Elegant way to do this mentioned in the comments at: 11 Ways to Locate Part Zero on Your CNC Machine - CNCCookbook: Be A Better CNC'er
There’s also a very simple, practical technique for zeroing your Z-depth against a piece with a flat top, using a parallel:
- Lay the parallel on top of your workpiece, so that about 1/4 of it is hanging off over one edge
- Position your cutter over the overhanging section of the parallel
- Get down so you’re looking directly across the surface of your piece, and slowly lower the cutter
- As soon as you see the parallel start to tip up, adjust your Z-depth until you’re just about to see daylight at the far end of the parallel that’s lying flat on the piece
- Subtract the thickness of your parallel, and you’ve got your zero
This is all about lever angles, so to do this as sensitively as possible, you want to keep a couple of things in mind…
- The longer the section of parallel that’s lying flat against the surface of your piece, the better, since that’s going to give you the most sensitive first sign of daylight
- Conversely, the closer the edge of your cutter is to the edge of your workpiece — without getting too close — the more aggressively you’re going to raise that far edge. Ideally, you want the edge of the cutter to be just a little farther away from the edge of your piece than the thickness of your parallel