Ballscrew alignment...

How good does it need to be?

Obviously the simple answer is "perfect" but, how many here can honestly say their ballscrew / nut / motion system is perfectly aligned??

So, what sort of tolerance is there, clearly if its way out then the nuts will bind, motion will lose steps and things will fail quickly but if only a little off of perfect?

When assembling my new build i have the rails on one side of a 45x90 beam and the screws on the other side, I aligned the rails and BK/BF blocks carefully with a vernier to the same reference surface of the beam to within 0.05mm.

However, there is naturally some discrepancy when it comes to tightening the nut mounting blocks down to the travel system, I have used some shims to get shaft deflection as close as possible to zero when tightening them down.

Or is it better to float the BK/BF blocks and then clamp the nuts, wind the travel to each end and tighten the BK/BF's up then??