Intro to Guerrilla CNC Machining and Resin Casting

Building mechanical devices is hard.  Unlike software, there are no compilers to effortlessly convert your design into a functioning system.  When I first decided to try building hardware, I was hopeful that hobbyist 3D printers were good enough for prototyping machines.  I purchased an Ultimaker and spent a few months trying to realize my mechanical ideas.  Although they are great for turning out larger models, as well as frames and mounting hardware, the current wave of affordable 3D printers don’t really have the precision to make intermeshing components like gears and joints.

Fortunately, I discovered Michal Zalewski’s astoundingly detailed Guerrilla Guide to CNC Machining and Resin Casting.  Michal has developed a workflow for quickly generating high precision (micron-scale) components using a desktop CNC mill.  He machines master models out of soft modeling board and then casts production parts from the masters in silicone and high-strength polyurethane.

Until now, I have had absolutely no experience with CNC machining, CAM software, or resin casting.  Nonetheless, after digesting his guide for about a week, I decided to try to replicate his process.

First order of business was a tool shopping spree.  I went through Michal’s guide and compiled a shopping list of all of the equipment and materials that he used.  I’ve posted my GCNC shopping list on Google Docs for public consumption.  A special thanks to Michal for providing extensive feedback on the list and patiently answering my questions.

It seems like a lot to buy, but aside from the CNC mill, most of the items are relatively inexpensive.  Additionally, good used CNC mills are widely available.  I was able to purchase a used Roland MDX-15 mill on ebay for $1500, and my total cost for everything on the list was around $2500.  This is comparable to the costs of a high-end hobbyist 3D printer (e.g. Ultimaker, Replicator 2).

After buying the mill, I started ordering everything else I would need.  Daily visits from UPS soon followed, and in short order, my casting workshop was complete.

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For my first project, I decided to make a simple gear set: 2 gears and a frame with axles.

I designed the parts in Autodesk Inventor and then arranged them in a positive mold cavity.  Although I’m new to CAM, the Roland’s included CAM software was very easy to follow and quickly generated the tool paths for the mill.  After a few hours of cutting, I produced a positive master mold in machinist wax.

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I then made a negative mold in silicone resin.

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And finally, I used the silicone mold to cast parts in rigid polyurethane.

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For reference, each gear tooth, at its root, is only 0.4mm wide.  The spacing between each gear and its axle is 0.02mm.

As you can see, the gears mesh smoothly and turn easily.  There were a few air bubbles in my first batch of cast parts, but premixing the resin with a small amount of catalyst (as suggested by Michal) improved the results greatly.

Overall, I’m really impressed with both the quality of the finished parts and the ease of the process.  Generating CAM toolpaths takes only slightly longer than preparing .stl files for 3D printing.  The machining is mostly automatic, though it’s good to pause the mill once in a while to vacuum up the wax shavings, and the casting only takes a few minutes of actual work.  Due to the time needed to cure the silicone and polyurethane resins, it takes about a day to get a finished part.

Now that I’ve got a solid grasp of the workflow, I’ll be making and posting lots of other mechanical things.





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