I also see a possible problem with the rails bowing slightly, and this will also put presure on the ball screw.
May be a good idea to either use some profile rail along the top of the 50X50 box section, or upgrade the top rail to 25 or 30mm.
Also how are you driving the gantry along X axis you mention rack, but are you looking at driving it from one side or both? if you are driving from one side, you may encounder some crabbing?
Looks very good overall though.

Quote Originally Posted by Smiler View Post
HI Kip,

The box sections are 50 x 50 x 3 steel box section, welded to 6mm thick endplates. The frame is bolted to the rigid PVC endplates. I can't detect any flex at all in that. My only concern is the rails (20mm) and the ballscrew(25mm) are in effect taking the torsional load but whether it will be a problem only time will tell. I've seen larger machines than mine where the rails are also the gantry frame (i.e. no box section )and they don't report problems with them.

On reflection, maybe I should have gone for a supported rail like on my X axis but it was an impulse buy from EBay so I'm stuck. Still there is nothing to stop me adding supports and open bearings later or even flat rails and trucks mounted face on to the box section.

Thanks for the input, any ideas for removing any flex?