Are you asking this because you are having problems holding tolerances with your ball screw equipped Taig, or because you're thinking of buying a Taig mill? There are lots of factors that will affect accuracy, like tool deflection, cut quality, temperature variation, and any looseness in the adjustable parts of a machine. A ball screw equipped Taig mill should be able to hold +/- .001" reliably, but all these things factor in,