A ball bar test the accuracy of a machine, or to certify the positional accuracy of a machine. Unless you have some need to prove teh accuracy of the machine you won't need the ball bar. You will want to test all axis against a good dial indicator at first, you may find that you are a step or 2 off based on whether you are using english or metric screws/gear racks/etc. and you may find that you have to make small adjustmants on each axis because minor inaccuracies in you system.
What kind of machine are you running, and what are you trying to do with it?