They are made with a 3.2mm shank (ideal for Dremel type rotary tools) but the BuildBrighton router on the cnc machine has a 6mm shank. So we made sure we also ordered collet/adapter, which worked perfectly.
So last night, Mike, Stephen and a few other cool kids had a play about with the cnc routing software and actually got it cutting. Exciting stuff!
We used ExpressPCB and Inkscape to create an isolation path around the outside of the tracks we wanted on the board, using this earlier technique. The final board was pretty impressive. The tracks were drawn at 0.5mm wide. The cutting bit used was 0.2mm and had a 60 degree angle.
(hot off the cutting bed - quite literally, at 35,000 rpm, the cutting bit gets hot and some of this can transfer onto copper surfaced board)
Cutting was aborted after a few minutes. Not because it's slow and laborious (though cnc routing is!) but because the sparks and really bad smell coming out of an increasingly noisy router eventually told us that it was time to consider replacing it!
But before the router gave up, it cut some really fine traces onto our copper clad board.
If anything the traces are maybe a little too fine. This is because our tracks were drawn at 0.5mm wide. We used a 0.2mm wide cutting head and plunged it 0.15mm into the board. Obviously, the deeper the cut, the more of the blade that goes into the material and even with a relatively sharp 60 degree angle on the cutting tip, a deeper cut means a wider cut. We didn't get the software to draw on the outer edge of the tracks, the cutting head followed the line exactly.
So where we had a 0.5mm trace with cutting lines running parallel on either side, the first cut reduced the effective width of the track to about 0.35mm and when the cutting head passed along the other side of the track, it reduced it by about 0.15mm further - so our final trace was more like 0.2mm wide, instead of the 0.5mm we'd planned on.
But that aside, the pcb it produced looked very, very good. (It should be a simple fix, in the software we simple need to tell the cutter to follow the outer edge of the cutting line, rather than the following the dead centre of the cutting line)