Size, weight, power needs

We're almost done: the last thing to do is a quick reality check. Benchtop mills span from units no larger than an inkjet printer, to ones weighing in excess of 200 kg and taking up almost 1 x 1 m of desk space. When shopping for the larger models, be sure to account for their physical characteristics, and make sure you have a way to get them in your workshop to begin with (some doors are barely 70 cm wide).

For heavier mills, it is also important to have a piece of sturdy furniture; it's not just the static load that you have to worry about, as the machine may also produce horizontal shear forces due to acceleration and deceleration of the cutting head. Not every wobbly desk from Ikea can handle that - but any proper workbench should.

Benchtop mills usually run on standard, single-phase 110 / 230 VAC power supply, but of course, make sure to double-check. They may require several amps in peak, so you don't want them to share a single circuit with a vacuum cleaner, an electric kettle, or a space heater - especially in an older home.

2.1.10. And now, all the things you don't have to worry about

Okay - that sums up the list of parameters that are worth looking at. There are also some characteristics that sound important, but usually aren't - so to help you decide, here's a quick list to consult when in doubt:

Well, that's probably it. If you spot any other puzzling parameters, please let me know.

2.1.11. So, which one should I buy?!

That really depends on your budget and the scale of the projects you want to be working on. Here is a list of some of the fairly popular, inexpensive mills, along with their catalog prices. Many of them sell for around 15% less if you talk to the right distributor:

2.2. Stocking up on end mills

Ordering a CNC machine? Well, the next stop is getting some cutters. The selection available on the market is quite overwhelming, so to save you time and money, let's talk about some of the properties that set these tools apart. Oh, before we dive in... here's a drawing of a typical end mill, and all the lingo you will have to memorize soon:

All right - so here are the differences you will see:

Phew! My favorite tool manufacturer is Hanita (now a division of Kennametal, confusingly sold under the brand name of WIDIA): they have an unmatched selection of metric tools at reasonable prices, and are available all over the world. If you are in the States, Sierra Tool is a good reseller; Centerline Industrial is a bit cheaper, but for some reason, refuses to ship to residential addresses. And if you are anywhere in Europe, I can strongly recommend ordering Hanita products with ITC.

Hanita aside, Harvey Tool has a very interesting selection of imperial system miniature tools in the US - and I found K&H Sales to be a dependable distributor. Other US manufacturers include OSG Tap & Die, Monster Tool, Micro100, and Microcut, but their catalogs are not as impressive. Readers in the EU may want to check out Nachreiner.

Here are the catalogs of the three most interesting manufacturers:

As for practical recommendations, I would suggest starting with Hanita 401403000, 402403000, or Harvey 73118-C4, as the baseline "3 mm" cutter ($15-$30); Hanita 7N2201021 or Harvey 76440-C3 for 1 mm work ($30-35); and Hanita 7N2200410 or Harvey 992515-C3 as a long-reach ~0.4 mm tool ($35). If you are not on a very tight budget, it makes sense to order two of each - it's easy to mess something up in the heat of initial experimentation.

With the tools selected, you also need to make sure you have the right set of collets. For ER16, if you are not desperate to save a few bucks, try Rego-Fix "UP" (ultra-precision) collets; they retail for about $45 a pop, and are carried by K&H; another good option are "DNA" collets from Techniks (about $30, require a custom nut). You can find lower-cost collets from many other, more obscure brands - but they are not always particularly good.

Oh, one more thing: for ER16, every collet has a specified clamping range - for example, 3.00-2.00 mm. It is always preferable to use the upper value: a 3.00-2.00 mm collet is better than a 4.00-3.00 one when holding a 3 mm tool.

2.3. Periodic testing and troubleshooting

Before embracing any complex or high-precision projects, it is important to understand the performance of your mill, and see if anything needs to be fixed, adjusted, or compensated for. While CNC mills don't require constant tuning, making several simple measurements after unpacking the device can save you a lot of time. If you neglect this step, you will find that troubleshooting mill accuracy issues in complex, real-world projects tends to be a daunting task, simply due to the sheer number of variables to look at.

The essential tool that you will need to perform the initial measurements is a micron-resolution dial indicator with a magnetic base. You can get a no-name unit on eBay for about $50 (link), or go with Mitutoyo or other reputable brand for about $220 (indicator, base). For lower-cost mills with a rotary tool acting as a spindle, you may be better served by a 0.01 mm indicator, though; in this case, you can get a Mitutoyo one for about $80 (link, base not included).

2.3.1. Spindle TIR

The first thing to check is the runout of the spindle and the tool holder. Wipe clean the internal spindle taper and the collet (use WD-40 if there is any excess grease or other accummulated dirt; a small brass brush works well for any stubborn gunk), and install a tool that extends at least about 20-30 mm from the collet. Tighten until you feel definite resistance, but don't overdo it - excess force may deform the collet, and is not essential in lightweight work.

Next, affix the dial indicator to the table or other sturdy surface, make the tip of the indicator touch the tool near the spindle, and observe the change in readings as you gently turn the spindle by hand, preferably at the top (the mill should be turned off, of course). Be very careful not to exert any unnecessary pressure on any of the parts.

Let's call the result of this measurement Rcollet. Move the indicator about 10 mm lower (stay clear of the flutes) and repeat the test; we'll refer to it as Rmiddle. Finally, if possible, remove the tool and the collet, and reposition the indicator to make contact with the internal taper of the spindle (the measuring tip now pointing up). Repeat the procedure, and write down the result - Rtaper. Here's how to interpret the data:

For a quality machine with a dedicated spindle and ultra-precision ER16 collects, after some adjustments, TIR should preferably stay within 2 µm or so; and for any mill, it is good to have runout within 0.01 mm. If you are seeing something much worse, poke around and see if you can improve it: it's usually simpler than it may seem. For example, the factory spindle that came with my MDX-540 mill had a TIR of about 6 µm; using a hook spanner to adjust the tension of the internal spring by one tenth of a turn reduced the value to 1 µm. In more complex cases, switching around or rearranging the existing bearings inside the spindle, or replacing them with new ones, will often do the trick.

Tip: try to quickly repeat the Rcollet measurement after every tool change, even if you are not doing high-precision work. Trapped dirt will affect the concentricity of the tool holder, and even a tiny difference can easily reduce tool life by 50%, and increase cutting noise by more than that.

2.3.2. Axial alignment of the tool

Spindle eccentricity aside, it is also useful to verify that the tool is truly perpendicular to the X-Y plane; if that's not the case, for example because one of the screws that attach the spindle to the rest of the mill is not tightened to the same torque, you may see somewhat perplexing dimensional errors in machined parts.

The test you should perform is exceedingly simple: you need to mount a cutter that offers at least around 2-3 cm of clearance between the collet and the flutes; programmatically lower the spindle by 2 cm or so; and set up the dial indicator as shown on the previous drawing, in section 2.3.1. When done, gently rotate the tool to find the mid-point of its TIR, and then programmatically move the spindle up by about 1 cm. The same procedure should be repeated after repositioning the dial indicator to make contact with the side of the tool (rather than the front).

If everything is fine, you should see no appreciable change in the values shown by the dial indicator; a few microns may be fine, but if the difference is getting close to 0.01 mm, you should definitely investigate. The issue is almost always trivial to fix: you may need to loosen the screws that hold the spindle in place, and perhaps insert a shim made out of aluminum foil on the offending side to straighten it out.

Caution: before operating the mill, be sure to read the safety tips provided by the manufacturer, as well as the advice included in section 7 of this guide. Small CNC machines are not particularly deadly power tools, but they are still power tools - and it's your responsibility to use them safely.

2.3.3. Spindle vibration

The spindle assembly is typically fairly heavy, and under normal operating conditions, will be rotating rapidly. At these speeds, any poorly balanced rotating part, any malfunctioning ball bearing, and any damaged transmission belt may easily introduce significant vibration - and that vibration will inevitably propagate to the end mill or the workpiece.

It is difficult to accurately measure high-frequency vibration without the help of specialized tools, but this shouldn't stop you from performing two rudimentary checks. Try this:

If any of these tests reveals excessive vibration, the first thing to do is disenage the motor from the spindle (there's usually a belt or some sort of a clutch involved); if the problem doesn't go away, you know that the problem is with the motor itself, in which case, it may be useful to have it serviced or replaced. If the spindle is to blame, replacing the internal bearings would be the obvious next step.

Whatever the cause is, fixes shouldn't be too expensive, but pinpointing the issue may take a while.

2.3.4. Repeat accuracy

Repeat accuracy is the single most important factor limiting the precision of the parts you can make. Even if you are not planning to machine anything particularly intricate, this parameter is still worth checking: if it's alarmingly poor, it may be indicative of a problem with the mill.

To estimate the accuracy of the machine, brace the tip of the dial indicator against the side of the spindle assembly, and then programmatically move the spindle away in the X axis, in 0.01 mm increments (or whatever the nearest multiple of your mill's mechanical resolution is supposed to be). After about 5-10 steps or so, reverse the direction, and gradually move back to the starting point. Here's what to look for:

Of course, try to repeat this procedure for all axes. If in any of them, repeat accuracy is worse than 0.01 mm, it may be good to talk to the manufacturer.

2.3.5. Deflection under load

We're almost done! The last parameter of note is the loss of accuracy you can expect if the mill is braking or accelerating rapidly, or aggressively plowing through a difficult workpiece. The value depends on the rigidity of mill frame, and the type and quality of its linear motion systems. If it's poor, there is no reason to despair - but it means that you may have to slow down when doing precision work.

The test here is extremely simple: with the mill on but the spindle turned off, brace the dial indicator against the side of the cutting head, and then use your hand to gently press the spindle from the other side - along the tested axis. Don't overdo it: the goal is to exert may be 20-50 g of force, and not to overcome the holding torque of the motors.

In a quality mill, the momentary deflection should stay under 5 µm or so - but up to 0.02 mm is something you can live with.

2.4. Ongoing maintenance

Once you know that your machine is behaving correctly, there isn't that much that needs to be done on an ongoing basis: it's a pretty sturdy piece of machinery, and it's usually not subject to heavy wear. Consult the manual for manufacturer-specific advice - but in most cases, the rules are pretty simple:

Some of the entry-level mills may be using low-cost brushed motors to power the spindle; such motors are a consumable, and may require a replacement after anywhere from 100 to 2,000 hours - but typically don't cost much. Higher end machines usually rely on brushless motors that should last a decade or more.

Linear drivetrain motors, bearings, and so on are typically not subject to substantial wear when doing lightweight hobbyist work; if properly maintained, they should last pretty much forever. Insufficient lubrication or contamination with abrasive materials (ferrous metals, glass, etc) are about the only things to watch out for.

2.5. Cutter management

If you are planning to do high-precision work, or simply wish to ensure high-quality surface finish when working with organic shapes, it's useful to measure and document the diameter of every new end mill in your collection. Although the tools are usually manufactured with micron-level accuracy, the specifications can sometimes be wrong, and on top of that, manufacturing variations may occur from batch to batch - for example, due to changes in the thickness of applied coatings or the gradual wear of the grinding tool. Case in point: some of my Harvey's 0.04 inch cutters actually measure around 1.022 mm, rather than the expected 1.016 mm. At these scales, such differences can bite.

To perform the measurements, you will need an accurate micrometer. This tool, along with quality calipers, is one of the most important investments you will make, so don't fret: $50 will get you a decent no-name brand, while $140 is enough for Mitutoyo. For two- or four-flute tools, the idea is to gently tighten the jaws of the micrometer around the flutes, while simultaneously rotating the cutter (in the direction opposite to its normal operation) to find the maximum diameter. You need to stop as soon as you feel any resistance, to avoid breaking the tool or gouging the jaws; practice on larger, sturdy end mills before moving on to sub-mm ones.

Three-flute tools are a bit harder to deal with. If the flutes are long enough, you may be able to grip the cutter so that one face of the micrometer is touching the peak points of two flutes, and the opposite face is touching the remaining one. That said, with stub-length tools, you may be essentially out of luck; doing a careful test cut and measuring the result may be the way to go (you need to account for TIR and repeat accuracy).

Beyond the initial measurements, it is also a good habit to re-check your tools after every 10 hours of cutting or so, preferably measuring the diameter near the very tip. When doing heavy cutting, you may see some reduction in tool diameter as the outer edge of the flutes gets a bit more dull; for this reason, I suggest keeping your primary roughing tool separate from the finishing ones.

Tip: even when working with plastics, applying several drops of a cutting fluid to the workpiece can improve finish and limit tool wear by keeping the tool cool and helping remove chips more efficiently - give it a try! For lightweight work, sulfur-free oils, such as Oatey 30200, work best. They are also easy to clean up with a detergent and don't interfere with resin casting work.

Cutting fluids or no fluids, it makes sense to examine your tools for damage and excess wear every now and then. You can't trust your naked eye, but a simple 7x magnifier, selling for under $50, should do the trick. If you have a microscope with magnification between 10x and 50x, that's even better. Here are the two most common cutter failure modes that aren't visible with naked eye - significant wear (left) and a chipped flute (right):

Both of these tools will still work, but the one on the left will not hold tight dimensional tolerances in particularly demanding applications; while the one on the right will produce crummy finish in machined parts and will be prone to gumming up.

Tip: good bookkeeping is incredibly important in CNC work: computer-aided design and creative chaos simply don't mix. Cultivate good habits starting with end mills: make sure that you have a spreadsheet (or even a flat text file) listing all the tools you have, outlining their geometry, and making note of the measurements you have taken.

Having such a list will not only help you avoid surprises, but will also make it easier to maintain a healthy stock of tools - so that you never have to put a project on hold for two weeks after accidentally breaking your last 0.4 mm end mill.

3. Mastering CAD and CAM

By now, you should have a good idea of which mill to choose, where to find the cutters, and how it all fits together... or perhaps you own a 3D printer, and didn't read the previous chapter at all. Either way, the next step is getting comfortable with the software needed to bring your ideas to life. In this section, we'll go over some of the basics, and then proceed with a simple starter project of our own.

3.1. Computer-aided design (CAD)

The primary function of CAD software is, quite simply, to let you design 3D parts. Your CAD application may be just about any general-purpose modeling program, such as Blender - but in the long haul, it makes sense to settle for a purpose-built tool. "Real" mechanical design software offers better control over part accuracy, and comes with powerful data input and analysis tools that streamline engineering work. On the flip side, they usually have less impressive rendering capabilities - and may come with no support for animation, physics, and other perks taken for granted in general-purpose 3D apps.

For now, though, the distinction between general-purpose modeling tools and CAD isn't that important. You simply need to get comfortable with any 3D design software of your choice - and that takes a bit of work.

3.1.1. Wait, but which application to choose?

A-ha, that's a good question indeed! In the previous edition of this guide, I tried to give an impartial overview of the market - but in the end, there is plenty of choice, and very few genuinely bad CAD tools. You can just look around, find the one you like and can afford.

If you just want a simple recommendation - and are willing to spend some money on software to begin with - Rhino 3D is probably the best CAD package that you can get on a hobbyist budget. Students can purchase a fully-featured edu license for under $140, so if you are still in the academia, it would be foolish not to go for it. For mere mortals, there is a heftier price tag attached - $750 - so it's a more difficult call. Still, it's a mature and user-friendly tool that runs well even on low-end systems, and it's just done well - so you probably won't regret it.

Now, if Rhino is priced outside your league, many people in the community are also fond of Alibre Design PE, which sells for about $100. Or, if you prefer not to spend any money at all, and can live with a somewhat clunky app, then FreeCAD looks fairly OK for simpler work.

What else? Several accomplished DIYers use general-purpose 3D modeling tools such as Sketchup or Blender. The free-of-charge general-purpose tools are pretty diverse and mature; the only problem is that they may be less suited for complex work later on. If you are willing to cross that bridge when you come to it, they are definitely worth a try as a starter option.

Last but not least, cash-strapped DIYers may also want to check out one of the "demo" editions of commercial CAD tools. For example, Creo Elements has a modest limit of 60 parts per document, but otherwise, should do the trick.

3.1.2. Some existential CAD advice

We should probably start the lesson with a gentle warning: CAD programs tend to have a fairly steep learning curve. This is in part because you are forced to manipulate 3D objects using a 2D input device and a 2D display - and it takes a while to master that skill. The other problem is that these applications tend to use unfamiliar UI paradigms and obscure terminology - and even something as simple as right-clicking an object may have an unexpected result. It takes some effort to start using the software in a competent way - and if you're just banking on your innate abilities, you will probably learn to do things exactly the wrong way.

If you want to make real progress, here are some rules to live by:

Before you start, there's just one more thing to do - you need to customize the program for precision work. Needless to say, the tolerances needed to design a building aren't the same as when designing a gearwheel - and CAD applications are used for both. For now, simply go through the configuration pages, and try to do the following:

Done? Then let's roll...

3.1.3. Drawing and manipulating simple planar shapes

Your first job is to figure out how to draw several simple, two-dimensional shapes on the X-Y construction plane (i.e., using the "top" viewport of your CAD app). Try do to sketch all of the following:

Practice a bit; perhaps sketch a simplified, boxy outline of a car, complete with wheels.

When you are comfortable with these 2D primitives, it's time for your next exercise: try to draw a smiley face without using your mouse at all. Type in the required commands and specify coordinates by hand; the display grid should be of great help. Oh - for some extra credit, add a hat!

With your drawing in place, it's time to get familiar with several important operators. Figure out how to select one or more objects with your mouse, and then find the commands that perform the following tasks:

Play around with these operators until you are comfortable with the way they work; pay special attention to scaling and rotation operators, and the way they are affected by the choice of the origin, reference point, and the viewport.

