Thanks Muzzer for your insight on EMF interference. Yes, I agree with what you mentioned. As I am an Electronics engineer from the 1980's, I do have a good understanding of these issues.

I thought about the need for resistors, caps and opto to reduce the stray currents - it would be over kill for a small hobby cnc machine like what I have got. In my case I am powering the controlled using a 24vDC power adaptor as opposed to maybe others who might be using the high wattage Mains PSU's to power their CNC machines and my be succeptible to such noise.

The spindle motor I am using is a basic 2 wire 24vDC 8000 rpm type so it is unlikely to interfere too much. I might solder a capacitor across the terminals to reduce and startup spikes.

Cheers