Some spindles are grounded which causes problems when connecting a Z probe. This is because the Z probe pin is normally at 5v and the sense plate is grounded, so if you connect the pin to the spindle the probe routine will show an error since it is already connected before it can try to move.
My solution is to use an optocoupler based paper sensor from a printer. The bit presses on the device which then breaks the light beam. No actual connection to the spindle is needed.
Details here
Optoisolated Z probe
Solve the earthed spindle problem when probing
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MaryD and Mark Carew like this.