If a delrin coupler is the only thing supporting the z axis weight it can gradually slide down on the motor shaft with each reversal of the stepper direction. The motor shaft is very smooth and the set screws in the delrin coupler has trouble staying tight enough to hold onto the shaft if there are no other means of supporting the z axis weight. I had to rough up my stepper shaft with fine sandpaper to get a better grip. Thrust washers and proper support would help my problem.
CarveOne
CarveOne
http://www.carveonecncwoodcraft.com