Some time ago, I noticed that the machine was cutting parallelograms instead of squares, and it turns out that the gantry wasn’t square to the rest of the frame. The motors are wired together but of course they can be forced to move independently, especially when not powered.
The obvious fix is to shove the carriages against a pair of stops, one on each side, carefully positioned to bring the axis into square when shoved against. The problem is that putting these stops at the front of the machine would interfere with homing.
This skew problem just bit me again today and ruined some expensive stock so I did some tinkering on the machine and realized I could easily put my squaring stops at the rear of the machine, just past $131 Y max travel (643 in my machine). Since the machine never goes there (I have soft limits enabled), it’s never gonna hit them on its own.
To speed up the shoving operation, I first G53 G0 X300 Y640 which gets me rapidly within kissing distance, then turn off the power supply (or just $SLP) to disable the stepper drivers. Nudge the gantry back so both carriages hit my stops (just a few mm beyond Y640), then power back up (or Reset and Unlock), then G53 G0 X0 Y0 to get rapidly back somewhere very close to home. Of course I’m a few mm back now, so $H home the machine and ta-daa, I’m now square, true, and zeroed.
(I have a big fat hotkey for that G53 G0 X300 Y640 command in my sender, btw. I call it GTFOTW. It helps massively when loading a new workpiece.)
The stops are just nylon spacers on a #6 screw. I found that bog-standard #6 square-nuts will tilt into the 20mm rail slots without needing to be fed from the end, and seem to hold just fine. Lots cheaper and more convenient than purpose-built slide nuts!