Using a flexible connection between stepper motor and threaded rod and a better mounting part for the ball bearing of the
z-axis wobbling can be suppressed, STL file in download section. Further information will follow on this site.
The stop for the z-axis reference is not easy to adjust. The original screw is reinstalled with the head up and secured with the lock nut (1) from turning. At the bottom end, a spacer bolt with M3 internal thread is mounted with a lock nut (2). An M3 screw with a large head, which is secured by a small spring (from ballpoint pen) allows easy adjustment of the z-axis zero level reference. When the metal plate (4) of the switch is bent downwardly right below the screw head there is a very precise stop.