Troubleshooting ''Stairs'' on one side of model

After three attempts to mill out this piece, just practicing on some insulation foam, a set of “stairs” or “steps” appears on one side of the model.
I am assuming it is the X-axis and have increased tension on the belt with no change to the severity of these stairs.
I am new to this machines, perhaps it is an error from not assembling the Shapeoko 3 correctly?
Any advice is much appreciated!

If belt tension is adequate, it’s probably backlash, probably from a loose pulley set screw. Please check that:

If that’s not the case, other things to check:

  • workholding — is the piece shifting? (probably not for foam)
  • endmill deflection (probably not for foam)
  • check the belt for damaged teeth or a stretched section
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