Zig- Zag Prints- How do I fix this?

I have a Lulzbot Mini, and my prints come out slightly offset between layers. One layer prints fine, but the next layer shifts to the right about 1 mm, then the next layer corrects itself, and the process repeats until the print is complete.


Two of your pictures are missing (not your fault, its a problem with the forum software), but based on your description of alternating left/right I would check the X belt tension and the set screw in the X motor pulley. The belt should make a good “twang” like a guitar string when plucked, and the set screw should be tight against the flat on the motor shaft.

Which are the set screws? I don’t want to make things worse.

The toothed pulleys that drive the belts each have two small grub screws that secure them to the motor shaft. One should be aligned to the “flat” of the motor shaft, and both should be snug, but be careful not to strip them (Lulzbot’s instructions say torque to 5 inch pounds). You’ll need a small (1.3mm as I recall) hex wrench to check/tighten them.

Can you post a picture of what they are supposed to look like? How do I know if they are too tight?

Study the picture below, which came from step 3 of https://ohai.lulzbot.com/project/mini-104-y-axis-assembly/mini-v104/

One of the toothed pulleys is shown, already mounted on the motor shaft. One of the grub screw holes is visible, just below the teeth, in the wider part of the pulley closest to the motor. Looking at the shaft below the pulley, you can see the “flat” part of the shaft is aligned with that visible grub screw hole. The other grub screw is out of sight in this photo, on the back side of the pulley.

As for how tight… The only way to be 100% sure is use a torque driver and torque to 5 inch pounds as the instructions say. The screws just need to be “snug”; 5 inch pounds is NOT much force and they can be stripped if over-tightened so be careful or get some hands-on help from someone mechanically inclined if you’re not sure. One screw must be aligned on that “flat” part of the shaft, and both just need to be snug enough that they won’t loosen from vibration.

If the screws are loose, or one is not tightened onto the shaft’s flat, then there can be “play” or “slop” between the shaft and pulley – meaning the motor shaft can turn slightly without imparting all of that motion to the pulley – and that can cause shifted print positions.