Best way to detect touch on a metal surface?

Question

Arduino

Greetings!
In general, a head with a milling cutter rides on the CNC machine over the desktop
on the desktop, the workpiece being processed is wood,
it is pressed by heavy metal blanks,
once overlooked, it broke the cutter, and buying them is an expensive pleasure = (
Processing lasts 5-6 hours and look closely all this time it’s boring behind the machine =(
Since the feed rate is small, I would like to somehow make some kind of sensor
with which it would be possible to determine that the cutter touched the clamping blank and urgently stop it.
If wires could be connected to the clamps ,
thus, upon contact with the cutter, a closed circuit would be created,
but alas, the cutter can catch on the wires.
Maybe you have an idea?

Answer 1

There are many options you can think of, for example, an alternating current connection:
The simplest battery- powered generator is connected to one metal part, to another it connects a filter (coil + capacitor in the simplest case) of the same frequency, a diode (better, of course, an amplifier, albeit the simplest one) and a relay that cuts off power to the cutter motor.
But keep in mind that this scheme will only work when the cutter is already scratching the metal.

Answer 2

The cutter does not cling to the wires that feed the cutter motor? So what's the problem with running a wire along the same route?

Answer 3

it is pressed by heavy metal blanks,

Replace steel with LEAD.
Smelt a couple of dead car batteries into ingots, wrapped in cling film for safety.