You could set up a tachometer with that unit. Maybe even mount the sensor in the dust shoe. There might be an issue with the dust affecting the readings, but no way to tell without hooking something up and trying it.
It could be done with simple electronics and an Arduino, or your favorite microprocessor.
how would I do it with the arduino?
and am I going to have to use a hall effect sensor?
I would want something that is robust and not going to give me problems
To make it simple you can use an optical sensor and paint a white spot on the spindle shaft or collet nut.
The Arduino (not the one that runs grbl) is to read the sensor and calibrate for rpm and also to run a display if that’s what you want to do.
I have done this with my Makita and it works well. The Makita top speed is 30000 rpm. I have not used it in a dusty environment yet so I don’t know if that will significantly modify the resulting reading.
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You need something like this:
There is a small electronics shop in The Netherlands selling a rpm kit.
rpm display kit
If Grbl is controlling the spindle rpm you can use the pwm% as input.