Once your mills are ready and your design converted to G-Code, the milling process is pretty straight forward. Level the front side, apply a few layers of Sharpie or layout fluid, run the mill, then do the same for the back side. Then etch the board, clean it, tin it, and put it back on the mill for drilling. Most of the optimizations are to save steps, ensure accuracy, or just make it harder to screw up.
The only mod you will need to do to your mill is set up a probe for auto leveling. GBRL (and the firmware for most milling machines) is already set up to allow probing. Most CNC boards have pins for this as well. The probe needs to be set to ground, and the copper board itself will become the positive side of that circuit. Lowering the mill head will trip the circuit once it touches the board, and at that point you’ll have exactly Z=0. Autoleveling just step this same probe around the board until you have a height map, and then bCNC will automatically compensate for slight variations in the height while milling. This is a crucial step for circuit milling — this ‘slight variation’ in height is only in the sub millimetre range, but so it the copper layer itself!
The main change you’ll want to make to the Nomad 883 (or any mill without a way to probe the PCB surface) is set things up for auto leveling. The Nomad has a tool probe already, but it detects the bit with a switch, not electrical contact (a good thing considering how dirty bits can become!).
You can easily add a second probe to the existing probe line, you just need some wire and a few passive components. I wanted this mod to be 100% reversible, so I made a crude splitter and plugged that into the probe slot. Then I plugged the original probe (green and white) into one end, and the new conductive probe (yellow and black) into the other. I can revert by just disconnecting the splitter and plugging the original probe wire back in.
The last thing to deal with is noise on the line. For this, add a 0.01uf capacitor across the wire, and a 100 ohms resistor along the 5V line. You can place this right on the wire, just make sure it isn’t shorting and wrap it in heat shrink. Place it towards the end of the probe nearest the PCB, but leave enough room for the platen to move around without disconnecting. I used twisted wire for most of the length (shielded wire would be even better I assume). The circuit is as follows:
Hooking this all up now is just a matter of connecting the positive wire to your PCB, and the ground to either the tool probe peg, the bit, or up in the motor area. Direction matters here – the PCB needs to be the positive side of this connection. I attach magnets to the ground wire and cover that with conductive copper tape – it then easily attaches to the tool probe peg, and is easy to get out of the way when not needed. Lastly, attach a spade connector to the positive wire, and screw that into one of the corners of the PCB.
I like this setup because the probe tool works as normal — I don’t usually disconnect it even when working with regular milling. Also can stay hooked up throughout the process – there is never a need to disconnect the GND side, and you mill with the positive attached to the board. I did add a connector for the whole probe (red, in the foreground). Also the two ends are different lengths, and there is a small cover to the live end so it doesn’t easily short out when removed. Yes, it does look hackish – and indeed it is -but it does work remarkably well :).
You should now be able to autolevel from bCNC. This leveling is an essential step for any PCB milling. I do mill the wasteboard flat, but slight variations in screw tightness, dust, and other gremlins mean you rarely will have a perfectly level board all the way across. The good news is auto-leveling only takes a minute or two with this setup.
Potential Screw Ups
If your probe isn’t working, it may be that things are hooked up backwards – polarity matters. Check with a multi-meter, or just switch wires and test. You can test by just bringing the positive wire to the tip while it is probing – better than needing to emergency stop.
You shouldn’t ever have to remove the copper ball side from your tool probe peg, however make sure your spade connector is screwed in before probing. Otherwise, I’ve heard from a friend, that your scriber bit will probe deep into your PCB.
No need to disconnect the positive wire from your PCB once it is leveled – this will only change the exact level of that corner. Also, be very careful probing a board that has already been milled or etched. Areas in the middle may no longer have a connection to the edges, so there may be no connective path there even if the tip lands on copper. Autolevel will no longer work.
Next: Etching and Tinning