Hi Jonathan

I've only had a very small think about this, but the ball screw moves it, the bearings hold it. You say the linear bearings ar not supporting? They should though? They look like they are hanging free from either end? Anyway, can you can mount a dial gauge on the bearing rod, or the bearings themselves and pick up exactly where the flex is. I bet it's the x bearing rods flexing? It's probably not the screw at all, in fact, moveing the screw closer may allow the bearings to flex more (or the flex at the tool will be greater because the screw forms a triangle?). Mine has an ali extrusion that supports the rod, the bearings have a break in them, they cover about 270 degrees of the rod. So the bearing is supported all the way across. You could upgrade and squeeze some rails in there but ££££s. To me, if you dont want to spend much, and your relying on the screw as an anti twist device of sorts cuz the bearings have play, move it away from teh bearings if you see what I mean?

You could add a third (or fourth) bearing, pref at the bottom, other side of ball screw so although the top still deflects, this will be minimised.. Whats the travel? I think there is a ground rod about a meter long and bearing (bearings if I can find them?) on the floor of my LR (Land Rover) (about an inch dia) (don't ask why!). You can have it for the postage mate if it's of any use?

Tom