Can't print circles lulzbot mini

My Lulzbot mini doesn’t seem to be able to print circular objects properly. It produces noticeably flattened sides. In the picture below the negative x direction has one indentation, the negative y axis has two.

So, it seems that as it’s rounding off around the y axis it’s travelling further along x than it ought to. These lines seem parallel when I pop the piece into the calipers. The x axis is about 20.1, the shortest width is about 19.7mm and the max width is about 20.7mm.

The inner circle seems to be off at a greater angle, and the direction seems different.

Any ideas on what is causing this and how I go about fixing it?



The scad code for this print is:
cylinder(d=20, h=4);
cylinder(d=5, h=9, center=true);
translate([-7,0, 4]) sphere(d=2, $fn=10, center=true);
translate([0,-5, 4]) sphere(d=2, $fn=10, center=true);
translate([0,-7, 4]) sphere(d=2, $fn=10, center=true);

The usual cause of out of round circles is a loose belt in the X or Y axis or your drive coupler is loose on the stepper motor shafts in the X or Y axis.

Go and tighten your belts first and give it another go. If that doesn’t solve it, check and see if the stepper motor shafts are slipping.

Most likely it’s a loose belt.

Is there any other advice or common solutions? I have pretty much the exact same problem as OP, tightening the belts didn’t help. Is there a guideline on the tightness of the pulley on the steppers? The set screws are really small so I don’t want to strip them with over tightening.

I did notice that my mini was QA’d over a year before i got it, do the belts stretch and lose precision over time?

It would be unusual for the belts to stretch at all over the entire lifetime of the 3d printer. It is possible the belt mount point slipped though. to tighten that, grab the end with needle nose pliers and start rolling it over the tip. Then loosen the belt anchor and keep turning to tighten. The belt should make a sound like a low guitar string being plucked when you pluck it.

If the motor pully is tight enough, it will not move on the motor shaft with the printer on if you try to manally manipulate it. If it moves back and forth a small amount that indicates a loose pulley.

Heat can also cause this issue if your mini is in an improperly vented enclosure or the main exhaust fan has failed.

I recently had a similar issue along the Y-Axis and it was my bearings alignment on the bed. The bearings are mounted to the underside on the HBP and there is a little flex there. So if your belts are tight enough try to slowly move your HBP all the way to the back and wait to hear the end stop click. Then move it to the front till the switch clicks. If it feels sluggish towards the back or front then your bearings might be out of alignment. That tension can cause your belt to flex and make shapes flat. What I did and YMMV was remove the HBP and loosen the 4 screws on the left and the 4 screws on the right (leave the 3 screw in the middle alone) Then slide the platform to the back and tighten the two screws on each side (4 total) towards the back. Then slide it to the front and tighten the remaining screws. It should slide easily now.

Thanks drandolph ! I think that’s my issue, the bed is much easier to move at one side than the other. Unfortunately I wasn’t able to find a ‘sweet spot’ for the bearings; I think the bars might be pretty far out of alignment. With the heated bed off and several screws loosened, I could get it to where the base plate was stiff on one side, and even 1/10 of turn on a loose screw would cause it bind, yet the bed was on the other side it was visibly loose.

I’m going to reach out to lulzbot support, this sounds atypical. Thanks for the help!