Fabricating Leadscrew Nut Questions...

[JAndrew]

HMEM,

I had to stop working on my first IC engine and tear down my Diamond horizontal mill's table. The leadscrew nut for the y axis is shot. There's no chance of me finding a replacement anywhere as it has an odd outside shape so I was going to go ahead with turning one myself (using the lathe and not tapping).

I've done internal acme threading once before though it was over 10 years ago now.

My first question would be:

Is it brass or bronze and what alloy should I try to buy. I'll need a piece of 1.75" square stock 2.25" in length so it's going to be pricey

The next question:

Do I need to lap the nut to the leadscrew? I read something about it online but don't know if that's something I really need to do or not. If so should the threads be cut slightly under finished....? I guess that wouldn't work though because the leadscrew wouldn't thread in....? Or is that only for leadscrew nuts that have adjusting screws on them...?

Thanks in advance for any advice or guidance you can offer.

-J.Andrew

P.S. Almost forgot the leadscrew is a 3/4"-10RH Acme thread.

 

[Goldflash]

Try these guys they make ACME nuts for lead screws including 3/4 x 10 tpi

http://www.roton.com/Engineering_Data.aspx?line=Lead

 

[JAndrew]

Goldflash,

Thanks but unfortunately nobody makes a nut with a large enough OD to allow for milling down the bottom alignment boss.

Here's a quick model of the nut since I don't have it here to take a photo:

No commercially made nuts have enough meat on them to mate up.

Also not enough clearance on either side of the existing nut's bore to allow one to press in a new nut. It's going to have to be made.

Thanks,
-J.Andrew

 

[crankshafter]

Hi JAndrew

Check this out: http://bbs.homeshopmachinist.net/th...-leadscrew-nuts-the-easy-way?highlight=EVAnut

CS

 

[Goldflash]

Try these guys they make ACME nuts for lead screws including 3/4 x 10 tpi

http://www.roton.com/Engineering_Data.aspx?line=Lead

Maybe they can point you in the right direction to get a tap.
Might be a lot easier than having to screw cut the Thread

 

[JAndrew]

Crankshafter,

Thanks a bunch for the very interesting link! I don't think it will work for this application because the material between the flat mounting surface and the bore is way too thin to be made of plastic. I will be filing this method away for later use though on other leadscrew restorations! I can't believe how good the backlash numbers came to be!

Goldflash,
I did find a tap on ebay already but it's $95 plus shipping! The internal threading won't be too much of a problem for me assuming I can find out about the whole lapping question.

Thanks all,
-J.Andrew

 

[bb218]

I am not an expert on the topic but I would not consider lapping, I would have doubts ever getting all of the lapping grit out of the nut. I assume you would have to make a smaller threaded lap and how much smaller would that need to be ?
I have cut a couple of Acme nuts and it is not an easy task as they end up deep in the material when you get close to a full thread, would be nice if you could lathe thread part way then finish up with a tap.
Mike

 

[JAndrew]

Thanks bb218.

I guess I will consider buying that $95 tap. The thread depth for this nut would be 0.074". That's quite a bit of rubbing for a 2.25" deep hole...

Did you use brass or bronze? And which alloy?

-J.Andrew

 

[R Degen]

When I made one for my lathe I used 660 bronze Randy

 

[Tin Falcon]

J:

As far as lapping probably not.
in lapping an abrasive is added to a soft material like copper or lead and it smooths the hard material.
some new parts aer lapped together with an abrasive that breaks down this would be better, but you have an old sooth lead screw and will have a new part.

The actetal method is an interesting one I have not seen before.

Anther option is Molguise (SP) or other machine repair epoxy.
bore out the damaged threads and cast new ones from epoxy.
But IIRC a epoxy lit will run as much as the tap.

I have a friend that went through a similar fix IIRC he did end up making a new part.
as far as material choice


http://www.mcmaster.com/#standard-bronze-alloy-sheets/=t9eq3d

IMHO 936 aka C93600 would be a good choice easy to machine and you can solder or braze so you can add the boss and buy a much smaller piece

You may be able to call 1-888-Dura Bar and they should be able to provide the materiel and advise for the best material.

http://www.dura-barms.com/dura-bar/index.cfm
Tin

 

[tornitore45]

I would make it out of bronze and consider a small modification.
Make it a couple of threads longer if possible, then slit two threads radially 3/4 of the way and place two screws squeezing the saw kerf.
That provide a backlash adjustment.
Make sure the slit is located such that the entire nut is loaded in the positive direction of movement and the 2 threads are loaded in the back direction.

