Irregular Circles on UMO+

[brentwerder 13]

Recently I've had quite a bit of trouble getting technical prints (especially very small ones with tight tolerances) to print right on my UMO+. It looks like my printer isn't printing circles right at all. As you can see, the tolerances here are off by about a millimeter. The irregular circles are very repeatable, too - the odd oval shape is the same through the part.

Even just looking at the part, it's obvious that it's not completely circular:

I've tried every suggestion I could find on 3DVerkstan, which helped the problem a bit, but it's still there and it's enough to keep very small parts from printing correctly.

I turned my acceleration down to 1800mm/s^2, and I printed the part pictured at 15mm/s just to make sure my speed wasn't a factor. My Ultimaker has an E3D V6 with a mount that holds it pretty tightly. I don't think that's likely to be the issue.

I might just want to order some new timing belts for the motors, but those seem pretty tight to me.

Any other suggestions?