Pratt and Whitney Lead Screw

I have recently gotten a Pratt and Whitney #3 horizontal mill. There was about 20 thou backlash in the y axis lead screw, so I am looking for a replacement nut. Does anyone have them for sale?
The screw itself looks worn as well. All of the other screws have their trapezoidal acme shape, but this screw has the trapezoids almost worn to a point along the pitch. I might try and machine a new one. Does anyone know where I can acme screws with some material on the end to work with?

Thanks,
-Matthew

You can buy acme threaded rod through Mcmaster Carr and other sources. You will likely have to weld pieces on for shoulders and bearing seats, etc.

Here is a picture of the nut:



I can machine the basic shape. Would it then work to press fit a cylindrical bronze nut into it?

Not unless you made a counterbore for a flange and some flathead screws, or somehow pinned it or the like. If you replace the screw you could maybe get away with splitting the nut to make it adjustable. I would make a new nut (that is adjustable) and borrow or rent the correct left handed tap. Or single point the female thread. I've made a 3/4-10 double-start (2- 5tpi threads indexed 180° apart) left hand acme before, just have to go slow.

If Brian Miller was willing to make you one, I expect it would be a winner

https://www.practicalmachinist.com/...p://millermachineandfabrication.com/products/

MatthewL said:

Here is a picture of the nut:

View attachment 233190

I can machine the basic shape. Would it then work to press fit a cylindrical bronze nut into it?

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So as to use a "store bought" nut? Yes, but.

I'd want to pin, key - even, if a similar Bronze were to be used for the body as the nuts are made from, Silflo solder/braze it so it dasn't spin in the block.

Collars for the ends of store-bought threaded stock can be threaded-on ones as well.

"Static" threads will sort the management of thrust at the ends just fine and allow fine adjustment as to shoulder position or such as part of the making-up of it.

Once positioned, just lock those to the rod stock, too, before final finishing up of their shape, etc.

Here are some better photos






The lead screw has one hole for a taper pin. If I try and make the entire shaft, what type of steel should I use?
It is difficult to measure where the threading is on the nut, it isn't centered.
Also, I can't find bearings on McMaster like the one pictured. Does anyone know where I can get them?

-Matthew

MatthewL said:

If I try and make the entire shaft....

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How good is your follow-rest?

It is difficult to measure where the threading is on the nut, it isn't centered.

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Take measurement from the old shaft, threaded into it. Several measurements. Do some math to balance-out the errors. The stamped number hints it is the OEM part, not a later shop-fab, and one would "expect" it to have been on the same CL at the round pin. But the photo hints otherwise? Could that just be camera positioning? You might dig deeper into that by turning it 180 and seeing if perhaps the threaded bore SHOULD have intersected the axis of the pin-shape.

Also, I can't find bearings on McMaster like the one pictured. Does anyone know where I can get them?

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Lots of places. Once you get proper size and/or ID and can cross-reference them.

The acme thread is only four inches long. If I take light cuts, could it be done without a follower rest?

I would be doing the cutting on a South Bend 9 lathe, which I don't have a follower rest for. I do have a follower rest for my small Sherline lathe, but I highly doubt it would be rigid enough to cut the thread.

Richard King said:

Green Bay Manufacturing

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I have a Green Bay screw on the crossfeed on my Holbrook C13, it is amazingly accurate according to the dial and what it cuts.

There's something missing there. The bearing outer race is missing and the screw is worn in the middle wear the inner race rides it looks like. How about some more photos of the casting where the nut bolts to and a few pictures where the screw feeds through a casting. It is difficult to believe the bearing rides against the bronze nut like that. On post 4 picture it looks like the spacer is pinned to the screw and it would have a 3 part trust bearing ride against that or a bronze bushing at the least then it is in a hole in the casting with another 3 part thrust bearings or bronze bushing and then an adjustable nut to set thrust.

The bearing your showing is a spherical bearing and not a thrust bearing. The worn low area on the shaft and the nut rides on the thread independent to each other. The threaded area to the right of the worn area would have a nut that is not shown or is missing. The nut would have a brass set screw so when you tighten or adjust the trust bearing it lock the nut on the threads.

Do you have a manual or have you looked for a manual for the machine? John O do you have a manual?? Or if you put a bunch more photo's that would help here so we are nut guessing. The problem with the .020 backlash is not the screw and nut, it's thrust bearing or bearings missing issue IMHO.

