Screw threads for really small thin tubes 5

[GuyNamedKevin]

I'm working with a very small, thin metal tube, on the order of .75" ID and .90" OD. I need to design a retaining ring or end cap of some sort to keep stuff inside of it. I checked the UN thread standard and even though there are a couple threads that will just barely fit in there, it's obviously not going to be practical to make because there's barely any material left. What is the general rule of thumb for the minimum amount of wall material left over when you cut a thread? And how many threads do you generally need to make to ensure two parts are decently fastened together?

Thanks,

Kevin

 

[Tmoose]

how hard is the stuff trying to get out?

 

[SnTMan]

Odd, I was just looking at threads in Machinery's Handbook...

I quote, "Fine Threads for Thin-Wall Tubing: Dimensions for a 27-thread series, ranging from 1/4 to 1 inch nominal size are also included in Table 4. These threads are recommended for general use on thin-wall tubing. The minimum length of complete thread is one-third of the basic major diameter plus 5 threads (+0.185 in.)."

Better get yourself one.

Regards,

Mike

 

[dhengr]

You might be better off using a cap rather than a plug. And, you better get yourself two, not just one, or the stuff will just get out the other end.

 

[kingnero]

Have a look at silencers... It's been done before.

 

[MadMango]

Can you ignore the wall thickness of the tube and thread directly into the end cap material?

"Art without engineering is dreaming; Engineering without art is calculating."

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[TheTick]

How about some sort of compression fitting?

 

[MintJulep]

 

[GuyNamedKevin]

Hey guys, thanks for your replies. I probably should have clarified a little more - the retaining ring or end cap is only there to keep the stuff in place, and is not subjected to any loading. Uniform wall thickness is important, which is why retaining rings are usually used instead of end caps. We actually decided on "flaring" out the ends of the tube to get a large enough OD to cut a thread in, only because the tube is made of steel and is going to be machined, although I'm not sure how much larger than the thread's major diameter the OD would need to be.

 

[Cockroach]

Theoretically you could scratch in a 13/16 - 16 UN-2G thread and utilize the tubing bore as your minor diameter within tolerance. That would only leave you with 0.044 inch wall, so it depends on what thread performance you would require for your design.

In your position, I would probably re-write my own thread form and base it off a Stub Acme geometry. Noting you only have 0.075 inch wall, I would make the minor diameter of the Box equal to the tubing ID minimum and go for a 1/32 inch thread height. That would put you at least even with the UN configuration above, but the Stub Acme geometry is a much more robust thread. So you would recover somewhat on strength. The equations for the Stub Acme thread can be found in the Machinist Handbook, but I would probably go with the ANSI standard so that I could pick up the miscellaneous threading information for the complete case. You could make the Pin the gauge for the Box and have QC/QA do a three wire measure on the male threads for some sort of machining control. Again, not sure of your application so the thin wall may be an issue. For machining the Box, I would wrap the OD in surgical tubing to limit the vibration during cutting of the major thread diameter. This is just an old machinist trick.

The only other thing I can dream up is talk to a watch maker, if you can find one these days. They're pretty sharp cookies with miniture parts, doing a web search may ferrit out more information.

Pretty tough problem, mechanically speaking. Good luck with it.

Regards,
Cockroach

 

[GregLocock]

Cross threading (especially) and galling strike me as likely problems

Cheers

Greg Locock


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[KENAT]

Can you use a 'bottle' (I think that's the term) formed thread as I believe Mint may show rather than a cut one? That is rather than removing material you deform the walls to take the form of the thread. There are standards for this though I have no idea of the number. You may be able to see this on some metal water bottles.

This was used for some 'cartridges' at a former employer back in the 70's or around then.

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[ornerynorsk]

0.075 wall is a lot of material to play with, unless you're looking at it from a perspective of having worked with very large pipe thread. Optics is an industry that uses almost exclusively thin wall sections and very fine thread. You also have enough material to effectively use spiral retaining rings.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[GuyNamedKevin]

The "bottle" formed thread is an interesting idea and it looks like it will probably have to work. I am working on different scaled down versions of a larger assembly and this isn't even the thinnest wall I'm going to have to work with. Any idea what those threads are technically called or what the standard is?

 

[btrueblood]

I have a part made of 1/2" o.d. aluminum tube that I make at home, and goes out for general use in the public, with .05 wall and 7/16-32 UN thread for an end plug...and there's room enough left over for a thread relief to allow an internal o-ring seal outboard of the threads. I had the luxury of picking a tube that would work with commercially available very fine thread taps and dies.

You just need to find a thread that will work for you, even something fully custom like a 0.900-32 (external) or .790-32 (internal) could work, you just need somebody to make a tap & die set for you and go to town. There are plenty of shops that do such work, google "custom taps and dies" - they may even have stuff "on the shelf" that the supply houses don't.

 

[SnTMan]

Well here's one: Machinery's Handbook, "Rolled Threads for Screw Shells of Electric Sockets and Lamp Bases"

Regards,

Mike

 

[imagitec]

I'm working with a very small, thin metal tube, on the order of .75" ID and .90" OD.

One engineer's very small is another engineer's average is another engineer's enormous. As ornerynorsk noted, such threads are common in optical systems for lens retainers and the like; also, custom bearing retainers and ferro fluid seal sleeves in motion systems. Size and weight are paramount in many of these applications, so walls are thin and aspect ratios (width of the ring to its diameter) can border on the ridiculous. The diameter of the optics are also non-standard, and you have vignetting to contend with on the ID. I don't remember the last time I was able to use a "standard" thread for one of these applications, but I've never had trouble getting parts that work either. UNS-40 threads of arbitrary diameter are common. Try to source the mating parts from the same vendor.

Bolt Planet is a nice online calculator for UNS and other thread forms.

Rob
Imagitec: Imagination - Expertise -Execution

 

[KENAT]

If you decide you need a 'special' UNS thread then you may find this faq404-1480 useful.

Many folks don't know how to call out UNS threads properly - it requires more detail than standard threads.

(No I'm not intentionally being a know it all, I only learned because my old checker pointed it out to me.)

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[GuyNamedKevin]

Thanks for your help guys, it is much appreciated.

-Kevin