CoreXY Carriage moves on the Y when moving the X I have a custom designed and built CoreXY printer. I have noticed that when I order the X to move from one side to the other (right to left) that the Y axis moves back about 0.6&nbsp;mm as well. When I move the X back (from left to right) the Y also moves forward approximately the same amount. 

What property of the CoreXY system could be causing this? Note, I am using 608 bearings instead of toothed idler pulleys; which I hope to rectify soon.

Steps per mm from the Marlin configuration.h:

DEFAULT_AXIS_STEPS_PER_UNIT   { 475.79, 482.87, 468.66, 188.1 }


Note, they are not the same for X and Y. This is because I am trying to run a calibration cube test on the machine.
For a CoreXY printer to move an axis (X or Y) it requires both stepper motors to turn. If both turn the same direction (at the same speed with the same pulleys), the X-axis will move, if they rotate both in a different direction (at the same speed with the same pulleys) the Y axis will move (see image of CoreXY kinematics below). Rotation of a single stepper would cause a 45&deg; printing pattern.



All four corner points (two top pulleys and two bottom steppers) are fixed to the CoreXY frame

It is therefore highly illogical that you need to use different values for your steps per mm (X = 475.79 steps/mm, Y = 482.87 steps/mm). If you need to use such values, this implies that your mechanical layout/mechanics is/are incorrect (skew frame, different pulley diameters, slip on pulley, incorrect tension of belts, etc.). It is advised to make them equal and check the mechanical layout and inspect all parts (pulley diameters) and make the tension equal (e.g. using a Belt Tension Gauge). If the head still moves in the perpendicular direction as commanded, you could try to calibrate from there.