I was thinking slipping couplings. My ball screws don't have any flats on them so they are effectively only held by the tightness of 2 M4 screws in the coupling, where as the steppers do i think.

I would of thought if it had slipped, it would of been one side or the other rather than both Y and A.

As you say - I will mark and see what happens.. Do you think my 1500mm per min feedrate / 4mm depth cut was fairly conservative with a 6mm cutter ?
My cutter is only a straight two flute and it was screaming a little. Might be time to invest in some better cutters.

I read compression bit for plywood and cut the whole 18mm at once! Sounds like that might cause me a problem.

Cheers
Martin