https://www.vabs.com/2020/08/thread_rol ... d_cutting/

This is a site that sells threading equipment and gives some data vs the two methods; rolling being prefered.

I was always told Wrought Iron was used as original equipment however various modern materials can be substituted.

Thank you J.David for your thoughts. While I do enjoy "enginerd discussions" on stay bolts and boiler work in general there comes a time when the "nuts and bolts" of what works and has worked is of more importance. Years ago a midwestern contractor once told me in a discussion about boiler welding and boiler repair that the tried and true practices often trumped some of the newer technology simply because of the steam locomotive being a steam locomotive vs a stationary or near stationary boiler on wheels. Re-inventing the wheel often reveals flaws that were not expected. Being I am not a working boiler contractor with years experience in the trade I was reluctant to bring up some of your points. By the way that medium size "American" engine lathe at the Valley made some very good staybolts for #97. Hope they are still in working order? Regards, John.

PS think that lathe used was the gray "American", thought that looked like a very useful lathe for RR work. But maybe Dave used the other one?

Some errors and misconceptions here. UNR threads can be rolled and most are. The root radius is much more conducive to rolling than the flat specified in the UN profile. There is no such thing as an internal UNR. UNR and UN are certainly compatible or they would just be an ornament.
A very common misconception is that UN means Unified National. It does not. The whole idea of the unified system came about in 1949 when the US, Canada, and U.K. agreed on the unified system to make wartime production easier by abolishing national systems.
Exotic materials , special profiles and rolling are really of little value in a staybolt. No way can just one pull through the sheet with out an adjacent one pulling at least partway through. Once a failure of that magnitude starts there will be no stopping it.

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My understanding was that the rolled-thread 'work hardening' was grain realignment, similar to the effect produced in forging, and this wouldn't cause a particular issue on subsequent slight cold-working. (Hot-working as in the reported UP practice is of course less of an issue.

Were the threads to be burnished, or given a work-hardened martensite layer as for rails, then there would be an issue with subsequent working 'breaking up' the hard coating, with the same potential consequences as hard coatings like boron nitride applied over softer substrates that deform under load.

While I worked out, in principle, ways of rolling threads in the plate (really a finishing pass once the initial threads were cut 'aligned' with a traditional double-diameter staybolt tap) this would only be a practical consideration in "restoration" boilers, where long life with minimal degradation is a strong consideration. Back in the day, a boiler was a commodity with a relatively short expected service life, and it could be expected there would be less 'interest' in low-distortion tapped thread in the sheets...

The mention of tit snaps reminds me that, years ago, when I was looking at full-pen-welded staybolts, I cvonsidered that an inside 'head' with centered hole would permit relatively easy staybolt extraction with the equivalent of a centering jig with concentric hole saw on a feed mechanism. This would center on the staybolt, be set coaxial with it, and the saw then used to cut cleanly through the staybolt OD to leave a clean hole at 'datum' diameter in the plate.

tdmidget writes "A very common misconception is that UN means Unified National. It does not."
Just curious, what does "UN" mean?

"UN" is one of the classifications in ANSI UTS. I always naively understood it to mean 'United Nominal' but it might as easily be 'UNited'.

Some of the history of American threading, some of which is unexpectedly interesting (did you know the ALAM had thread standards for its members?) is here:

https://www.ring-plug-thread-gages.com/ti-N-vs-UN.htm

"UN" absolutely stands for Unified National. "UNR" stands for UN form except it has a radiused root on external threads only. The radiused root reduces stress risers as a sharp V or flat root would have and it reduces fit issues due to the effects of wear of the cutting tool. There is no UNR internal because UN taps already had a root radius. Both UN and UNR external threads are assumed to have crests flatted to .125P however rounding is optional. Cresting inserts can be a blessing depending on the material. About 100 years ago, they figured out the crests on fasteners don't do the holding and were causing more issues than they were worth thus flatted the crests. On staybolts, you have to hammer bolt material to expand the bolt threads into the sheets which provides a water/steam tight seal. This is not the same as a head bolt on a chevy 327 which does it's job by being under tension. I would use the same thread form on the bolts as is on my staybolt tap. I'm betting the staybolts installed in 1924 closely matched the tap and were properly hammered. Water chemistry probably didn't suck as bad as your typical logging or industrial railroad. The most common staybolt tap was/is 12-UN. As Kelly mentioned, he has seen Whitworth. Reading used 2" Gas Pipe thread instead of 11.5 NPT on at least some whistles. Never assume, measure your threads with gages. Proper design, materials , wokmanship and operation go a long way.

Of course, Adam. Three governments met to abolish "National" thread standards and called there agreement a National Thread Standard. Makes perfect sense. To you.

I believe UN stands for simply, Unified. Long hand is "The American National Standard for Unified Screw Threads". (Machinery's Handbook, 21st Edition, page 1256.)

Below from the 21st edition of Machinery's Handbook, page 1261:
"American Standard for Unified Screw Threads. - American standard B1.1-1949 was the first American standard to cover those Unified Thread Series agreed upon by the United Kingdom, Canada, and the United States to obtain screw thread interchangeability among these three nations. These Unified threads are now the basic American standard for fastening types of screw threads. In relation to previous American practice, Unified threads have substantially the same thread form and are mechanically interchangeable with the former American National threads of the same diameter and pitch. The principal difference between the two systems lie in: (1) the application of allowances; (2) the variations of tolerances with size; (3) difference in amount of pitch diameter tolerance on external and internal threads; and (4) difference in thread designation." <snip>

Kent

You're right on that. One of my handbooks is the 29th ed, toolbox version. Low light and reading fast with dirty glasses. I just reread it. I agree, UN stands for Unified. I even read the Declaration of Accord in the 1950 supplement to H28. It just says Unified. Well, that's settled.

Adam, do you have an actual published reference that indicates the ANSI UTS "AN" prefix contains the specific word 'National'? [EDIT: never mind. I see things have changed while I was typing.]

Meanwhile, is this actually so:

My understanding was that the steam seal on the fireside was accomplished by calking and seal-welding the periphery of the driven bolt. In order to increase the physical diameter of the staybolt you'd have to hammer it hard symmetrically, and yet buck it effectively all the way at the outer wrapper without inducing any bending moment in the portion of the bolt in the waterspace... which according to Tross had some interesting bending-moment behavior just under thermal expansion stresses between the inner and outer wrappers.

I might buy expanding one or two of the threads at the very end of the bolt on the fireside of the inner wrapper, but not really more than that. Not that you couldn't do it with a glorified tit snap up inside a tapered bore in a hollow staybolt, but then you'd need something like the grandmother of all slide hammers to get the snap out of the deformed taper bore...

Matt, I can't load anything in your post. Do you have a source or link to the reference? I'd like to read the whole thing.

Maybe I have been shadowbanned...... [sarcasm}