The new z-axis is very stable and I am confident enough to take the z-axis opto endstop into use for calibrating the height automatically at the beginning of a print. A very simple bracket to hold the opto endstop was printed and fixed to a vertical rod. The endstop is mounted with bolts with springs to allow for easy calibration. Three nuts are trapped in the plastic. The bracket looks like this:
The final endstop installed looks like this:
The printing procedure now is to set the extruder in the middle of the area where an object should be printed, with height close to zero (0-2 cm high). The extruder than will be lowered 2 cm, but the movement will stop when the opto endstop is activated. This is the point where the height is calibrated to zero and the extruder can move up 2 cm to warm up and do a test extrusion.