Suggested thread size?

[PepeLapiu]

Hey guys. First post here.

I have two titanium tubes I need to join with threads. Both tubes are titanium grade 9 and they have an O.D. of 38mm (1.496") and an I.D. of 34.8mm (1.370"). So that means tube wall is 1.6mm (0.063").

I would like to join these two tubes by threading the inside of one and the outside of the other. So that they look like one continuous tube. Can you suggest what thread specifications I should put on my plans?

I think 40 tpi should work. But on prints, what would be the thread diameter? Is the thread diameter considered at the minor diameter of the female thread? Or at the major diameter of the male threads?

I'm not sure how I should write all of that down on my plans for my machinist to understand.

Thanx for your help.

Cheers

 

[winger]

That thin, my first choice would be gluing in an insert, either one longer plug or 2 plugs that screw together.

How round is this tubing, if oval you won't get good threads.

Dave

 

[Barry Weeks]

How would you thread two tubes of the same diameter to thread the outside of one into the inside of the other? They would have to be different diameters to do that. Same size tubes are usually joined with a threaded coupler.

 

[PepeLapiu]

The tube nominal OD is 38mm( +/-0.005).
The tube ID is 34.6mm but will be machined to 34.8mm.
Glueing or heat shrinking the parts is not an option. I need to be able to pull the two tubes apart.

If the threads are too weak due to their size, I'll just have to extend the threads to 25mm or 50mm.

But please tell me. If I write down thread diameter to be a 100 mm, does that mean the diameter of the major male threads is 100 mm? Or does it mean the minor diameter of the female thread is at 100 mm?

 

[PepeLapiu]

How would you thread two tubes of the same diameter to thread the outside of one into the inside of the other? They would have to be different diameters to do that. Same size tubes are usually joined with a threaded coupler.

What I mean is that the tube with the male threads would have the OD reduced where the threads will be. And the tube with the female threads will have it's ID reduced where the threads will be.

We will have to experiment on what thread pitch would be best.

But what thread diameter should I pick to have an equal amount of 'meat' left on both sides?

 

[keithmech]

If you are unsure as to dimensioning ,make a note.
Spec the thread OD and pitch and length.(ie a 1/2 nc bolt is 1/2 inches
in diameter).The minor diameter can be calculated or use a handy online
calculator.UN imperial screw thread calculator
as far as glueing,if parts are made with a small clearance fit ( +.001-.002").Heat will kill the
loctite so parts will come apart.Reglue to reassemble.

 

[keithmech]

you also may find threads that small and that material will gall
and weld together.

 

[Porschefix]

The tube nominal OD is 38mm( +/-0.005).
The tube ID is 34.6mm but will be machined to 34.8mm.
Glueing or heat shrinking the parts is not an option. I need to be able to pull the two tubes apart.

If the threads are too weak due to their size, I'll just have to extend the threads to 25mm or 50mm.

But please tell me. If I write down thread diameter to be a 100 mm, does that mean the diameter of the major male threads is 100 mm? Or does it mean the minor diameter of the female thread is at 100 mm?

Think about a thread, say a 1/4"-28. The diameter of the thread O.D. is .250.
Making a titanium thread longer, say from .5" to 1" or 2" as you propose will not make the combined assembly stronger, the strength issue will be the thickness of the tube at the root of the connecting threads. Also you need to be careful of Ti on Ti galling. You don't have a lot of meat to work with there, but the finer the thread the more material strength you'll have, to a degree.

 

[Limy Sami]

+ another on galling, if it has to come apart and go back together ''regularly'' you will be in big trouble, even with antiseize compound if you can use it.

In my albeit limited experience with Ti galling is a big problem.

If I were you I'd fine another solution to your problem - dissimilar metals are a good start.

 

[PepeLapiu]

If you are unsure as to dimensioning ,make a note.
Spec the thread OD and pitch and length.(ie a 1/2 nc bolt is 1/2 inches
in diameter).The minor diameter can be calculated or use a handy online
calculator.UN imperial screw thread calculator
as far as glueing,if parts are made with a small clearance fit ( +.001-.002").Heat will kill the
loctite so parts will come apart.Reglue to reassemble.

That went right over my head. I have no idea what you are talking about.
I'll try to read and re-read until I understand.

So let's say you have a 20 tpi x 1-7/16" thread.
Where in the thread is the 1-7/16" dia? Would that be the major dia. If the bolt? Or the minor dia of the nut? Or somewhere in between?

 

[PepeLapiu]

Yeah,I am looking at the tubes now.
Trying the thread together such small walls will not work now that I have the parts to look at.

 

[Limy Sami]

Yeah,I am looking at the tubes now.
Trying the thread together such small walls will not work now that I have the parts to look at.

Back to the drawing board then? ........you'll crack it - we machinists always do

 

[EPAIII]

Just playing with some real numbers here and making seat-of-the-pants assumptions. And I am not including anything for clearance and tolerance here. Just a theoretical thread form.

