Need a diagnostic with problem on leadscrew assembly on Bridgeport Series 1 please?

[rons]

If you read the OP he has a gap it the collar that changes. The change in the gap of the graduated collar has nothing to do with the screw threads, that can only be the thrust bearing set up. Now if he showed some back lash: the collar rotating .020"-.060" then that would be play in the screw/nut combination.
Dan

Wouldn't hurt if you correct your English.
If the thread has a point of resistance while turning into the nut then a equal and opposite reaction is for that bearing to move outward. There could be loose metal pieces in the nut location that stick occasionally to a greasy screw. A screw/nut combination plus crud equals bearing movement.

 

[jhruska]

Should there be axial play in the left side bearing? In other words, does the axial play I'm feeling in the bearing exist when the bearing is brand new.

There should be no free motion by the left bearing. A new bearing is unlikely to have a different width that would tighten things up.The resources available to me when this problem would crop up was a pile of parts to interchange or just make a brass shim(s) to fill the void.
The outer bearing retainer ring needs to seat flush against the table end bracket. When a bolt or cap screw as in this case is over tightened it tends to pull the thread out of the tapped hole. Check both parts for a burr or raised spot. Steps for a tapped hole are center drill-tap drill-chamfer drill-tap. The chamfer reduces the chance of a raised thread. The holes in the retainer are slightly over size so it should not be a problem but take a look.
A lathe could be used to face the retainer up to the shoulder to restore the fit.
Usually I just used a shim that came with the Servo-feed units as there were a lot of spares and they were of the correct dimensions.
Measure the distance the bearing moves as close as you can. The fix should not put a heavy preload on the bearings and the screw should rotate with out perceptible resistance.
Table trick; build or place a very sturdy work bench on one side of the mill and under the table. With sections of pipe on the bench lower the knee so the table is supported by the work bench.
John

 

[Randalthor]

This thread is a bit old, but I wanted to try to bring it full circle, in case it might help someone else.

Basically, as a recap, I was getting ready to tear the base apart, and I was curious why the X axis crank seemed to jump about .010.

jhruska, and others, pretty much diagnosed it right. The X screw was worn, which when the brass feed nut hit the "loose" threads, it caused the bearings at the end to jump back into place. The bearings on the left side of the table were old and behaved strangely. When I took them out, they would "click" back and forth laterally (thrust wise), which caused the jumping of the dial away from the table.

I ended up putting all new bearings on both sides and a new X axis screw and new brass feed nuts.



It was interesting the left side captures the bearing firmly in the bearing retainer ring, but the right bearing retainer ring allows a little movement. Only one side need be rigid. It's a good design. The right side "free" allows a little more tolerance in screw dimension, as well as expansion/contraction of the screw for temperature variance.

The new screw/bearings/nuts removed most of the backlash. It's a consistent .007 backlash all the way through travel, which is the thrust play in the two new stacked bearings on the left side of the table. I'm surprised Bridgeport didn't use thrust bearings for this side. I think real thrust bearings would remove even more backlash.

I was a little disappointed with the .007 backlash because I had heard new screws/bearings/nuts should result in about .003 backlash, but the backlash is all in the new bearings on the left hand side. They are Japanese NTN bearings, but I think the design just isn't much of a thrust bearing.