Calibration cube, top and bottom have different widths


I noticed during some cuts that the wall widths at the top and bottom of a cut were slightly off. So I cut out a 30mm * 30mm cube (because it’s quick) to test out what is going on.

The short story is the the bottom of the cube is winding up ~0.015" / 0.381mm wider than the top (so roughly an extra 0.0075" / 0.1905mm on each size), measured via caliper. This is fairly consistent regardless of endmill, having tried 1/8 upcut and straight, and 1/4 upcut.

Any thoughts on troubleshooting? I’m trying to machine finger-joints, but the variable width from top to bottom of the cut is throwing my tolerances off. It’s not like my cuts are skewed, it’s more that the width from face A to face B (parallel) is increasing as the endmill cuts around them.


initial thoughts, stepper motors might need calibrating, belts, check for square.

What spindle are you running? I had a similar problem when running the 24V spindle – it was causing interference with the steppers. Try attaching a marker to your X-axis gantry and running a job (with the spindle off) to draw a square several times and see if you experience the same thing.

I’m running a Dewalt 611. Belts have been tensioned with a hanging-scale (type with a hook), steps/mm calibrated on all axis using a combination of the wasteboard grid and a depth gauge, and everything checked for square.

I’ll give the marker thing a try at different heights.

My initial thought was maybe something to do with bit deflection…

maybe your spindle is not square to floor/wasteboard ,
i managed to square mine up by adjusting the v wheels to close the gap when i held a square against x/z carriages