So…..I found the same problem with the current thingiverse design as the new one I was working one. Makes so much sense now. It explains why people were getting machines that weren’t square, why there are those parts on thingiverse to start your cnc square.
If you tighten the hell out of the long 3″ bolts on the center assembly you will pivot the middle end in a bit pushing the whole thing out of square. I never had these issues because mine are super loose like hand tight. I have never tightened mine enough to torque it out of square, I never had the issue, I just assumed bad prints or something else causing it. Or in other words. if you were to put to large of conduit in your z axis you will push the bearings open too far, and that will flex the middle end the other way, opposite of over tightening it.
I made the the new center assembly very adjustable to see if I could tighten it up and carefully square it at the same time. Not easy.
-Leave it, now that we know about it we can easily check it during assembly. If it’s worked this whole year why bother? (not my favorite)
-Change the design and add 4 more bolts. 4 too hold it all together, 4 to use as the z axis tensioners. (better) (maybe make it monolithic instead)