As soon as you are done moving, flipping, and cloning stuff, locate and play with the analytic tools that let you do the following:

This is a good opportunity to experiment with object snaps, too: enable them temporarily, and check the various options they offer. In particular, be sure to give tangent snaps a try: draw three random circles, and then try to connect them with tangent lines, like this:

Piece of cake? Thought so! You should be now ready to master several more complicated modeling skills.

Note: Somewhat surprisingly, 2D drawing techniques are more important than any 3D sketching tools; in fact, you should resist the temptation to play with 3D primitives at this point. The bulk of mechanical modeling work is almost always done with spline curves, which are later converted into 3D objects with the help of operations such as extrusion, revolution, or lofting. We'll get to these operators soon.

3.1.4. The inner workings of splines

Your CAD application probably stores every curve as a non-uniform rational B-spline. The visual representation of this mathematical model is not very easy to grasp, but getting a hang of it is essential to any sort of serious modeling work.

In essence, every NURBS curve is defined by three parameters:

An example of a degree 2 spline with no kinks is shown below; control points are marked in yellow, and knots are red:

To practice a bit, try to locate the command that lets you draw a curve of a given degree by specifying subsequent control points. Get a hang of its behavior particularly for curve degrees 2 and 3. There should be also a separate command for creating interpolated curves that go through any number of specified "via" points that you click on, which may be useful for tracing around bitmaps and so forth. Oh, one more thing: using both of these tools, try to create a proper kink!

When you are comfortable with drawing, experiment with the following curve-editing tools:

Last but not least, be sure to familiarize yourself with the "explode" operator that splits curves at existing kinks (see what it does to a rectangle, circle, etc); and the "join" tool that merges adjacent curve endpoints to form a curve with kinks.

3.1.5. Curve trimming

As should be apparent by now, NURBS curves can be separated and joined at kinks in a completely lossless manner: the underlying coefficients do not have to change, and there is no gradual reduction of accuracy if you repeat these operations 100 or 1,000 times.

Unfortunately, in many cases, you will need to truncate curves at locations that do not correspond to any existing kink. In these situations, the CAD application will be usually forced to create a new curve with a different set of control points in the vicinity of the cut. This fitting process usually works very well, and the resulting deviation should be much smaller than your configured tolerance, but you should be aware that the operation does not come completely free.

With that warning in mind, find the command that let you split a curve at a point where it intersects with another one, and experiment with it. For degrees higher than 1, you should be able to see the effect it has on the set of control points for the curve - and if you ask the program to calculate the deviation between the input and the output curve, it will probably give you a tiny but non-zero result. Examine the deviations for a couple of input curves to see what to expect in normal work.

In addition to the curve splitting tool, there may be also a second operator that works roughly the same way, but lets you immediately delete (trim) the unwanted parts. It may sometimes save you a click, so if possible, get familiar with it.

In any case, it's time for a brief exercise. Remember the command for drawing radial arrays? Try to combine that with splitting and trimming to arrive at a result such as this:

Nice, eh? It's not quite a proper gearwheel, but we're pretty close.

Note: as mentioned earlier, it is always preferable to use the simplest editing tools that still do the job. In particular, you should always favor the join, explode, refit, split, and trim operators over any features that, for example, automatically compute unions, differences, or intersections of 3D objects.

Why? Well, these advanced tools quickly go bonkers if you have something as simple as two closely touching, parallel walls - not to mention accidental self-intersections, non-watertight curves or solids, etc. It's one thing if they just fail in an obvious way... but if the resulting error is subtle and goes unnoticed, you may end up having to redo a good chunk of your work later on.

3.1.6. Other curve tools

We're almost done with curves - hooray! But before we go, there are three other, immensely useful editing tools that you should probably learn about:

The fillet operator is applied to any angle (kink) on a curve, and replaces it with a specified radius. The trick is often used for engineering purposes: fillets serve as a stress relief in sharp corners and drastically improve their strength. But even more interestingly, filleting lets you capture the real-world result of machining a corner with an end mill of a given size - which is important for certain types of snap fits.

This illustation shows the use of filleting to determine the actual shape of a machined part:

Note: Of course, it's not that you can't make a competent acute angle with a CNC mill. For example, refinishing the one on the left with a 0.4 mm cutter would result in a deviation of barely 0.3 mm. Still, in many cases, it's more convenient to account for such intrinsic fillets in your designs, than it is to refinish the workpiece with a separate tool.

Oh, one more thing: in situations where you don't control the shape of the mating part (lighter blue in the picture below), you can always resort to bone fillets, too:

Chamfering is a similar operation, but it replaces the selected kink with a straight segment that starts at a specified distance from the vertex. Compared to fillets, chamfering may have a more desirable appearance, depending on the effect you are aiming for - and produces a much simpler 3D mesh.

The last operator, offsetting, offers an interesting way to resize any shape: it creates a derived curve that consists of points at a particular, constant distance from the original geometry. It is quite different from traditional scaling, in the same way that gaining weight is different from growing bigger - and it comes quite handy for generating walls, undersizing or oversizing parts, and so forth.

Well, that's really it! You may also want to explore tools that let you extend or close curves, or blend them with more fine-grained controls - but for most intents and purposes, we are ready to talk about something a bit cooler than that.

3.1.7. Creating simple surfaces

Let's start building three-dimensional solids. It's simple: draw a closed curve, and then find the command that creates a planar surface from it. Examine the result, then check out what happens if you select two curves, one within another. What about separate or intersecting input curves? Examine failure modes of this operator, too: what if one of the input curves is at a slightly different Z level? What if the curve is open or self-intersects?

Of course, the planar surface that you have just created is still, well, pretty flat. The next important operator to play with is extrusion: select your curve, and using the side view, select the extrusion height. Disable the creation of top and bottom "caps": we just need the side walls. Try to use the join operator to merge the flat base surface with the extruded wall. If it worked, you have pretty much mastered 3D work. Lets try a slightly more complicated exercise:

That's pretty easy, eh? In fact, it's a lot simpler to construct shapes this way than it is to start with 3D primitives, and clumsily piece them together. But don't let it go to your head. Spend a few more minutes playing with extrusion: see what it does to open or self-intersecting curves, to multiple curves that overlap or aren't at the same Z level, and so on.

You should also take this opportunity to experiment with the explode, split, and trim operators that you remember from your curve editing days. Draw two solids and make them overlap, then see if you can use the trim operator to remove all the overlapping internal sections, and join the resulting outer shells into a new, closed shape. Can you do the same to "subtract" one solid from another, or only keep the intersection? Find the analytic tool that lets you calculate the volume of each of the solids, and use it to double-check that your results check out. Oh, one more thing: can you trim surfaces with curves? Why not give it a try!

These tools aside, there are several other advanced surface editing operators that you should be aware of; they are particularly important if you want to create organic shapes without having to think too much:

The list of useful operators goes on - for example, it's sometimes useful to extract edges from surfaces or solids, to place or remove holes, or to orient something on an oddly-shaped surface - but you can discover these on your own. For now, let's just have a quick look at how to manage your work.

3.1.8. Working with multiple objects

It's one thing to sketch a nice 3D box - but any medium-scale project may easily consist of several hundred of such solids, often meshing closely and stacked in creative ways. If you don't manage your virtual workspace well, you will get overwhelmed, make mistakes, or both.

The document management tools at your disposal will vary from one program to another, but you should familiarize yourself with at least the following:

As soon as you are reasonably comfortable with object management, we can actually make a simple mold!

3.1.9. Practice time: let's show some love!

Okay, it's time for some fun. Let's start by trying to draw a heart:

Here's what's going on in that illustration:

Voila! Easy, right? Now, let's join all the curve segments, and turn the whole thing into a mold:

See if you can figure out all the steps on your own. The final result should be a watertight box with dimensions around 33 x 30 x 8 cm. Its bottom left corner should be at coordinates (0, 0), and the top surface should be at 0 mm, likewise. The mold cavity should have a 4 mm clearance around the heart, and should be 7 mm deep. The heart itself should be resting at the bottom of the cavity, and should be 4 mm high (its top surface should be at -3 mm).

If everything checks out - well, the good news is that you have yourself a master mold (aka a pattern). Now, we cut some corners with the moldmaking process, given the simplicity of this project, but this is how the entire thing usually plays out:

Be sure to save that project - we'll need this file soon.

3.2. Computer-aided manufacturing (CAM)

CAM software reads the geometry created with your favorite modeling application, and turns it into toolpaths that can be sent to a milling machine, a 3D printer, or some other automated tool. All the computer-controlled manufacturing technologies use a common set of underlying concepts and have a comparable degree of complexity, but there are many details that remain specific to a particular tool. Since we have to choose one way or the other, the rest of this chapter will focus on the software designed for CNC mills.

3.2.1. Shopping for the right application

When I first published the original version of this guide, the makers of CAM applications catered almost exclusively to commercial users. The software was ridiculously overpriced, shipped with archaic UIs, and always included a collection of mind-boggling bugs. Thankfully, the emergence of low-cost mills and serious hobbyists is slowly changing that. Still, there is no single package that would be a sure bet for all users, and you need to understand what sets them apart. Here's a quick list of the things that matter the most for everyday CNC work.

3.2.1.1. Support for fourth axis

If you own a computer-controlled rotary axis, or plan on getting one, you should figure out which 4-axis machining modes are supported by the program - and decide if they are worth the extra price. The three fundamental choices are:

What to buy: your call. If indexed cutting is all you need, you may be able to live with lower-cost three-axis CAM.

3.2.1.2. 3D milling strategies

Vendors of CAM software tend to be pretty creative about the range of specialized and obscure machining modes that they offer, but there are only three options that really matter; the rest is either redundant, or doesn't work well in complex molds.

Here are the three methods in question; the first two really must be supported by the application you want to go with:

What to buy: the two first strategies must be supported unconditionally, and should come with no strings attached: you should have full control over how they are used and when. The third one is a plus. Advanced features of these modes, and other toolpath generation methods, matter a lot less.

3.2.1.3. Region selection capabilities

Efficient machining of complex models requires the ability to decide which tool and which strategy will be applied to various regions of the model - and in what order. To facilitate this process, your CAM application should let you define a freeform region for every machining operation, either by selecting an existing boundary curve imported alongside with the 3D model - or by drawing something by hand. It should let you exclude regions within an existing selection by picking a second curve inside the first one, too.

These capabilities are pretty much all that matters. You may notice that certain programs have the ability to automatically generate selections based on some property of the model - for example, to only machine flat surfaces, vertical walls, or something in between. These features are sometimes nice, but are usually more limited or less reliable than what you can do in CAD - so don't pay any special attention to this.

What else? Oh - several programs offer "residual machining" of regions left over after the previous operation with a larger tool. This sounds great in theory, but is typically both fairly coarse and extremely slow; I tried two applications that supported this feature, and it misbehaved or ran out of memory more often than it proved to be useful.

What to buy: curve-based selection is a must. Pay little or no attention to the rest.

3.2.1.4. Cut optimization

Some CAM applications do not put any special thought into organizing the toolpath segments that are spewed out by the underlying geometry-analyzing algorithm. Depending on your model, the result of that can be pretty inefficient; for example, a waterline cut that sequentially machines every outline on every layer before moving further down will produce a total of 15 operations when machining these two separate holes (left):

The image on the right shows a more sensible ordering: instead of going back and forth between the machined regions, finish one first, and then move to the other. The result is 9 operations, consisting mostly of short movements. Quite a few programs are capable of such common-sense optimizations - but not all.

What to buy: it is useful to shop for CAM applications that offer cut optimization, especially for waterline machining. It's not an essential feature, but it's a major time saver. If in doubt, consult the manuals or ask the vendor before buying.

3.2.1.5. Arc interpolation

G-code - the language supported by most of the CNC machines on the market - has several commands that can be used to execute circular, spiral, or helical movements of the tool. While support for these opcodes is not required, quite a few embedded controllers recognize them, and translate them to motion optimized for the hardware they are hooked up to.

CAM programs, however, usually convert input files to polygon meshes, and generate toolpaths where arcs are broken into thousands of small, linear movements. This practice is not just a waste of electrons, but may result in lower machining speeds, because it hinders the ability for the controller to plan ahead and pick the best acceleration and braking strategy. Perhaps more annoying is that if the pitch of the generated 3D mesh doesn't overlap with the hardware resolution of the motors in your CNC mill, needless vibration may be introduced, too.

What to buy: If your machine supports arc interpolation, it is good for the CAM program to know how to generate G-code that leverages that. It's not a deal-breaker, but something really nice to have.

3.2.1.5. Cut direction control

There are some brief moments in the cutting process where the tool may be pluging head-on into uncut material. Most of the time, though, the tool simply works to widen existing pockets, and engages the material one side at a time. There are two principal configurations possible during that task:

In the conventional (aka upcut) mode, contact with the workpiece is made on the left side of the tool, in relation to the direction of movement. Toolpaths generated this way allow the material to be engaged very gradually, starting with a near-zero chip load, and ramping it up. This strategy is less dependent on the rigidity of the tool and the workpiece, and results in superior dimensional accuracy and surface finish with rigid plastics and many other types of easily machinable materials.

In the climb (downcut) mode, the orientation of the tool in relation to the workpiece is reversed, and every flute plunges into the material to start at the maximum chip load, and then have it taper off. This approach puts greater strain on the tool - and if the tool or the workpiece deflects, it reduces the dimensional accuracy of the final part. That said, it may offer better cutting speeds or improved surface finish in certain annoying materials - such as metal alloys prone to work hardening, or soft, malleable thermoplastics.

In waterline cutting, maintaining a specific direction of cuts comes naturally, and at no extra cost; in fact, it takes special effort to alternate the type of cut between each layer. Overhead projection strategies are more problematic: if the projected pattern is just a bunch of parallel lines, the only way to achieve consistent direction is to cut the first line, raise the tool, move back to the starting side, lower the tool, and make the next cut - in short, there's a lot of non-cutting movements. And that's the major advantage of "offset", spiral-like projections that we have mentioned earlier on: they permit the direction to be maintained without having to raise the tool.

What to buy: you should make sure that the software gives you full control over the direction of cut for every machining strategy of note. Not having this ability will make your life miserable, especially when working with miniature, long-reach tools that flex easily.

3.2.1.6. Input and output formats

Last but not least, the application you end up choosing needs to support the file formats that can be written by your CAD package. The most common interchange formats include IGES and STEP (both of which are vendor-agnostic); DXF and DWF (originating with AutoCAD); 3DM (Rhino3D); and STL (3D Systems); support for at least one of these is good news - and the more, the better. NURBS surfaces are supported natively in all of these except for STL and DXF, where approximate tessellation may be required.

Input formats aside, you also need to confirm that the application actually supports your CNC mill, and can generate suitable output files. If your machine speaks G-code, and the application offers the ability to create custom G-code postprocessors, you may be able to find a third-party converter or write your own one with relatively little effort; otherwise, you need to make sure that the mill is supported out of the box.

3.2.1.7. ...and all the things you don't have to worry about

Here's a short and incomplete list of things that tend to appear in product specifications and may seem important, but typically don't matter much:

Of course, feel free to ping me if you stumbled upon any other cool-sounding feature, and need help figuring out what it's worth.

3.2.1.8. All right - so which CAM package is good for me?

Well, it's complicated. For starters, your CNC machine may come with a basic CAM application, and that package may turn out to be good enough for starter jobs. For example, Modela Player 4 - the application that comes with Roland mills - is pretty decent. That said, in the long haul, you will probably want to upgrade to something more featured and flexible.

Somewhat lamentably, there aren't that many free-of-charge applications that would be easy to use yet powerful enough; FreeMill is one potential choice, but it only supports a single, rudimentary machining strategy - and really, isn't worth much.

This leaves you with commercial tools. If you are on a tight budget, you may want to check out MeshCAM ($200), which is actually a very competent package with good technical support. Other alternatives within that price range include Cut 3D, or the hobby license for DeskProto. I have tried both DeskProto and MeshCAM, and both of them are pretty good - so these would be my top picks.

Other than that, there are quite a few other reputable choices with prices hovering around $1,000 and more. The pricing in the "pro" segment, especially for more featured packages with 4-axis machining, is still rooted in the era of strictly-industrial CNC - but deep discounts are available to students, and sometimes, to those who buy the software along with the mill itself. Notable applications in this category include VisualMILL / RhinoCAM, Alibre CAM Mayka, madCAM, and quite a few more. If money is no object, or if you can get a good discount, you may want to dig deeper. If you need advice, I've seen VisualMILL in action, and it looked pretty good.

Tip: many of the commercial packages have demo versions. These demos are either limited to a 30-day trial, or lack the ability to write G-code, but are otherwise fully-featured - so be sure to check them out before you buy.

3.2.2. First minutes with CAM

CAM programs can be counterintuitive; you will probably need to read the manual carefully to even understand how to create a toolpath, or write the resulting NC file. These applications are also often finicky, unforgiving, and buggy - so it's important to take it slow, and check everything twice.

Before loading your 3D models, you need to configure the software for your particular machine, either by selecting the appropriate postprocessor, or - in the worst case - by writing your own based on an existing config file for a similar mill (not as scary as it sounds, as long as your machine came with a code reference manual). You should also go through all the configuration options, and make sure that units are set to millimeters, that the tolerance is 1 µm or better, that arc interpolation is enabled if supported by your hardware, and that the G-code coordinate system is set to G54 (the first, default user-configurable coordinate system, and the only one you will be using in hobby work).

With this out of the way, you should load the geometry to be cut. Let's use the heart-shaped mold created earlier on; if you need to export it from your CAD application to a mesh-based format, such as DXF or STL, be sure to configure meshing tolerance to 1 µm or better for that step, too, as this setting is often separate from the global value.

Once the model is loaded, you need to verify its position. For three-axis work, the mold cavity should be facing up, and the top surface should be aligned with the Z=0 plane in the CAM application; when looking from the top, the bottom left corner of the mold needs to be at X=0, Y=0, too. If it wasn't loaded this way, you can move or rotate it as needed, but also try to figure out what went wrong: is your original CAD model oriented correctly? If that's not the reason, is there a CAM-level option you should have toggled at import time?

