Colin, Ref. your above post (#8) by flex coupling I hope you mean bellows or similar. The type of flex coupling that permit radial "torqueing" will introduce lost motion between the axis motor (and encoder/resolver) and the ballscrew. Not good for .0001" stuff. The bellows type coupling compensates for limited angular and axial misalignment without changing rotational displacement, as long as appropriately sized.
Dick Z
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