Slipping pulleys - Solution

I’m new to the forum as I recently acquired my X-Carve. I used to own a Shopbot PRT-96 that had a heavy steel Y-Axis and Z-Axis gantry and used a 3 HP router for a spindle. The PRT-96 had stepper motors were keyed and used a rack and gear instead of a belt. When I was looking at the Pulley connection on the X-Carve Stepper Motors, I was surprised to see that the shaft was smooth and didn’t have a key or at the very least a flat spot on the shaft. Stepper Motors create a lot of torque.

To test if your Pulley is slipping;

  • Place a mark on the outside shaft with a marker.
  • Use a piece of left over drive belt so you don’t damage the shaft and carefully tighten a pair of Vice Grips on the shaft to hold it.
  • Then hold the drive pulley with your fingers and gently rock the Vice Grips back and forth.

You shouldn’t feel any play at all on the Pulley. See pictures below.

I couldn’t tighten up the Set Screws enough to make the Pulley hold on the Shaft.

The Solution is to Grind a Flat Spot

  • Loosen the belts and make a mark where the Set Screw contacts the
  • Use a hand held grinder, I have a Ryobi but a Dremel works just a good.
  • Grind a flat spot on the shaft and then reassemble.

I used a little Lock-Tite on the Set Screw, but I’m not sure that was necessary and will make the Pulley difficult to remove later. After you put it all back together, do the Vice Grip test again.

Other references can be found on the web discussing this issue with stepper motors, one listed below;


I do this with all my stepper motors as well. I actually use a cut off wheel to carve a groove parallel with the shaft so the set screw enters the groove. Guaranteed not to slip! I hope everyone sees your post - it will solve a lot of “mysterious problems” for new owners.

Take care to not use too much pressure. Motors are usually not designed for side loads.

You don’t use pressure with a cutoff wheel - this wears it down too quickly and they are too expensive. You basically set it on the shaft and let the cutter do the work. As a bonus you get a small fireworks show, so it’s best to cut with all the lights off for maximum enjoyment.

The Nema 23’s in my kit have a flat spot. I may consider adding my own for the 2nd grub screw though for added assurance. I just had my pulleys slip on me and it took me an entire weekend to track it down letting my newb shine through.