Originally Posted by
awerby
That could be due to backlash. Backlash is the error you get when a screw changes direction. Since there's a little gap between where the screw threads bear on one side of the nut's threads and the other, the motor will turn a little before the threads engage. Expensive machines have ball screws using double or preloaded nuts that have little or no backlash; cheaper machines have to deal with it. CNC control programs have ways to compensate for it, but you need to measure it accurately first, before telling the program to add a certain number of steps when the screws change direction.
It could also be that your steps-per-unit settings are a little off - that would be the first thing to check, before dealing with backlash.