Thanks Shane! I have a Duplicator i3, so the bed does move on the y-axis. I already print fairly slow: 45mm/s for infill and slower for perimeters and everything else. I’ve turned the jerk and acceleration down as well. That’s not to say that it might not be wobble causing the over-sized cube. It’s defintely not over-extruding; I’ve calibrated it down to where it was clearly under-extruding, and the cube was still over-sized.
I’m going to check belt tension, and make sure there’s no play anywhere else. I’m not sure what else to check.
I measured an older print of my middle-joiner, and there’s definitely some sizing issues. For example, the slot for the bearing on the connceting bolts is 8.6mm where according to NetFab it should be 9.07mm. The height of the total heigiht of the inner opening is 61.6mm where it should be 62.65mm. Now all the middle pieces fit together fine, and they fit on the conduit without issue, but I’m guessing this HAS to be the root issue.