Bridgeport Knee lift very tight

Hi,

i have a promblem with the knee lift on my Bridgeport, when winding the table up towards its maximum height, the crank handle is nearly impossible to turn, it gets progressively tighter

its get so tight i think i will break something

we had the knee gib out, that comes out easily and looks good

the auto way oiler is working fine, with nice clean oil on the dovetails

the brass columm lifter nut seems too tight and after lifting from the middle of its travel to where it gets tight the brass nut gets very hot, the nut does not have an oiler or grease nipple, and is fed with a pipe dropping oil on the side of the screw

i am convinced its not the gib or the ways and must be the screw as the nut runs freely from mid position to the bottom

we,ve jacked the knee up with a 15 ton machine jack with the pedestal unbolted and the knee glides up and down, so conclude the problem is with the screw / nut

has anyone else got any experiance of this problem

also just to say this is a metric machine made in the UK IN 1980's

i can source a new inch screw and nut, but am struggling to find a new metric screw and matching metric nut

one thought we had was that the screw is strectched and that effects just the bottom of the thread

has anyone the actual dimensions for the original metric screw ? we think it m32 x 5mm pitch left hand single start

or suggestions as to the cause

many thanks

Andy

I would disassemble the knee lift screw mechanism and bench test the screw and nut. If it is problematic on the bench it sounds like wear or stretching of the screw pitch.
Joe

It sounds like the problems people have when they replace the X or Y axis nuts and adjust them for the worn part of the screw. When they wind the nut over toward the unworn end of the screw, it jams up. Is it possible the knee nut has been replaced? If it was replaced and the replacement was cut to fit the worn part of the screw, you would have this problem. If you have a lathe and the ability to cut a thread, you could either make a new nut with more clearance, or chase the existing nut to provide more clearance. Additional clearance in this nut shouldn't make any difference, as gravity always pre-loads it in the same direction.

If you get it running and find the amount the knee comes up per revolution varies along the length of the screw, then the screw needs replacement.

While you're working on this, if your lube system manifold has an extra unused port, you could add another meter unit and line direct to the pedestal nut.

Dennis

When I needed a coule of parts for my Bridgeport I got them from

Braithwaite

Home


Have you tried them for screw and nut?

Are you saying it jams at only one end of travel? It sounds like the screw is worn in the middle, but I would expect it to be tight at both ends not just one end of travel. I would also expect the table ways to be worn a little if the screw is worn.
Bill D.

Hi guys

thanks for all your advice

What i didn't mention in my original post, is that i've only had this machine a month or so, and I was systemstically going through any issues as i identified them

I'll try and pickup on some of your comments

Bill D, yes it does get tigher the higher you lift the table, in fact the screw get rigid once it has cooled down, following a brisk wind up to near the top

lionel T, no not tried them yet as there may be a new twist, as i have just been under the table, and with the knee screw now exposed i am not now convinced its a metric screw, as it measures 1-1/4 and and conviced 6 threads, so my assumption because of metric dials the screw would be metric.

Dennis, Before removing the screw and nut at the weekend will, measure the lift per turn throughout its available travel, one to establish any stretch, but now also to establish inch or metric !

also once out will put it on some rollers and measure any deflection in case its bent

The machine currently has a two axis Sony DRO, which i plan to reuse on the Churchill lathe and i have already sourced a 3 axis for the BP, so if the knee. screw is inch, i can read just off the new DRO, once installed, and or also fit an inch scale.

bit like my drawings, not uncommon to have both inch and mm on the same drawing...

so the plan is inline with Dennis' suggestion to get the screw and nut off and inspect where the problem is and probably scim the nut on the lathe, be fun if we have to, pick up the original thread...

also plan to pipe a new oil feed to the nut as suggested by Dennis.

so will let you know later what i find

best regards

Andy

andy_m said:

Dennis, Before removing the screw and nut at the weekend will, measure the lift per turn throughout its available travel, one to establish any stretch, but now also to establish inch or metric !

Click to expand...

I fail to see how a screw which spends its whole life in compression can "stretch." However, the thread can wear. This normally shows itself as backlash on the X and Y axis screws. I once had a beater Bridgeport that had over .080" [2mm] backlash in the X axis. The acme thread in the bronze nut looked like a little knife edge spiraling around the inside. That machine was used mostly for drilling, and with a DRO the backlash didn't make much difference; dial to the numbers on the DRO and lock the table, and all is well.

