ah, looks like I wasn't paying attention at school during maths.

1 / 5mm/rev = 0.2rev/mm of ballscrew shaft

0.2rev/mm x 2.5 belt ratio = 0.5rev/mm of motor spindle

0.5rev/mm x 400step/rev = 200steps/mm of motor spindle

therefore 1 step = 1 / 200 = 0.005mm/step or 0.00019"

I would, therefore, agree that 20 steps would need to get forgotton about to realise such an error. I'll have to have a closer look at mechanical backlash as I would agree that this now seems like the obvious problem.

thanks for the lesson Sterling

...Sweep