I found out one of my stepper motors in the K40 had worn bearings and it was chattering when doing fast moves. And then I found a video talking about repairing them by replacing the bearings when I was looking for information on this worn bearing chatter.
10 625zz bearings are $10 or less and the process is pretty painless. I did end up making a couple of slotted jigs to help with the pressed on belt drive gear and sticky bearing removal. So don’t throw those stepper motors away and just rebuild them.