The fix was a new nut, that brought the backlash to about .030". Splitting the nut and spreading the halves to take up the clearance would reduce it further, but only to a point, because the ends of the screw aren't worn, so the "expanded" nut won't fit the unworn thread, and you can't crank the table the full length. Adjusting the nut so it is a workable fit at the ends of the screw means you have to live with the extra lash caused by the wear in the middle of the screw... or replace the screw.

This is what I think has happened on your machine; someone made a new nut and made it to fit the worn part of the screw they use all the time, so it won't fit the unworn thread at the ends of the screw. Since backlash isn't normally an issue on the knee (gravity always keeps the knee loaded against the screw, adding more clearance in the nut will allow it to run the full length of the screw.

However, this does nothing for accuracy. On a Bridgeport with inch screws, one rev. of the screw will raise the knee .1 inch, so twenty rev.s will raise it 2". However, if twenty rev.s takes the nut from a place where there is no wear on the screw to a place where there is .005" worn off the thread flights, the knee will only come up 1.995", which kind of sucks, because most people, unless they have a DRO on the quill, rely on the knee to have accurate movement.

If you take the screw and nut out, you should be able to determine a lot of this on the bench. First, make sure the screw isn't bent. Next, make sure the nut will run the whole length of the screw by hand. If not, determine why. If the nut has been re-made and the thread is so oversize that it won't run on the unworn ends of the screw, you might want to consider replacement of both nut and screw while you have them out.

Dennis

andy_m said:

also once out will put it on some rollers and measure any deflection in case its bent

Click to expand...

When you get it apart, pay particular attention to the minor diameter of that nut. Its not uncommon for the flank that carries all the load, to extrude or throw a burr into the minor diameter. Not a problem where it does all its travel, it just wears the minor diameter of the screw. But will bind up on an unused section of the screw.

You would be able to feel it with a finger nail. If you were to clean and blue up the screw. Drying blue / lay out stain. Not bearing blue or Engineers blue to you. And run the nut. If its rubbing in the root of the screw, that's your tight spot.

I've often found they can have a hand full of backlash, but still be tight. An expanding reamer, run threw the nut until it just touch's the crest of the nut brings them back.

Regards Phil..

Hi Guys,

Thanks for all your advice, which we have found invaluable

last weekend we got the knee screw out and discovered, as Phil suggested that the brass nut did have an excessive burr.

we put it in the lathe and removed 3 thou off the minor diameter, then the nut could be freely hand rotated along the length of the screw, which itself showed no damage.

in fact we tried the screw in the cross screw nut and it was a nice fit in there too

as part of our strip down we discovered that the top bearing was badly worn and very notchy, having, been caked in solid grease and debris.

sourced a new bearing for £28.00 through , Brammer, which arrived on Tuesday

We've put everything back together today and the table now lifts effortlessly.

as discussed earlier we decided to drill and tap the brass nut and feed with a new pipe from a spare port on the oiler, however it now seems to get over-oiled, and is probably robbing some of the oil from other feed points
I.ve made a reducer / restrictor so that the feed to the brass nut is now only 1.00mm

i dont understand why this feed not gets so much oil, when the feed to the top of the knee screw, is just a push fit in the centre end of the screw above the gear, which pushes its oil down the centre of the screw where it meets a 2mm cross driiled hole so feeds oil to both sides at the top of the screw

so any ideas how to further restrict the flow of oil to the knee screw nut and rebalance the flow to the original oil feed points ?

many thanks

Andy

The factory way....

http://www.bijurdelimon.com/fileadm.../Datasheets/35470_B_FIT_Meter-Units_DS-R2.pdf

If you have a line with no meter unit, likely none of the other lines are getting oil. The open line acts just like a broken line, and the pressure pulse from the pump may never develop, and the other meter units won't open.

The purpose of the meter units are threefold; they give a metered amount of oil, which allows sizing proportional amounts to different points. They also act as check valves, so the oil doesn't drain back through the pump. This ensures there is actually oil in the line at the meter unit when it is needed. Not particularly useful on a Bridgeport style mill, but on more complex machines it allows the lube points to be in series on a single line, and still all get the proper dose of oil.

Buy a meter unit for the new line. Whatever size are on the other screws should work OK.

Dennis

My fault with Bijur lube system is if you have 10 meter units and 9 of them plug up you wouldn't know it because it will still pump oil, just not as fast, now for the knee nut I'd use a 2-0 you'er still going to give it more oil than ever gets