Sunday, 12 August 2012

ExpressPCB to NC Drill files

As part of the CNC drilling machine challenge, one of the things we have to be able to do is parse NC drill files. The idea being that using an industry standard file format makes the machine compatible with a much wider range of PCB layout software.

One of the problems we have is that Eagle sucks.
Yes, that's quite an inflammatory comment but, compared to ExpressPCB, we've seen loads of people have trouble with Eagle-drawn circuit boards.

The first thing is those stupid lozenge shaped pads. And the default hole size seems to be too small. And the pads are ridiculously small. And when you've finally etched your board and drilled it, it's all too easy to ruin a pad because your 1mm drill bit has ripped up all but the tiniest thread of copper left around the pad (right-most-pad, below). One slight wobble with the drill or mis-aligned pad and the whole board can be ruined!

Over at Nerd Towers, we defy convention and refuse to get drawn in to the everyone-uses-Eagle-so-we-must argument. Although it's less of an ideological standpoint and more to do with the fact that it's just so complicated to use when no-one has ever shown you how!

Our tool of choice is ExpressPCB. It's not only free but it's simple to use.
For the hardcore gerber-loving geek crowd, the very things we laud it for may well be it's Achilles Heel too - but it is very simple to use and you can get a PCB thrown together very quickly, all with 2mm pads with 1mm holes (ok, the default is 0.89mm but what's a tenth of a millimetre between friends?). No worring about mirroring, or not mirroring, or which-do-I-mirror before printing for toner transfer - just draw on the top (red) layer and print it out!

To produce PCBs for etching, we usually print to a virtual printer, such as CutePDF and make a PDF file for editing in Inkscape but one thing we recently discovered was the "export to DXF" option

This is quite exciting, as it allows us to generate a file which can be parsed and turned into a drill file. The export to dxf option in ExpressPCB can send just the pad data to a single drawing. Simply loop through the text-based dxf file, find all instances of CIRCLE and write the co-ordinates out to a NC Drill compatible file format!

After a cursory glance at the generated dxf file we can see all our pad data quite easily:

Every pad is a circle entity so we fiddled with a few values and loaded the resulting dxf into Inkscape until we found which entries corresponded to the X and Y co-ordinates. Ultimately it is these values that we'll be interested in to create our own NC drill file.

(comments in the above image were added one we'd identified which values did what, they were not present in the original, generated dxf)

To try out our idea, we picked a circle and set the X/Y to zero and the radius to 4
Interestingly, Inkscape does not position circles from their centrepoint, but from the bottom left corner of the shape. So we expected to see our shape at -4,-4

Inkscape seems to include the stroke (shape outline) width in the X/Y co-ordinates for each shape. So we reduced the stroke width and indeed the X/Y co-ordinates updated accordingly.

We can only assume that with a stroke width of zero, the shape would indeed line up to -4,-4 and thus prove that the values we changed in our dxf file were indeed the correct x,y and radius values.
With this in mind, we're off to write a simple script to convert metric x/y value pairs into an NC Drill file.....