Add alligator clips to the touch plate and another wire to the tool, and set up Mach3 to auto zero. Three springs help avoid damage to the tool.

You will need:
The 2 printed parts
3-2" x 1/4" springs
3-5mm x 30mm hex head screws
3-Percision shims
1-2" Fender washer
Two part epoxy
Alligator clips and wire.

Use pictures to assemble. Solder a short wire to the fender washer so it will go through the hole in the top printed part. Not necessary to feed through the bottom part. Glue Fender washer to the top part using epoxy. Hook up to digital input on your stepper control and setup the port to be z-zero. measure the actual height of the block and setup Mach3 to offset the height of the block. Then when you need to zero your z, connect one lead to the tool/bit and the other lead to the block wire.

Customize using the 123D files included.