Z banding / ribbing may caused by:
1) 2 bolts on the lead screw nut too tight
Loosen slightly of the 2 bolts on the lead screw nut, so that it can have slight allowance for the lead screws to move and compensate the banding.
2) Spring couplet doesn’t provide enough compensation
Replace with another spring couplet.
3) Gantry plate (Extruded side) not 90°
Add a washer to one side of the lead screw nut to compensate the angle, it may need a modification on the Z motor mount as well.
Some user try to band the plate to 90º.
Share with us in the comments if you facing the same problem, what is the cause and the fix.