On CoreXY the X gantry can sit at a slight angle and still move perfectly smoothly. Nothing feels wrong, nothing binds — but every print comes out as a parallelogram, and no slicer setting corrects it. This has no equivalent on a bed-slinger, which is why it catches people moving up to their first CoreXY.
Phase 0 — Mechanicalabout 20 min
How to do it
Power off so the motors are free to move.
Push the gantry gently to the very back until both sides hit their end stops or the frame.
Hold it there for a moment — this lets both belts settle to the same reference.
Some machines want the belts loosened for this and re-tensioned afterward. Check your printer documentation before forcing anything.
Print a large flat square and measure BOTH diagonals. Equal diagonals means square; a consistent difference means the gantry is still off.
How to read the result
The gantry meets both Y rails at the same point on each side, and a printed square measures the same across both diagonals.
Redoing this invalidates
Calibration is a chain. Change this and the results below were measured against the old value: