The wire I use (simply because I had it spare) is a grounding wire from an anti-static wrist strap. It has a banana plug pre-moulded on it and is the extra-flex type with a very flexible silicone rubber sheath. If you make a tool height setting block, let me know and I will send you the Mach3 Macro and some instructions on how to adapt it for your own needs. I did not write the macro, just furtled it a bit. I might just post it on the forum, but don't hold your breath, I've never been so busy since I retired.
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BTW, you need to check that the tool in the spindle connects through to ground on the BOB, otherwise you will need to put a croc clip on it with a connection to the BOB ground.
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Cheers,
Rob