Large Custom Thread 1

[DRoam]

We are designing a couple of parts which will thread into each other. The OD of the internal part is 8.75 inches. From I've seen, standard UN threads only go up to 4 inches maximum.

We need a thread with a nominal size of 8.75 inches, and a relatively fine pitch, say 16 threads per inch or more (I'm not really sure what's a reasonable value for such a large diameter).

My question is twofold:
1. What method would the machinist use to cut such custom threads?
2. How do I call out custom threads in a way that communicates everything the machinist would need to know to cut them?

This topic was discussed some in another thread (thread404-226452) but the resolution was that the OP's desired thread designation turned out to be a standard size. User KENAT gave some valuable information in the thread and in a related FAQ he created (How Do I Specify Unified Thread Form Custom Threads?), but I'm not quite sure how to apply it to my situation. I've seen some slightly contradictory information which indicates I should not have to call out the pitch diameter or even the major/minor-diameter tolerances, but rather those should be dictated by the UN standard.

Any information and expertise would be very appreciated. Thanks!

 

[KENAT]

If nothing else they should be able to do it by single point threading. I believe that's how some of our larte 14" dia Acme threads were cut at former employer.

https://en.wikipedia.org/wiki/Threading_(manufacturing)

Looks like threadmilling may be an option too.

Look again at my FAQ, it is very closely based on the ASME B1.1 which if you're using unified thread form is correct.

For custom threads you do have to give extra information on the diameters per section 6.4.

Brief look and I don't see an upper limit stated for use of unified thread form. Tables go up to 6-16 UN, 20 UN drops out at 3".

However, I can imagine relatively fine thread on large dia may be tricky.

(Thanks for the kudos btw)

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[JStephen]

My approach would be to try to locate the shop/machinist first, then work off of their requirements as to detailing, tolerances, etc.

That seems like awfully fine thread for such a large piece. My concern, is that even if you were able to machine that just absolutely accurately, when you went to assemble the two parts, they'd be heavy, and the merest bump of one against the other would goober the threads up at that point, and then what?

 

[EdStainless]

We both thread mill and single point turn stuff over 4" (depends on the production volume).
I don't have the reference in front of me but I think that 8.75" we use a 12 pitch.
8" pipe is 8.625" x 8 pitch.

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P.E. Metallurgy, Plymouth Tube

 

[dvd]

Think about these -

http://www.nord-lock.com/superbolt/multi-jackbolt-tensioners/nut-style-tensioners/introduction/

 

[Jughandle]

2nd question - Between the above links and posts, you should be able to spec out a thread. (Don’t forget to account for length of engagement.) The machinists will do the rest.

1st question. If you are only making ‘a couple of parts’, a single-point cutting tool will prob be used. These consist of inserts, with the thread profile and pitch, that clamp into a holder. Also machinists usually like to have access to the mating parts to ascertain the fit, assuming you won’t have gauges made.

Personally I prefer Acme threads, esp for large parts with fine pitch. The flat crests appear to be less damage prone. Lastly if these are aluminum parts (or something else that galls, e.g. Ti), 16TPI just seems too fine. I’d go with around 8TPI.

 

[drawoh]

DRoam,

Call up 9-16UN. The UN specification describes the thread form, and it establishes all the tolerances you need.

Fabricate both pieces at the same vendor. They will check to ensure everything assembles okay. If this is production, your machinist(s) probably will fabricate inspection gauges. There are all sorts of convenient ways to inspect external threads. They can inspect the male thread, and they can inspect their plug gauge.

--
JHG

 

[DRoam]

Thanks all for the input. I think I should be able to designate the thread okay with the information in KENAT's FAQ. Thank you for that, KENAT.

My question now is, how do I go about making sure my threads will not strip under load? The threads are holding the lid onto the end of a pressure chamber. There is a piston in the chamber and the hydraulic cylinder driving the piston is attached to the lid in question. So sealing isn't a problem--the pressure will be retained by the piston. The only thing the lid has to do is not fly off.

We know the pressure and therefore the axial "pulling" force that will be on the lid. So, how do I select the threads-per-inch and length of engagement so that the threads will withstand the force trying to pull the lid off of the chamber?

My first thought is to select a thread designation and then find the shear area of the threads per unit length of engagement, and then multiply that by the shear strength of my material. That would tell me shear strength of the threads per inch of engagement. I could then determine how many inches of engagement I need to withstand our pulling force, plus a safety factor.

Is that a reasonable approach? Or am I over-simplifying the situation?

 

[tbuelna]

Large diameter, fine pitch threads like the 8.75-16 UN thread described can be difficult to machine with sufficient accuracy to allow engagement without interference. ANSI B1.1-2003 provides some guidance on specifying dimensions/tolerances for large size threads. But the best approach is to define all of the dimensions and tolerances in a table, rather than simply calling out a standard.

With this type of large diameter, fine pitch thread the best method to manufacture them is by thread grinding. Thread milling might work OK, but single pointing them on a lathe would be my least preferable option.

If only a single set of mating parts is required, the threads should first be machined on one part and then the threads on the mating part should be machined to provide the required fit. If several sets of interchangeable parts are required, a set of master thread gauges that are used to check both halves of the thread joint will ensure proper fit.

 

[kingnero]

Why would you prefer such a fine pitch? What exactly are you trying to accomplish with that?

I concur with the posters above, and would also suggest a coarser thread than 16 tpi. I'm in metric country, and would say that 2mm would be a practical minimum (12 tpi), and starting from 3mm (8tpi) would greatly affect the production and handling in a positive way.

 

[israelkk]

Look for ASME B1.1-2003 (Revision of ASME B1.1-1989 (R2001)) Unified Inch Screw Threads (UN and UNR) and search for UNS threads.

Here is a quote from paragraph 6.4 Method of Designating UNS Threads:
"UNS threads are special combinations of diameter and pitch with tolerance to unified formulation. UNS threads have the basic form of designation set out above, supplemented always by the limits of size."

 

[tbuelna]

Interestingly, the ANSI B1.21M-1997 standard for metric J form threads have extensive tables for pitch diameters up to 200mm.

 

[KENAT]

Per ASME B1.1-2003 Standard series only goes up to 6" diameter (Table 1 invoked by para 3.1.1). So there is not a 9-16UN thread. My FAQ which the OP is familiar with gives detail on UNS thread with explicit referent to ASME B1.1 and select extracts from it.

faq404-1480

My FAQ also has a link at the bottom to boltplanet where you can check your math not just on diameter calculations but also strength. (Note I say check.)

To get the numbers initially you can use Thread Yield and Tensile Strength Equations if not in a reference book you own you should be able to find online, maybe

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html

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[tbuelna]

Read thru MIL-S-8879 (UNJ threads) and it states in section 3.3.2 that diameter & pitch combinations not listed in the tables are considered "special" and should use the designation "UNJS". The section also provides one example of how the thread data should be shown on a drawing. Table VIII provides the special diameter-pitch thread computations.

The MIL-S-8879 16 pitch thread tables only go up to 6.00" diameter, so an 8.75" diameter 16 pitch external cl.3 UNJ thread is special and would be designated 8.750-16 UNJS-3A.