Screw the nut onto the shaft and measure the backlash in the by putting a dial indicator on the screw end and push and pull the screw. I would bet there is nut doesn't have whole lot of backlash. Rich

Edit: I found this old thread and it looks like a dial is missing and the outer nut I was thinking may be missing too. Look at post 16...can zoom in by pressing CTRL and the + button on computer
https://www.practicalmachinist.com/...al-universal-milling-machine-got-pics-264585/

Richard King said:

There's something missing there. The bearing outer race is missing and the screw is worn in the middle wear the inner race rides it looks like. How about some more photos of the casting where the nut bolts to. It is difficult to believe the bearing rides against the bronze nut like that. On post 4 picture it looks like the spacer is pinned to the screw and it would have a 3 part trust bearing ride against that or a bronze bushing at the least then it is in a hole in the casting with another 3 part thrust bearings or bronze bushing and then an adjustable nut to set thrust. . The bearing your showing is a spherical bearing and not a thrust bearing. The worn low area on the shaft and the nut rides on the thread independent to each other. .. And the threaded area to the right of the worn area would have a nut that is not shown or is missing. The nut would have a brass set screw so when you tighten or adjust the trust bearing it lock the nut on the threads.

Do you have a manual or have you looked for a manual for the machine? Or if you put a bunch more photo's that would help here so we are nut guessing. The problem with the .020 backlash is not the screw and nut, it's thrust bearing or bearings missing issue IMHO.

Screw the nut onto the shaft and measure the backlash in the by putting a dial indicator on the screw end and push and pull the screw. I would bet there is nut doesn't have whole lot of backlash. Rich

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+++ !!! Good eye, Rich.

It would be a NICE outcome if just getting that bearing and thrust issue sorted meant not HAVING to make a new screw or nut!

I do have all the parts you mentioned, they are sitting about ten feet from the photo. I just took the screw down to one solid piece to take measurements. The machine is full of those raceless bearings. There are two bearings for this shaft. There is a race inside the casting, but it is part of a bigger part (I will take photos).
Yes, there is a nut for the other threaded area.
I know the backlash is caused from the wearing on the screw and brass nut. When I thread the nut onto the screw, I can move it back and forth just a little without turning it. It is worn pretty bad.

Also, the raceless bearing is not part of the brass nut, I just used it to get a good angle for a photo.

I will go get photos

MatthewL said:

I do have all the parts you mentioned, they are sitting about ten feet from the photo. I just took the screw down to one solid piece to take measurements. The machine is full of those raceless bearings. There are two bearings for this shaft. There is a race inside the casting, but it is part of a bigger part (I will take photos).
Yes, there is a nut for the other threaded area.
I know the backlash is caused from the wearing on the screw and brass nut. When I thread the nut onto the screw, I can move it back and forth just a little without turning it. It is worn pretty bad.

Also, the raceless bearing is not part of the brass nut, I just used it to get a good angle for a photo.

I will go get photos

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Ah well. Worth chasing... nothing lost..

Even if.. you were to start with pre-threaded Acme stock and jsut finish the rest of it, I'd suggest you scout a follow-rest for your SB 9". Or fab one.

As to which steel.. I am sure we can all manage to come to a workable disagreement over THAT recommendation pretty quickly!

I avoid leaded-anything meself, but 12L would be easiest to cut. 4XXX pre-hard is not actually "terrible" to thread though.

I would prefer not to have to cut the acme, since that would require some tools I don't have. I may email Green Bay to see if they can leave part of the rod unthreaded, and a little over size. Does anyone think they would do this?

if I do cut it and use 12L, would I have to harden and temper it at all?

MatthewL said:

I would prefer not to have to cut the acme, since that would require some tools I don't have. I may email Green Bay to see if they can leave part of the rod unthreaded, and a little over size. Does anyone think they would do this?

if I do cut it and use 12L, would I have to harden and temper it at all?

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Greenbay surely "knows HOW", but you do not want to know what they would have to charge for that sort of one-off!

They - and competitors - mass-produce these threaded-goods in running lengths on automated machinery, and it is SERIOUS machinery, not trivial. So no, they won't offer any "half done" options.

The only heat-treat I'm aware of as being appropriate to 12L is sending it off to the scrapper for re-melt.

As said, I won't mess with it at all. Not a tree-hugger thing on the lead. There ain't really that much of it. Other nuisance-factors, rather.

4XXX, 8620, or the various "drill rod" stocks lengths, "A" (Air) or "O" (Oil) hardening rather than "W" (Water hardening) series are my personal go-to's for general-purpose "project metal".

"Hardened and ground" was not uncommon for OEM leadscrews, but is generally well-beyond a typical small-shop's DIY capability.

You won't wear out ordinary "pre-hard" 4XXX fast enough to have to worry about the difference.

Ready-made from Green Bay or similar might outlive YOU!

Either "Stressproof", or just get "1144" from McMaster-carr. Cuts great, and is a bit harder so it does not wear like 12L14, nor does it rust like it.

If I make a feed screw etc, that's my first choice unless it has to be something else.

Alright, I may be getting this a little too complicated now. Do you think it would work to buy regular acme from McMaster, turn one end down to .375, press a "sleeve" of new metal to the proper size, and hold it together with two taper pins? If that doesn't work, then I will just have to machine a new thread.