# Minimum wall thickness for stainless steel tube



## tgun829 (Sep 27, 2019)

Hi

I’m working on a project.  I’m considering to use off the shelf stainless steel tubes with 13mm OD and 11mm ID.  That gives 1mm wall thickness.  The idea is to lathe the ends of the tube, then thread them to make male and female.  Thanks for your help in advance.

Todd


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## JimDawson (Sep 27, 2019)

If you threaded the ends at M12x1 that would give you 0.5mm wall at the threads.  Or you could make them 1/2-20 or 1/2-28 and get a little more wall.

Would it work for your application?  I don't know. How much wall strength do you need at that joint?


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## RobertB (Sep 27, 2019)

JimDawson said:


> Would it work for your application?  I don't know. How much wall strength do you need at that joint?



That is the question, without knowing the application it is hard to give a good answer. 

Is the strength of the wall or the strength of the thread more important? There are examples of much thinner wall tube being threaded, for example the brass tailpiece on a sink drain. If they are just threading together you don't need a standard thread, you could use a very fine shallow thread leaving plenty of wall thickness at the expense of thread strength.


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## whitmore (Sep 28, 2019)

tgun829 said:


> I’m considering to use off the shelf stainless steel tubes with 13mm OD and 11mm ID.  That gives 1mm wall thickness.  The idea is to lathe the ends of the tube, then thread them to make male and female.



That'll be difficult; the 1mm wall is going to have to stay rigid when you apply the cutting tool.   An external
(collet) support can hold it for internal threads, but some sort of internal support (a mandrel, or 
maybe fill with low-melting alloy)
would be a good plan for external threading.   

Tapered threads (sounds hard, but it isn't) are tolerant of minor dimensional mismatches, and you can do the
internal thread without having to undercut the tube at the max depth.


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## nnam (Sep 28, 2019)

I don't think you can use that same size as male and female, unless the thread is very fine, so you would cut both tubes thinner. A larger female may work to connect them.  A section of larger female connecting two male works also.  Will it be strong? It depends what it's for.  Brazing larger ends that have threads (male and female) can be both strong and work.


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## nnam (Sep 28, 2019)

Also, flare coupling like brake lines work great too


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## Downunder Bob (Sep 28, 2019)

De[ends on what you are going to do with them. Is the ID or Od important to be smooth and at constant dia. How much strength is needed, is internal pressure involved. we need the full story to advise.


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## RJSakowski (Sep 28, 2019)

tgun829 said:


> Hi
> 
> I’m working on a project.  I’m considering to use off the shelf stainless steel tubes with 13mm OD and 11mm ID.  That gives 1mm wall thickness.  The idea is to lathe the ends of the tube, then thread them to make male and female.  Thanks for your help in advance.
> 
> Todd



With both internal and external threads fitting within the 1mm wall thickness, you will want to use a custom thread.  For more or less equal wall strength, the pitch diameter will need to be around 12mm.   

I would use something like a .5mm pitch which would have a major diameter of 12.36mm and a minor diameter of 11.71mm.  Going to a 1mm pitch would yield a major diameter of 12.65mm and a minor diameter of 11.54mm.  

This would be for perfectly formed threads.  Cutting tools with a radius smaller than the standard thread profile will produce slightly thinner walls.


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## RJSakowski (Sep 28, 2019)

If you don't want to single point the threads, 1/2-48 taps and dies are available.  This would be the equivalent of an M12.37-.53mm.


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## tgun829 (Sep 30, 2019)

RobertB said:


> That is the question, without knowing the application it is hard to give a good answer.
> 
> Is the strength of the wall or the strength of the thread more important? There are examples of much thinner wall tube being threaded, for example the brass tailpiece on a sink drain. If they are just threading together you don't need a standard thread, you could use a very fine shallow thread leaving plenty of wall thickness at the expense of thread strength.


The strength of the thread is more important.  Yes, I was thinking about finer thread.  In fact, for my application I really don't need more than 2 to 3 threads.


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## tgun829 (Sep 30, 2019)

RJSakowski said:


> With both internal and external threads fitting within the 1mm wall thickness, you will want to use a custom thread.  For more or less equal wall strength, the pitch diameter will need to be around 12mm.
> 
> I would use something like a .5mm pitch which would have a major diameter of 12.36mm and a minor diameter of 11.71mm.  Going to a 1mm pitch would yield a major diameter of 12.65mm and a minor diameter of 11.54mm.
> 
> This would be for perfectly formed threads.  Cutting tools with a radius smaller than the standard thread profile will produce slightly thinner walls.


Hi Robert,

Your suggestion sounds interesting.  I'm sending you couple sketches of the connected tubes.  Hope this illustrates my intent.  The idea is to connect the two tubes via thread.  My concern is the wall thickness and that could it handle threading.   I don't want the wall to collapse during threading of the two tubes.  Another thing I only need 2 to 3 threads.  The idea is really just to connect them.  There is no high pressure going through.  The only other concerns is the frequency of the usage.  I do anticipate high frequency.  hundreds of usage.


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## RJSakowski (Sep 30, 2019)

tgun829 said:


> Hi Robert,
> 
> Your suggestion sounds interesting.  I'm sending you couple sketches of the connected tubes.  Hope this illustrates my intent.  The idea is to connect the two tubes via thread.  My concern is the wall thickness and that could it handle threading.   I don't want the wall to collapse during threading of the two tubes.  Another thing I only need 2 to 3 threads.  The idea is really just to connect them.  There is no high pressure going through.  The only other concerns is the frequency of the usage.  I do anticipate high frequency.  hundreds of usage.


If you are planning to connect two tubes of the same O.D. and I.D. via a threaded joint, what I have proposed is the the best option.  There are numerous exaqmples of this type of threading in optical devices.  The key is a fine pitch thread.  Were I doing this, I would probably single point thread, the reason being that you can maintain concentrixcity, something that is difficult to do with a tap and die.  The next best option would be the 1/2-40 tap and die from one of the industrial suppliers.  They should set you back around $70 -80.    You could start the thread with single point threading and finish with the tap and die as well.

To support the tube, I would make a plug for threading the O.D. and either a collet or collar for threading the I.D.  These will prevent the tube from distorting under threading pressure.  I would also recommend more than two or three threads, especially if you plan on multiple assembly/disassembly's.


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## tgun829 (Sep 30, 2019)

Robert
Do you know manufactures who can work with me to design and manufacture prototypes?  Thanks.


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## JimDawson (Sep 30, 2019)

tgun829 said:


> Robert
> Do you know manufactures who can work with me to design and manufacture prototypes?  Thanks.



How many thousands of parts do you need?     These would be a good fit for my equipment, I can do small quantities.  303 stainless?


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## tgun829 (Sep 30, 2019)

Jim
Preferrably 316 for its corrosion resistance.


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