Many Thanks to the group & "Z axis counterweight

Hi Al,

Yes, I did replace the knee screw with a ball screw - from Rockford Ball
Screw. It's a 1.25" ballscrew with a single ball nut fitted. The
reasoning (based on judgement plus input from others) was that there
would always be some downforce load on the knee so backlash in Z axis
would be much less an issue than X or Y where I used double ball nuts
for zero backlash. I had to mill clearence to one side of the knee
support column in order to correctly fit the ball nut. I can post a pic
if anyone is interested. No, I didn't replace the bevel gears on the
knee (but did consider doing so) all I did was by trial & error add a
couple of thin shims under the large bevel gear to ever so slightly
pre-load the gears and reduce play. You know you have too much pre-load
when turning the knee crank causes a cogging sound. Mine is snug, with
minimal backlash less than a thou.

I chose to fit the gas springs near the front of the knee because by my
(misguided?) thinking at the time, I felt this arrangement would provide
a better balance of the various forces. I realize now the knee isn't
really cantilevered so my assumptions may be wrong. However, before I
fitted the gas springs, during a rapid Z move (over 48 IPM) I did notice
a horrible vibration sound which I suspected was the Z axis gib
vibrating. I played with the gib/way tightness and could reduce the
vibration but not eliminate it during a rapid move. After fitting the
gas springs, the problem went away completely.

Hope this helps.

Cheers, Peter