I’m building a 1000x1500 OX. I’ve squared the frame nicely, I think. I milled the gantry 20x60s and 20x40 to the same length and squared the ends. I set careful preload on the wheels; just enough to be able to turn them with my fingers. But across the ~1000mm Y axis, the gantry ran out of square by about 3mm end to end. Clearly I did something wrong. I tried loosening the gantry beams, holding it square against stops, and retightening it, but no luck. The left side consistently didn’t touch the stops.
I finally realized that the mill’s vice was probably a little bit misaligned, so I hadn’t quite managed to square the ends. arcsin(3/1000) is about 0.17, so it would only take about 0.17⁰ misaligned vice while squaring the stock to result in this error.
I loosened the gantry. I pushed the right side back a few inches to open up some gaps, and put 3-4mm wide strips of soda can wall in the gaps; 60mm long pieces in each gap on each side for the 20x60 beams, and 40mm long pieces in the gap on each side of the 20x40 beam. Because I was correcting for the left side being further back, these strips were on the front sides of the left ends of the beams and the back sides of the right ends of the beams.
I tightened it up, and now it’s as close to perfectly square as I can perceive by eye.