CNC machines have plenty of moving parts and if we want our routing and LASER cutting to be as good as possible the movements need to be as smooth and unrestricted as possible. When I built my cheap Chinese CNC it came with rigid shaft couplers for connecting the threaded lead screws to the stepper motors. That meant that the motor spindle and lead screw couldn’t be adjusted to be perfectly in-line, just because of tolerances in parts and mountings. So, for part of a full rotation the lead-screw was binding (i.e. becoming hard to turn) and the rest of the turn was fine.
If you find you get the same problem, you could just do what I did: change the rigid couplers for slightly flexible ones. In the photo below I’ve shown the difference between the two. The bottom one is the original rigid one and changing to the new one involved no more than loosening the grub-screws, moving the lead-screw to the side, then inserting the new coupler, pushing in the end of the lead-screw, and tightening the grub-screws. For just a few minutes work it made X and Y axis motions so much smoother. And, for only a few pounds/dollars per coupler it was a cheap job too.