Thanks for your take on this, JAZZ - I certainly don't have a practical way to measure over a significant length of the screw travel to the accuracy I would need! I'd have to base it on short segments, which it sounds would be far from ideal.

I just heard from Marchant Dice and they confirm the ballscrews I have should be no worse then C7. So to me it just feels like my issues are being caused by something other than screw pitch errors. Prime target must be lack of rigidity. But how to prove this....?

Cheers
Graeme