Well, all right. Once the model is in the right spot, it's time to create some toolpaths next.

3.2.3. Roughing toolpaths

To maintain sanity, it helps to split the cutting process into several phases. Each of these phases accomplishes a different task, and may use a different tool, feed speed, machining strategy, stepover distance, and so forth. The first phase of almost every project is known roughing; its goal is to remove the bulk of the material as efficiently as possible. Of course, astute readers may ask why this isn't also the final step - and the answer is that, quite simply, heavy chip loads and rapid tool speeds offer limited accuracy; it's better to leave a small margin of uncut material, and refinish the surface in a separate process later on.

The usual roughing strategy is a hybrid approach where the model is cut in Z layers, and on every layer, the application first uses a pattern of horizontal movements to clear the pockets, and then performs a waterline-type cleanup pass. Once the first level is done, the toolpath advances to the next one:

In some applications, roughing may not include that cleanup pass, in which case, you should probably configure a separate waterline step right after roughing. This is to ensure that the margin of remaining material is reasonably uniform, so that chip loads don't fluctuate wildly later on.

Tip: good CAM applications let you use pocket-shaped offset cuts, rather than linear movements; this completely eliminates the need for the waterline cleanup pass. If that mode is offered by your software, take advantage of it - it will speed up the process and also let you maintain a consistent direction of cuts.

In any case, for the materials used in this guide, you should configure the roughing process the following way:

Well, that's pretty much it. Once the roughing operation is set up, simply tell the program to calculate the toolpath, and examine the result closely. Make sure that it actually makes sense: is the tool staying within our virtual workpiece? Do the toolpaths look anything like what we discussed before? If the program can give you estimated machining time, is it less than 10 minutes or so?

If everything checks out, congratulations; now, let's finish this thing.

3.2.4. Finishing toolpaths

Finishing toolpaths typically refine the geometry in several consecutive steps:

  1. The vertical (Z axis) margin on flat surfaces is removed using a projection toolpath that has a zero tool height offset, but preserves the original tool diameter offset from the roughing process.

  2. The X-Y plane margin around vertical features is removed using properly spaced waterline cuts, configured with the true diameter of the tool.

  3. If there are any sloped surfaces, they are locally refinished using a tightly spaced projection toolpath or 3D draping, possibly with a ball end tool (this is not applicable to our model).

  4. If there are any holes or tight pockets that couldn't be faithfully reproduced with the current cutter, they are selectively refinished with a smaller one.

To take care of the first item on this list, you'd normally want to configure an offset-type, projection finishing toolpath with the following parameters:

This step cleans up the flats; now, we need to take care of the walls. This is done with a "pure" waterline toolpath that traces around vertical shapes, and does nothing more. The parameters for this step should be:

Everything else should be the same as for the previous step.

Well, that's almost it. After this toolpath, the heart should be looking pretty much the way we designed it - with just one minor blemish: center kink in the middle of the heart will have a slight fillet on the top, due to our relatively large 3 mm tool not being able to fully squeeze into that tight spot. To fix this problem, we can perform a selective waterline cut with a 1 mm end mill. Most of the parameters don't change from step #2, except:

And... well, that's it! Generate the toolpaths and pat yourself on the back.

For future reference: here's the list of maximum recommended cutting parameters for the tools you should have bought. These values are applicable only to several classes easily machinable prototyping materials that we're going to talk about soon - when working in more demanding stock, you will need to slow down:

Speeds reduced by 20-40% are recommended for finishing operations, especially with 3 mm and 6 mm tools.

3.2.5. Final sanity check

Measure twice, cut once. Before sending any data to the machine, it's always good to recheck your work; 5 minutes of that may save you several hours of troubleshooting in the physical realm. Here are some questions to ask yourself:

Try to use that checklist until you are reasonably comfortable with the process; it's not that mistakes are common, but they can strike at inopportune times.

If everything checks out, you need to get the toolpaths ready for cutting. In some programs (e.g., Modela Player), the output is done directly from the application - in which case, just sit tight. In many other apps, the data is written to a text file, and that file is then sent to the machine using a separate CNC utility; if so, export ("post") the toolpaths now. Of course, remember to save the toolpath for the 1 mm tool in a separate file!

3.3. Please, let's cut stuff already!

Okay, okay - but first, you need a suitable workpiece. We will cover more permanent options shortly, but for your initial tests, I strongly recommend getting something called machinable wax. It is a hard, rigid, wax-like substance, technically a blend of low-density polyethylene (LDPE) and paraffin. It is not the cheapest or most durable stock, but has two important properties: it machines quickly to a very good finish with excellent dimensional accuracy; and more importantly, because it's much softer and more fragile than common plastics, even fairly major mistakes won't immediately result in a broken tool.

In the States, the material is available cheaply from the folks at MachinableWax.com. I would suggest ordering their set #17-424215 (featured on this page). It's a box of 12 pieces, 42 x 42 x 15 mm, selling for $21. Don't overdo it - this really isn't the best or the most cost-efficient material for real work, so get just a single box or two.

The only other thing you need in advance is a way to secure the wax to the milling table. Your mill may have come with some sort of a clamping system, or at least slots or screw holes that let you rig something together after a quick trip to the hardware store. Alternatively, you may want to consider MWHS01 from High Tech Systems ($100), which is a very versatile and lightweight system of clamps that should work for almost any medium or large CNC mill. That said, when working with easily machinable materials and using small tools, you can also get away with strong, thin double-sided tape, such as 3M 444 or Tesa 4965. Simply place four short strips in the corners of the workpiece, and press it down; then lift one of the corners to detach.

3.3.1. Setting up a job

Assuming you have all the necessary supplies, we can mount the workpiece at this point. Wipe clean the table and the stock material, apply the double-sided tape as described earlier, and attach the workpiece to the table, making sure that it's entirely within the working area of the mill. Depending on the design of the machine and the length of the tool, you may need to put something underneath the workpiece to allow the cutter to reach it, too.

Next, confirm that the workpiece is actually attached securely, and install your 3 mm tool in the collet. Tighten the tool holder, quickly confirm the TIR, and make sure that there is some clearance between the tool and the top of the workpiece when the spindle is fully retracted. Assuming everything is fine, you can now turn on the mill and let it initialize.

Caution: I am assuming that, as requested earlier on, you have familiarized yourself with the operating manual of your mill, and with the safety tips information included in section 7 of this guide. As with any power tool, you can get hurt if you are careless.

For G-code mills, the next step is to set the coordinate system to G54, then move the tool to align it with the bottom left corner of the workpiece (looking from the top), plus about 5 mm. With the tool at this X-Y location, tell the machine to set its X and Y origins; this is done in a hardware-specific fashion.

Setting the Z origin is the last hurdle to deal with. Find a convenient location on the workpiece, in a region that won't be removed in the cutting process. If the machine has a tool sensor, simply place it underneath the tool, and follow the manual to perform the measurement. Without a sensor, you can use the trick we talked about earlier on: place a thin strip of paper or foil on top of the workpiece, and wiggle it back and forth while lowering the tool; use increments of 0.05 mm when you are close, and stop the moment the tool pinches the foil. Set the Z origin in that location; oh, and try not to pinch your hand instead!

Done? Well, in that case, it's time to put on safety glasses, perhaps put several dabs of cutting oil in the region to be cut, and then hit "send" and output the 3 mm roughing and finishing toolpaths to the machine. With any luck, several minutes later, the machining process should be wrapped up. Vacuum off a bit and inspect the result; is the shape in line with what we're expecting? If yes, let's fix the kink: install the 1 mm tool, check TIR, redo the tool height measurement (very important), and hit "send" to output the remaining toolpath on your list.

In case you are wondering, there are very few things that can go wrong with the cutting process itself, and all of them should be fairly self-explanatory. For example, if the machine starts cutting at an unexpected location, or too high / too low in relation to the workpiece, you probably messed up setting the origins, selecting the right coordinate system, or had the CAM model positioned incorrectly. Similarly, if the tool breaks while cutting in the expected location, this is probably due to grossly incorrect RPM, feed speed, or cut-in depth; or bad ordering of toolpaths. But chances are, you won't run into any issues just yet, so sit back and relax!

Note: as hinted earlier, for more complex jobs, it's useful to keep the roughing end mill and the finishing cutter separate, even if both processes call for the same diameter of the tool. This way, you don't have to worry about the loss of micron-scale accuracy as the roughing end mill begins to wear - and you will be able to use both tools for a lot longer.

3.3.2. Inspection and troubleshooting

As soon as you're done, detach the mold and vacuum in thoroughly. Machinable wax sometimes requires a gentle but firm cleaning with a brush and a touch of compressed air to get rid of the somewhat sticky chips - so do that, and then have a closer look. Is the surface silk smooth? Do you see any pronounced tool marks or gouges on vertical surfaces? What about horizontal ones? Do the dimensions check out when you use a caliper? (In more complex projects, pin gage sets are useful for measuring the diameter of small holes, too.)

Chances are, you're in good shape - but if anything is even slightly wrong, now would be the right time to track the problem down; don't wait until you're working on more time-consuming projects, or in less forgiving materials. Here are several causes of the problems you are most likely to see:

Still, in all likelihood, the result will be fine. If that's the case, let's wrap up by reviewing some common-sense rules that come handy in mold design, just so that you are well-equipped to work on your next CNC project - and then, let's cast a mold.

3.4. Things to avoid in mold design

There are relatively few real constraints on what you can cut on your milling machine, but there are several simple design strategies you should follow to keep the cutting process simple and quick, and make accurate, durable, and dimensionally accurate molds. Here's the gist of it:

Well, that's it. It may sound overwhelming, but once you catch a whiff of it, even remarkably complex multi-part molds are fairly easy to crank out. We'll cover this and several other advanced design topics later on - but for now, let's have some fun with polymers!

4. Resin casting and you

As discussed in section 1.3 of this guide, resin casting is a pretty amazing, simple, and user-friendly process that comes equally handy in CNC prototyping, 3D printing, and in manual DIY work; all its sophisticated uses aside, you will end up using it to replace broken parts in appliances or toys, make unique gifts, or even encapsulate backyard flowers or bugs (don't deny it).

Alas, the online market for moldmaking and casting supplies is dominated by several companies that cater chiefly to artistic users, and sell expensive products with poor mechanical properties and little utility in high-precision engineering work. In that spirit, even if you are familiar with artistic resin casting using epoxies, polyester resins, or polyurethanes from Alumilite, Smooth-On, and similar sources, you will be probably surprised by how much better your results can get.

4.1. Raw materials

4.1.1. Machinable stock

Well, before we dive into the world of casting resins, we should briefly revisit the choice of materials you can use to make master patterns. This part of this chapter is specific to projects that rely on CNC milling, so if you are interested in replicating manually crafted or 3D printed parts, you may want to skip ahead a page or two (this site provides a good overview of how to build patterns by hand - and if you have any other questions, feel free to drop me a mail or stop by /r/resincasting).

Still here? All right! Of course, milling machines are not particularly fussy, and will cut almost anything that is softer than tungsten carbide, but rigid enough to stay in one place; still, some materials are more predictable than others, and produce better results. Prime choices include rigid engineering plastics such as polyurethane, epoxy, polyester, ABS, polyamide (Nylon), or acetal (Delrin); many varieties of hard woords; aluminum, brass, and other soft metals and alloys; and more exotic choices such as printed circuit boards, hard waxes, plaster, etc.

Conversely, common materials that machine with greater difficulty or offer sub-par surface finish include stringy, low-melt thermoplastics (PET, polycarbonate, some grades of polyethylene); rubbers and other stretchy or squishy polymers (including some grades of PVC and most polystyrene foams); plywood and particle boards (including MDF); and exceptionally hard or highly abrasive stuff, such as steel, stone, or glass. Of course, many of these materials can be still cast or formed using CNC-machined molds and dies.

Poor choice of working materials is one of the most common mistakes made by hobbyist machinists; quite a few people stick to workpieces that offer poor accuracy, get damaged easily, gum up the tool, or simply cost way too much. For moldmaking purposes, your best bet is one of the little-known materials: an extremely accurate, low cost piece of plastic known as a medium-density modelling board, originally devised for the automotive industry. It's essentially a mix of medium strength polyurethane, and a combination of soft fillers such as calcium carbonate and aluminum hydroxide. There are many types of machinable boards, but the one we are interested in has a density of about 0.70-0.78 g/cm³, and vaguely resembles wood:

Prototyping boards of this particular variety include Huntsman RenShape 460 (or sligthly less dense BM 5460), Axson ProLab 65, Sika SikaBlock M700, BCC MB2001, Necuron Necumer 651, and several more. The material is typically sold in bulk, in sheets of about 50 x 150 cm, 25 mm thick. This may sound like a lot, but I recommend buying a full board, rather than grossly overpriced cut-to-size bits. The material lasts me for about a year, and costs about $12 per liter (roughly $250 for the whole thing). It's much less than what you'd pay for a similar slab of HDPE or acrylic - and it machines easier, too. (Planks of dense hard wood, when glued together and planed, may be a cheaper alternative for uncomplicated parts.)

Buying prototyping boards is actually pretty easy. If you are in the US, you can simply go to Freeman Supply, and order RenShape 460 online (search for item #075229). In other places, simply look at the manufacturers' websites and find local distributors, then send out several e-mails or make some calls (online ordering isn't common in the industrial world). Be aware that prices may vary significantly, so shop around.

Alongside with the board, you may want to order a matching board repair putty; it's a fast-curing, polyester compound that can be used to fix minor damage to your molds, or even completely fill a previously created cavity to reuse a particular workpiece for a new project. If you are ordering online with Freeman, go with their Quik-Fil; otherwise, ask the distributor for a matching product - they will be able to advise.

What else? Oh, about the only minor drawback of the medium-density boards is that they have a very fine but perceptible grain, as shown in this magnified image:

This grain normally has no appreciable effect on dimensional accuracy, but imparts a satin finish that will transfer to any transparent, water-clear parts. Of course, you can create high-gloss molds by coating the pattern with paste wax or a similar sealer (carnauba wax is particularly good); or you can always simply polish the final part - but both these options affect dimensional accuracy, and can be annoying when working on complex molds. The alternative is to use a more expensive material known as a tooling board - made out of solid, dense polyurethane, with no perceptible grain. Boards such as RenShape 5169 or BM 5272 cost up to 50% more, need to be machined 20-40% slower, and cause some wear to the tool - but they scratch this particular itch.

Note: RenShape 460 is relatively easy to cut with a hand saw; in fact, it's comparable to soft woods. That said, the extra labor may be annoying in the long haul, so it makes sense to have a decent jigsaw nearby. You can get one for around $35; blades designed for hard woods will cut the material very quickly and last for years.

4.1.2. Silicone rubbers

In order to replicate the parts laid out inside your pattern cavity, you need a flexible and durable substance to take an impression of the desired shape, and use it as a mold for the final product of your work.

There are several types of castable rubbers that could be useful for this purpose, but silicones are hard to beat. There are quite a few formulations that combine ease of use, excellent mechanical properties, perfect dimensional accuracy, no odor, no toxicity, and temperature resistance up to 300° C. On top of that, silicones come with an inherently non-stick surface, which helps greatly in casting work.

Almost all the silicone formulations you can find on the market come as a viscous goo consisting of long, linear, partly polymerized chains of siloxanes; that nominally non-reactive soup is then combined with a suitable cross-linker and a catalyst. The reaction between these components quickly turns the goo into a very bouncy solid; this can be initiated in several ways:

In other words, you almost certainly want to stick to RTV-2 platinum-catalyzed silicones, unless you are working on life-sized castings (at that point, the cost of silicone can become prohibitive).

Before discussing specific products, let's have a quick look at the notable characteristics that will come up in product datasheets for these rubbers, and review their significance to our work:

All right, that's it! Other parameters are either uninteresting, or are not advertised consistently. To help you with the selection process, my top recommendations for mechanical projects would be:

You should pick just one of these; if you're undecided, go with QM 262 or P-592. If you need other options... well, Silicones Inc and Quantum are the most interesting US-based companies I know of. Other choices include Polytek, GT Products, BJB, and Smooth-On - but in my opinion, their selection is much less impressive, and the options I have tried pale in comparison with the ones mentioned on the recommended list.

US market aside, globally, Bluestar Silicones (Rhodia) is fairly ubiquitous; you may also want to check out ShinEtsu, Wacker, Zhermack, Huntsman, Axson, or Dow Corning - depending on where you are located, they all offer some interesting choices. But then, one reader living in Norway reported that placing an international order with Hobby Silicone for QM 262 actually turned out to be much cheaper than locally available alternatives (whoa).

As far as pricing goes, platinum cure silicones cost around $35-$45/liter when bought in one liter cans, or about $30-$35/liter in 4-5 liter pails; for example, P-592 costs around $120 for 4 l, while QM 270 and QM 262 in 5 l quantities fetch $140 and $170, respectively. Unreacted liquids should survive at least 2-3 years without significant deterioration, as long as you keep them in tightly closed containers, away from sunlight, moisture, and excess heat - so getting a full pail is not a bad idea.

Note: resin manufacturers in the States use a somewhat confusing scheme for describing the size of their two-component kits: "1 gallon kit" usually means that you are getting about one gallon of whichever component is needed in greater quantity; and a matching amount of the other one. If the mix ratio is 10:1 (as is the case with most platinum cure silicones), the gain is minimal - but for resins mixed 1:1, you are actually buying two gallons or so.

To further confuse you, the same does not apply to sizes specified in pounds - "15 lbs" means that you are getting just enough to cast a 15 lbs blob of plastic or rubber. Be sure to account for these differences when shopping around: a lower price is not always a better deal.

