The accuracy is highly dependent on how hard the bit is pushed thru the material. The theoretical accuracy is 13 microns (0.0004"). However, there are several things that add to error.
- bit deflection
- belt stretch
- micro- stepping
- lead screw error
- frame rigidity
- bit size configuration in the g-code
With all of these, the well-maintained E3 or E4 CNC router will still be able to hold accuracy within a few thousands of an inch. (0.002" to 0.004"), or 50 to 100 microns.
The best way to hold tolerance is to make light cuts on the final pass. This reduces bit deflection, belt stretch, and frame flexing.