Hi Bryan, Connect one terminal of a ohm meter (or a buzzer etc) to the bit and the other to the metal strip. When the bit hits the strip it closes the circuit and you get an indication buzzer or needle swing on meter. If the metal strip is under the Maslow (which is the conventional way of doing it) the meter indicated the Z home position. I prefer to use the metal on top of the perspex, bring the maslow down until it just touches, set Z home temporarily, then remove strip and lower the bit the distance of the strip depth plus height of sled. And reset Z home.
You can also make it semi-automatic by bring the contacts out of the controller board which is described. Search for:
Instructions - How To Make And Set Up A Z-Probe On The M4
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