Hey Ross,
did you first check your machine for rectangularity, like → described here.
After you ensured that both diagonals are perfectly equal, then you can do the following:
- Shut down the CNC controller.
- Disconnect both Y stepper motors from the controller.
- Loosen slightly the four bolts on the top of every Y carriage that attach the X rail to them.
- Move both Y carriages towards front until they both touch the front end.
- Retighten the eight bolts.
- Restart the controller.
Which controller do you have, Buildbotics-derived Onefinity Original Series, or Elite Masso G3 Touch?