Source for oddball sized acme nut? 3D-print one instead?

[JeffThompson]

Maybe a long-shot, but I'm restoring an old machine with a strange leadscrew size: 3/4" x 3. The original nut was cast in babbitt and is really stripped and needs to be replaced. Re-casting the babbitt seems expensive (I've never done it before so I'd need the putty, a crucible, etc) and doesn't seem ideal anyway. I'd like to replace it with a new bronze, etc nut – I'd machine one myself but don't have access to a lathe that can single-point it and a tap is $$$$ even if I could find one.

I've looked on the usual places like McMaster but that thread is unusual and I can't find anyone that stocks that size. Any suggestions on places to look?

Alternatively, I'm considering a 3D-printed nut. The leadscrew is under very low load (it's a back-stop for paper-cutting machine) so I think it would hold up ok. Anyone had experience trying this?

Any help or suggestions appreciated!

 

[enginebill]

Are you sure on 3/4-3? That would make the minor diameter of the screw pretty small, only .417". If it is 3tpi and under little load, a 3d printed nut should work fine for a while.

 

[jmm03]

Hi Jeff, the size you have is not listed but these guys may be some help: Acme Bronze and Plastic Sleeve Nuts - Roton Products Inc. Jim

 

[atomarc]

Unless you're restoring this machine for posterity it seems sensible to replace the screw and nut with something more current and available. On the other hand, using the proper material, printing a nut for a low stress application could work..as long as it truly is light duty.

Stuart

 

[lazz]

I have used delrin for a couple nuts over the years. And I noticed they have a delrin filament for sale.

 

[Joe in NH]

Lining the existing with a new coat of babbitt is not as difficult as you think. It is why it was done that way in the first place - it certainly gives a "failure point" should one have both change feed and lead-screw inadvertently connected together.

About all you need is a camping stove, a stainless ladle from Walmart, and the babbitt. You can use plaster of paris to bed the bottom in and prevent "leak-out." (be sure to dry the pop in the oven at 225F.) I would recommend "nickel power babbitt" as this is the strongest and least likely to "separate" in the ladle. It is expensive, however. Magnolia Metal was and is the cheaper lead based alternative and might work for quite a while. More than likely the babbitt was of this lesser quality originally.

Babbitts - Magnolia Metal CorporationMagnolia Metal Corporation

Observe your lead screw thread. The portion of the screw up near the chuck is likely more worn than segments of the lead screw further away. Do your re-cast using these "better" sections lest the recasting "bind" in the worn inaccuracies of the lead screw.

Joe in NH

 

[magneticanomaly]

I agree that re-babbitting is not so hard as you think, but you could also form a nut over the well-greases screw with an epoxy putty, or even heat-soften Delrin or other thermoplastic and squeeze it around the screw.

You could also hand-carve a nut in two halves with a tiny gouge...blacken the screw with soot, press the halves against it, scrape away the black form the halves, and repeat.

 

[atomarc]

Many good options mentioned above, and I agree heartily, with a little work, pouring new babbitt for a nut would be simple, and fun!

Stuart

 

[Scottl]

Unless you're restoring this machine for posterity it seems sensible to replace the screw and nut with something more current and available. On the other hand, using the proper material, printing a nut for a low stress application could work..as long as it truly is light duty.

Stuart

My question would be how hard is the nut to change?

If not that bad just 3D print a few spares in case the first wears out.

If not, or original appearance is important babbitt isn't all that expensive. You don't need a crucible for something that small and even wooden forms faced with heavy duty foil would hold up fairly well for a single pour. If it is not a super critical item modern lead free plumbing solder will make a hard cast with some of the characteristics of babbitt.

EDIT: Would this work?

Bronze Acme Nut 3/4"-3 RH, Nook Part Number 20073

Inch Acme Nut Bronze | Acme & Lead Screw Products | Nook Industries

2nd EDIT: Sorry, took another look and it has 2 starts.

 

[JeffThompson]

Thanks everyone, this is super helpful! I'd love to replace both the nut and screw but don't have a way to machine parts that big. I've watched a bunch of babbitt videos online and have done some metal casting before – great to confirm it isn't too hard!

@Joe: plaster works ok instead of babbitt putty? And do you mean put the parts together, add the plaster, and bake the whole thing? I'm guessing that's to drive out any water to avoid explosions, right?

I'm seeing a lot of cheap babbitt pieces on eBay like this one: STONEWALL BABBITT INDIANHEAD ADVERTIZEMENT PAPER WEIGHT 2+ POUNDS | eBay

I know it's probably not the high-quality stuff nickel stuff but would that work ok?

Thanks again everyone!