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Are 100% sure there are no grubscrews on the side?
And if there is, that they've not been doubled up?

No, Panasonic (the brand). For example - https://www3.panasonic.biz/ac/ae/motor/fa-motor/ac-servo/a5/index.jsp

It is a little-known and barely documented fact that there is one version of Mach3 that is able to support two probe inputs with independently-set parameters (active low, etc). It might well be...

Great, well done! MOSFETs are nice devices.

Use an old phone charger with USB output, butcher a USB lead to get at the power conductors. Yes you MUST connect the earths. Probe is specced for 5V I think so you should use that.

It is actually a "solid state relay" so may be similar to what you were trying to use anyway.

For that one I linked to, typical turn on time is 0.1ms, and turn off 1ms.

On a typical home brew machine, those times are going to be negligible in the grand scheme of things.

That's the sort of thing, but you need to know more about the Gecko inputs. Their manual is silent on the input config except to say that they expect to be grounded by a switch which implies there's...

Something like this -
31681

These probes are basically to the original Renishaw design, which in effect have a pair of contacts that are closed before triggered and open when the probe moves the slightest amount. So this is...

That complicates things somewhat.
The diagram for the probe is just a basic functional diagram then, and there's a bit more going on in it.

Have you measured the output voltage on the probe pin?...

I'm not sure your treadmill motor would last too long, even if you are using it in short bursts.....
I'd be tempted to use a a step down transformer and then rectify and smooth the output.
Do you...

Tripping an RCD means you have leakage current to ground. This could potentially be fatal so disconnect the machine and test which of the power or signal wires are shorting to the chassis. On my KRV...

That's a perfect machine to drop modern electronics in.......

It depends on the voltage of the drivers you're using. Most drivers that come in the eBay kits are up to 48V, drivers like Leadshine AM882 (now superseded buy AM806 drives I believe) will handle up...

There are quite few Bridgeport conversions around. Might be worth having a search for some of those to look at what other people have done with a similar size machine?