What is backlash?
Backlash is strictly the axial free motion of the ball screw to the ball nut, it is not rotational.
HOW TO MEASURE FOR BACKLASH
1. Place a magnetic base indicator directly on the screw. Put a .0001 indicator point on the nut flange. Take a bar and press between the base and the table in both directions. It is not necessary to use much pressure because of the leverage. You could bend the screw or leave ball marks on the screw or the nut. There is no such thing as zero backlash.
OTHER CHECKS FOR TABLE MOVEMENT
2. Place the magnetic base indicator on the ball screw and put the pointer on the bearing support block. Press again. This will show you the movement in the bearings and bearing lock nut. Note, always change the bearings when you change a ball screw. They have as much wear as the ball screw.
3. Place the magnetic base indicator on the screw and the pointer on the ball nut housing. This will show you if the nut flange bolts are loose or bottomed out.
4. Place the magnetic base indicator down on the machine base and the pointer up on the slide (table). Press again and record your movement. Do this as close to the ball screw and again out on each end of the table up high and low. This can help identify if the gibs are loose, worn or contaminates are on the slides.
5. Do the same checks on any other points that are bolted or moving. Always use the same two pressure points to reduce variables.