4.1.3. Rigid polyurethanes

All right, let's talk about the materials you can employ to actually make final parts!

Polyurethanes are an incredibly interesting and versatile class of two-component, addition cure polymers. They use two principal reagents, mixed in comparable quantities: a non-volatile isocyanate and a complex alcohol (polyol). Some formulations trade some or all of the polyol for a polyetheramine, resulting in a material that is more properly called a polyurea. In any case, the two components are usually combined with variable amounts of chain modifiers, usually chemically similar to the primary polyol; and possibly surfactants, plasticizers, fillers, and so on. The whole thing is then catalyzed with a wide variety of organometallic compounds (bismuth, zinc, tin, zirconium, aluminum, or similar); with tertiary amines such as 1,4-diazabicyclo[2.2.2]octane (DABCO / TEDA); with diazoles such as 1,2-dimethylimidazole (DMI); or with something pretty close to that.

Modern polyurethane chemistry lets you manufacture everything from soft foams, to high-performance rubbers, to faithful, often superior imitations of many other rigid engineering plastics - all that without having to go bankrupt on injection molding equipment, and while using only fairly safe and predictable chemicals. They greatly outperform more familiar resins, such as epoxies or polyesters, and in hobbyist workshops, are much less dangerous to work with.

The only downside of high-performance polyurethane systems is that they generally require a basic vacuum rig - a small pump and a suitable container to remove any dissolved gases from the mix. Products that do not require degassing are readily available, but usually don't perform as well as their peers. (We'll talk about the required harware later on, but it's pretty compact and doesn't cost a lot.)

Anyway - if you are aiming to make functional prototypes, it is probably prudent to start by stocking up on a polyurethane resin that lets you produce hard, rigid, and shock-resistant parts. Once more, let's have a look at some of the key things worth highlighting in a datasheet:

All right, ready for some recommendations? Here we go:

At this point, unless you have a specific itch to scratch, it's perfectly OK to order just IE-3075 directly from IPI. Their "1 gallon kit" (actually around 6.5 l) sells for about $110. "Quart kits" are also available if you need to try it out first.

If you want to shop around, I don't think it makes sense to look beyond Innovative Polymers - not if you're in the States; they have a remarkable selection of unique, user-friendly products designed specifically for manual casting, and great customer service. They take direct orders, and also have several local distributors. If you're skeptical, you can have a look at products from Smooth-On, Alumilite, BJB Enterprises, or Freeman - but you will not find anything that even comes close to that selection. If you are in Europe, checking out Huntsman and Axson is not a bad plan.

As with silicones, polyurethane resins are pretty stable and have a fairly long shelf life - but they are fairly sensitive to sunlight, humidity, heat, oxygen, and moisture. It is a good practice to buy several 100-250 ml polypropylene or HDPE bottles (example) for intermediate storage of the amounts of resin you plan to use within a month or so - and keep the original containers sealed and blanketed with inert gas. The inert gas can be just a burst of "canned air" (difluoroethane or tetrafluoroethane), but if you want to save money in the long haul and have some room in your workshop, it makes more sense to invest around $150 in a small nitrogen tank, a regulator, and a hose. This setup will last for months, and it's only about $10 to refill at a nearby Praxair location or so.

In any case, if you store the resins properly, you can expect most of them to maintain their properties for at least 2-3 years; manufacturers usually give much more conservative guarantees, but take them with a grain of salt. Remember to agitate the containers if the components separate. Oh, some formulations may crystallize if kept below 10° C or so, but this process can be reversed easily. Prolonged storage in crystallized form is not advisable, as it may lead to the formation of insoluble dimers.

Caution: although most of the polyurethane formulations you will encounter are reasonably safe, there are some unfortunate exceptions. We'll talk about this a bit more in chapter 7 - but for now, definitely watch out for:

You won't find any of the problematic components in resins from Innovative Polymers - but other manufacturers sometimes show less restraint. Request and study material safety datasheets (MSDS) when in doubt.

4.1.4. Flexible polyurethanes

Flexible polyurethane elastomers are an interesting alternative to silicones. You don't necessarily need to buy any, but they may come handy if you wish to make functional rubber parts, and your silicone is not pigmentable, or has insufficient strength.

Key advantages of these rubbers include 30-50% lower price, and much better performance toward the upper end of Shore A (above 40 or so): several times higher tear strength, much less pronounced tear propagation, excellent abrasion resistance, lower coefficient of thermal expansion, lower viscosity, and the ability to pigment the system as seen fit. On the flip side, polyurethanes exhibit some exotherm-caused shrinkage in larger castings, and adhere to many other plastics, making them less desirable for creating negative molds; they also tend to have slightly worse rebound characteristics, and limited temperature resistance (they get soft around 70-90° C, and deteriorate somewhere between 150 and 200° C - so casting low-melt metals, for example, is out of question).

The parameters to look for in these compositions are similar to these for silicones, with the exception of pot life and viscosity - here, the advice provided for polyurethanes is more pertinent. When reviewing the datasheets, pay close attention to the ratio of tensile strength to elongation at break, because there can be striking differences in the rigidity of various products, especially those rated 70 Shore A or above; some of them are only somewhat flexible, and will not be suitable for making parts such as tires, rollers, or transmission belts.

Note: This goes both ways: not all Shore A polyurethanes are particularly rubbery, and not all Shore D polyurethanes are necessarily very rigid. For example, some 60 Shore D polymers are highly elastic and can be stretched up to three times their original length, even though their surface feels hard as nails. They resemble the rubbers used in certain garden hoses, shopping cart wheels, etc.

As for recommendations: you should go with Innovative Polymers HP-21xx series; I've tried quite a few other products, and nothing else comes close. They are relatively inexpensive ($25 per liter) and feature superb "true rubber" mechanical characteristics, long pot life, and good cure profiles. For example, HP-2170A is a super-stretchy 70 Shore A rubber with tear strength of 42 kN/m, far surpassing most silicones. There are also softer variants, down to 50 Shore A (HP-2150A); and more rigid but still surprisingly flexible ones, up to around 60 Shore D (HP-2160D). In fact, it's possible to blend them to achieve intermediate properties as needed for a particular project.

All the products in the HP-21xx line take several days to polymerize at room temperature; if that's too slow and heating them up is not an option, you can add some separately purchased catalyst to get overnight cure with no real trade-offs.

4.1.5. Epoxies, polyesters, and so on

If you have dabbled in resin casting before, chances are, you used epoxy or styrene-based polyester resins, rather than polyurethanes. These options are popular with hobbyists because of their broad availability, low price, and less onerous processing requirements (i.e., less sensitivity to mixing ratios, moisture contamination, etc). That said, I think it's a bad idea to use these materials in precision casting work, for a couple of reasons:

Of course, I don't want to demonize these polymers. Polyesters are sometimes useful for bonding and repair applications; and epoxies are extremely useful as high-performance glues and laminating or potting resins. Both epoxies and polyesters can be superior in lay-up composite applications, too, in part owing to their improved bonding capabilities. You can have a look at the products sold by Freeman Supply, or check out the low-cost, water-clear epoxies available from Polymer Composites Inc; just don't expect them to be a sensible match for PU for the processes discussed in this guide.

4.1.6. Pigments and dyes

Adding colors to your castings is fairly easy. One option is to find an artist store, and shop for dry, non-toxic organic pigments. Such pigments will work equally well with polyurethanes, silicones, and just about anything else - but may be relatively painful to disperse or to blend with any accuracy. The other choice is to purchase coloring pastes where pigments are already dispersed in a non-reactive (plasticizer) or reactive (polyol) medium - but these won't work in silicones.

For dry pigments, Kremer is probably the best source online. For reactive dispersions, you can ping Innovative Polymers (send them your picks from this RAL chart); Eager Plastics has a pretty good selection of non-reactive pigments and dyes, too.

Beyond that... selecting your palette is a matter of personal preferences, but here are some quick tips:

If you need inspiration, here are some nice picks:

A small bag or bottle of a pigment will cost somewhere between $5 and $20, and should last for years.

Tip: You can, of course, opt to paint your parts instead of adding pigments to the resin; acrylic and polyurethane lacquers can be used alike. For adding text or other ornamental elements to machined parts, you may also want to consider a low-cost vinyl cutter, such as Silhouette SD or Roland Stika SV-8. These devices are fairly affordable, and the results look amazingly good - especially if a layer of clear coat is applied on top.

If another cutting machine is too much, you can also simply equip your CNC mill with a specialized drag knife to get comparable results.

4.1.7. Other additives

Adding colors aside, many other properties of cured plastics can be altered in profound ways by introducing certain easily available, low-cost additives. It's almost impossible to provide an exhaustive overview of all the available choices, but several use cases are definitely worth calling out:

Of course, multiple types of fillers can be combined; in particular, it may be useful to add some glass fibers to lightweight materials filled with hollow glass spheres, to maintain acceptable flexural strength. If you are wondering what to buy up front, it's not a completely bad plan to get some Scotchlite iM30K, plus 0.8 mm glass fibers, and a bit of DPGDB (also useful for preparing pigment dispersions and such). Other fillers are not nearly as essential.

4.2. Casting workshop

Now that we have the selection of resins, pigments, and fillers sorted out, it's time to briefly chat about the workshop equipment you will need to get the ball rolling. The list isn't particularly long, but even when it comes to something as inconsequential as mixing cups, making the wrong choice will unnecessarily complicate your life.

4.2.1. Vacuum pump and chamber

Insufficient mixing may cause a range of problems with finished parts - but vigorous stirring will almost always introduce some air into the resin. This problem aside, bubbles of air may get trapped inside mold crevices as you pour the mixture in - even if your mixing skills are beyond reproach. Last but not least, some resins may simply liberate some amount of dissolved gases once the polymerization reaction begins; IE-3075 is an example of that. Vacuum degassing solves all these problems, and is not as scary as it may sound.

Even if you are on a tight budget, you should get a low-cost vacuum pump capable of getting pulling around -1000 mbar of vacuum (that's about 10 mbar absolute, or -29.5 inches of mercury). I ordered mine from VIOT: click here for an entry-level model that sells for around $100, and should work fine; I have a $140 model with a higher flow rate, and it served me well. You will also need some sort of a vacuum chamber to hold the mixing container and the mold itself; it's possible to rig something together on your own, for example using a sturdy cooking pot and a cover made out of thick polycarbonate - but low-cost vacuum dessicator chambers work fine in that capacity, and start around $50 or so. If you are planning to work on relatively small projects, or opted for a pump with a relatiely low flow rate, Bel-Art #420100000 is a good choice; otherwise, model #420430000 will accommodate larger pieces, too.

(For a bit more money, you can also find some good-looking purpose-built degassing chambers from several sellers on eBay.)

About the only other piece of this puzzle is a hose to connect the pump with the chamber. I use 1/4" Kuriyama K7160 Polyspring, which I ordered from these guys; it's about $4 per meter. Other hoses can be used, too, but they need to be vacuum-rated, which in practice means that they need to be either fairly rigid, or reinforced with a metal spring to avoid collapsing as soon as you turn the pump on. Beyond that... well, getting some vacuum grease for o-rings and other parts of the vacuum chamber is not a bad plan (link); it also comes handy for several other purposes every now and then - for example, for preventing caps on resin bottles from seizing in storage.

4.2.2. Auxiliary tools

It goes without saying that for any sort of precision work, you need a reasonably well-equipped workshop to begin with. I am assuming that you already have that - and that basic tools such as clamps, needle files, or several grades of fine sanding paper, are always within reach.

With that in mind, there are several less obvious, minor items that will be useful specifically when casting mechanical parts, and that we haven't mentioned before:

That's about it! For working with small quantities of resin and pouring it into complex multi-part molds, you may also find it useful to get a box of single-use, two-element 10 ml syringes with no rubber seals (e.g., from eNasco), and some blunt-tip dispensing needles.

4.2.3. Mold releases and other useful chemicals

Mold release is a material that forms a protective barrier between the mold and the resin you will be pouring in, and makes it easier to demold final parts. The use of mold releases is optional when casting silicones in polyurethane patterns, or polyurethanes in silicone molds, because these materials don't adhere to each other in a particularly strong way - but still, a properly selected release agent makes demolding easier, and prolongs the life of any mold by reducing its exposure to reactive chemicals. On top of that, if you ever want to cast polyurethanes in polyurethane molds, or silicones in silicone molds, a robust adhesion barrier is simply a must.

I have tried many different mold releases over the years, and my top pick is, unquestionably, Stoner A324; this spray-on agent beats most of the silicone, PTFE, or zinc stearate releases that I have tried before. They sell it for about $5 per can, and this should last you for about 1-3 months of hobby work. Stoner ships for free if you order a full box of 12 cans ($60).

If you can't obtain this product in your market, silicone-based mold releases are your second best choice, although some varieties may gradually permeate and swell silicone molds. Releases based on mineral oils, PTFE, zinc stearate, polyvinyl alcohol, and so on, usually don't perform well in high-precision, vacuum-assisted casting work. The manufacturer of your silicone rubbers may be able to recommend some specific, locally available picks.

That aside, you may want to also grab a wax-based mold release: they dry to form a hard, polishable, permanent layer that not only serves as a barrier, but should improve the surface aspect of polyurethane patterns. Low-viscosity brush-on formulations, such as Synlube 531, should work well; more viscous liquids, such as AdTech MR-1, may need to be diluted with naphtha when working with intricate shapes, but it's not a hard thing to do. Hardware store paste waxes that contain carnauba wax (link) work fine for simple shapes where dimensional accuracy is not critical, but high-gloss finish is desired - and if you dilute them to a water-like consistency, you can use them for complex models, too. In all cases, once the wax is dry, you can simply buff it with a soft cloth or a brush.

Note: waxes should not be applied to silicone molds, because they will inevitably crack and peel off - and the solvents used are usually damaging to silicone rubbers, too. In general, with any new mold release, always test it by soaking a piece of silicone in it for about 5-10 minutes; any substantial or permanent change in dimensions would be a reason for concern.

Mold releases aside, you may find it useful to get some of the following, largely optional chemicals:

4.2.4. Optional: temperature-controlled oven

Temperature-controlled ovens are not essential in casting work, but they let you perform several time-saving tasks:

Low-cost toaster ovens are the obvious solution, but somewhat unfortunately, they have extremely poor temperature accuracy and stability, especially on the lower end of the scale. If you are an accomplished DIYer, you may be able to grab one and simply equip it with a more accurate, digital temperature control circuit. That said, if you are willing to part with a few hundred bucks and have some floor space, you can also try getting a real laboratory oven, such as this one ($400); hot air sterilizers and dryers may offer a smaller and slightly more affordable alternatives in some markets, too (example, around $300).

If you don't have that much money, or simply don't have enough room, don't worry; you can do just fine without this piece of equipment, provided you are willing to wait a bit longer for your castings every now and then.

4.2.5. Optional: pressure pot

There are some complex, multi-part molds where it may be hard to consistently avoid air entrapment, even with the aid of vacuum; on top of that, there are some resins that tend to be difficult to fully degas, or that will develop bubbles of carbon dioxide when not mixed perfectly well, or when exposed to residual humidity. That latter set of problems is particularly evident in mercury-free water clear poulyrethanes, such as the vanilla version of Innovative Polymers OC-5086.

To improve your odds when dealing with such tricky cases, it helps to have a pressure pot; the idea is to increase ambient pressure surrounding the mold to about 3-4 bar, thus crushing and dissolving back any existing bubbles, and discouraging the formation of new ones. Sure, it's a brute-force solution - but can you argue with its results?

Pressure casting equipment is more bulky, more expensive, and somewhat more dangerous than vacuum pumps (due to much higher pressure differentials) - and for most part, isn't necessary; get it only if you have plenty of room, and you are either forced to work with water clear resins that don't tolerate non-pressurized casting, or you are willing to spend at least $200 to improve your yields a tiny bit (say, from 85% to 97%).

If you want to take this route, pressure pots start around $80 (link); nicer ones fetch as much as $300 (link). To operate it, you will also need a compressor - and these start from $100 from fairly noisy units from the hardware store, to $180 for relatively quiet-running ones; standard pressure hoses and fittings add another $20 or so.

4.3. Your first casting projects

All right, all right - enough with all the theory and shopping tips. It's time to dig out that wax pattern that you have made before, and turn it into a finished work of art! This section is all-text, but if you need visual aids, this photolog is probably good to look at.

Caution: similarly to many workshop and household chemicals, casting resins can be harmful if misused. In particular, they may react violently when mixed with incompatible substances; cause severe irritation or lasting damage if splashed into your eyes; and will emit dangerous vapors if overheated, burned, or intentionally aerosolized.

Please refer to product safety datasheets (MSDSes), and to section 7 of this guide, for an overview of known risks, material handling recommendations, and disposal procedures. Do not proceed with any hands-on experiments until you have done so.

4.3.1. Making a silicone mold

Resin casting is fun, but you need to remember that once the components are mixed, the reaction will proceed no matter what; it's important to plan accordingly: read this section fully and memorize all the steps beforehand, and have all the necessary supplies and information within reach.

First, you need to estimate the amount of resin you will need: for CNC-machined or 3D-printed shapes, simply ask your CAD application to calculate the volume of the master, and subtract this value from the volume of a dummy box of the same height, width, and depth. To have a comfortable margin, add about 15% to the result, or 10 ml, whichever is greater. Then, multiply that volume by the density of the resin (check the datasheet; it's usually between 1.1 and 1.4 g/cm³). The resulting figure is the weight of the material you need to prepare.

With these calculations out of the way, prepare the following stuff:

Although accidents are unlikely, you should still try to minimize potential damage: if there are any LCDs or other expensive gizmos in your workshop, consider moving them a bit farther away, or covering them with plastic sheeting. Don't wear your best clothes, and if you have a carpeted floor or expensive hardwood, cover the area that is most likely to suffer in case of a spill.

When you are ready to go, place the mixing cup on a scale, then tare it. Agitate both components of the resin in their original containers, and then use a clean tongue depressor or a spoon to pour about 20 g of silicone into the mixing cup. Add a suitable amount of catalyst (the ratio is usually 10:1, but check the datasheet), start the timer, and begin mixing thoroughly for honest 3 minutes; be sure to repeatedly scrape the sides and the bottom to avoid leaving any unmixed resin in these spots.

Next, place the cup under vacuum; the mix will initially rise as the bubbles expand, and then collapse back; you should keep it under vacuum for another 1-2 minutes past that point. If the resin gets dangerously close to overflowing during the initial rise, simply release the vacuum (i.e., yank out the hose), and try again; several cycles of that should do the trick - and next time, use a larger cup.

The next step is to pour some of the resin into the mold created in chapter 3; use about half of the required volume or so. Place the mold on a strip of aluminum foil, and put that under vacuum; this will help the resin conform to even the most intricate shapes with no effort on your end. After about 2-3 minutes, you can slowly release the vacuum, pour the remaining amount of resin (use a bit more than necessary to get a convex surface at the top of the mold), and set the whole thing aside for several minutes to allow any bubbles to rise to the surface (or collapse back into the resin). If there are any stubborn bubbles on the surface at that point, you can apply a gentle burst of compressed air to get rid of them.

Finally, cover the entire contraption with a flat, clean sheet of polypropylene; lay it down gradually, starting at one side, to avoid air entrapment. When done, weigh it down with something reasonably heavy - around 500 g should do - and brace the whole thing, so that the cover doesn't slide off. Be sure to check the timer at this point - has the entire process taken more than about 12-15 minutes? If yes, why?

In any case, give it about 12 hours or so, until the resin remaining in the mixing cup is firm and tack-free. If you are impatient, placing the mold in a temperature-controlled oven at around 50° C will cut the curing time down to 1-2 hours or so - but don't go too high, given that this particular master is made out of wax. When the rubber is ready to demold, pull off the cover, and then use a dental hook or a similar tool to pry the rubber off near the corner of the mold. Remove it fully and inspect the result. if it looks flawless - as it should - you may want to briefly post-cure it at around 100° C for 30-60 minutes, and in the meantime, pat yourself on the back!

Here are several questions that may be on your mind:

Okay, okay - time for some polyurethane fun!

4.3.2. Casting plastic parts

Polyurethane casting is not dramatically different from working with silicones, but you have to be swift, and pay more attention to detail. Quite simply, with a resin such as IE-3075 or TP-4052, you will probably only have about 6 minutes to go through all the steps - so there is no time to look around for paper towels or a mixing stick.

In general, before starting, you should go through all the preparation steps outlined in the previous section; and when done, you should also do the following:

With all the preparations taken care of, you are ready to roll. Place the mixing cup on the scale, tare it, and pour the required amount of isocyanate; in our case, 10 g will do. Next, gently pour the appropriate amount of polyol (for IE-3075, this will be 8.9 g), start the timer, and begin mixing very thoroughly, frequently scraping the walls. Most polyurethane resins, IE-3075 included, require at least 90 seconds of mixing to progress from a dispersion to a proper solution when mixed in a small quantity - and if you stop sooner, the cured material may not cure properly. Do it right.

When done, place the container under vacuum, and turn on the pump. The resin should take no more than about a minute to rise and collapse back; if it's taking much longer, your vacuum chamber may be too big, or the pump may be inadequate or malfunctioning (e.g., due to an open gas ballast valve, or due to contamination). If, on the other hand, everything went as expected - and the bubbles have collapsed in a timely manner - you should now pour some of the resin into the mold cavity (to fill it roughly halfway, taking care to cover any detail where air entrapment is likely), and place the mold under vacuum for another minute or so. Don't worry if it never stops bubbling at this stage; that's OK.

After a brief round of degassing, release the vacuum gently, place the mold on a previously prepared flat surface, and add the remaining resin, until it overflows and forms a convex surface (this is important - otherwise, air entrapment is a lot harder to avoid). Grab the polypropylene cover and carefully lay it on top of the mold, using the technique outlined for silicones. You should brace the cover against something, so that it doesn't slide off, and weigh it down with around 200 g (larger molds can be clamped with several kg of force, but this particular one is relatively easy to squish).

Well, that's it! Before you go, check the timer; if the process has taken more than six minutes, you should figure out how to improve it. In any case, leave the mold alone for at least 3 hours (or more, depending on the resin used), and when you come back, confirm that the material left over in the mixing cup is tack-free and hard as nails. Next, gently flex the polypropylene cover to detach it from the part, and extract your casting from the mold.

Hopefully, the result is perfectly fine; that said, the likelihood of mishaps is higher for polyurethanes than it is for silicones - so if something isn't right, don't despair. Here's a quick summary of the most common issues I have seen:

If you are seeing any issues, it's important to narrow the problem down right away, while the number of variables is still fairly low. If you are out of ideas, don't hesitate to ping me at lcamtuf@coredump.cx; I may be able to help. A good place to discuss your experiences or showcase your work may be /r/resincasting, too!

Anyway - if everything turned out to be just fine, you may want to briefly post-cure the part and any leftover material. Place it at around 100° C for one hour, and then play with it to get a sense of its physical properties: try drilling a hole in or scratching the surface of one of the leftover bits, and see how hard it is to break it.

4.4. Miscellaneous polyurethane casting tips

This section is just a quick a collection of random notes that should come handy in real-life projects, but that I wanted to keep out of your first casting job. Enjoy!

4.4.1. Meta: not all resins are alike

Before we dive into various advanced topics, you should know that there are significant differences in the handling characteristics of various polyurethane resins, even if the advertised cure times and ultimate physical properties of the material are roughly the same. In particular, be aware of the following:

Because of all these striking differences, don't take everything you see in this guide as universally applicable to every formulation on the market; and in the same vein, don't expect your own experiences with product A to be fully applicable to product B. If in doubt, request a sample of any new product you are considering, or simply ask.

4.4.2. Adding pigments and fillers to resins

Let's start with something simple. Non-reactive (i.e., plasticizer-based) liquid dyes and coloring pastes can be mixed into the working amount of isocyanate, before adding any polyol; when taking this route, just try to stay under 2% by volume (around 8-10 drops per 10 ml); if you find yourself routinely having to add more, consider switching to dry pigments or a higher-yield dye or a reactive carrier - because past this point, solvents used in the dye will be affecting the properties of the part.

Reactive coloring pastes designed specifically for polyurethanes use a polyol as a base; that's the case for pigments from Innovative Polymers. Their main benefit that they can be added at much higher levels without completely messing up the properties of your parts. In principle, you should subtract the weight of the added dye from the required weight of polyol - but in practice, this varies from one formulation to another. In a quick experiment with IE-3075, I found that using the nominal amount of polyol results in improved strength. Results in other resins may vary.

When working with dry pigments, there is a bit more legwork involved. If you simply dump the pigment unceremoniously into the liquid, it will probably clump together - and stay that way. To avoid this, you need to place the desired amount of material in an empty mixing cup, tare it, and start adding isocyanate drop by drop, mixing constantly, until you end up with a homogenous, runny paste (siloxane surfactants can make the process easier, too). Once the paste looks good, you can gradually add the remaining isocyanate while constantly mixing - and you should be all set.

Tip: if you have a high-yield, hard-to-disperse powdered pigment that you keep coming back to, it may make sense to make a custom coloring paste for future use. Simply disperse it thoroughly in an inert plasticizer (e.g., dipropylene glycol dibenzoate, discussed earlier; silicone oil works for platinum cure rubbers) or in a suitable polyol, and pour that into a dropper bottle.

Of course, as noted earlier, you should ensure that the material is moisture-free; in tricky cases, premixing the resin or adding zeolite should help. Glass-based fillers, such as Scotchlite or milled fibers, may benefit from being pre-treated with a silane coupler, too.

4.4.3. Premixing the system

Premixing is one of the simplest and lesser known tricks that can help solve many of the problems that crop up in some polyurethane casting jobs. For example, it can dramatically reduce shrinkage without affecting cure time; lower the risk of cure inhibition; halve the time needed to fully mix the resin in small batches; and greatly reduce the sensitivity to moisture, to the point of making many systems suitable for surface coating applications. These benefits stem from the reduced reactivity and improved compatibility of a partly polymerized liquid. It's not a silver bullet, but for many formulations, it's pretty close to being one.

The only price to pay for premixing is an increase in viscosity, which typically isn't a big deal if you have started with a low-viscosity system such as IE-3075; and the added expense of about 5 minutes of work and about one hour of waiting per every batch that you intend to cast.

If you want to try it out, the recipe is very simple: measure the desired amount of isocyanate, add all the fillers and dyes you want to have, and then introduce between 10% and 20% of the necessary amount of polyol; this mix needs to be stirred thoroughly, degassed - and then stored in a covered cup, blanketed with an inert gas, for about 1-2 hours. At that point, the remaining polyol can be mixed in, and the resin can be cast.

Keep in mind that premixed resins will have short shelf life: the viscosity will keep increasing, and in presence of a catalyst, the isocyanate will more aggressively react with ambient humidity and deteriorate. You should premix only the amount you intend to use right away.

Oh - in case you are curious, this graph shows the impact of premixing on the exotherm for 10 g of IE-3075, and how it compares to the use of fillers. The measurements were taken using a thermocouple submerged in a small, insulated polypropylene cup, approx. 30 mm in diameter:

The X axis is time in seconds. The Y axis is temperature in °C. The resin and the room are initially at around 20 °C.

4.4.4. Adjusting room-temperature cure speed of existing resins

Here's another tidbit you won't find in any other hobbyist reference on resin casting: it is possible, and in fact fairly easy, to chemically slow down many polyurethane systems to significantly reduce shrinkage; and to accelerate slow-curing ones to get your parts sooner or have fewer artifacts in thin sections of your molds. Why bother, you may ask? Well, it not only saves you money, compared to buying several resins for different applications - but perhaps more interestingly, it enables you to come up with custom-tailored cure profiles that are of no commercial interest to the manufacturer.

In essence, there is a wide variety of catalysts used in castable polyurethane resins. Every catalyst behaves differently: some are highly active at room temperature, some kick in only later on, when the resin has warmed up due to exotherm. Some are better at driving the early stages of polymerization, but stop shortly thereafter; some have a sustained effect until the very end. Some are highly selective toward the desirable isocyanate-polyol reaction, and some don't mind catalyzing the isocyanate-water reaction - which leads to the formation of bubbles of CO2. Some are very stable, and some deteriorate when exposed to open air and other substances, which may cause inhibition or poor surface cure. But there is no single product that gives you the very best on all fronts.

For this reason, manufacturers combine various catalysts to reach a compromise that makes sense for their intended customers - but these parameters aren't necessarily ideal for your needs. For example, OC-5086 is a resin designed for larger castings; when dealing with tiny parts, it will cure too slowly, and with far too much sensitivity to ambient moisture.

Thankfully, you can fix this on your own.

4.4.4.1. Speeding things up

Ideally, if you wish to use OC-5086 or HP-21xx - or accelerate any other finicky resin - you should get bismuth neodecanoate from Santa Cruz Biotechnology or Krackeler. The cost is around $35 for 250 g, and that amount will last you forever. The catalyst isn't dangerous, but both of these places have a blanket policy of shipping only to commercial addresses. If you can't have it shipped to work, ping the folks who run Chemsavers.com - they should be able to get it for you and ship it to your home for a very modest premium.

Bismuth neodecanoate is a syrupy liquid which needs to be diluted with a plasticizer (e.g., DPGDB), a suitable polyol, acetone, or something else of that sort. Depending on the resins you are working with, you may have to experiment with dosage, but typically, levels between 50 and 500 ppm will be enough. For example, to "fix" OC-5086, you can prepare a 4% solution in plasticizer, and add it at about 1-2 drops per 10 ml of isocyanate as you are getting ready to mix it with a polyol. To speed up HP-21xx, you will need a solution closer to 50%, added in similar quantities.

Now, if this particular bismuth compound is hard to find where you live, don't despair! A decent alternative is tin(II) 2-ethylhexanoate, also known as stannous octanoate. This substance is commonly sold as an accelerator for condensation-cure silicones; for example, a nearly pure form is available under brand names such as Smooth-On Accel-T, Quantum Silicones QSil STO, or Bluestar VICURE #2. Just be careful not to buy anything based on dibutyltin dilaurate, dimethyltin dineodecanoate, or a similar tin(IV) compound: they will work great, but also happen to be a lot more toxic.

As with bismuth, the appropriate dosage varies depending on the resin; for OC-5086, a 4% solution, added at 1-2 drops per 10 ml of isocyanate, is a good starting point. Note that the compound is a bit more harmful than bismuth - handle it with care.

Bismuth and tin aside, there are several other, more exotic options to choose from. They may offer very specific benefits, such as improved curing characteristics in particular systems, or no subsequent inhibition of platinum silicones (which are somewhat sensitive both to bismuth and tin). If you need additional guidance, click here to expand a section with some rough notes.

Random rant: as noted above, most suppliers of lab chemicals are no longer willing to ship to residential addresses. Such restrictions make some sense for haz-mat materials - but the companies simply won't do any business with you, even if all you're trying to buy is salt or glucose.

There are two reasons for this. First, there is a growing number of government agencies - ranging from DHS, to DEA, to CPSC (yes, that's right!) - that don't want people to make anything ranging from illicit drugs to bootleg fireworks. Companies that sell to individuals face a hodgepodge of regulations and vague reporting requirements, and risk police raids and other serious consequences if they mess up. Second, there are liability concerns: if a kid loses an eye and his parents sue - well, even if the manufacturer prevails in court, there are still legal expenses and bad PR to deal with.

Because of this, it simply makes no sense for most of them to cater to the hobbyist market at all - shipping to a commercial address creates a pretense of due diligence, no matter how weak it may be.

To keep chemistry alive as a hobby, I urge you to support the remaining few places that did not succumb to this trend; in particular, consider going with eBay sellers or friendly outlets such as Chemsavers even if you have an opportunity to order certain chemcials directly from the manufacturer for less. Just stay away from ScienceLab.com.

4.4.4.2. Slowing down the reaction

In many types of polyurethane formulations, it is possible to slow down the reaction by converting the catalyst to a less ractive complex. In particular, systems that rely on amine catalysts (and do not contain reactive amines as crosslinkers or any other vital components of the formulation) can be slowed down with strong, non-oxidizing acids that react with the catalyst to form a largely inactive ammonium salt. In the same spirit, some of the less obnoxious thiols and certain other substances can chelate a variety of organometallic catalysts.

In products such as IE-3075 or OC-5086, you can get good results with p-toluenesulfonic or methanesulfonic acid, both of which are available from Chemsavers for around $20. Methanesulfonic acid is slightly more convenient, because it is liquid at room temperature; but p-toluenesulfonic acid is pretty easy to directly dissolve in polyol. Levels around 0.1-0.5% by weight should have a very pronounced effect; just be careful not to go overboard: excess acid may react with isocyanates and mess things up.

This graph shows the impact of p-toluenesulfonic acid on the curing exotherm of IE-3075, using the setup outlined earlier on:

If PTSA or MSA are not easily available in your market, a much less potent but possibly still acceptable alternative is sulfamic acid, a common cleaning compound available on eBay and Amazon for just a couple of bucks. The main problem with this compound is its relatively poor solubility in polyols and in most organic solvents. A saturated solution in n-methyl-2-pyrrolidone (NMP) may be your best bet; it will need to be added at a level closer to 1-2% by weight, which isn't exactly ideal. The solution is also not stable in the long haul, so prepare only as much as you intend to use in a couple of days.

What else? If you really can't get any of the above, you can try tartaric acid. Along with several other weak, aliphatic hydroxy acids, this compound will inhibit the reaction to some extent, although it shows some interest in side ractions that may liberate bubbles of carbon dioxide or impart a yellowish hue to your parts. On the upside, it's a common food additive, available pretty much everywhere; and it can be easily dissolved in acetone.

Note that PTSA, MSA, and sulfamic acid are all highly corrosive; use gloves and eye protection whenever working with concentrated solutions.

Oh, one more thing: keep in mind that while adding catalysts to a resin is guaranteed to make a positive difference, adding a particular inhibitor is not. Of the fast-curing resins discussed in this guide, I never found a way to significantly slow down TP-405x, but almost everything else seems to be a fair game.

4.4.5. Blending several resins together

Every now and then, you may be hoping to modify the properties of a resin in a way that goes beyond what's possible with non-reactive fillers, plasticizers, and so on. Other times, you may be interested in changing its cure speed in a situation where the methods outlined in the previous section are impractical or simply don't work.

Well, the good news is that you can do quite a bit without resorting to making your own formulations from scratch. First of all, if you have two resins with comparably reactive isocyanates or polyols, and similar catalysts, you may be able to simply mix them together as-is. For example, let's say that you own HP-2150A and HP-2160D, and want to create a range of tough elastomers. The mix ratio is 100:43 for the first resin, 100:20 for the second one, and you want to blend them at a ratio of roughly 2:1 to get a rubber around 70 Shore A. In this case, suitable mixing amounts may be:

Of course, if the systems are based on dissimilar chemistry, the resin may cure prematurely, not cure at all, or have disappointing mechanical properties. Even in this case, not all is lost: you may be able to get somewhere by starting with a single resin, and then partly or completely substituting one of its components with that belonging to another system. There are situations where it won't work, and situations where it will give you useful materials with faster or slower cure profiles, and mechanical properties somewhere between these of its constituents.

