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Not printing round circles

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Our Ultimaker is not printing round circles any more. I've checked if the pulleys are screwed tight, if there is not to much or to less tension on the belts, the small belts as well. I have checked if the pulleys of the small belts do not touch the wood. We even changed the pulleys, because we thought that they might be excentric and changed the belts as well. After changing these and adjusting the axes, the problem didn't change. I also tried printing on half the speed, that didn't change a bit in the roundness. For as far as I can see is the casing straight and not damaged or misformed.

What else could cause the problem of not printing round circles anymore? Any other suggestions?

Help would be appreciated,



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9/10 it is the pulley on the stepper motor the small belts are connected to. You could test that by making an index mark on the pulley and stepper motor shaft with a pen and seeing if it has slipped after a print (obviously just do a small one to see so you aren't wasting plastic).


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I doubt your pulleys are slipping as this would cause a shift on layers above. Please post a photo of the part. How much error are we talking about?

Sometimes the bearings in the head of the printer get loose. With power off, try pushing the head back in forth in X and in Y (with the maximum force where the head does not yet move). Is there any play?

If you have "play" or "backlash" then you would also see issues where the infill isn't meeting the edge of the part and the infill is in a pattern (sometimes) where there are 2 lines together, then a gap, then 2 lines, then a gap.

Please post a photo showing the problem.


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Looking at the photo - I can get a print like this with a perfectly functioning printer. In other words this can be caused by either:

1) Something wrong with one or two X/Y axis.

2) Other things.

I'm going to assume #2 for a minute.

The first layer is critical - it's hard to get the bed the exact right height. Often when laying down a circle for the first layer the filament is place don the tape at the right spot and then 1/10 of a second later it gets pulled and slides across the tape towards the center and you can get a "flat spot" in the circle. This is fixed by printing the first layer very slow in Cura (20mm/sec) but more importantly getting the Z height perfect. If you are too high by even the width of a sheet of paper, then the filament doesn't get pressed down enough. On the first layer the filament needs to get pressed down more. Look at the first layer carefully as it is going down - if it isn't much wider than tall then grab the z screw coupler and twist it away from you 4 or so clicks. Do this while the skirt is being printed so it is perfect before the part is being printed.

The second problem is cooling - on small parts like this you need at least 3 seconds - preferably 5 seconds between layers. This is small enough that the UM can do a layer in only 2 seconds. So make sure the fan is on high and use the "minimum layer time" feature and set it to 5 or 7 seconds and don't print faster than 100% when printing parts this small.

I could be completely wrong - it might be something with your X or Y axis.


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Another idea. It could be that the belt tension for the x/y axis is different. If there is too much of a difference I can imagine that circles would be printed like this (not round).

I think this issue is slightly related to one of my problems of "walls delaminating" (link).

tl;dr: Make sure the belt tension is consistent (for x/y and sepparately for the shortbelts).


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