I have been prototyping PCB boards for some time using a small engraving CNC. I bought the Longmill in hopes of doing larger projects while still maintaining my abilty to do PCB work.
I screwed down a piece of 1oz copper PCB to my spoilboard and manually established zero. I wrote a macro to increment Z in minimum increments (.005mm). I used a multimeter to establish my zero.
I then proceded to cut a small square in the PCB with a 10d .1mm engraving bit. I ran the code in .01 mm depth steps. At .03 mm, still continuity. At .04mm it broke through the copper completely. This is great since 1oz copper has a thickness of .0348mm.
Kudos to the Longmill designers!
I may elaborate more with a video.