Hi Graeme,

I have that sneaky feeling that you need to check the whole geometry of the machine making sure the rails are set parallel and at 90 deg everywhere they should be, if you follow that rather contradictory sounding statement.

It doesn't sound like an electrics problem to me as it is consistent, so I understand. I doubt it is ball screw accuracy. You could do a 3,4,5 triangle and move x and y the 3 and 4 then measure the hypotenuse and with a bit of swapping x,y, 3,4 directions, you might work out which axis is inaccurate (proportionately) and then alter steps per to compensate.

Hope it is a hobby and not a source of income for you !

Cheers

Rob