Quote Originally Posted by Norvic View Post
.....The few times I've been there to actually see it happen, the motor makes a high pitch noise then carries on.

....
Sam
Hello Sam, how annoying this sort of thing can be! My mill X power feed is done with a stepper motor and I get the high pitched squeal when and if it stalls. There is a tight spot in the table towards one end and if I try to drive the table too fast that will cause this. It sounds as though you have tried different feeds so may not be that.

One other thing that can happen is that a stepper can have a fragment of swarf in it which gets caught in the "teeth" and jams it. This may go away but then come back in a different place as the bit gets "circulated". I've had this once on a new NEMA 8 motor from a far east supplier. Take the stepper off and disconnect the wires, then try spinning it with your fingers. You may need to try for quite a while but if you suddenly feel the motor jam you've found it. There's no cure really, better to scrap the motor and replace.