# Bridgeport X & Y feed screw nut replacement ...



## FOMOGO

Tearing into my B-port and trying figure a few things out. While I enjoy the work, I have a ton of other projects to do and would like to have it operational sooner than later. I've looked at parts, and for under $250 I can replace both feed nuts and the cross feed screw. Most of the end play is in the Y axis. In the parts break down they show both one and two piece nuts. I checked past threads and found some info, but would like to know from those who have been there which is best, and why. There was also mention of a two piece nut with a spring used on a lathe, which remembering after I removed the nuts from the casting, led me over to my collection of misc. springs. Low and behold, there was one just the right diameter and length that in my tiny mind seemed just right for the job. My thinking is, get the two piece nut for the X axis and install them with the spring in-between to maintain Zero backlash everywhere on the length of the screw regardless of regional wear. I realize one of the nuts would have to be lapped to allow a slip fit, and both would have to be shortened some. Does this sound at all workable? Would it cause premature wear, or affect machining operations in any way? Just throwing it out there to see what those more experienced than I might think. Thanks, Mike


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## cathead

MIke, 

A shim might work but a spring would allow for some free play I think.  On my clone mill, there is a screw on  the nut that allows one to compress
the nut and pull it together slightly.  I don't know how much play this will take up but I remember adjusting it twice so far since it was new 20 yrs. ago. It has a lot of use so it wouldn't surprise me that it will need to be replaced eventually.  Maybe talk around a bit and see if an adjustable nut
can be had.  My mill is an ENCO 9x42,  now taken over by MSC.


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## 4ssss

They do split those nuts and add a cap screw adjustment to take up backlash. If you're set on replacing, I'd go with a ball bearing leadscrew, though they are pretty pricey.


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## Cadillac STS

Ball screws are required with a CNC mill. While CNC control is on the servo or stepper holds the screws tight all the time.

In manual though the ball screws will move (creep) when not intended. The original acme screw has more hold on the non moving axis. So if you have ball screws you need to always tighten down the non moving axis.  For manual use not a good idea.


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## Bob Korves

Cadillac STS said:


> So if you have ball screws you need to always tighten down the non moving axis.


You should be doing that anyway...


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## chips&more

As said and needs repeating. Ball screws on a conventional mill is not a good idea.


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## tfleming

I bought the "Bridgeport Rebuild Manual", and the recommendation in there on the cross feed nuts is to actually cut them in half where the split is.  Face them off in your lathe.  re-install them with a heavy felt spacer between them.  The felt allows oil to wick in for lubrication.  This gives a significant amount of adjustment to take up backlash.  I have not done this yet, but plan on using this technique.  this approach is what the book terms "upgrade" in design.  Again, I have not quite yet gotten to this point, but I plan on using the approach.


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## MBuechle

I split the nut on my x-axis feed and reduced the backlash from .097 to .007.  My lead screw has very little wear getting only .002 tighter at the ends verses the center.  Not difficult to do at all.


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## expressline99

MBuechle said:


> I split the nut on my x-axis feed and reduced the backlash from .097 to .007.  My lead screw has very little wear getting only .002 tighter at the ends verses the center.  Not difficult to do at all.



Mine has .070 with everything tight. lol I'll be making replacements soon!


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## tfleming

Expressline99, replace or modify?  The modification makes sense, and is relatively easy to do.


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## FOMOGO

I went ahead and cut both the X & Y nuts and faced them off on the lathe. Used a Dremel to clean up the thread to match the factory end, which you can see was done by hand originally. Mocked up the Y axis (which was the worst) in the screw casting and it appears to have taken out all of the backlash. Will adjust them to spec when it all goes back together. Pulled the metal seals off all the feed screw end bearings including those for the knee crank and cleaned out the old dried grease and repacked them and replaced the metal seals. They turn nice and smooth now. Just got the ferules and nylon tubing in to replace all the oil lines. Will have about $50 total in the rehab. May get it all together before we head down to PR for the winter. If not it will have to wait until spring. Going down to Denver over the weekend to see our new grand daughter. Being my first, I'm pretty excited. Mike


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## expressline99

tfleming said:


> Expressline99, replace or modify?  The modification makes sense, and is relatively easy to do.



I'm going to make a new set. My X axis leadscrew has some wear. I'm going to mount that on my lathe and attempt to make the wear more conformed along the length. Once I've got that done I am going to make a nut to match the fit. 

On my y axis I found a new screw for cheap so I'll just make a new nut for that. 

I'm very new to chasing existing threads so it'll be fun! But if I ruin any of it I know there are parts that can be bought. So I'm going to do my best with help from the board here.

Paul


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## gi_984

I'm with chips&more, no ballbearing lead screws on a manual mill.  For CNC, yes but not a manual machine.


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## FOMOGO

Almost forgot. Obligatory pic of grandchild. She was born on the anniversary of the attach on Pearl Harbor.


