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This tutorial describes my experiences with milling PCBs on the MAXNC.
I have used the following software:
Eagle 5.1 Layout editor (Light Edition) to create PCB Layout
Pcb-gcode v316 (free) by John Johnson to create G-Code
NC-Plot v1.1 (free) by Scott Martinez to verify Gcode created by Pcb-Gcode
And Hardware:
MaxNC 10CL Mill
Drewtronics 60deg spade mill, Micro carbide drill bits, End mills
Aluminum Tooling plate (self made), clamps
Video showing how to measure cutter runout. I've used a Mitutoyo .0001" Test Indicator which is around $200 but a cheaper .0005" Chinese indicator would also work for $70. Runout should not be more than .001". Runout is a combination of spindle bearing, spindle taper, collet inaccuracy.
Normally I would cut the traces first then parting off the board but this time the PCB was not perpendicular which is very important when it comes to milling double sided boards. The cutter was a 3/32" 2 flute carbide endmill. Z zero was measured to the tooling plate so the final Z height was .001" to ensure that the endmill cuts through the pcb I placed a sheet of paper between the tooling plate and the PCB. The paper was .003" thick thus the endmill was cutting through the PCB but did not cut into the tooling plate.
 For zeroing X and Y I have used the milled edge of the PCB. It is very important to zero X accurately otherwise the traces and holes will not line up properly on a double sided PCB. With the method shown on the video .001" repeatability can be achieved. Zeroing Y does not have to be that accurate because Y zero does not change when the PCB is flipped. Zero X to the left side of the PCB. Cut the traces, drill holes then flip the PCB and this time zero X to the right edge of the board. Y is zeroed to the bottom of the PCB. It is a good practice to use always the same endmill for zeroing X,Y.
Rolling pin method is an easy and cheap way to zero Z with practice betten than .001" repeatability can be achieved. A 1/4" precision ground dowel pin is perfect for this purpose. If you do not have one at home any friendly machinist would give you one for free. Here I am using a 3/32 endmill but the same technique can be used for zeroing the Drewtronics spademill and drills.
Note: In this video I am zeroing Z to the top surface of the PCB. In the previous video I was zeriong Z to tooling plate.
Before cutting the PCB make sure that the board is perfecly flat. With the rollling pin method check Z height on the four corners and in the middle. If the difference is more than +/-.001" between the readings place shims under the PCB. Thin alufoil can be used as shims (.001-2"). For cutting traces I use Dretronics 60 deg spade mills. Regularly check the quality of the cut because the spade mill becomes dull quickly. PCB fiber glass is not only a tough material to cut but toxic too make sure to wear respiratory mask. To save cutter life and improve surface finish clear chips regularly wich a brush or vacuum.
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