Improved X stage movement

I had noticed that the X stage threaded rod was a little off from the motor drive shaf, and it didn't move very easily. To correct this, I moved the nut to a new position, carefully measured against the actual rail when mounted between the roller skate bearings. After adjusted the nut (it can rotate slightly due to a stupid bend in the holder), it is now near-perfectly centered and movement is much smoother. This is one of the few things where accurate measurement is actually important.