I watched Keen's (Threadexpress) youtube videos on 'Tormach Z - quick adjustments) with great interest and afterwards decided to check my Z axis linear accuracy. Using a Heimer (and checking with another DTI) I have around 0.1mm (.004") 'lost motion', so decided to adjust.
After following Tormach's procedure for both the gib and ballscrew bearing pre-load, I can't improve below about 0.08mm, or 0.0031". The specification is 0.035mm (0.0014") and the test sheet for my mill (s/n 70971) shows 0.001mm or 0.0004". Keen managed after his adjustments to get 0.001mm so that's what I'm after, or thereabouts.
I've been over the procedure at least 3 times for both adjustments and can't improve. I removed the Z axis stepper and substituted a 'shaft and handle' to feel the ballscrew rotational friction as I don't want to go over-tightening the pre-load unnecessarily. The problem I guess is 'what feels right' in these circumstances. The gib adjustment is 'certain' as the lost motion can be seen to increase on the DTI when it's too tight, so the 'sweet-spot' can be determined. The ballscrew friction, however is down to 'what feels either free or tight', and for me too subjective. With the Z axis stepper (and hence brake) removed and the gib properly adjusted, should the mill's head drop (spinning the ballscrew) under it's own weight? Mine doesn't, so is the pre-load too tight??
EDIT - Update: I again removed the Z axis stepper, coupler and loosened the two adjusting nuts so the ballscrew bearing friction reduced to allow the head to drop under its own weight, then reassembled and applied preload. No change, still no better than 0.1mm! Tormach state in the manual that the ball-nuts are pre-tensioned for backlash (not user adjustable) so the lost motion HAS to be in the ballscrew angular contact bearings, right? Maybe the bearings are shot, but in less than a year's light use??
Any experienced help would be appreciated!
Cheers, Andy
Oxford, UK