So I’m in the process of assembling my machine and for the life of me can not figure this out. The left and right side plates are not square on the machine (about 1/4 inch difference). I have measured the work area and it is square. Not sure what direction to go to.
 need to loosen the wasteboard and the frame ends and them possibly using a clamp or ratechet strap, pull the shorter diagonal (in your case the left rear and right front diagonal) until they equal the opposite diagonal measurement and then tighten down all of thr fasteners again and verify the frame is square after by measuring diagonally and coparing the 2 measurements and also repeating this gantry side comparison for good measure.
