My short daydream about hobbyist-level tool-changing CNC was associated with the project to build a CNC from a 2-axis motion-controlled optical inspection table. Shortly before pandemic lockdown it reached the milestone of engraving a scrap sheet of polycarbonate plastic ("Lexan"). A lot of things have changed since then, putting the project in limbo, but I have a few partially written entries languishing in my "Drafts" folder. Following the precedence set by that tool-changer post, I intend to do a bit of polish and publish them for the record. First up: an idea for milling custom PCBs on the machine.

After that successful engraving test, I started considering PCB milling. It is another largely 2D task with tighter precision requirements than engraving cosmetic details, without increasing requirements on physical side loads and machine rigidity. I bought a batch of single-sided copper-clad boards (*) and started thinking about how I might design a fixture to mill them. This was the first draft, which I never got to test.

The idea was to create a small 3D-printed object that I could bolt to the MDF working surface of my XY Stage CNC. The object's top surface is intended to be milled after bolting in place, to make sure it is flat/parallel with machine motion axis. The side overhangs are designed for 3D-printed clips that will hold the board in place while milling.

The first draft of the clips had very little holding power, because I anticipated physical forces would act a certain way and reality worked differently than how I thought. I had curved it so it clamps to the top surface as well as the rail, but doing so meant less force on the actual PCB holding claw.

The second draft was more successful, the PCB holding claw became part of the topside holding force. I also extended the shape downwards so it could brace against an additional surface for more strength.

PCB milling fixture installed

The idea was to put double-sided tape on the board's back side before sticking it on this fixture. The clips would then exert downwards force on the double-sided tape to improve its resistance to sideways cutting forces. Would the tape add too much variation to the height? Would it flex too much for accurate cutting? Would the clips help or hinder the operation? I fully expected this first draft to not work as expected but looked forward to learning exactly how. Once this prototype fixture was bolted to the surface, I started looking into using FlatCAM to generate PCB-milling G-code. I never got far enough for a test run but I did make some mechanical refinements.

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.