You have a wall thickness of 1.6mm. If you are going to thread these together as you suggest, you have to divide that into three fractions: the thread depth, the amount above the female thread, and the amount below the male thread. The second and third ones would probably be about equal since you would probably want equal strength on the male and female thread. That means that the thread height would be selected first and subtracted from the 1.6mm figure. The remainder would be divided by two to reach the second and third fractions.

OK, for a first guess, I would allow about 0.4 mm for the thread height. Lets play with that. The thread height of a sharp Vee thread would be 0.5 x square root (3) times the (metric) pitch (P). But a standard thread form usually has a crest flat and some root fill so it's height is only about 3/4 of that. So we now have 3/8 square root (3) which equals 0.6495 P. I rounded that to 0.65 P.

Working backwards from a 0.4 mm thread height I get 0.615 mm metric pitch. That's totally non standard
so I round it to 0.6 mm metric pitch. Working forward again the thread height will be 0.6 mm x 0.65 = 0.39 mm. For you English system guys, that is about 0.015" high. In TPI, the thread pitch would be 42 1/3 TPI. So this is comparable to a 40 TPI thread which is common. In fact, even finer threads are used for things like camera lenses. This is NOT a nightmare situation.

Finally subtracting that 0.39 mm thread height from your 1.6mm wall thickness you get 1.21 mm and dividing that by half you are left with 0.605 mm for the meat left above and below the female and male threads. Again, for the English units people that is about 0.024". Not a lot, but it can work if the loads on the tube are small.

As for specifying this thread, that is simply given by the major or outside diameter of the male thread and the pitch. So the OD would be 38 mm - 2 (0.605) mm = 36.79 mm. And the thread would be a M 36.78 x 0.6. The male thread would be made to that number, +0 / -0.xx mm and the female thread would be made a small bit larger to provide an allowance for the fit. You would have to determine what that allowance would be and the tolerance on the female thread would be on the opposite direction (+0.xx / -0 mm). This determination would be made based on the needs of your parts/assembly.

I see these parts as completely doable. Weather it would work depends on the needs of your parts and device that they go into. My numbers can be adjusted for either more thread height or more meat left above and below the threads, but I would not go too far in either direction. You may have to make a prototype and see if anything needs to be changed. The 0.6 metric pitch that I used is a standard one, but perhaps little used. I don't know how many metric lathes can cut it. I do know that I can cut it using metric compound gears on my South Bend lathe which has an 8 TPI, English lead screw. 0.5 and 0.7 mm pitches are perhaps more common.

 

[Illinoyance]

If you can tolerate it you might make a coupling out of a dissimilar metal, say steel. That should take care of the galling problem. The Ti tubing would be internally threaded. The coupling would be externally threaded.

 

[SeymourDumore]

they have an O.D. of 38mm (1.496") and an I.D. of 34.8mm (1.370"). So that means tube wall is 1.6mm (0.063").

Can you suggest what thread specifications I should put on my plans?

I think 40 tpi should work. But on prints, what would be the thread diameter? Is the thread diameter considered at the minor diameter of the female thread? Or at the major diameter of the male threads?

I'm not sure how I should write all of that down on my plans for my machinist to understand.

Cheers

OK, so you are the engineer and it is you who has to decide if it works or not for the intended purpose, so I'll leave that for you to munch on.

Now some answers for UN threads:
Threads are defined by the nominal OD of the male thread, the required pitch and the required tolerance class.
There are standard threads and there are special threads.
For example, UNC is course, UNF is fine, UNEF is extra fine.
Typically, one tries to specify a standard thread ( for obvious reasons )
At the same time, there are also cases where no standard thread is available for the intended purpose, and this is likely one of those.

So, the "proper" definition would be calling out the thread as:
Nominal OD - Pitch - UNS - nA(B), where the S is indicating that the thread is Special.

Now, any time an "S" thread is specified, it is also specified what makes the thread "Special"
Sometimes it's the major or minor dia, sometimes it's the pitch, and in some cases Everything!!! in the thread is special.

So, let's take your example here.
You have a tube that's 1.496 OD and 1.370 ID.
Your wall thickness is .063, so you may want to make the thread major nominal to be 1.370 + .063 = 1.433
Now that leaves you with .0315 thickness total, so you want to select a pitch that has a thread height considerably less.
You think 40 pitch works, which has a thread height of .0153 on OD and .0135 on ID.
Cool! We now have something we can perhaps test, and then put on the drawing.

Here is how I'd indicate it on the drawing:
OD
1.433-40-UNS-2A
Major dia: 1.4319 - 1.4268
Pitch dia: 1.4157 - 1.4120
Minor: 1.4048 (max)

ID
1.433-40-UNS-2B
Major dia: 1.406 - 1.412
Pitch dia: 1.4168 - 1.4216
Minor dia: 1.433 (min)

Now I'm all open for flames from anyone, but this is how a special thread properly defined on any and all drawings created by competent people that I've ever seen.

