Drilling
I 3D printed a support for the PCBs, holding them a couple of mm off the bed. This allows me to drill through the PCB and mill around the outline. Designs on thingiverse.Milling
I previously mentioned milling bits, and there are lots you can buy from Amazon and the like. However, the best one I have found so far is one from RS components (part 382002). It is not cheap (£40), but don't be fooled - it works, and works, and works. I did manage to finally blunt one after a lot of PCBs, but really it is very good. The small end means it does not snap off easily either.I am using a 0.5mm drill to make holes, and a 1mm drill to mill around the edges.
Design rules
I am now milling for the finer pitch used by the ESP32-WROOM-32 SoC modules. So I have to be able to mail quite fine tracks with 1.27mm spacing.
What I have found is that, with that bit, the simplest rule is to ensure isolation tracks are at least 1mm apart. This is quite easy, using inkscape, I just set the tracks to 1mm wide and see if any over lap. I have a layer for the parts, a layer for the drilled holes, a layer for the edge mill, and a layer for the isolation tracks. I can then simply save each layer as EPS and convert to an NC G code file.
Faster milling
I also updated my code to take the EPS output from inkscape and make a mill file. It now sorts the lines, and makes paths with a much more optimal routing. On GitHub.
Have you looked into conventional PCB manufacture using etch resist and acid? I knew people that did it at home in the 1980s with stencils or an etch resist pen. Maybe these days it is possible to print the etch resist onto the PCB?
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