I have 2 nema 23 motors (X,Y), one of which (Y) started 'slipping', (for want of a better word) today. Its holding torque when not moving seems to be a good as ever, but when the y axis goes some distance, then stops and reverses, it makes a sound which was familiar to me when I ran it off a much lower current when testing. The motor has worked fine on the odd occasions I've used the machine in the last 4 or 5 years, I recently mothballed the machine, moved it, and reassembled it, and suddenly this happens.

It's a belt driven axis, I checked and rechecked everything there that could slip, nothing wrong.

Running the motor at 1/6th the acceleration didn't help, in fact, when I set it back to full acceleration, it had got worse.

The x axis motor, which works fine, feels very stiff and notchy to turn by hand, off the machine.

Are either or both on their way out? I hope not. I hadn't expected it to be such a sudden demise if so. I have a spare motor, which may not be that good either. I had expected a longer lifespan; the Z axis is not light by any means, and the Y axis does do a lot of small moves, and a few large ones. Also, the Y axis motor's bearings take the full side force of any acceleration/decceleration, although on this machine, there are no cutting forces or sideways forces for it to cope with. But as I said, the Y axis motor feels smooth to turn by hand., it's the X that's 'notchy'

Could it be a Mach problem? I doubt that, so I haven't reloaded Mach to test that idea.

M21

On a stepper notchy is good smooth is bad. Notchy is because the permanent magnets line up with the poles on the stator. So a smooth stepper has probably gotten demagnetized. That happens if you drive them with too much current, or overheat them.

Check the wiring connections, after long periods of non use ( a couple of weeks) my machine shows the same symptoms as yours in the X axis. I only have to touch the connectors on the top of the X stepper drive and it corrects its self. I finally cleaned the connectors and the problem has gone.  Low voltage supplies are prone to missing pulses caused by bad connections with the resultant noise from your motors.

I rechecked all wiring, substituted in a spare motor, and set the acceleration to 1/50th of its previous amount; that last change solves the problem, but leaves me with a very very slow machine. I don't think the motor is up to the task, maybe I need it to move too fast for it to maintain enough torque to avoid missing steps. Need more current, so may need to upgrade motor, maybe driver too. More money will need to be spent, it seems...