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## expressline99

FOMOGO said:


> Almost forgot. Obligatory pic of grandchild. She was born on the anniversary of the attach on Pearl Harbor.
> 
> View attachment 250424
> View attachment 250424


50 gamer points! Awesome. The baby is awesome too!, Paul


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## sidecar580

chips&more said:


> As said and needs repeating. Ball screws on a conventional mill is not a good idea.


A rolled thread ball screw has less resistance than a acme nut and screw ....but much more resistance than a ground thread...in my opinion...a rolled thread ball screw is an upgrade for a manual mill.. They do not move as easially as a ground thread...    JOHN


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## tfleming

If anyone is interested, I just did the "split nut" modification and installed it.  Worked like a champ.  Only cost was about 30 mins of machining time, and $1.98 at Advance Auto for the battery felt washers (which I placed between the 2 pieces after splitting).  I recommend this to anyone on a budget that is battling mucho backlash on their Bridgeport feed screws....................


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## FOMOGO

Finally got a little time away from home repairs here in PR, so a little update on the Bridgeport rehab. Pulled and cleaned all of the knee screw, elevation crank, and gear assembly. Repacked all the bearings, used some valve spring spacers to adjust up the ring gear for the knee crank and adjusted the the pinon gear to optimize the back lash. The bearing for the knee screw is a shielded (not sealed) bearing and would normally get its lubrication from whatever finds it's way from X/Y screws above. It didn't seem to be getting much, and I added a sheet metal shield to protect ring and pinion gears from swarf (got that idea from someone here, thanks). So I did a back yard modification to the Knee screw bearing. After repacking it placed a suitable o-ring and slid it up against the bearing shield and then used RTV to seal it in place creating more of a sealed assembly. I decided I'm going to drill and tap the knee for a grease zerk with a soldered on copper tube to give the occasional shot to the ring and pinion assembly as plan on powering the knee, and using it more, and extending the quill less to preserve rigidity. Also considering running a separate oilier line to the Y axis as it relies on oil dripping down from the X axis above, and though it would normally see less use than the X, it shows considerably more wear. 
  While I had it apart, decided to do a little polishing on the cranks and related assemblies. Which according to e-bay analysis should convert my $2500 mill instantly to a $15000 wonder machine.  Looking forward to getting home in April and finishing up. Mike


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## Firestopper

looking good Mike. The frosting still looks remarkable.


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## FOMOGO

Back to where I left off last Dec.. Spent an hour or so in the shop and implemented my idea for a grease zirk and line to supply grease to the knee lift gears. Will let the pics tell the story, pretty straight forward. If you do this modification, be advised that less than a quarter stroke of the grease gun handle, twice 180 deg apart is more than sufficient. Probably 1-2 a year for most of us. I put mine on the left side of the knee to keep the right
side free for DRO scale. Cheers, Mike


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## Richard King 2

I have been replacing Bridgeport screws and nuts for 1/2 a century.  The older models the nuts were split or cut like the OP's first picture.  Then around 25 years ago when you ordered nuts from Bridgeport they came as a 2 piece that helped reduce the backlash on the entire thread and not the old ones that had a Philister screw bending one side in.   The newer ones had 2 Bijur tubing goes to the top and bottom nut, the old ones you lined up marks on the front of the table and saddle so a set screw plug above the nuts could be removed and oil was squirted down the hole and the hole in the bracket to feed the Y nut too.   But many people never knew about the set screw so the Y axis never got oil.  Most squirted oil on the X axis screw only.   I would never use grease on that Z axis bevel gear assembly as it will get contaminated with crud and chips and it will wear the gears 3 times as fast IMHO.

One thing too.  If the ways are worn remove the pins that align the end screw brackets and with the cap screws finger tight, crank the feed screw to the extreme in position and then tighten the bracket.  Then re-drill the pin holes and install spring pins.  If You don't do that the close you screw the bracket to the center it will bind.        Never hurts to re-engineer a machine as long as it gets better.   Rich


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## FOMOGO

Thanks for the info Richard. If you look in the last pic above, you will see that I installed a shield above the gears to keep contaminants out. When I disassembled the machine there was a fair amount of swarf in the gear assembly and considerable backlash. I shimmed out the back lash after cleaning things up. I think with the shield in place it will be fine. The Y axis on mine had considerably more wear than the X, most likely for the reason you mentioned, and I will be adding a separate line for it when it goes back together. Mike


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## Richard King 2

Yeah I saw the guard but I still would do it.   Hey it's your machine and if it works for you, I just figure in all my years I have only  seen 1 machine with bad gears.   Have a great day.  Rich


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## Janderso

I have read, seen and heard to never use an air gun to clean out chips when milling for obvious reasons. When I saw FOMOGO's pics for the tray to protect from contaminants I was impressed. Good idea. 
When I took my step pulley down far enough to inspect the bevel gear in the knee, I was surprised to see no chips. The bevel gear was dry. I can only assume who ever had this BP before me, must have performed some sort of clean up/refurbish on the lower end. Regarding grease, I will never question Mr. King's advise but I don't see any way to oil/grease the bevel gear.
How do the rest of you lubricate your bevel gear??
Thanks.


