Friday, 17 June 2011

Building a drilling milling machine

With the arrival of our CNC machine, all kinds of ideas starting springing to mind. But the idea behind getting a machine like this was to help speed up the production of PCBs - to easily create kits of components for workshops, like those run by BuildBrighton, or even to produce products for an online shop or to sell on eBay.

So that's where we're starting - to get the CNC machine to drill a series of holes that match up with a PCB circuit/design.

Following Tom's excellent research into g-code files, we're pretty confident that we can get the CNC to move to specific points above a sheet of copper-clad board, and activate the z-axis (up-and-down axis) to get it to thrust a Dremmel drill into the board. The most challenging part is to create the g-code to begin with!

So far, we've resisted all attempts to learn Eagle for designing and producing PCBs. But it looked like this was on the cards, since it already includes export options for drilling and milling PCBs. The only trouble with Eagle is, well, it's just not very nice. The software we use for schematic design and PCB layout is the excellent (and free) ExpressPCB.

It's just easier and all round nicer to work with - no confusing menus, sub-menus, things hidden inside other windows and so on. It has a really neat library of existing components, and if you can't find what you're looking for, creating your own is a doddle. Because we do a lot of home-etching, you can make big fat pads and traces really easily, and the final boards produced are easy to solder and work with. Eagle, on the other hand, was just confusing and produced boards with nasty lozenge-shaped pads and thin traces that were easily broken during etching.
The problem with ExpressPCB is that although the software is free, it's provided by a fab-shop who want you to send your designs to them for manufacture. As a result, there are no export options supported.

To date, we've always printed a PCB layout either straight to the printer (where the press-n-peel is already loaded and ready to do) or - using CutePDF - to a PDF file for editing.

Inkscape is a great program for editing graphics-heavy PDF files. So naturally we loaded our PCB into Inkscape and choose the "ungroup" command to break the board up into lots of lines and circles.
Then we selected the (black) centre circles found at the centre of each pad and copied them all to the clipboard. By creating a new image and pasting the copied shapes into the top-left hand corner, suddenly we had the basis for our drill file:

From within Inkscape we saved this new image as an .xaml file

This created an XML document, filled with plot and curve commands.
The curve/shape commands are of little consequence to us, but the plotting commands are just what we're after:

<?xml version="1.0" encoding="UTF-8"?>

<Canvas xmlns="" Name="svg5395" Width="744" Height="1052"><Canvas.Resources/><Canvas Name="layer1"><Path xmlns:x="" Name="path4253" Fill="#FF000000" Data="m 44.142826 61.717113 c 0 -1.05 -0.75 -1.8 -1.65 -1.8 -1.05 0 -1.8 0.75 -1.8 1.8 0 0.9 0.75 1.65 1.8 1.65 0.9 0 1.65 -0.75 1.65 -1.65"/><Path xmlns:x="" Name="path4255" Fill="#FF000000" Data="m 44.142826 70.717113 c 0 -1.05 -0.75 -1.8 -1.65 -1.8 -1.05 0 -1.8 0.75 -1.8 1.8 0 0.9 0.75 1.65 1.8 1.65 0.9 0 1.65 -0.75 1.65 -1.65"/><Path xmlns:x="" Name="path4257" Fill="#FF000000" Data="m 44.142826 79.717113 c 0 -1.05 -0.75 -1.8 -1.65 -1.8 -1.05 0 -1.8 0.75 -1.8 1.8 0 0.9 0.75 1.65 1.8 1.65 0.9 0 1.65 -0.75 1.65 -1.65"/><Path xmlns:x="" Name="path4259" Fill="#FF000000" Data="m 44.142826 88.717113 c 0 -1.05 -0.75 -1.8 -1.65 -1.8 -1.05 0 -1.8 0.75 -1.8 1.8 0 0.9 0.75 1.65 1.8 1.65 0.9 0 1.65 -0.75 1.65 -1.65"/>

As you can see, every hole drawn in the .xaml file begins the Data tag with "m" followed by two digits. This is the "move" command. The digits that follow are the curve commands to draw a filled circle. We're not bothered about that - but what is of interest is the co-ordinates of the m command (i.e. where the hole is placed).

In fact, because every single hole on the board is drawn by the same filled circle shape, it should be possible to simplify this process in future. If we save the original PCB as .xaml (without first selecting all the hole points and copying them to a new image file) we should be able to pick out the drill holes, even from a complex PCB image.
As you can see from the .xaml example above, every hole has a move command, co-ordinates to place it, then exactly the same sequence of drawing commands - "c 0 -1.05 -0.75 -1.8 -1.65 -1.8 -1.05 0 -1.8 0.75 -1.8 1.8 0 0.9 0.75 1.65 1.8 1.65 0.9 0 1.65 -0.75 1.65 -1.65"

So in theory we should be able to parse the original PCB image file, even with all the traces and filled plane backgrounds and everything else, and just look for any shapes that include "c 0 -1.05 -0.75 -1.8 -1.65 -1.8 -1.05 0 -1.8 0.75 -1.8 1.8 0 0.9 0.75 1.65 1.8 1.65 0.9 0 1.65 -0.75 1.65 -1.65" in the Data tag. Any shape with this data in it we can say is a black filled circle (a 1mm hole that needs drilling) so our parser can strip out the co-ordinates from the "m" command. All other shapes can be ignored.

Results of a test app will be posted here soon.
If all goes to plan, maybe we won't have to shell out on an Eagle licence that we didn't really want in the first place.....