I concur with Nigel, that you cant use RC servos to move a stage on a CNC machine. The easiest way is to use stepper motors to drive linear ball screws (or cheaper threaded rods). Alternatively, you can use regular motor and a distinct encoder (optical, hall-effect, etc) on each shaft in a closed loop system. You would probably need encoders to feedback to your control system anyway. If you use threaded rods, be certain that you get the type with lots of threads/inch as that will give you the best precision. However, they have very high friction since they are not really designed for continuous motion.