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?