Rectangle carved out out of spec

I created a larger project that included several rectangular cutouts. I sized them .480 x 2.000. The cutout came out to .625 x 2.1. The remainder of the project was within spec, so I don’t suspect product movement. I was using a 1/4" spiral upcut. This was cut on a new Shapeoko 3XXL. Is this too tight of a tolerance to ask for from the XXL?

No, you can do a lot better than that.

ShapeOko 3 specific questions can be found on the Shapeoko forum, Inventables sells the X-Carve which has the same origin the ShapeOko 2. As the machines arre quite similar they share te same problems.

Is the machine sound?
Motors make pullys turn, pull the belt and move the system. A slipping pulley will make you lose steps. If your square is rectangular I do not expect that to happen.
There will be some slack in your system so you need to send a little more steps for each mm to your machine than theoretical.

For a good machine calibration I suggest you take a look into the calibration process documented by Robert Rieke Calibration pattern

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It sounds as if you didn’t get the CAM correct in terms of taking into consideration endmill dimensions and runout.

Suggest cutting a diamond-circle-square pattern to calibrate:

Electromechanical things to check: