Here are the assembly pics, I didn't take machining pics because it's all standard stuff.
If something is not clear just ask and I'll mock up the set-up for you.
These are the parts,
The buzzer is a 3 to 15 volt DC type
The pack with the grey and white bits are a TO-3 transistor insulating kit. You only want one of the white bits (insulating sleeve)
The plumber thread seal tape is 0.1mm thick (not critical)
The other parts are a standard 9V battery and a battery snap.
This is the spring I got from the auto store, it was 100mm long and had a diameter or 15mm. I is quite stiff.
I heated and cut it at 20mm and then bent it to form a flat bottom and ground the base level.
The springs free length ended up to be 15mm long and the cavity space is 12.9mm so there is a bit of force on the button holder when assembled. This is what you want.
The following pics are the assembly steps
The wire was and old USB cable that I cut the ends off and used two out of the four wires.
The black wire is sandwiched between the button holder and the insulating sleeve cap.
The red wire attached and seated
The top button wrapped with one layer of Teflon tape. Use two or more layers if you have the thin tape.
The button installed in the button holder. I ran a blade around the tape to trim off the excess.
The inside view as assembled.
Finished the assembly. now to show the wire connections.
Well that's it
Oops, forgot to explain how to use it.
Just sit the Z-axis zero setter on top of your part and raise the mill knee or lower the spindle until the stationary
cutter contacts the button top. when contact is made the buzzer will sound. Then just set your DRO to minus (or plus, depending on your DRO) the height of the Z-axis zero setter and use that as you offset above the job.
Note the Z axis DRO readings
It could be used on the lathe as well, but you would have to hold it in place on the part face with you hand while you touched the tool tip to it.
The button is spring loaded so if you contact it with a bit too much force, it won't get damaged.