View Full Version : Half Nut Repair

06-20-2010, 12:47 AM
I don't know how to fix the Gulf leak and it looks like there's no shortage of opinions so I'll leave that to the ones that know more than I do about it. :rolleyes:

But I did have a job repairing half nuts on an old lathe and since that hasn't been beaten up recently I'll take whack at it.

This is the half nuts (what's left of them) in situ on the apron of a 16" South Bend. You can only about make out where the threads once were. Leadscrew is 1-1/8-6 Acme.

So they're mounted on the cross slide to be bored out. Not shown is the dummy for a leadscrew section to make sure of the spacing and registration. It also had a boss on it to indicate center, though in the event I was loaned a centering microscope which worked nicely. They've been indicated square and centered and a between-centers bar helps with straight.

After boring they're also counterbored both ends to lock the new thread insert.

... to be continued ...

06-20-2010, 12:51 AM
The new thread is bronze, threaded first and turned on a mandrel to get the outside spool section and keep concentricity. Skipping a few steps including sorting out the tool bit grind for an Everede boring bar with a triangular angled (two directions) insert here are the parts ready to mate up.

The (de-nutted) half nuts are put together with the new sleeve and lubricated liberally with Loctite sleeve mount.

And finally when sufficiently cured, sawed apart and trimmed up to go back in service.

06-20-2010, 01:01 AM
Very nice work.

Doc Nickel
06-20-2010, 01:15 AM
Nicely done! But, I have to ask, why a chemical adhesive rather than, say, solder? I'd imagine even a soft plumbing solder would be considerably stronger than the Loctite.


06-20-2010, 11:27 AM
I don't have strength comparison numbers between solders and Loctite so can't comment on that directly one way or another. I do use silver solder in some applications when I think it's the best choice.

In this case it would have taken a lot of heat because of the size of the half nut castings to be sure everything was heated evenly so the fit-up was also even. That is, the two soldering approaches would be to tin surfaces first and then heat everything and get it into position. Or to place the parts in position first and then hope to wick solder end to end and half way round. Either approach seemed problematic.

The significant force is longitudinal which is why there are flanges on both ends. I can't imagine the rotational frictional force, even with no lubricant would be sufficent to shear the bond and rotate the threaded sleeve sections out of place. Other approaches? Sure. A secure solution for what's required? I'll stake my reputation on it and repair it if the customer ever has a problem.

06-20-2010, 11:54 AM
Beautiful job Jan! The flanges on the ends were clever.

One advantage I see to using (blue) Loctite is that if you ever wear out the half nut, you can heat it up to 350 with a heat gun and replace the nut.

06-20-2010, 11:56 AM
I think that people are too skeptical of modern ahesives. Over that area, I doubt you'd get that nut out of the casting withut damaging it even if you wanted to, without the application of heat.

06-20-2010, 12:09 PM
I would have used two screws in each half to hold them in along with the locktite. I am not comfortable using locktite or epoxy in that manner but time will tell as to how well it holds up.

06-20-2010, 03:16 PM
I have the same lathe (16" South Bend) from the 1920s w/ the same worn out half-nut... I've gotten mine bored out, but the project stalled some years ago (I've got a 15" newer machine now instead of the 10" Atlas). Thanks for the kick in the pants!

- Bart

06-20-2010, 03:56 PM
Great looking project, Very interesting job. I especially like the way you set the thing up on your lathe. I learned a lot from the photos.
Thanks for sharing

06-20-2010, 04:24 PM
Nice repair. I'd have glued the part together as well, but probably would have used PC-7 epoxy. I'm not knocking your use of the Loctite product- I don't know it- but I do endorse the epoxy one because I've used that and had good luck with it.

For those who are skeptical of epoxying parts together, remember that epoxies are not all the same. The commonly found 5 min ones are great at what they do, quickly securing parts together, but they are not what you might call an industrial adhesive. Some are better than others, and I don't have a recommendation right now of a good 5 min, but there probably is one. Of the slower varieties, I have found a few I like, and they are normally ones that don't cure to a brittle state. PC-7 does cure quite hard, but it's very strong and the bond is tenacious, as long as you leave it undisturbed to cure. A couple of the coffee table epoxies are also in my good books, and I believe that's largely because they remain slightly flexible once cured. That has to help them stay molecularly attached to materials that have a different rate of expansion with heat, and so will remain bonded for a long time. One I use now (because I can easily get it) is called Nu-Lustre-55. The usual disclaimers apply- I don't have stock in the companies, etc., and I don't receive compensation for advertising these products. I'm just a happy user.

I share some of the concernes of others regarding pinning or otherwise securing the parts together mechanically. One thing I've often done is drill and tap for one or more bolts, then install those at the time of epoxying. The epoxy gets squished into the bolt threads and leaves the whole assembly as one rigid piece. Even if the epoxy bond failed, the parts of the assembly would remain in alignment.

06-20-2010, 05:24 PM
Many thanks "TGTool" for the original post and others such as darryl who add helpful and useful details in their postings.

