Yeah that canted circle with the distinctive “spot” in the curve where it shifts. I see it all the time on my 3D printer.
On the printer it is caused by one of the belts being loose.
I am not entirely sure how being lose generates that shape. It has something to do about movement in on direction in the axis being slightly off. I think it has something to do with the looser belt stretching? Sort of the belt equivalent of backlash? Not sure.
Double check the set screws on the pulleys, they can work themselves loose.
It could be that one side is too tight not too lose.
Double check the alignment of the pulley with the idlers, make sure one belt isn’t being pinched.
Double check the v wheels that they are not too tight or have some bits of plastic stuck in the groves from your acrylic crave.
Belts - Belts. I 'm posting this pictures again in case someone want to do what I did. I have no belt problem what so ever.
If you find insert nut difficult to put, you can insert any metal piece same thickness. Target is jointing belts teeth together to hold each other.
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I had problems with out of round circles, stiffened X axis, got better, then it got worse, bad winding on one of the Y motors and a weak pot on the g shield, all good now, doing 3d carving in soapstone!!
One of the windings on one of the Y motors had about one half of the resistance of all of the other windings (you need to totally unhook the leads to check), Also the controller on the g-shield for the y axis kept over heating. A friend who is much better with electronics than a am suggested the winding probably had a weak spot in its insulation somewhere and it was beginning to short out. This led the g shied to become over heated and caused temperature spikes that eventually killed it.
I honestly don’t understand it all but I swapped out those two components and everything is good. You might start by testing the motors with a multi-meter.
I’ve been having a very similar issue. Tried everything. I THINK my pulleys are slipping on me even though the screws are tight (they’re so small that I think they just don’t have the holding power). I’m going to take a leap of faith and put a dab of loctite 609 (green) on the shaft just inside of where the pulley sits so there’s minimal adhesion and hope that helps but doesn’t completely screw me if I ever have to take the thing off. If it works, I’ll let you know. If you don’t hear back from me… assume the worst! 
@JoshuaLMabry get it figured out? I have same shape as you.
Best idea is you can take pulley off, use your dremel or something to make flat surface for the set screws on the stepper shaft.
Update: Whew. Mine turned out to be V-Wheels were too tight this go around. Adjusting those gives me perfect circles again.
I have taken off my pulleys to grind a flat spot for the 2nd Grub Screw (mine have flat spot for 1 grub screw already from inventables) I then applied some Sleeve Retainer and reinstalled. hope that solves that slipping once and for all.
@WillMcleod i have not had a chance to fix this problem as i had another problem right after that. Can't connect my xcarve - #59 by JoshuaLMabry
I have been in the middle of a few big projects, including moving into a house with my own shop, so I’ll be getting my xcarve over here this weekend and hopefully can get it up and running soon. I’ll keep you posted
What do you mean by bad winding? Just got back to dealing with this issue.
The motors have coils of wires in them that make electro-magnets. One set of coils is powered by a pair of wires, another set by the other pair. One pair of wires had less resistance than the others as measured by my ohm-meter, implying that the insulation on one of the coils had broken down inside the motor, allowing the current to take a shorter path through one pair of wires than thought the other. This winding was the only one of all of them (I checked all of the motors) that was different. When I checked them I disconnected them from the controller so that they were electrically isolated from any other conductors.
As I understand it this can be caused by over heating
What is the resistance for a good steper? I have yet found my out of round problem.
Maybe the moderator can give the manufacturer’s specs.
The bad winding on my stepper motor had a resistance that was about 1/2 of the others.
This interacts with the controller because the lower resistance allows more current to flow.
