According to Joe AutoDrill :

[ ... ]

Something like that... The drill I provided has a dry contact proof of
return switch so I'm looking for something I can put in series with that so
that BOTH (or actually ALL since there are more than one clamping locations)
have to be made for the circuit to be completed and thus the signal received
at the PLC to proceed.

Hmm ... the burglar alarm switches which used to be in Radio
Shack (and may still be, for all that I know) were in plastic and had a
pair of screw terminals on them

They would close whenever a magnet was close enough to them.

The real problem might be a buildup of chips on the magnet over
time, if he is drilling steel. If aluminum or plastic -- no problems
there. I would still coat over the screw terminals with an insulating
tape or some kind of insulating coating.

Good Luck
DoN.
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