Have you calibrated your steps/mm?
You have to remember, the machine can only move in micro-step increments — so 1/40th of a mm w/ the default belt, pulley and 8x micro-stepping — if you calibrate, then that number become some odd number, and the machine can’t get to certain positions which would otherwise be on the grid.
It’s a trade-off — not calibrating for steps gives one nice numbers in the interface which don’t match where the machine is actually at, calibrating has the machine actually where it is (more-or-less, belts don’t stretch exactly evenly) but you can’t get it to some numeric positions.
Ways to improve this:
- get motors with more steps/rev. (I rather miss my SO1 which had 400 steps/rev NEMA17s)
- implement a pulley reduction system (may introduce backlash)
Folks who get really serious about this will machine a part, measure it, then adjust the CAM file to adjust for how things are off.
The other possibility is it’s getting pulled into the cut — leave a roughing clearance and take a finishing pass, see: Adding geometry to cut as a pocket with a finishing pass