The challenge with this second approach is figuring out the correct mixing ratio for isocyanate coming from product A, and polyol coming from product B; the manufacturer won't tell you how many reactive NCO and OH groups are there in each of the components, and without this information, you have to resort to trial and error. The correct ratio is usually between 100:30 and 80:100, and you can pinpoint it by doing several tests and selecting the range that resulted in the highest indentation hardness (Shore D durometer costs about $25-$50 on eBay); guessing the ratio within 5% should be fine in most uses.

4.4.6. Heat-accelerated cure

As noted earlier, polyurethanes and silicones begin polymerizing the moment you mix the components; by the time you reach the demold time, the reaction is mostly over - but some cross-linking may continue for many days or weeks at an exponentially decaying rate. As this process goes on, the properties of the part will keep approaching these advertised in the datasheet.

If you are impatient and want to demold your castings sooner than normally possible, but don't want to sacrifice pot life or deal with chemical catalysts, placing the mold in a temperature-controlled oven will typically cut the time in half per every 10° C over ambient. Alas, the combination of significant thermal expansion of silicones (0.025%/°C), and the somewhat lower but still noticeable expansion of rigid polyurethanes (0.005%/°C), will probably affect dimensional accuracy of the part - so if you are aiming for snap fits, it makes sense to keep the mold at room temperature for as long as you can, and then bake it at no more than perhaps 40° C.

For an already demolded part, post-curing is a valuable process that involves fewer trade-offs, and lets you reach the final properties of the material in hours, rather than weeks; since the resin is already largely polymerized, and is not confined in an expanding mold, its own thermal expansion is less likely to have lasting effects. It's important to ramp up the temperature gradually, though, so that the part doesn't get too soft. I suggest one hour at 40° C, followed by 30 minutes at 60° C and 80° C; the cycle can be wrapped up with 1-3 hours at 100-110° C. Note that many polyurethanes begin to deteriorate around 150° C, and that for transparent formulations and flexible rubbers, this limit may be even lower.

4.4.7. Multi-part molds

All right, all right - enough with chemistry. But there's one more topic that may help you in casting work. Sooner or later, you will need to make parts with complex features on multiple sides. When replicating hand-made shapes, the process is usually very intuitive; for example, the geometry can be submerged in a blob of silicone that is carefully dissected with a scalpel or a box cutter, and put together later on.

The process for designing accurately meshing multi-part molds in CAD software isn't much more complicated, but may take some effort to wrap your head around it. Let's say you want to make a part with a cross-section as shown on the left (and some additional features that prevent us from simply laying the shape on its side):

The first step is to make a regular mold similar to what we would do for one-sided parts, but also add a small pedestal around the geometry - this will serve as a registration mark. The second mold is simply the same part and its pedestal, flipped around; this top mold will neatly slide into the bottom one. Voila!

In more complex molds, the parting line may be located less conveniently, and may not allow all the air to escape on its own. In these cases, the mold will typically have a sprue through which the resin is poured in, and strategically placed vents to allow the air to escape from tight spots; a reservoir of resin on top of the spruce will offset for shrinkage in large parts, too. All in all, it's not hard, but when it comes to that, you will need to practice a bit.

5. Essential parts for robot builders

Righty-o. Now that we have the basics of machining, moldmaking, and resin casting covered to a good extent, it's useful to discuss more practical designs and part geometries of interest to robotics and other mechanical work. But before we dive into the inner workings of custom-designed gears and drivetrains, it's important to take a brief detour, and go over some of the prefabricated components that may come handy in your projects. Without a good source for tiny screws, springs, dowel pins, or cheap electronic sensors, you simply won't be able to get far.

Rest assured, this chapter will merely scratch the surface of it all. If you are looking for inspiration, get "Machine Devices and Components Illustrated Sourcebook" by Parmley, or "Mechanisms and Mechanical Devices Sourcebook" by Sclater and Chironis; and if you need a primer on electronics, my concise guide to electronics for geeks may come handy, too.

5.1. Mechanical components

5.1.1. Screws and threaded rods

Machine screws, also known as bolts, are one of the most important items to put on your shopping list: if you want to create durable, serviceable designs that can't be approximated with indiscriminate use of glue, suitable fasteners are simply a must. In tough plastics such as polyurethanes, the use of screws is particularly easy: simply machine a slightly undersized hole and drive the fastener into it, impressing its own thread onto the part. Much of the time, you don't need a nut!

If you want to shop for useful sizes and quantities of machine screws, skip your local hardware store. There are three particularly good online sources for miniature and subminiature fasteners in bulk: Micro Fasteners, Fast Metal Products, and Amazon Supply. Micro Fasteners is a good all-around source for low-cost screws in diameters over 1.5 mm or so; FMP offers decent pricing on fasteners smaller than that. Last but not least, Amazon Supply (formerly Small Parts) tends to be a tad more expensive - but Amazon Prime customers get free two-day shipping on every single nut and bolt, so especially for small orders, it's quite a good deal.

The exact selection of fasteners depends on the projects you intend to pursue, but I recommend starting with a good assortment of 0.8 mm, 1.5 mm, and 2 mm screws (000-120, 0-80, and 2-56 designations in ANSI UTS, respectively), 100 pieces each. You should grab the cheapest variety of steel or brass screws, aiming for lengths around 4, 6, 8, and 15 mm; drive type doesn't matter a lot - could be slotted, Phillips, or hex. Expect to pay around $2-$5 per 100 pieces for common diameters, and closer to $10-$15 for sizes under 1 mm. Getting some nuts and washers, especially for 2 mm screws, is not a bad plan - but as noted, you won't be routinely needing them.

You may also want to look into threaded rods, available from sources such as Amazon - the diameter around 2 mm probably being the most useful. Their more boring use is an extended-reach screw (with one nut at each end); a more interesting possibility is creating extremely compact and simple linear motion systems, like so:

Another possible arrangement is using a motor to directly rotate the shaft. In both cases, the transmission enjoys a very high ratio, because every turn of the motor moves the nut by a distance equal to the pitch of the shaft - often in the vicinity of 0.5 mm or so. The downside is poor efficiency - likely under 20% - due to significant friction under load.

Note: when it comes to online retailers, many hobbyists also love McMaster-Carr as a source for screws and other mechanical components. That said, they are almost always significantly more expensive than specialized distributors, and often more expensive than Amazon. They do have good deals on some otherwise hard-to-find stuff - say, miniature timing belts - but it's more of an exception than a rule.

5.1.2. Dowel pins, rods, and tubes

Traditional dowel pins are rather unassuming: they are just pieces of featureless, cylindrical steel, machined to tight tolerances. For their appearance, they find a truly surprising number of uses: as axles for spur gears and other rotating parts; as registration pins for molds and multi-part assemblies; as movement limiters and contact sensors; as serviceable torque couplers; and so on. You just need to have some - trust me on that.

Non-tapered, solid metal dowel pins are available from many sources, including Small Parts / Amazon Supply, and cost very little - usually in the vicinity of $4 to $8 per 100 pieces. I suggest stocking up on 2 mm diameter pins in several lengths ranging from 4 to 20 mm. For high-precision work, 1 and 1.5 mm diameters may come handy, too.

Dowel pins aside, it's also good to have some vanilla steel rods or tubes: they are very cheap (usually $1-$2 per meter), and can be cut to size with a hand saw to build anything from long-reaching axles (left) to fairly complex frames (right, also showing threaded rods used as linear motion systems):

Metal bars with rectangular, hexagonal, or L-, I-, or T-shaped cross-sections are particularly useful for torque transfer, because you can simply slide components onto them, and there is no risk of slippage under radial load; perfectly round profiles may require the application of glue or the use of a lock screw.

5.1.3. Springs and spring wire

Similarly to dowel pins, springs have quite a few uses; many of them are obvious (wheel suspension and other pre-tensioned mechanisms, energy storage, etc), but some aren't. For example, springs are indispensable for transfering rotary motion at an angle - a process that otherwise requires complex bevel gears or universal joints.

Perhaps the most common sort is a compression spring: it has generous spacing between its coils, and is meant to contract under load. You can find them inside many types of pens, spray bottles, and so on. The other popular type is an extension spring: it is tightly wound, and offers little or no compressive action - but stretches very well.

It's difficult to recommend a particular selection of compression and extension springs up front, but it's definitely a good idea to have a robust variety always available in your workshop, simply to prototype stuff easily. Possibly the best and least expensive assortment I have seen so far is this set - 200 reasonably sized springs for less than $9; comparable kits are also available on Amazon. A great selection of individual springs with specific diameter, pitch, and length, can be also found on Amazon, usually in packs of 10.

Traditional springs aside, you should also grab some spring wire (also known as music wire). It comes handy for making contact sensors (especially whiskers!), for creating simple tensioners, and for designing other devices where you want to use a straight piece of elastic material to deflect effortlessly, and then spring back to its original shape. There are many low-cost assortments you can find on the Internet - and as usual, Amazon isn't bad.

5.1.4. Ball bearings

There are many situations where it is desirable to constrain rotary movement to a particular axis of rotation, and support it so that the part doesn't wiggle back and forth, or snap under load. Sleeve bearings are the simplest solution: you can route the rotating part through a round, slightly oversized opening, and perhaps use a bit of grease to minimize friction.

Alas, this approach has its limits: if the part is rotating very quickly, or if it's subject to significant radial forces, sleeve bearings will result in significant power losses or excessive wear. In particular, sleeve bearings for propellers and wheels may have a very limited lifespan.

Because of this, you should get a decent assortment of ball bearings, and use them when appropriate. There are many sources of bearings on the Internet, but most of them tend to be pricey; VXB.com is a notable exception to this rule. They ship internationally and have an amazing selection of 10-, 20-, 30-, or even 100-packs at sensible prices - often hovering around $1 to $1.50 per piece. Some comparably good or even better deals can be found in the $0.99 discount bin or the 10-pack-section of Boca Bearings, too - although their "regular" prices are higher than VXB.

Some of my favorite bearing sizes (ID x OD x H) are: 3x6x2 mm (link, $1 a piece, only for miniature projects); 6x10x3 mm (link, $1.50); 8x12x3.5 mm (link, $1.50); and 8x16x5 mm (link, $1). For larger projects, 8x22x7 mm bearings are a bargain, too - trading for about 50 cents a piece or less (link).

If you don't have any specific designs in mind, but plan to work on small to medium-scale projects, grabbing a set of 6x10x3 mm or 8x12x3.5 mm bearings is not a waste of money.

5.1.5. Gears and timing belts

When building precision mechanical assemblies, it is usually much more convenient (and a lot cheaper) to make your own gears than to source them online. Part of the reason for this is that most of the time, you will want to use compound gears with very specific diameters, heights, tooth counts and ratios, axle bore sizes, and so on. Chances are, you won't be able to find just the right ready-made options online, at least not on the cheap.

Timing belts are a different story, in part because they are tend to be simpler. Whether you need timing belts is a good question; they do offer an alternative form of gearing in applications where other solutions are inconvenient to use because of size constraints or the distances involved. For example, they may offer a space-saving way to couple a motor to a wheel without reducing ground clearance. Another popular use is in linear motion systems, where they can acutate any assembly clamped to the belt (and mounted on some sort of a rigid slide to keep it from flopping around).

Historically, timing belts used to be fairly expensive and difficult to source in miniature sizes, but this has changed over the past few years. One of the best sources for small timing belts, many for around $1.50 a piece, is McMaster-Carr; check out the MXL series for some very good picks. Some of their smallest ones are just about 10 mm long and 3 mm wide.

Timing belts run quiet and have very little backlash, but they are less durable and less rigid than plastic gears; they may be somewhat less efficient, too.

5.2. Robot-related electronics

5.2.1. Motors

The selection of motors at your disposal is definitely the single most important factor affecting the ability to bring your electromechanical designs to life. It's also something very easy to get wrong - or get right, but grossly overspend on.

It is probably safe to assume that you are interested primarily in small, low-voltage DC motors; if so, the choice is roughly as follows:

As you can see, there is no perfect solution. I personally prefer sticking to vanilla brushed motors and creating my own gearboxes, but if your patience can wear thin, servos or geared motors may be a better choice. For any of these motors, you should definitely look at the following characteristics when shopping around:

Possibly the best source to find a great assortment of low-cost brushed motors (both vanilla and geared) is Kysan Electronics; they have a $100 minimum on all online orders, but seem to be willing to make exceptions if necessary. Good deals can be also sometimes found at various surplus outlets, including All Electronics, Electronic Goldmine, Surplus Shed, BG Micro, or HSC Supply - but their inventory can change rapidly, so your mileage may vary. Last but not least, for servos and brushless motors, Hobby King is hard to beat - they ship from Hong Kong, but do so promptly and cheaply; on orders under $200 or so, you are unlikely to run into import duties.

Whatever you do, I'd recommend avoiding robotics-oriented sources such as Solarbotics, Robotics Connection, Pololu, Acroname, and many more. They are good people, but they usually sell exactly the same low-cost motors, and simply charge you more for the privilege of shopping with them. Case in point: this motor costs $23 when bought from Robot Marketplace, or $16 when you go to Solarbotics - but Kysan Electronics carries it for $8 a piece... or just $3 on orders over 1,000 (which is probably the price that the first two shops have paid).

In any case, it makes sense to find 2-4 models that are best suited for your needs, and then buy 10-20 pieces of each; having a steady supply of well-performing motors beats having one or two of every mediocre product available on the market. My personal recommendations are:

If you need inspiration, here's a video of Mabuchi FF-N20PN powering a miniature planetary gearbox:

5.2.2. Useful sensors

Sensors are essential in almost any electromechanical design, helping interact with the outside world, and providing internal feedback about the state of mechanical assemblies. This section covers some of the most useful, low-cost choices to consider in your work:

5.2.3. IC glue and MCUs

If you have a favorite brand of microcontrollers, there is probably no need to revisit this topic; but if you are looking for advice, it's pretty hard to go wrong with AVR chips such as ATmega1284P ($8). This particular 8-bit MCU, for example, operates at speeds up to 20 MHz (internal oscillator is provided), has 128 kB of Flash memory for program storage, 16 kB of data memory (SRAM), and 4 kB of non-volatile EEPROM. It's essentially a complete computer-on-chip, complete with 32 bidirectional I/O lines, 8-channel 10-bit ADC, hardware PWM channels - all that supporting a wide range of supply voltages, from 1.8 to 5.5 V; there are precious few external components required to operate it in most real-world applications. ATmega chips have a mature GCC-based toolchain with tons of useful libraries, a nice emulator, and a pretty good IDE - and unless you are doing complex image processing or working on something else data-intensive, they will serve you well. (In more demanding tasks, you may need to spend quite a bit more on 32-bit ARM or AT32 chips; I also like Intel Edison.)

For ATmega, the only other gadget you need is a simple USB ISP dongle (e.g., AVRISP mkII), costing somewhere between $15 and $30 - and even that can be avoided if you opt for a chip with a built-in USB controller.

Note: some people love AVR-based development platforms such as Arduino or Teensy. I am personally wary of these boards, because I find them to offer very few real benefits over the AVR chip itself; you are essentially charged a 1000% markup in exchange for someone soldering the chip to a PCB, and then adding several components that are completely unnecessary in many uses, but make it look sophisticated (e.g., voltage regulators, external crystals).

Especially when developing more complex software, you may find it useful to add a way for the MCU to communicate essential information in an easily readable way. Tethering it to a computer is one option, but you may also consider getting an LCD module based on a well-known HD44780 chip; for example, NHD-0216K1Z-NSB-FBW-L ($11) is a very user-friendly device with ample display space. It can be controlled with as few as 6 data lines, and is pretty trivial to interface with - its dedicated controller maintains its own display memory, and even stores editable font data and track of the cursor for you, so you just have to send ASCII data to the appropriate port.

In addition to the MCU itself, you should also have a good assortment of standard "glue" chips that are useful for example in multiplexing and demultiplexing applications, and will allow you to extend the I/O capabilities of your chip almost arbitrarily. Probably the best IC family to stick to is 74HC - they are widely available and fairly cheap ($0.10 - $0.35 per chip), and offer respectable speeds and good load driving capabilities. You may want to grab basic logic gates (74HC00, 02, 04, 08, 32, 86 - NAND, NOR, NOT, AND, OR, and XOR respectively); line drivers (74HC240, 241, or 244); multiplexers, demultiplexers (74HC164, 165); line selectors (74HC137, 42, 151); and flip-flops / latches (74HC175, 75, 259). Some projects may also have uses for counters, timers (e.g., 7555), external oscillators, assorted op-amps, etc.

About the only thing you can't do with all these parts is driving any power-hungry loads, such as motors: the tiny transistors inside most MCUs and 7400 series chips can output at best around 20-40 mA per line - enough for a LED or two, but not much more. It is possible to use discrete power MOSFETs (e.g., BUK7510) to control high-current devices, but doing so is not always space- and cost-efficient - so you may want to look into IC-based motor drivers. FAN8082 is probably the cheapest ($0.40) full H bridge (i.e., bidirectional) driver capable of delivering up to 1.5A at 18V to brushed motors and bipolar steppers; it even comes with rudimentary speed control. The disadvantages of this chip are its reltively high voltage drop (almost 2V), and the fact that it doesn't support "freewheeling" (high impedance) mode. Somewhat more expensive TA7291P ($0.90, 2A peak at 20V) supports all four output states: forward, reverse, brake, and freewheel; TLE52052 chip ($3.50, 6A peak at 40V) can drive even larger motors with ease. Several dozen similar products exist - shop around, and grab at least around 10 pieces or so.

For driving unipolar steppers, solenoids, and other power equipment where you don't need to change polarity, you can also save some money by going with simpler devices: ULN2003 ($0.30) can drive up to 6 devices at 500 mA and 50V (or one device at 3A); while ULN2065 ($2) has four outputs capable of delivering 1.5A at 35V, adding up to 6A total.

