[Thomasio]

Forgive my novice-ness...

I have an old '40s Bridgeport knee mill (32" x 9" table).

The backlash on the x screw was terrible (close to 100/1,000 ths), so I removed the table, and removed the brass nut. It was the single piece leadscrew nut w/ the pre-cut section. I followed the procedure to cut the nut in two (at the pre-cut section), cleaned and regreased everything and reassembled.

Now the adjuster screw works (it is very sensitive), and the backlash has been cut in half or better... but, as the leadscrew turns, it gets tight then free, tight then free... approximately once every 360 degrees or so. It almost feels like it is binding once per revolution or so...

Any thoughts on what could be causing this?

I apologize for the newbie description...

[79TigerPilot]

I also had some significant backlash and was told cutting the acme nut into two sections could help. Even built a tightening ring system to put even pressure on the nut. The ring tightener worked fairly well and I had good control over the compression force but the backlash actually got worse. Like you indicated the adjustment was very sensitive. I didn't have the same tight/free condition every rotation as you. In my case if I got the backlash down in the center of travel the screw would be too tight when moving the table away from center. I think two things happen, first when you cut the nut the net nut key-way distance is only half as long resulting in more key-way slop. Second we were cutting a worn nut and who knows what that does to the the thread profile. I was going to buy a new nut but decided to upgrade to ball screws and never did get to test a new brass nut.

Craig

Forgive my novice-ness...

I have an old '40s Bridgeport knee mill (32" x 9" table).

The backlash on the x screw was terrible (close to 100/1,000 ths), so I removed the table, and removed the brass nut. It was the single piece leadscrew nut w/ the pre-cut section. I followed the procedure to cut the nut in two (at the pre-cut section), cleaned and regreased everything and reassembled.

Now the adjuster screw works (it is very sensitive), and the backlash has been cut in half or better... but, as the leadscrew turns, it gets tight then free, tight then free... approximately once every 360 degrees or so. It almost feels like it is binding once per revolution or so...

Any thoughts on what could be causing this?

I apologize for the newbie description...

[machintek]

Possibly a bent lead screw?
Put the ends on V blocks and a dial indicator in the center and spin it.

George

[Thomasio]

Possibly a bent lead screw?
Put the ends on V blocks and a dial indicator in the center and spin it.

George

That's interesting George... it never did that before and I can't imagine that I bent the leadscrew yesterday, but maybe there is a slight bend that was masked by the extremely sloppy / worn leadscrew nut. Interesting idea.

[Thomasio]

Craig -

was the ball screw retrofit a direct "drop in" replacement, or did you have to modify things? I don't want the machine down time or the cost... but if there is either 1) an economical leadscrew & nut drop in replacement, or 2) an economical ballscrew drop in replacement, I'd like to hear about it.

[Thomasio]

Possibly solved: I believe that when I reattached the power feed on the right hand side of the x-axis leadscrew, I didn't get it concentric w/ the leadscrew axis.

MAJOR THANKS to Kirk at MajesticMachinery.com

Kirk is a class act. He patiently fielded several phonecalls from this newbie, he was never annoyed or impatient. He recommended that I try something BEFORE I spent money on a new nut that I may not need. (His integrity is unheard of these days.)

THANK YOU KIRK!

[Thomasio]

Problem solved: my powerfeed on the right side of the x-axis must have been slightly off-center. It is mounted via 3 bolts, but does not have guide pins (like the left hand side handwheel does). Therefore it was mounted slightly off-center, causing the leadscrew to bind about once per 360 degree rotation.

[79TigerPilot]

Basically it was a drop in. Because it was an ebay special I did have an issue wiht the Y axis nut mounting on the yoke. You can buy a kit from Rockford that has it all ready to go. Bridgeport Kits Configuration Settings

Craig

Craig -

was the ball screw retrofit a direct "drop in" replacement, or did you have to modify things? I don't want the machine down time or the cost... but if there is either 1) an economical leadscrew & nut drop in replacement, or 2) an economical ballscrew drop in replacement, I'd like to hear about it.