Guerrilla guide to CNC machining, mold making, and resin casting
Copyright (C) 2013 by Michal Zalewski (lcamtuf@coredump.cx)
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1. Preface

Hello there, stranger! Let me start by explaining what it's all about - and that should help you decide if this page is what you were looking for.

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 the 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 parts. Which brings us to...

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) with a drill-like rotating cutter (end mill) through a set of multiaxial, 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 are largely useless for low cost, high quality hobbyist work; this may change in the next few years with the advent of affordable SLA printers, but that revolution is yet to come. 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 a 3D printer, 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 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. So wait, why am I even bringing this up?

Well, despite sounding needlessly complicated, the overhead is actually pretty modest - and the process has some interesting advantages over directly machining (or printing) the desired shape:

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.

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