About the only way Ive ever seen a stepper motor go bad is overcurrent burning out a motor coil, which then.stips the motor from.workimg at all.
You can check the grub screws for slippage by trying to stop the leadscrew from turning while commanding movement. If you can make the screw slip, something is loose. This is orders of magnitude more likely than a motor going bad.
You can hear the stepper motors skip. It’s not a subtle sound. The firmware counts pulses, and if you tell it to move back to its start position, the motor will return to the same position to within 1/2 of a full step.
A bad stepper driver could have this effect. You can check that by swapping the driver with one on a different channel. I would suggest swapping the Y and Z drivers. If the problem moves to the Y axis, then the drivers are confirmed as the problem.
An intermittent wiring problem could also do something like this. A wire that sometimes doesn’t make connection. This sometimes happens with solid core wiring. This would be harder to catch, but this will also make an unsubtle sound. The sound that it makes when it misses a full step because of a disconnected wire is a loud thunk.
Are they moving 10mm when asked. As in are you sure the steps are correct?
Is the problem only one side or both?
Take the lead screws off and observe and feel each Z steppers strength. matched?
Are you wires series or parralel? Parallel is no good at all.
At this point you have said everything is perfect. So something we have asked is not perfect.
The drivers power the stepper, voltage under and over is a problem.
Physical binding, or loose grub.
Firmware steps correct.
Moving them at the right speed. At your 3mm/s they should easily lift 20-30lbs each. But a slightly binding nut will stop them.
Wire connections, loose and they will move funky.
If they are moving at all it is 99.9% not an actual stepper issue. They are just loops of wire if one was broken they will not move.
when i tell them to move 10mm they move 10 mm but for some reason when i run the gcode whenever it goes to lift the z axes one moves the correct amount but the other does not i thought it might be an acceleration issue so i lowered the acceleration but that did not fix the problem.
its not doing it this badly any more but it basically still does this to a certain extent. (and yes i checked if i was giving it enough power and i was)
ya i know that is way to fast in the video so i dialed it back buy a lot but it still does it, they both start moving but then one of the stepper motors stops for a short period and then starts up again.
i think i figured it out after reading threw the marlin documentation it says that if G0 or G1 commands are not given a federate they will use the previous feed rate given and in my case after looking though the gcode it is trying to move the z axes at 1650mm/min because that was the previous federate.
Just so you know here, we ask that you specify your feedrate on every line because that part of Marlin has not worked correctly many times. Any of the post processors here, in our esltcam basics and milling basics page are set up that way.