5.2.4. Power sources, watchdogs, regulators

About the last major set of electronic components that you need to think about are the power sources you will be using in your work. To make the right call, you need to consider several factors:

Today, the best all-around option for robotics are rechargeable lithium-polymer cells, simply because of an excellent balance between capacity, weight, and cost. My favorite source is Hobby King. They have good products, and although they are in Hong Kong, they ship cheaply, quickly, and with no hassle whatsoever. If you browse their site, you can find a 7.4V 5 Ah cell, weighing around 300 g, for about $25; a smaller 1.6 Ah cell fetches $10 and tips the scales at 90 gram; while a tiny 800 mAh one weighs barely 50 g and costs $5.

Of course, nothing comes free: lithium batteries have two drawbacks that you should know about. First of all, if they are charged improperly or badly damaged, they can overheat and catch fire - so you need to store and handle them with some care. The other issue is that they shouldn't be discharged past a certain minimum voltage to avoid altering their chemistry; using a voltage cut-off IC, such as MAX8211 or MAX8212, is a very good idea.

Of course, there are many alternatives to Li-poly; a typical AA battery is nothing to sneeze at, and delivers up to 3 Ah, with peak current as high as 10A; your usual 9V battery is closer to 500 mAh and can't source more than 1A. If weight is not an issue, you can also go with lead-acid batteries, of course: they are cheap, but weight a ton ($20 will get you 15 Ah at 6V, but be prepared to haul around 2 kg). Ultracapacitors are also of some interest in recent years - but right now, they tend to be fairly expensive, especially if you are interested in supply voltages over 2.5V or so. Last but not least, solar cells deserve a honorable mention - although similarly to ultracapacitors, they are not that practical in everyday uses. Because of their lamentable power capabilities in function of their size, they are useful mostly as a way to conveniently recharge a chemical battery or a capacitor, and not as a continuous primary supply.

Oh, one more thing: for prototyping, I recommend grabbing an adjustable benchtop power supply, such as Mastech GPS-3030D ($90); convenience of being able to quickly adjust voltage aside, their huge benefit is that you can limit the current to a safe value, so that an accidental short-circuit will not destroy everything in its path. The same can't be said about most batteries.

Tip: the reason why you should match the supply voltage with the most power-hungry components in your circuit is that high-current DC voltage adjustments can be pretty tricky. It's easy to lower the voltage supplied to a low-current device, such as a microcontroller or a couple of LEDs: just grab a linear regulator such as LM317T ($0.25) or L7805 ($0.50) and be done with it. For efficient regulation of higher currents, or for stepping the voltage up, you generally need switched regulators, however.

Such regulators are fairly complicated to build on your own, and get expensive if you want a plug-and-play solution. For example, ICL7660 - a chip that can handle up to 20 mA - goes for $2; Murata OKR-T3-W12-C - a hybrid device that can deliver up to 3A - retails for $7; and a 6A variant of the same Murata product will fetch $14.

5.2.5. Other components

Well, it goes without saying that you will also need an assortment of generic electronic components to get anywhere: make sure that you have a bunch of resistors and capacitors, a handful of PCB mount potentiometers, some medium-power MOSFET transistors (n- and p-channel), a good selection of terminal blocks and ribbon connectors, a solderless breadboard or two for prototyping, perforated boards in various sizes, and so on. In fact, if you need any help with selecting the right components and using them in a circuit, check out my short primer on electronics in your spare time.

What else? If you want to make your own PCBs for finished projects, you can of course print and etch them - although keep in mind that it's also quick and easy - and often more precise - to machine them on your CNC mill: you can simply selectively remove copper plating from a blank board with a cutter - and drill mounting holes at the same time.

And of course, don't forget about installing a conveniently located and obvious self-destruct switch!

6. Practical part geometries

To wrap up the discussion of technical topics, let's go over a collection of practical approaches that should help you create useful and reliable mechanical designs for small- to medium-sized projects - no matter if you are using CNC machining, 3D printing, or still carving stuff in stone. The failure to get even the most rudimentary understanding of these topics is one of the cardinal sins of hobbyist makers - so don't let this happen to you.

6.1. Basic part design tips

The mathematics of material engineering are fairly obtuse, and the finite element modeling software that can be used to model the dynamic behavior of real-world part geometries is prohibitively expensive. Nevertheless, if you are designing small- to medium-scale components and can afford to learn from mistakes every now and then, there is a couple of simple rules of thumb you can follow to get great results without resorting to any of that.

(You can also find quite a few good engineering tips on this page.)

6.1.1. Rigidity management

Perhaps the most profound cognitive dissonance you can experience during your first adventures in DIY manufacturing is making the first part that happens to be long, thin, and flat. Let's say you grabbed IE-3075 and cast a piece that is 2 mm thick, 1 cm wide, and 15 cm long; you will be probably impressed by how difficult it is to break this sample - but also shocked by how easily it deflects. It just doesn't seem to make any sense: the datasheet says that the material is ought to be 50% more rigid than the plastic used to make Lego bricks, and several times less flexible than the commodity plastics used in everything from toaster ovens to computer cases. But your toaster doesn't flex nearly as much, right?

To understand what's going on, consider what would happen if you applied the same bending force to the side of the part, rather than the top:

If you do that, you won't be able to flex the part even the slightest bit. The cross-section of the material is still the same - 2 x 10 mm - but all of sudden, it is a lot more willing to resist your efforts.

That's because flexing the material amounts to stretching the polymer chains farther apart on one side of the part, and squeezing them close together on the other end. If the material is thin, the effect is relatively modest, and you don't need to invest a lot of energy to make it happen. For thicker segments, the displacement is a lot more dramatic, however:

The effect is pretty significant. Simplifying slightly, the rigidity of the part is proportional to the width of the sample - but to the cube of its thickness.

This behavior brings us to the manufacturing trick that is being used in almost every item made out of plastic or sheet metal - including toasters and Lego bricks. In essence, you can greatly improve rigidity by increasing the maximum span of the part in the direction subjected to bending loads - without adding any significant bulk. The most common approach is the addition of a couple properly oriented ribs:

The average thickness of the part - and thus its weight or the amount of plastic used - hasn't changed significantly. But because each of these ribs contributes to rigidity in proportion to the cube of their height, their impact is more significant than it may seem.

The following picture is a good example of the use of ribs to control the rigidity of a thin-walled chassis of a medium-size robot. Note how the ribs are routed around any locations where additional clearance is required, and not interfere with the mechanical design. In fact, they probably improve the overall aesthetics:

It is also possible to achieve the same effect without having to introduce any "standalone" ribs; for example, if the part uses curved surfaces or happens to have outer walls, this can provide a comparable degree of reinforcement:

Properly reinforced thin-walled parts weigh less, cost less, provide more clearance for other components, exhibit much less shrinkage - and often simply look better. Because of this, try to learn and use these approaches as often as possible. Stick to 1 to 1.5 mm as a baseline wall thickness for all the larger parts - especially for chassis components, covers, etc - and just sprinkle them liberally with ribs, gussets, and other features of this sort. A handful of ribs extending 2-4 mm from the surface of the part is usually all you need to make it work.

6.1.2. Stress distribution

So far, we've talked about the need to reinforce thin, long, planar surfaces; but there is one more situation where a small amount of extra plastic routinely saves the day.

From the discussion of casting resins earlier in this guide, you may recall that the resistance of these materials to non-uniform loads (e.g., tearing) is dramatically lower than the ability to withstand uniformly distributed stress. For this reason, you should pay close attention to sharp corners, jagged edges, holes, and similar features in locations subjected to significant loads - and incorporate simple reinforcements to ensure that the stress isn't concentrated in any single spot.

For example, when designing a coupling for an axle that may be subject to strong radial forces and shock, adding a fillet or a couple of load-bearing gussets can save the day; all other things being equal, the "vanilla" approach shown on the left is much more likely to fail:

Given the negligible cost of these features, it typically makes no sense to spend too much time trying to figure out in which exact spots they happen to be absolutely necessary. Whenever it is clear that a localized stress may occur, throwing in a rib, a gusset, or a fillet is simply a good plan.

6.1.3. Fitting tolerances

Another problem that a novice designed may bump into is the question of fitting tolerances: when one part needs to mate with another, what should their respective dimensions be to form the desired type of fit?

The correct answer depends on a number of factors, including the accuracy of your CNC mill, or the operating conditions of the assembly (temperature, risk of contamination, running speed). Nevertheless, when working with a reasonably accurate milling machine and dealing with normal circumstances, the following rules of thumb tend to work pretty well for rigid plastics:

When in doubt, be quick to experiment: it takes just a couple minutes to machine a rectangular or cylindrical test piece and play with it. Save yourself the time and frustration of having to debug issues in more complex parts.

6.2. Creating spur gears

Gears are an extremely versatile tool that will come handy in all sorts of mechanical assemblies. Depending on your needs, they can reduce or increase the speed of movement, adjust torque, or change the direction, axis, and type of motion - all that with high precision, excellent efficiency (95%+), and great load-bearing capabilities. Alas, unfortunately for hobbyists, much of the freely available information about the geometry of common types of gears is misleading or incomplete - and even the articles posted on Wikipedia are littered with inaccurate images or animations such as this.

The key property of a well-designed set of gears is that they transfer motion almost exclusively through a rolling action, with very little sliding (and thus friction) involved; and that they maintain constant velocity and a constant angle of contact through the process. If you violate these principles, your gears will become inefficient, noisy, and prone to vibration. The optimal design is exemplified by the ubiquituos involute spur gear:

The blue lines shown on this picture are the theoretical pitch circles of the gears; if you replaced the gears with idealized rubber rollers of these diameters, the behavior of the transmission would not change. The linear speed of any mating gears is the same when measured along the pitch circle, and the ratio of angular speeds depends on the ratio of their pitch diameters. For example, if one of the gears has a pitch circle with a diameter of 10 cm, and it's turning another gear with a pitch diameter of 4 cm, one turn of the former will rotate the latter by 10 / 4 = 2.5 turns.

Before getting any real work done, you need to make up your mind about several key parameters that will define the gears used in a particular project; except for tooth count, these values should be the same for any two gears that are supposed to mesh with each other.

6.2.1. Tooth count

This parameter depends chiefly on the transmission ratios you are hoping to achieve. The only constrain here is that with very low tooth counts, you will see a behavior known as undercutting: the two meshing gears will collide, necessitating the introduction of some additional clearance - and thus weakening the part:

This is not a big deal if the undercut is small; but when it begins to extend significantly into the area where power transfer takes place - that is, the region above the so-called base line, the gray circle shown in the earlier animation - the performance of the gear may be compromised. To avoid having to heavily compensate for this issue, unless exceptional circumstances apply, try to keep tooth counts over 8-9 or so.

Note: most of the gear generators you can find online can't deal with undercuts at all: quite simply, if you try to specify fewer than a certain number of teeth, they will reject your input or generate an incorrect result. Avoid these like a plague - and scroll down for better alternatives.

As for the maximum number of teeth - there is no real limit to how far you can go, but to keep your models simple and the cutting process rapid, it's preferable to keep the count under 100 or so.

6.2.2. Tooth width

This parameter is measured at the pitch circle. The tooth must be wide enough to allow the mating region to be machined with the tools you happen to have at your disposal; in fact, it's advisable to use tooth width at least 30% greater than the diameter of the smallest end mill in your collection.

In precision applications that do not involve extreme torques, the value will typically range from 0.3 to 1 mm - and 0.6 mm is a good starting point. You may want to increase the width when working with larger models, to keep the number of teeth sane.

6.2.3. Pressure angle

When you look back at the animation shown earlier in this section, you will notice a slanted line that goes through the center. This is the path along which the gears come into contact, and the actual torque transfer takes place. The slant of this slope can be selected to suit your needs, but it needs to be reflected in the geometry of the resulting teeth:

In general, low pressure angles result in quiet-running, precise gears that are easy to machine due to generous clearances - but will be weaker, and have more pronounced undercuts. High pressure angles result in thicker tooth profiles which need to be machined with smaller tools, and can be more noisy and prone to backlash and slippage - but also survive a lot more abuse.

Sensible values for this parameter range from 12° to 25° or so - and my personal favorite for miniature assemblies is 17.5°.

6.2.4. Addendum

The addendum is the distance by which every tooth sticks out above the pitch (contact) circle. The recommended value that ensures continuous and smooth torque transfer is tooth_width * 2 / π, with some wiggle room if you are pressed for space or are running into other constraints. Going below 70% of the recommended distance should be done at your own risk.

To minimize undercuts when dealing with low pressure angles and low tooth counts, you can use a larger addendum on the smaller of the two meshing gears, and a smaller addendum on the other one. This technique, known as profile shift, is particularly useful for gears with fewer than 12 teeth, where it can significantly reduce undercuts without compromising machinability (right):

Of course, profile-shifted gears will not mesh correctly with regular ones - the mating gear always needs to have exactly the opposite shift. This is seldom a problem, but you need to keep this constraint in mind.

6.2.5. Putting it all together

All right! Equipped with all this knowledge, you can compute the profile for your first set of involute gears. There are many different approaches to this task, but my favorite method is unique in that it comes without the limitations that plague simpler techniques - and if you are so inclined, it can be easily generalized to deal with a variety of unusual gear shapes and tooth profiles later on.

The process begins with calculating the circumference of the pitch circle for your gear: cpitch = tooth_width * 2 * tooth_count. From this, you can trivially calculate its diameter of the circle you need to draw (c / π). Once you have it on your screen, you need to add a straight line tangent to the pitch circle; this will be used to build a hypothetical mating gear with an infinite diameter. If we can mesh with this, we will be also able to mesh with any smaller gears encountered in real life.

On this "pitch line" of our infinite gear, we can draw two lines perpendicular to it, precisely tooth_width apart - and rotate them around the point where they intersect the infinite gear by an amount corresponding to the pressure angle we want to maintain (18° in this example). To turn this into a proper tooth, we need to add a horizontal line that is normally placed addendum = tooth_width * 2 / π below; in fact, draw the line about 5% farther down to create some clearance for any dirt or grease caught in between gears. When done, trim off the excess - and voila:

The next step is to simply perform a simulation of how this tooth would engage and then disengage with the actual gear we are trying to create. Recall the mention of rubber rollers: for any two meshing gears, the linear velocity at the pitch circle should be the same - so if the infinite gear (aka rack) moves by n millimeters, the other gear will rotate by 360° / cpitch * n.

The rest should be fairly obvious; this is the pattern you will see if you keep creating copies of the tooth at successive offsets along the rack, and then rotating them back by a matching angle around the center of the pitch circle of our gear:

The process can be automated easily in almost every CAD tool, although for the initial try, it makes sense to do it by hand; in any case, once you have this pattern generated, you simply need to trace a curve around it, replicate this curve using the polar array tool to create the preselected number of teeth, and offset the pitch circle by the addendum distance to trim off the unnecessary bits:

Try to do this exercise on your own. With rare exceptions where backlash is completely unacceptable, you should draw the outline about 0.05 mm apart from the trace left behind by the simulation; without this small amount of play, the teeth will engage on both sides simultaneously, producing unnecessary friction.

6.2.6. Bonus tricks

The beauty of the simulation-based method of constructing gears is that it's WYSIWYG, and that it takes most of the usual constraints out of the equation: for example, you can produce gears with any number of teeth, because the possibilty of undercuts is automatically accounted for. On top of that, it can be modified in several fairly intuitive ways. You can:

Sounds more fun than it should be? You bet it does!

Tip: of course, there is no need to repeat all these steps by hand. The most flexible option is this web-based generator developed by Rainer Hessmer. The tool is very user-friendly: simply enter several parameters and get a DXF file in return. The tool properly handles undercuts, internal gears, profile shifts, and so on - so take it for a spin!

If you're using Rhino, you may also enjoy my experimental Grasshopper module, which provides a comparable degree of flexibility, but is much faster, with instant feedback right within the CAD environment. It comes with fairly detailed instructions and features important for machineability, too. To run the add-on, you will need to install the Grasshopper plugin itself, open the plugin with the namesake command, and then load the gear_calc.gh file into the app. Note that the internal gear functionality is not fully operational yet: undercuts are not properly compensated for if the diameter of the meshing gear is close to that of the internal one.

Last but not least, some Rhino users may want to grab a copy of my earlier semi-automatic script. Its main advantage is that it can handle arbitrary tooth profiles and very accurately handle internal gears. It is activated by entering Hob4 in the command prompt and answering some simple questions about the gear you want to generate. It will then draw the appropriate circles and - if instructed to do - a standard trapezoidal tooth profile. You can modify the profile or simply select the one produced by the script, and run Hob4 again to finish the job.

As noted, the script also allows you to generate internal gears; to do that, you must provide a mating pitch circle and tooth profile extracted from a previously generated external gear. This circle needs to inside, and must be touching the top of the pitch circle for the internal gear.

6.3. Other types of gears

Involute spur gears are remarkably versatile, easy to model, and easy to manufacture - but they are certainly not the only type of a gear you can use. Let's go over some of the more exotic but still useful options that may come handy in your work.

6.3.1. Bevel gears

In comparison to planar spur gears, bevel gears have a very important property: they mesh at an angle, and therefore, can be used to change the axis of rotation. Although this goal can be achieved by other means - for example, with simplified crown gears or with worm drive, bevel gears do so with remarkable precision and efficiency.

The process of constructing a set of meshing bevel gears is considerably more complicated than what we discussed before, and involves a brush with non-Euclidean spaces; nevertheless, if you follow these instructions closely, you should be able to pull it off.

As with spur gears, the first step is to choose tooth counts, width, height, and pressure angles; you should also think about the exact gear ratio you want to achieve, as one pair of gears will be not interoperable with any other differently configured set.

