Screw threads for really small thin tubes
Screw threads for really small thin tubes
(OP)
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
Thanks,
Kevin





RE: Screw threads for really small thin tubes
RE: Screw threads for really small thin tubes
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
RE: Screw threads for really small thin tubes
RE: Screw threads for really small thin tubes
RE: Screw threads for really small thin tubes
"Art without engineering is dreaming; Engineering without art is calculating."
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RE: Screw threads for really small thin tubes
RE: Screw threads for really small thin tubes
RE: Screw threads for really small thin tubes
RE: Screw threads for really small thin tubes
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
RE: Screw threads for really small thin tubes
Cheers
Greg Locock
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RE: Screw threads for really small thin tubes
This was used for some 'cartridges' at a former employer back in the 70's or around then.
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RE: Screw threads for really small thin tubes
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.
RE: Screw threads for really small thin tubes
RE: Screw threads for really small thin tubes
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.
RE: Screw threads for really small thin tubes
Regards,
Mike
RE: Screw threads for really small thin tubes
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
RE: Screw threads for really small thin tubes
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.)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Screw threads for really small thin tubes
-Kevin