I had a problem with Mach3 crashing/freezing when spindle is on.Last two days I spent trying to find solution and read alot of threads on this forum,almost every thread said that you need shields and special groundings.I tried everything but no success.Then I took multimeter and saw that every power supply in the room is going crazy 0-50V no matter if its 12V or 36V power supply it still goes 0-50V like crazy, I've came to conclusion that my whole router table is sending electrical noise. Before all of this I had my spindle on wooden plate and it was working but when I put it on metal one it just crashes everything.

So I realized that the easiest solution is to make non metal isolation between spindle and Z axis plate and it worked.I conected it without any grounds and shields and it works great.
Just make sure you also isolate screws where those are touching spindle and connect ground for the safety purposes.

I hope this will help someone.