Internal thread on large diameter thin wall tube

[winflex]

I am designing a part that requires an internal thread on the ends of a thin-wall 5" OD 60" long welded 304 stainelss steel tube.
The current design has a UN 24 TPI thread in a 0.065" wall thickness tube. The major diameter of the thread penetrates about half way through the tube wall thickness. Tube OD mill tolerance is pretty tight at +/- 0.02.
I imagine tube end roundness (ovality) and deflection will be problematic during single point threading on a lathe with a steady-rest. Is it possible to force the tube ends into round by clamping a collar around the tube OD during machining?
If 0.065" wall thickness is unachievable, what minimum wall thickness & thread pitch would you recommend? Available tube wall thicknesses are 0.065, 0.083, 0.095, .109, .120, and .134”.
Are there any special fixturing tricks or other threading techniques that will make this job easier?

Thank you!

 

[DDoug]

How about swaging ?
Swaging down the O.D. a wee bit would:
1. Thicken up the wall.
2. Bring the area to accurate size.

 

[Triaged]

How about swaging ?
Swaging down the O.D. a wee bit would:
1. Thicken up the wall.
2. Bring the area to accurate size.

Swaging sounds like a good idea. If you do it over a threaded mandrill you wouldn't need to worry about machining the threads if they are formed. Could be fun...

 

[thermite]

I am designing a part that requires an internal thread on the ends of a thin-wall 5" OD 60" long welded 304 stainelss steel tube.

Fold the paper, Skywalker. Shortest route between two points drops to zero when they are made to touch.

Give the designer a swift kick to motivate him to find a design that does NOT "require" threads at all?

 

[Limy Sami]

Just a heads up, ……...if the male threaded part is stainless steel as well - all bets are off.

 

[LKeithR]

...Give the designer a swift kick to motivate him to find a design that does NOT "require" threads at all?

Either no threads or a thicker tube. I'd be willing to bet that just the pressure of the tool on the inside of the tube is going to
be enough to cause it to flex and distort. Keeping the tube round and the thread depth consistent is going to be fun.

...Just a heads up, ……...if the male threaded part is stainless steel as well - all bets are off...

Yup........be fun to watch the first time the thing goes together though...

 

[Gordon B. Clarke]

I am designing a part that requires an internal thread on the ends of a thin-wall 5" OD 60" long welded 304 stainelss steel tube.
The current design has a UN 24 TPI thread in a 0.065" wall thickness tube. The major diameter of the thread penetrates about half way through the tube wall thickness. Tube OD mill tolerance is pretty tight at +/- 0.02.
I imagine tube end roundness (ovality) and deflection will be problematic during single point threading on a lathe with a steady-rest. Is it possible to force the tube ends into round by clamping a collar around the tube OD during machining?
If 0.065" wall thickness is unachievable, what minimum wall thickness & thread pitch would you recommend? Available tube wall thicknesses are 0.065, 0.083, 0.095, .109, .120, and .134”.
Are there any special fixturing tricks or other threading techniques that will make this job easier?

Thank you!

I hesitate to comment on this but it'd help if we knew why you're going for a very thin walled pipe especial when there is a thread involved. To be very blunt then a 24 TPI thread on a 0,065" wall thickness is what I'd describe as suicidal unless you have a very good reason.

I'd also hope your single point has a nice radius.

My advice? Go for as much wall thickness as your function and design reasonably allows.

 

[winflex]

Some background: The tube forms a drum that is routinely filled and emptied in an industrial process. Keeping the wall thin makes it cheaper and easier to handle. The drum rotates inside a pressure vessel so the end caps have to be well located as they are the hub between the bearings and the tube. Hence the thread.

1 - What minimum wall thickness would you recommend?
2 - Wouldn't forcing the tube end into round with a clamp collar prevent any deflection during threading?

 

[winflex]

How about swaging ?
Swaging down the O.D. a wee bit would:
1. Thicken up the wall.
2. Bring the area to accurate size.

Swaging sounds like a good idea. If you do it over a threaded mandrill you wouldn't need to worry about machining the threads if they are formed. Could be fun...

I like the idea of making an under-sized hardened steel plug (mandrel) with male threads and then rotary swaging the tube onto the mandrel.

1 - What equipment would be required for this swaging operation?
2 - How much of a tube OD decrease would you recommend?
3 - Would removal of the mandrel after swaging be problematic?

 

[winflex]

Just a heads up, ……...if the male threaded part is stainless steel as well - all bets are off.

Great point - The male threaded end plugs will be brass to prevent galling. Nitronic 60 may also work well!

 

[magneticanomaly]

Household submersible well pumps are often in a thin-walled SS "can" around 4" OD, closed at both ends by bronze heads, The connection is fine threads..I do not have one in front of me, but wall looks to be well under .100, and threads are finer than 24 TPI. The ID threads look cut, not swaged.

 

[litlerob1]

Per the original design and the description on the OP. This feature/part is going to be slightly less painful than getting drawn and quartered. 304 is not compliant, especially when you are dealing with large diameter thin wall parts, that need to be round. Single point Threading is a heat generating operation, especially in 304.

R

 

[magneticanomaly]

A consideration I missed earlier is the OP's statement that the vessel will b filled and emptied...of what material he does not say. Large-diameter fine threads are very intolerant of dirt or gunk. If space allows, I'd go for a big flange, thick rubber gasket, and over-center clamped lid

 

[winflex]

A consideration I missed earlier is the OP's statement that the vessel will b filled and emptied...of what material he does not say. Large-diameter fine threads are very intolerant of dirt or gunk. If space allows, I'd go for a big flange, thick rubber gasket, and over-center clamped lid

Thank you all for the excellent input.
Based on the options presented I'm going with a threaded insert that is welded into the ends of the pipe. Space does not allow for clamps, so I have elected for a 4.625-10 UN 2B thread with 4 starts. This way the end caps can be un-twisted in 1/2 turn. Thread pitch can be increased further if things gunk up during operation. The end caps will be Nitronic 60 to prevent galling while the insert and tube will be 304SS.

 

[Joe Pie]

If this unit requires the removal of these end caps after filling it up with something, and it will be trapped between some type of restrictive centers, I'd personally eliminate the threads all together and go with expanding end caps utilizing o'rings. The threads are a recipe for disaster and cutting them will be time consuming. I posted a YouTube video on a variety of expanding arbors and a variation of the o'ring type is ideal for this application. Just my opinion based on limited information. Good luck.

 

[HuFlungDung]

Thank you all for the excellent input.
Based on the options presented I'm going with a threaded insert that is welded into the ends of the pipe. Space does not allow for clamps, so I have elected for a 4.625-10 UN 2B thread with 4 starts. This way the end caps can be un-twisted in 1/2 turn. Thread pitch can be increased further if things gunk up during operation. The end caps will be Nitronic 60 to prevent galling while the insert and tube will be 304SS.

I don't think you understand what happens when shit gets into threads.
I'm thinking you'll be better off with a redesign, perhaps a bayonet type mount instead of threads.