This is the best way for me to learn and have half a chance of achieving successful repairs of a similar nature :cool:

John Stevenson
06-20-2010, 05:58 PM
I have done these before as well, not a SB but similar lathes,
I'm loath to say how I did it so as not to hijack TG's very informative thread but I did use loctite and some dowel pins, roll pins actually.
The final job wasn't far removed from TG's


06-20-2010, 06:13 PM
When I was in my late teens I took a job working for a company developing brushless DC motors. Old-hat today but then it was still developing technology. One of my jobs was building the rotors with rare-earth magnets around the circumference. All sizes from a 60KW traction motor with huge magnets to 100,000+rpm dentist drills. They were just fixed with epoxy on one flat surface and held up just great. They did manage to throw the magnets off one of the dentist drill motors by running it up to full speed and throwing it into full reverse on the drive circuits - it jumped a foot off the bench. Other than that I never heard of a failiure.

The adhesive I used for most was an oven-cured grey paste, I don't recall the manufacturer. I guess things must have developed a long way in the 20yrs since.

06-21-2010, 01:06 AM
I like the boring bar. Looks like a set screw of some sort to adjust the height of the cutting tool? How to you get just the bit to advance, Is it just a standard screw and the half threaded cutting tool holder?

Nice job and great pics and description BTW.

06-21-2010, 02:47 AM
Yeah, I'm with fishfrnzy. I too would like to know more about that boring bar.

Very nice looking work! Looks like it ought to outlast the original.

06-21-2010, 03:49 AM
Looks like the boring bar has a hole drilled all the way through it for the cutting bit and an adjacent hole part way through it for an Allen set screw. I think both holes have no threads. The bit appears to have threads on one side that engage with the Allen screw. As the Allen screw is turned it can't move in as it's tip is hitting the end of it's hole. Since it is engaged with the cutting bit on one side, the bit is pulled out of the hole and projects proud from the surface of the boring bar. Turning the Allen screw right or left sets the cutting depth of bit. I would guess you would need to both make a jig to cut threads into the side of the bit and both anneal and reharden the bit to make it work. Did I get this right?

06-21-2010, 08:42 AM
Really nice rebuild!

About the Loctite. Ever since I got a small bottle of 270, which was required for fixing steel tapered bushings to pivot pins on my motorcycle swingarm, I've been trying it out on non-critical assemblies and am amazed at it's strength.

I'll continue to use less on higher stress applications until I get a failure.
Maybe not scientific enough for some of you but...

Secrets seem to be:
large area
tight fit
very clean

06-21-2010, 11:03 AM
I was guessing he'd soldered a piece of HSS into softer steel to make the threaded tool we see in the boring bar.

The big mystery was the exact layout of that adjustment screw. You may be right 914Wilhelm, but I'm having difficulty visualizing what's keeping the screw from pulling out ...one way or the other.

06-21-2010, 11:25 AM
I think Wilhelm has it right. It's much like the way a taper hub locks the taper. He probably has a set screw on one side to lock the plug in place after he sets the protrusion of the cutter.

He did a nice job on the repairs.

06-21-2010, 11:34 AM
I like the boring bar. Looks like a set screw of some sort to adjust the height of the cutting tool? How to you get just the bit to advance, Is it just a standard screw and the half threaded cutting tool holder?

Nice job and great pics and description BTW.

There was an article in MEW a year or so back giving the basic layout. I've modified it somewhat from the original. This is a smaller bar (3/4") which makes it harder to shoehorn everything into the space so I used an Everede triangular toolbit and soldered pieces together to get the toolbit cross hole and the whole cartridge. It has to be a larger piece so it can be tapped, then turned down to expose the half-thread. The advancing screw just lies in the adjacent hole, though I did have to make a cutter to come in sideways to form the recess for the circlip (photo following). The screw has a head on one side and a collar formed by the circlip so when inserted sideways through the larger hole first it's trapped. Then the cartridge can be backed in. In this case I think I did use a LH tap so "advancing" the bit would be a right hand turn. It is a 10-32 LH which doesn't come out right graduating the screw head, but it's usually only reference anyway. For a 1" or larger bar you can machine a collar on the screw as shown in the original article which is a little nicer and a 1/4-40 thread makes graduations sensible. Even LH taps can be easily found from industrial suppliers.

914Wilhelm's proposition for a setscrew would also work and be a little simpler. In that case the set screw would just lie at the bottom of the counterbore and a smaller diameter hole would give access for the allen wrench, though it wouldn't have any calibration indication and wouldn't actually retract the cartridge, not a particular problem.


06-21-2010, 11:41 AM
I have done these before as well, not a SB but similar lathes,
I'm loath to say how I did it so as not to hijack TG's very informative thread but I did use loctite and some dowel pins, roll pins actually.
The final job wasn't far removed from TG's



If you have alternative ideas or photos of a different approach, pitch them in. There's never a better time for readers (including me) to get more informed. I've seen the roll pin retention illustrated and that would be important to know about in the absence of some current excellent adhesives, or if the double-ended flanging wasn't an option.