This cockamamie contraption has a multimeter directly clamped to the probe, and to a wad of aluminium foil held in contact with the slide's tin oxide coating with a miniature clothes peg. The whole lot held down with various clamps. The probe tip is only just touching the surface - 1/50th of a turn of M3 with a 2:1 reduction will make or break contact. M3 thread is 0.5mm, so that's in the order of some microns (and a lot of undetermined experimental error).
As you can see, the contact resistance is around 370K. This is well within the ability of a microcontroller input pin to pick up.
So now all I need is a part-coated miniature slide, a redesigned slide holder with a contact clip (hey, I have a pogo pin collection .... somewhere), wire on a non-solderable probe, more bloody wires, and Z touch sensing on the GRBL board. A piece of cake 😉
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