So Neale

One motor stalls for some reason, or sometimes when I hit the e-stop button. If the two motors do not stop at the same time, or come to a halt at slightly different rates, you have lost sync.

When you first power on the stepper drivers, the motors may or may not be at a full-step point. If not, it's likely that the driver will initialize it to the nearest full step, although I believe that some drivers are supposed to remember the last state when powered down. It's possible that if this happens often enough, the gantry may drift slowly out of square, a tiny amount each power-down/power-up cycle.
whether it is motors jumping sync/steps, and/or the movement of a less than stiff gantry/endplate/bearing setup, the TWIN homing switches correct the gantry squareness.

Makes sense.

And presumably, within a single motor/twin belt system, the "stiffness" within the drive belt arrangement tends to hold the gantry square.


Great got it thanks.

-Martin