Hi Hood good news you got me thinking, been down the workshop and had a look, I did wire up a couple of the limits in series so I have input5 spare which is linked to pin 15 so I should be able to use it. Once again thanks for you help mate. Jim

Yes, should work fine for you
Hood

Just connect that 5v to a free input on your BOB and then go to Config menu then Ports and Pins then Inputs and under the Index  entry put the port and pin number you are connected to. Go To General Config and set the Index Debounce to zero if its not already.
Thats you done
Hood

Going to attempt this myself. But I have a question or two.
How does Mach know that this is an input to read the spindle speed? Also, if Mach understands this is the actual spindle speed, does Mach intelligently use this to control the spindle speed or is this simply a feedback for visually checking.

The Index input is where Mach looks to see what the spindle speed is, ie 1 pulse per rev so it can work out what the RPM is.
There is a setting which can affect that, it is the ratio in Pulley Setp page, if you have a ratio of 1 then each pulse is one rev, if however you had the sensor on the motor and there was gearing between the spindle and motor then you would set a ratio so Mach can work out the amount of pulses it sees before it calls it a rev.

The index is used for readout and if you are in Feed Per Rev mode (G95)
It is not used to correct spindle speed unless you have closed loop spindle control selected but not sure how well that works, have heard reports from some people have said it doesnt. I have never used it as my spindles are step/dir servos.
Hood

As always, thanks for the quick response.

I have a Belt Drive Conversion kit installed, so I'd planned to mount the rare earth magnet under/on the pulley that's set on the spindle shaft itself.

Since the feedback doesn't reliably control the spindle speed, I'm not sure now whether I need to bother with all this since I have a handheld tachometer. (other than for "Coolness" sake).

I would be nice though to have it right there on the screen.