Mini not printing circles

My wife bought me a lulzbot mini from you about a month ago. In the last week or two it stopped printing circles correctly (see image).

It seems that as it’s rounding off around the y axis it’s travelling further along x than it ought to. The flat areas seem parallel when I pop the piece into the calipers.

Printed from dimensional accuracy test.stl_ that comes on the lulzbot usb using white pla (filaform) using .
Here are the settings:

Don’t have the gcode, but if you tell me how I will export it. However, there’s nothing special about this stl, it happens with every curve I print.

The belt drives seem tight to me, although this is my first experience with a 3d printer, so I’m not sure exactly how tight they are supposed to be.

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



Hi Brendan,
You’d be hard pressed to get a belt on your printer that is too tight. It should almost sound like a banjo if you were to pluck the belt, anything less can result in this exact issue.

With that in mind, 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 your stepper motor shafts are slipping. Let me know how you go.

So, if there’s slack then when it changes direction the head doesn’t move while the slack is taken up? That’s why things are skewed?
The x belt is noticeably more musical than the y, so I will tighten the y.

Do you have a guide to belt tightening?

Hi @Brendan22876,

This guide should help you check and adjust the belts.

Thanks for the responses.

Trying to tighten those belts was unreasonably difficult. It is seemingly impossible to keep hold of the belts (tried two sorts of pliers) while tightening the screws. I have tried to tighten both x and y belts. There is an improvement in the prints, but I had an issue and need to redo them before I can tell whether the circles are printing properly.

I assume I could tell that whether stepper motor shafts are slipping by whether the set screws stay aligned with the flat part of the shaft?

Hey Brendan, you are correct. Make sure your set screws are aligned after tightening the belts.

Actually, I think it’s one of the pulley supports that is broken. It has been printing, but it was always the same flatness to the curves. I only just noticed this:

Is this part in the lulzbot production parts on the usb key? If so, which one? If I print it in abs would that be appropriate?

Edit: apparently it’s the y axis idler



Oh wow, it takes a lot of force to do that! The part can be downloaded here.

Mine has rounded ends. I think I want the earlier version.
Once I print it do I need brass heat set inserts in order to connect it?

Hi Brendan,

Personally, I haven’t been down that pathway. In this scenario it would be best to get in touch with Lulzbot support directly (they are available almost 24/7) via

I recall that there was a black line there. It must have been a crevice that has since separated.

Digging up my earliest round prints (a keycaps removal tool and a round keyring), the flattening at the end of the y axis travel is there, but not as noticeable.

Nice, perhaps perfect timing to get a reprint underway of that part. Please do let us know how it goes Brendan!

Aleph objects are sending me a new y idler. It is supposed to arrive today. As a stopgap I glued the original back together with acetone. Prints are much better:

Since it was working more or less I tried printing my own y idler. However, there seems to be something going on with my z axis - the first 2 or 3 mm of layers are printing untidily (see A below, x to the right, z is up the page) and circles in the x-z plane seem to be flattened in z (B):

What might be causing this?

Hi Brendan,

Great to hear you are on track with the fix! Given the Y axis is iffy, until you have the replacement I wouldn’t worry about the B issue.

Re A: if there was any warping what-so-ever then that can also lead to the edges appearing like that (as the material is pushed into a warped surface and is squished off the edge, appearing as it does above). A brim will help hold down those edges, especially if using ABS. Also bear in mind the layer height you’ve chosen might not be ideal if there is a camber. A thin layer of ABS printing up on an angle will encourage warping.

If a brim doesn’t help with similar prints in the future, then perhaps do an extruder calibration which can help fix any material / machine nuances.