 

[Hopper]

Is there enough "meat" on the original nut to bore it out, turn up a threaded sleeve and insert it in the hole? That is the way I reco'd the half-nut on my old Drummond lathe. Soft soldered it in position and it's worked for years.

Don't be scared of screwcutting Acme threads. It is pretty easy in brass or bronze 660. Use the biggest diameter boring bar that will fit in the hole (.600 or so for a 8 tpi 3/4 Acme). Grind a HSS cutting tool much like a parting tool but dead flat on top, no back rake.
Make sure the leading flank on the tool has the extra clearance to clear the helix angle of the thread (test this by inserting the bit into the thread on the lead screw and eyeballing it.
Make the width of the tool tip to the width of the thread, plus a thou for clearance.
Hone the cutting edge to a fine finish on an oil stone. Taper the width of the tool narrower as it comes back from the tip, to give chip clearance. Let the very tip do all the work.


Have the cutting tool sticking out just enough to cut the full depth of thread plus a few thou for chip clearance. Then take .010" cuts until you get down to little finishing cuts. Test the thread by screwing the leadscrew into it before taking it out of the lathe, and take more tiny cuts if needed.

No need to lap.

Brass vs bronze. It depends. If the leadscrew is hardened then bronze is fine. If it is hardish, then 660 bronze maybe ok. If it is not hard or hardish, you can use brass so it wears but does not wear the lead screw.

 

[abby]

I think Hopper has told you everything you need to know but I will add that I have repaired large nuts by casting white metal around the lead screw.
This gives a good fit and lasts a very long time too.

 

[Chiptosser]

If your nut is worn out, you should consider making a new screw.
You most likely, will not like the feel of the loose and tight spots in the travel.
That is what I have found over the years. Just trying to save you some grief.
Good luck

 

[BaronJ]

Hi JAndrew

Check this out: http://bbs.homeshopmachinist.net/th...-leadscrew-nuts-the-easy-way?highlight=EVAnut

CS

Hi Andrew,
I was going to point you in this direction as well. Crankshafter beat me to it

I've made several nuts based on this technique and it works very well. The only issue is that the nuts are very tight to get moving on the shaft. Two or three layers of PTFE tape help somewhat particularly if you can persuade the tape to come out of the nut when you have got it off. I did try making an adjustable nut but made a right mess of splitting one side and machining flats on it. The idea was so that it could be loosened with a thin wedge in the split.

HTH.

 

[Omnimill]

I think Hopper has told you everything you need to know but I will add that I have repaired large nuts by casting white metal around the lead screw.
This gives a good fit and lasts a very long time too.

I always wondered if this would work, sounds like a quick fix worth trying!

 

[MachineTom]

The depth of the thread at 2.25" long is tough with a bore of. 670" or so. A tap is the tool for this type repair.

Get a chunk bronze 660 and mount in a lathe, or use a boring head with a reversed cutter to mill a stem, then bore the hole and run the tap through.it will be a long tap 6".

 

[JAndrew]

Thanks all for the input. I'm probably going to attempt without a tap. The only tap in that pitch I can find is $95 used on ebay. No sign of long taps either.

For now I cleaned everything and re-installed the nut with a slightly offset bushing on the tail end of the leadscrew. It misaligns it just enough to take the nasty slop out. It's a temporary fix until I have some time to make a new nut.

It's a funny coincidence the only time I ever did acme threading was in the short lathe school in the Navy and I happened to do the exact same thread pitch (3/4"-10) for my final project. It was also in brass. At least I know it can be done.

Thanks again. HMEM members seem to be a good deal more helpful than any of the other machinist forums out there.
-J.Andrew

 

[Hopper]

Good lad. Have a go. All you have to lose is short piece of bronze worth a few dollars. A .625 diameter boring bar should well be up to the job, and provide enough chip clearance. The secret is all in careful tool grinding and honing, making sure there is enough clearance on the front, sides and top of the tool. And that it is oil stoned sharp sharp sharp. One of those jobs where the preparation will take longer than the job.