When I first assembled my machine I found it to be out of square by about 0.1” or so. I tried to get into square first by homing it in and then by shims, neither of which worked to my satisfaction.
I eventually discovered that there is a down side to building such a stiff frame, if it ain’t square to begin with, it’s about impossible to rack in into square. I eventually found out for sure that I could rack my frame by about +/-0.05” using homing, not quite enough and even then it put a lot of stress into the frame.
I tried this several different ways, the one that got really close was when I loosened the trucks to allow the frame to pivot on them. At that point I cracked a bearing and decided to go back to the drawing board so to speak.
I decided to reengineer the gantry uprights so I could rack more easily. Previously, I was basically trying to move the A axis in an arc and the trucks, rail, and frame protested every time. With the new design I will be racking as a parallelogram instead of an arc which doesn’t induce stress into the frame.
Here is a picture of the new design:
I cut one of the gantry uprights to make a 6” riser because the 8020 is easy to work with. The 6” isn’t quite enough so under that is another 2” riser block made of hardwood. The wood riser bolts to the 8020 riser through the truck plate. The gantry beam is attached to the 8020 riser via two 3/8 x 8 carriage bolts on each side. The hardest part of all this was drilling 4 half inch holes in the gantry. The 3/8” carriage bolts fit into the 8020 with only minimal grinding on the head and shank by the way.
With this, I’m able to loosen four nuts and rack the gantry into square very easily. Once Square, it stays that way. Although it will be harder to change the gantry height now, I like the design better, it’s visually smaller as well.