With the half nuts engaged with the lead screw, set it for let's say 8 threads per inch. Set you up a 1" travel indicator to indicate carriage travel, mark you a point on the chuck that would be at 12 O'clock position or what ever, so there is a reference point to stop at. Make sure the slack is out of the half nuts and lead screw, mark "zero" on the 1" travel indicator. Rotate the spindle 8 times and stop rotation on the reference point at 12 O-clock position. Take reading on your 1" travel indicator. It should read 1.000" If not, the difference will be the lead error in your lead screw. I would suggest doing this for several inches starting at the headstock and advancing out toward the tailstock for about 12-18" or more. Record each reading to establish a pattern of wear in your leadscrew. A worn gear train will not create lead error, only if the lead screw is worn. Ken