Quote Originally Posted by JAZZCNC View Post
That then would just be a bodge to hide poor workman ship.
Or cheap ballscrews - hence why I think my measurement of the backlash might be misleading.

Quote Originally Posted by JAZZCNC View Post
Plus it wouldn't preload the nut it would twist the nut.!
Maybe, maybe not. It depends what's twisting. If you assume the rails are rigid (not quite true for supported rails as they bend upon the support, but not much) then it must be the gantry in between twisting and any force not parallel to the X-axis is resisted by the X-axis linear bearings leaving only the axial force for the nut. On James' machine, clearly the weakest point on the gantry is the joint between the sides and 6" wide cross piece, so that and the cross piece will bend. The ballnut mount and gantry sides are solid so won't bend noticeably. .. etc it's obvious I don't need to tell you.

Not suggesting one should actually do this intentionally. The force ratings of the ballnuts is huge, so I think you'd get away with it. I'd be most worried about the linear bearings.