Next, draw the appropriate pitch circles for both of the gears you want to create, placing them on the X-Y plane; when done, add two lines perpendicular to this plane and going through the center of each circle. Finally, rotate one of the circles (and the associated perpendicular line) in the Y-Z plane, pivoting the operation around the point where touches the other one. In our example, we will use 90° between the mating gears, but any other angle is a possibility:

With this out of the way, draw a line that starts where the two perpendicular lines meet each other, and ends where the two circles touch each other (Q). Next, using the Y-Z drawing plane, add a line perpendicular to this newly created one. This final line is denoted as l:

To minimize clutter, we'll focus on constructing the gear profile for circle a, and hide the other circle for time being; the steps needed to construct a suitable gear profile for the second gear will be analogous to what we are doing past this point, so you should have no difficulty figuring it out.

The next task is to mark two locations on the line l that are in equal distance from point Q; these newly created locations are denoted as C and D. The distance must be a bit greater than the addendum for your gears, but beyond that, the exact value doesn't matter. With these two points in place, draw a circle in the X-Y plane that has the same center as circle a, and passes through point C; a similar circle should be drawn for D. Lastly, use the loft operator after selecting the newly created circles c and d to build a conical surface:

The next challenge is to check the manual for your CAD application, and identify an operator that allows you to "unroll" this surface to a construction plane; in Rhino, UnrollSrf is what you need. Select the surface and the original circle a, and apply this transformation; you should get an object resembling the one on the left:

Extract the two "broken" edges of the surface as straight curves, and draw a circle that has the same center and diameter as the arc produced by unrolling the curve we selected alongside with the cone. This will be your virtual pitch circle, on which you will have to construct a regular tooth profile using the approach outlined earlier on.

You can start by simulating the appropriate tooth profile (center, above), and then by making a planar array to complete the gear (right). The only difference is that before making the array, you need to rotate the profile by 90° to touch one of the extracted edges; and then make a radial array consisting of tooth_count copies that fill an angle of 360° / length_of_virtual_pitch_circle * length_of_circle_a, rather than the usual 360°. When done, truncate the parts that stick outside the edges, and you should be all set.

To wrap up this process, you need to position the created gear profile over the unrolled surface again, making sure that it is aligned correctly. Next, use an operator that will map the curve back onto the original conical surface, using the unrolled geometry as a reference. In Rhino, this is accomplished with FlowAlongSrf. Just make sure that you select matching locations on the reference and destination surface, and if you did it right, you should be able to get something of this sort:

You can delete the cone and all the helper surfaces at this point, and repeat the same procedure to generate the other gear. In the end, you should be seeing a result that resembles this:

Neat, eh? You can now create a scaled copy of each outline along the appropriate axis, using a 3D scaling operator with the origin set at the point where the axes intersect; be careful with this step, as using an incorrect origin will result in gears that would not mesh. With this step out of the way, the scaled copy and the original can be then lofted together, and each of them can be individually extruded toward any point behind the gear to create a cap surface. A few extra moves to trim and cap everything neatly, and you should have a final result in front of you:

Sweet!

6.3.2. Worm gears

Worm gears are an interesting animal. On one hand, they offer extremely high reduction ratios, which makes them quite attractive for robotics: every turn of the input shaft advances the output gear just by a single tooth. On the other hand, they generally boast very poor efficiency due to significant friction - 30-40% being the norm.

From the design standpoint, these gears are fairly unremarkable; the output gear is generated the same way you would make any other spur gear, while the input gear - or more accurately, the input screw - is essentially a rack tooth that is wrapped around a spiral path, with a tooth_width * 2 pitch between the turns:

Depending on the thickness of the output gear and the diameter of the input screw, the profile used to generate the output gear may need to be corrected slightly, based on a 2D projection of the trimmed, meshing section of the screw. You should be able to create all this with no help by now; operators such as Sweep1 and Silhouette in Rino are about everything you need. The only complication for CNC machining is that as opposed to spur and bevel gears, the input screw needs a two-part mold.

6.3.3. Helical gears

Helical gears are essentialy spur gears - but (literally!) with a twist. Instead of having their teeth extruded along the axis of rotation, the top and bottom profiles are rotated in relation to each other, and then lofted to form a twisted gear. The twisting may be done in a straight line (left) or in a curved fashion (center):

The point of this modification is that each gear will now engage more gradually, thus reducing the amount of noise and vibration produced under load, this matters particularly in automotive applications and other situations where steel gears transfer high torques at high speeds. On the flip side, this geometry means that the gears act like a wedge, and exert substantial axial forces on the assembly they are coupled to; a double helical layout, also known as herringbone (above, right) works around that to a large extent.

Helical gears are relatively difficult to make using simple, CNC-machined molds; for optimal results, the mold would need to be split into four to six parts. It is easier to machine them directly using a rotary axis - but in any case, the benefits of using this class of gears are negligible in small-scale robotics work.

6.3.4. Pulleys for timing belts

Gears that mesh with timing belts are constructed in a manner fairly similar to a normal gear meshing with a rack, but their shape is not entirely symmetrical. MXL timing belts use 40° teeth with a pitch of 2.03 mm. Addendum of the timing belt is approximately 0.5 mm; the dedendum is zero. The width of base of the tooth (at the pitch circle / line of the belt) is around 1.2 mm, leaving just around 0.8 mm in between the teeth. The solid section above the gears is usually around 0.7 mm thick.

Of course, the belts are made of rubber and will easily conform to pulleys even if the dimensions are slightly off - so you don't need to obsess over it as much as you'd have to with rigid gears.

6.4. Getting started with gearboxes

It is possible to adjust the speed or torque of a motor by employing just a single pair of gears, but doing so is often impractical: for example, if you have a motor that nominally runs at 10,000 RPM, and want to get down to 100 RPM - a reduction of 100:1 - you would need to mesh an input gear with about 10 teeth to an output gear with no fewer than 1,000. Assuming that the width of a single tooth is 0.6 mm, the diameter of the latter gear would be almost 50 cm - oops!

To solve this problem, it is common to build gearboxes that employ multiple stages; each stage reduces the speed by a more reasonable factor - usually between 2:1 and 5:1 - and then powers the next stage that performs the same trick again. The gains are exponential: if one stage gives us 4:1, then two will offer a 16:1 reduction; three will yield 64:1; and four would get us down to 256:1.

The most rudimentary type of a gearbox is the offset-gear arrangement shown on the left; you can check out a practical example of this design here. This approach works well, but tends to take up space; a simple modification, shown on the right, is to stack the gears alternating between two shared axles, instead (example):

Offset gear transmissions of this sort are simple and adequate for most needs; for instance, Tinybot mk III uses them exclusively. Having said that, every now and then, you may wish for something more compact and self-contained. If you are pressed for space, it makes sense to consider a planetary gearbox. There are several possible ways to design such a mechanism, but the most practical arrangement relies on a small, centrally placed sun gear coupled with the input shaft; that gear is surrounded by several planets, and these planet gears in turn roll inside a stationary internal gear known as annulus or ring:

Of course, this design makes little or no sense if the planets just run freely; the output stage is created by creating a carriage that slides onto the planet gears, often using a sleeve bearing. The transmission ratio of such a mechanism will be tooth_cntsun / tooth_cntannulus - and that's quite a good result in such a small envelope. Just as importantly, additional stages of a planetary gearbox can be packed tightly on top of each other, resulting in a form factor that is pretty hard to beat.

To better illustrate the practical aspects of making a planetary gearbox, you may want to check out this rendering of the design I have been using in my projects for quite a little while:

Of course, planetary gearboxes are no silver bullet; they are about 2-3 times more compact than offset gears, and outperform them in some ways, but it's possible to run into situations where an even higher ratio would be desirable - but where the poor efficiency of worm gears and other types of screw-based actuators is hard to swallow.

Of all the more exotic transmission systems, the most promising one is probably the cycloidal drive. This device easily attains ratios of 100:1 or more in a single stage by brilliantly exploiting eccentric motion of a large gear in a comparably-sized "cage":

I have a separate page describing this particular design in more detail, so I won't repeat all the information here. Rest assured, cycloidal drive systems have some substantial downsides - for example, they tend to introduce vibration, and require subminiature ball bearings to isolate the input shaft from the eccentric gear. Nevertheless, when out of mainstream options, they are worth a try.

6.5. Overview of drive systems

By now, you should be well-equipped to get ahead with almost any electromechanical project you can think of; the only question is where to start. In the world of robotics, the single most important choice is probably the mode of locomotion; everything else - from the selection of sensors, to power sources, to data processing capabilities - follows from that. In the last section of this chapter, it makes sense to quickly run through the options you have.

In all likelihood, your initial projects should probably be wheeled. It is tempting to try something bolder, but until you are familiar with the dynamic properties of plastics, and have several precision robots under your belt, it's really the best choice. Some of the common approaches to wheeled robots include:

Wheeled robots aside, legged creatures have some undeniable appeal. These robot designs tend to pose a range of challenges related to space management, torque requirements, obstacle sensing, and the creation of practical movement algorithms. Sure - it is relatively easy to bolt a bunch of expensive servos on top of a simple frame - but going beyond that point is usually fraught with peril. Still, if you want to give it a try, your basic choices are:

These movement modes aside, there is a long tail of exotic inventions that are worth at least reading about. We have hopping pogo stick robots; several well-publicized ball-balancing units; a bunch of levitating designs that leverage air cushions, magnetic fields, or vibration; insanely cool crawling robots; and a large selection of flying and sailing ones. And hey - why not add something of your own to that list?

7. Workshop safety topics

We're done with the technical stuff, but stay with me for a bit longer: even for DIYers, workplace safety is no laughing matter. You will be dealing with power tools and reactive chemicals, and so if you want to pursue this hobby, there are inherent risks you simply have to understand and accept. It's entirely possible to lose an eye or set your house on fire; there are certain common-sense steps you can take to minimize the probability of mishaps, but the odds will never go down to zero.

It is also your sole responsibility to investigate and follow all the applicable HOA rules, city ordinances, and other regulations - especially as they relate to purchasing laboratory supplies and chemicals, the use of power tools, and the disposal of unreacted chemicals. In many places, the laws are sensible or non-existent; but exceptions happen. If you accidentally ignore a piece of obscure, boneheaded legislation - well, nine out of ten times, nobody will know or care, but do you really want to be that one unlucky guy?

OK. With all that in mind, this chapter provides a quick but non-authoritative overview of some of the most obvious safety considerations and health risks for CNC machining and resin casting. There are no guarantees that this information is accurate, complete, or up-to-date, so do your own research. Do not blindly trust one random dude on the Internet!

7.1. Working around milling machines

Small CNC mills are fairly safe, compared to most other power tools - but you still need to read and follow the instruction manual, and in general, exercise common sense in everything you do. The primary risks are getting your hand pinched between the rotating tool and the workpiece, or between any other moving parts; having loose clothing or hair caught by the spindle; or being hit by any debris ejected during the cutting process.

Becaue of this, you should really stay clear of the machine while it's running a job - after all, you have no business fiddling with it. If you really need to make some sort of an adjustment, or perhaps simply vacuum off accumulated shavings, it's imperative to pause the cutting process first. It is also a very good idea to wear basic eye protection at all times; although the odds of a broken tool or other sharp material hitting your eye are very low, the damage can be significant.

All that aside, high-speed CNC lathes and high-power CNC mills (over 1 kW or so) have their own safety rules that I will not cover here. These devices are often powerful enough to break bones, sever limbs, or eject a loose workpiece with a speed sufficient to kill or maim. If you have one of these, the use of robust safety covers made out of sheet metal, polycarbonate, or wood, is definitely a must.

7.2. Dust considerations

Any type of dust can aggravate existing respiratory conditions; prolonged and heavy exposure may also lead to the development of brand new ones. That said, most of the noteworthy CNC stock materials give off relatively little airborne particles when machined in a sensible way - and so, your overall exposure to nuisance dust will probably not change in a major way.

Lifelong exposures to wood dust are associated with nasal cancer, so reasonable dust management is advisable for wood-related projects. When working with prototyping boards such as RenShape 460, airborne dust is produced in modest quantities and has no known, specific health risks, but it's still probably good to keep it in check. Covering the mill with plastic sheeting for the duration of the job (well clear of any rotating parts and inlet vents) is a simple dust management trick; building or buying a proper enclosure is an option, too. When containment is impractical, a simple N95 particulate respirator (link) should offer protection.

Of course, there are materials that pose an elevated risk; to avoid really unpleasant surprises, always investigate the substances you are milling, sanding, or otherwise dispersing in the air. You should be particularly wary of anything that contains crystalline silica (quartz), asbestos, elemental metals, toxic pigments, and any other harmful substances that may be absorbed through the lungs.

Note: whie crystalline silica (quartz) is a significant danger when inhaled, there is no compelling evidence that amorphous glass products, such as milled glass fibers or 3M glass microspheres, share the same risk. You still don't want to breathe any of that stuff in, but these fillers should be safe to use in their intended way - and a tiny amount of dust should be OK.

7.3. Vacuum and high pressure

Vacuum isn't particularly dangerous to work with, but you should be aware of the risk of implosion. Plastic vacuum chambers should be inspected for cracking or crazing, and should never be exposed to high temperatures, substantial quantities of solvents, strong acids and bases, and so on. Sketchy-looking or homebrew chambers can be secured with packing tape, shrink-wrap foil, fabric, and similar materials that will retain debris if things go wrong. Other than that, you have very little to worry about; oh, if you are working in a confined space, it may make sense to vent the vacuum pump to the outside, so that the relatively small quantities of gases liberated from the resin - as well as the fine mist of mineral oil from the pump itself - don't end up right under your nose.

Pressure casting is more dangerous; that's in part because the pressure differentials are much higher than for vacuum casting (especially if something goes wrong), and because all the energy is released outward if the container fails. Because of this, you should never try to rig your own pressure chambers or any other parts of the system, and not even think about removing any safety valves, regulators, and so on. Follow the instructions religiously, especially when it comes to draining the compressor; do not exceed manufacturer-provided pressure ratings; always double-check that the system is depressurized before trying to open the chamber; and disconnect everything completely when not in use. With common-sense precautions, catastrophic failure is unlikely - but rest assured, bad things have happened to quite a few people who should have known better.

Oh, one more thing: if you are inclined to get a nitrogen tank for blanketing resin containers in storage, go for a small one - capacity of 1 cubic meter (40 cubic ft) is more than enough. Store it horizontally or attach it to the wall to make sure it can't fall over, and always use a regulator.

7.4. Silicone resins

Platinum cure silicones are believed to be essentially harmless; both the siloxane resin and the catalyst are non-reactive and show virtually no toxicity in animal studies. You should always read material safety datasheets for the specific product you are using, but chances are, the only risk you have to worry about is that the material is sticky, and spills can be annoying to clean up (naphtha and other nonpolar solvents can help). Beyond basic workplace hygiene, no special precautions should be necessary.

Now, I don't recommend using condensation cure silicones, but if you decided to choose that option, you should be aware that they often use a small amount of dibutyltin dilaurate, dimethyltin dineodecanoate, or a similar tin(IV) compound, to catalyze the reaction. The substance is corrosive, and more troublingly, exhibits some chronic toxicity. It is present at around 5% in the catalyst component, and usually at less than 1% in the finished product - but since it will leach out in favorable conditions, you should probably not be using condensation cure silicones to make anything that is routinely worn or handled by humans, or that comes into contact with food. When mixing these resins, it's best to use latex or vinyl gloves; basic eye protection is not a bad idea, too. Storing the catalyst in a suitable cabinet and out of reach of children is important, too.

7.5. Polyurethanes

Polyurethane resins consist of several very distinctive components, and it's probably useful to discuss them separately. Here's what you can typically find in the MSDS:

As with any chemicals, you should avoid mixing polyurethanes - and isocyanates in particular - with potentially incompatible substances; as hinted earlier, this includes alcohols, water, but also strong bases, oxidizers, amines, and metal salts. Rapid polymerization, decomposition, or other funny developments may ensue. Isocyanates also decompose in a matter of minutes in dimethyl sulfoxide (DMSO), so do not use this solvent as a carrier for dyes, catalysts, and so forth.

In the same vein, don't mix unreasonable quantities of polyurethane resins in a single batch. Runaway exothermic reactions can become dangerous if the resin gets so hot that the mixing container melts away, or something catches fire. If you need to dispose of a large quantity of unwanted resin, polymerize it in batches; or at the very least, use a large, shallow pan and work outdoors, away from other humans and flammable stuff.

Last but not least, because of their irritant properties, uncured resins should be stored safely, and kept out of reach of children and other creatures other than the intended user.

These considerations aside, cured polyurethanes are one of the safer, more stable, and least controversial plastics out there. Even if you mess up the mixing ratio, the unreacted isocyanates will simply eventually react with moisture, unreacted polyols will not pose a threat, and nothing horribly bad should be leaching out of the material otherwise. Like most plastics, and organic materials in general, polyurethanes release a fair amount of toxic substances during thermal decomposition - carbon monoxide being by far the most significant problem - so try to resist the urge to burn them if at all possible.

7.6. Other chemicals

Throughout this guide, I have mentioned quite a few chemicals that won't be covered in this chapter in special detail. If you plan on getting any of them, you should take care to obtain and read the appropriate material safety datasheets, and understand all the associated risks. In no particular order, here are some of the things you should know:

It hopefully goes without saying that you should store all the materials discussed in this guide away from sources of ignition, excessive heat, and so on; that you need to keep them away from children; and that if you are pregnant or breastfeeding, you should avoid any non-essential exposure to exotic, industrial chemicals - no matter what the MSDS says.

8. Closing notes

Well, that's it! I'm hoping that the content included here inspired you to some extent, or helped you solve a specific problem or two. Since this guide is incredibly niche, if you made it this far, your feedback would be greatly appreciated. If you are so inclined, drop me a mail at lcamtuf@coredump.cx.

In any case, good luck in your future work!

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