Note to everyone: The above numbers are courtesy of ThreadPal from Close Tolerance Software

 

[clarnibass]

I don't know how many metric lathes can cut it. I do know that I can cut it using metric compound gears on my South Bend lathe which has an 8 TPI, English lead screw. 0.5 and 0.7 mm pitches are perhaps more common.

I think it's common enough that most (or almost all) metric lathes would have it and many other lathes would have it with change gears (mine does and the metric gearbox version of it has it too).

I didn't notice your post at first and I did pretty much the same but with 0.5mm pitch. For similar tubes, I guess it's easier for me to think of it as just leaving half the wall thickness plus the thread tooth height (before considering clearance). For 0.5mm pitch and 38mmOD/34.8mmID tube, approximately 36.7mm OD and 36.1mm ID, before clearance (too lazy to not do this in my head... hope I'm not confusing anything).

OP, can you have an outside or inside sleeve or does the outside have to be "smooth" (i.e. same diameter throughout the length) or the inside can't have a smaller section?
When you say you need to be able to pull them apart, do you mean all the time? How often? Without any tools? Do you need to be able to remove them, reassemble and immediately use it again? etc.

 

[PepeLapiu]

Hey guys.
I had forgotten all about this thread.
Now I see all the replies which I somehow missed.

Here is what I was trying to do.

I am designing solvent traps for firearms, also known as DIY gun silencers.

So the solvent trap is basically a tube with a thread adapter at one end to plug into the gun barrel. And an end cap at the other end of the tube. And inside, a bunch of baffles and spacers.

Threading the end cap to the inside of the tube is what everyone else has been doing. But this greatly limits the baffles and usable space inside the tube.

So the solution would be to thread the end cap to the outside io the tube. Problem is, that would not be what my customers would consider esthetically pleasing. They want a tube that looks like a can, not something with a cap on the end of it.

So I had envisioned a end cap that would have the same OD as the tube itself and thread it onto the outside of the tube. I think you understand what I mean.

But I found a workaround to this problem.

The end cap will be threaded to the inside of the tube. But I am designing it so the end cap itself will be able to be used at a baffle. Never mind this right now.

Your help is greatly appreciated.

Once I got home and I saw how thin 1.6mm really is, I realized that my idea of end cap threaded to the outside with the same OD as the tube just wouldn't work. Just not strong enough for our application and the high pressure a silencer is subjected to.

I am an inches and feet guy. Millimetres mean nothing to me. So I couldn't imagine how thin 1.6 mm really is.

But again, I figured out am other way to solve this end cap space wasting problem.

 

[PepeLapiu]

OK, so you are the engineer and it is you who has to decide if it works or not for the intended purpose, so I'll leave that for you to munch on.

Now some answers for UN threads:
Threads are defined by the nominal OD of the male thread, the required pitch and the required tolerance class.
There are standard threads and there are special threads.
For example, UNC is course, UNF is fine, UNEF is extra fine.
Typically, one tries to specify a standard thread ( for obvious reasons )
At the same time, there are also cases where no standard thread is available for the intended purpose, and this is likely one of those.

So, the "proper" definition would be calling out the thread as:
Nominal OD - Pitch - UNS - nA(B), where the S is indicating that the thread is Special.

Now, any time an "S" thread is specified, it is also specified what makes the thread "Special"
Sometimes it's the major or minor dia, sometimes it's the pitch, and in some cases Everything!!! in the thread is special.

So, let's take your example here.
You have a tube that's 1.496 OD and 1.370 ID.
Your wall thickness is .063, so you may want to make the thread major nominal to be 1.370 + .063 = 1.433
Now that leaves you with .0315 thickness total, so you want to select a pitch that has a thread height considerably less.
You think 40 pitch works, which has a thread height of .0153 on OD and .0135 on ID.
Cool! We now have something we can perhaps test, and then put on the drawing.

Here is how I'd indicate it on the drawing:
OD
1.433-40-UNS-2A
Major dia: 1.4319 - 1.4268
Pitch dia: 1.4157 - 1.4120
Minor: 1.4048 (max)

ID
1.433-40-UNS-2B
Major dia: 1.406 - 1.412
Pitch dia: 1.4168 - 1.4216
Minor dia: 1.433 (min)

Now I'm all open for flames from anyone, but this is how a special thread properly defined on any and all drawings created by competent people that I've ever seen.

Note to everyone: The above numbers are courtesy of ThreadPal from Close Tolerance Software

Hey! That was actually a really cool, helpful, and interesting reply. Too bad you decided I am a smuck and not legit.

Still, I am scared threading two tubes this way will be too weak. And the chance of galling too great.

But I just might try a prototype and see how that works. Thanx.

 

[TeachMePlease]

Hey! That was actually a really cool, helpful, and interesting reply. Too bad you decided I am a smuck and not legit.

Still, I am scared threading two tubes this way will be too weak. And the chance of galling too great.

But I just might try a prototype and see how that works. Thanx.

No, he called you a schmuck.

And so far, you are proving to be.

 

[PepeLapiu]

No, he called you a schmuck.

And so far, you are proving to be.

Okay. So who you been talkin' too? This wife or the last one?