Worn lead Screw loosing travel?

[dkmc]

Cross slide on 1954 LeBlond lathe. Dial indicator secured in the dovetail on the carriage, aligned with slide travel.
Error varies, .0005 on .200 revolution, then another .400 and the indicator comes up .0017 short. Max error several inches farther is aprox. .0026. Can/do lead screws 'loose' travel like this when worn?? Not grasping the concept.

 

[Erich]

Wear is a function of use. More use, more wear.
The cross slide does NOT travel full distance back and forth uniformly.
Soooo, wear is going to vary down the lead screw.

 

[hvnlymachining]

If you were to travel from one end to the other of the screw it would likely show zero loss of travel, but as you travel through the warn area it shows lost travel distance from the wear on the threads. If you continue past the wear it will actually appear to gain travel as it pulls back onto the un warn portion. Another thing that may show this is to operate the cross slide backwards since the wear is likely all in the pushing direction and pulling will use the un warn side of the threads.

Try indicating near the far end of travel of your cross slide, you'll likely get a much better reading.

One more thing is if the screw actually became stretched, but I highly doubt it.

 

[dkmc]

Starting to make sense. This stems from installing a DRO on the machine, and confusion why dial movement didn't match DRO reading, a bad approach due to the above problem. Better approach is getting the DRO to match an indicator showing actual distance traveled. Error mapping should correct the display discrepancies.

 

[michiganbuck]

Still, the zero will be the same on the second part made..and the first part often needs measuring to get size..so not a big deal.
I ran an al Bridgeport that had the long travel screw wear listed along the table

 

[dkmc]

Still, the zero will be the same on the second part made..and the first part often needs measuring to get size..so not a big deal.
I ran an al Bridgeport that had the long travel screw wear listed along the table

I was just confused why I was dialing a number and then ending up big by .0015 to .0025- ish on different diameters. I get it now.