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## FOMOGO

Almost finished with the lower end. I'm getting pretty good at putting this thing together, and taking it back apart, due to forgetting a piece, or changing my mind on how to go about something. When I went to install the X&Y nut, it became clear that it was going to be problematic for the metal gear cover to clear the nut when moving the Y axis, so out it came. Looked around the shop and settled on a piece of reinforced rubber sheet from the gauntlet of an old bead blast glove. Drilled and tapped a hole for a 10-24 screw and made a small clamp to hold the grease tube and rubber sheet in place in conjunction with a few of the ever popular zip ties.




Going back a bit to the split nut issue of how to shim the two sections to get maximum screw engagement.  This was established by pushing the two nut halves together and using a depth mic to measure from the outside machined surface of the casting to the end of the nut, and then threading the acme screw into the two halves of the nut until both sections of the nut were on the least worn part of the shaft, and then taking another depth measurement at the same place. This process gave me a space of .097 between the two nut halves. I searched my stash and not finding anything appropriate, I lite on the idea of trying an o-ring. I found one that measured .123 and cut it to fit the casting bore tight to both sides of the key that holds nut sections in place. After assembling the nuts, o-ring and nut adjusting screw, I used the adjusting screw to compress the o-ring to the .097 I required, and threaded the acme screw into place. I did a final adjustment to get a snug but movable fit. Installed the assembly on the mill, adjusted the Y gib for minimal side to side movement (.0003 at the outermost end of the saddle), and ended up with .0025 backlash on the most worn section of the screw, where there was originally .049 . Pretty happy overall, and learned a lot in the process. Will post some more tomorrow, time for bed. Mike


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## Janderso

I think you are doing a fine job. I have enjoyed your journey so far. Maybe when I retire I will follow your lead. Meanwhile my old BP does what I ask of it.


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## FOMOGO

Finally pretty much got my mill back together. Still have a few things to polish up, and reinstall wipers and way covers. Ended up with zero backlash on the X axis, but it is a little tight at the extreme ends. Still movable, but tight enough that the power feed clutch will disengage at the very ends. I could pull it back down and take .015 or so off the thickness of the o-ring and I'm confident that all would be well, but I've decided to wait until I get the big lathe set up in the new shop. Then I will pull the X axis lead screw and single point it so the thread is uniform along it's entire length. Spent an hour cleaning up the X axis lock handle, the last one thankfully . It looked like it had been run through a hammer mill. A lot of nicks and gouges. Took the worst out with a file, then medium, and fine 3M Rolocs on a 90 deg die grinder, and finally emery compound on the buffer and a coat of wax. It's starting to look good enough that I just may have to break down and paint it at some point, but for now its just good to have it in service again.  Mike


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## Firestopper

Looks great Mike!


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## Richard King 2

I suspect the table is worn in the middle and bent so the outside ends are lower plus the screw is unworn on the ends. .  The table is bend because they stretch when you tighten the Tee Bolts.  Many call it peening.  But it's a reverse peening inside the T-slots.  I like what your doing but hopefully you will trust my years of experience on building new machines at the factories and rebuilding machinery for over 50 years.   Did you remove the end bracket alignment pins like I mentioned before?  When I assemble a Mill (Bridgeport or other brands)  I leave out the pins and leave the bracket screws loose and with the gib loose so it has .0005" per side then screw the table to each end as far as the travel will let you and then tighten the end brackets and then try putting in the pins.  If they are not aligned I drill them to the next size and put in new ones.  If you had a straight-edge I would bet money the table top is high in middle of convex (bent) and that means the bottom is bent too.  That contributes to the ends getting tight too.    I come on the forums to help and teach.

I have written this 2 times as separate posts, but it doesn't  show up.....but disconnect both ends and try pushing the table by hand end to end.  That would tell you if its the screw or table.     Rich


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## FOMOGO

Thanks for the input Richard, always looking for the best way to go about things, and I will certainly check into that the next I take it back down to cut the lead screw.  Thanks, Mike


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## Richard King 2

May not have too, if the brackets are not aligned right.   You might be smart to remove the table brackets and with the gib set at .0005" to .001" loose and hand push the table to the ends and see it it gets tight.  I suspect it will get tight on ends.


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## Richard King 2

Thought I wrote this before... Guess I hit delete and not post.   Remove the table brackets and with the gib set at .0005 to .001" looseness for oil and then push the table by hand and see if it gets tight on the ends.


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