Hi Stan.
Having built my own smaller set up, it was a bit of a learning curve to discover just how flexible my aluminium mounting plates were.
So I'm a bit concerned by the plates you have at the top and bottom of the z-axis where the bearings for the lead screw are mounted. They appear to be about 5mm thick ?
Is there any chance that the whole lead screw assembly can flex up and down by any small but measurable amount ?
Other than that, I'm more than a little envious of what you have achieved.
Regards
John
It's like doing jigsaw puzzles in the dark.
Enjoy today's problems, for tomorrow's may be worse.