Calculating thread from tube ID

[RifledAir]

Hello everyone

I have a question that should be a snap to answer but for some reason I struggle with it.

I do a lot of work threading the ID of CrMoly tube (various OD and wall thickness) then machining a matching spigot to thread into the end of the tube.

Starting from the ID of the tube is there a handy calculator or rule of thumb for figuring out the OD of my spigot.

For example today's tubes are 0.745 ID and the thread pitch can be either 1.00mm or 24tpi

whats the OD of my matching spigot?

[TGTool]

If you had a sharp crest, sharp root thread, then the calculation would be something like this.

Tube ID + 2*pitch*.866 = OD of thread and thus of spigot. If you're turning the threads on the tube to leave a flat on the bottom that will reduce the OD slightly, and if your spigot also has flat topped threads it will be reduces slightly more.

Both metric and UN threads call for a .125 * pitch flat at the top of male threads and flats or radii at the root of the threads so you have some latitude for variation.

If you're single pointing the ID of the tube AND you're not taking it out to a sharp point on the thread ID you may have to just make up your own rule of thumb from the theoretical and then machine to it.

[henrya]

Good answer above, but I'll make it more complicated, time consuming and better.

Buy a copy of Machinery's Handbook (Machinery's Handbook - Wikipedia, the free encyclopedia) and read up on threads.
Even for simple jobs like you're doing, make a drawing on paper of the two parts, including the threads. Use the book's charts or the calculation given above to figure all the dimensions. Write all the numbers on our drawing. Study the drawing a minute and double check. Now you're ready to set up your job. This seems slow and complicated but what you get is - job done right the first time.

[RifledAir]

Good answer above, but I'll make it more complicated, time consuming and better.

Buy a copy of Machinery's Handbook (Machinery's Handbook - Wikipedia, the free encyclopedia) and read up on threads.
Even for simple jobs like you're doing, make a drawing on paper of the two parts, including the threads. Use the book's charts or the calculation given above to figure all the dimensions. Write all the numbers on our drawing. Study the drawing a minute and double check. Now you're ready to set up your job. This seems slow and complicated but what you get is - job done right the first time.

That's the way I do it now. I'm trying to save some precious shop time and was hoping someone had created a handy dandy calculator for the task. Given that I may use 5 or 6 tube sizes a day. compound that with a nearly infinite variety of wall thicknesses and pitches and then compound that by the fact that every batch of tube is slightly different. I spend a lot of time doing math.

[TGTool]

That's the way I do it now. I'm trying to save some precious shop time and was hoping someone had created a handy dandy calculator for the task. Given that I may use 5 or 6 tube sizes a day. compound that with a nearly infinite variety of wall thicknesses and pitches and then compound that by the fact that every batch of tube is slightly different. I spend a lot of time doing math.

Do you have easy access to a computer in the shop? If so, there are programs such as Shop Calc where you can put in numbers and let it do the calculation for you. If not, but you have a programmable calculator, you can do the same thing with whatever assumptions you prefer about your threads. Failing that, what kind of thread do you want if it's not sharp crest/sharp root? The formula can be made up so it's easy to calculate but there are undefined factors you haven't yet specified.

[RifledAir]

Yes I have a computer in the shop (typing as a barrel turns.....slooooooowly) Shop Calc sounds good I will look into that.

To be honest I'm using matching In/Ex inserts and there is not much info on the packet. I shoot for an 80% thread because there is some losses during the polish and colouring process

[mrainey]

ThreadPal might be able to help you out.


ThreadPal

[Clive603]

Assuming that you are using constant % engagement, truncation flats and crest / root clearances what you really need is a nomogram / nomograph (never did figure out which name is right). You know. Those things with several vertical lines marked out in suitable scales that you lay a ruler across to get which value on one scale goes with what values on the others. So you have one scale with threads, one with tube ID and one with spigot diameter. Could do separate inch - metric scales but its probably OK to mix by putting inch one side and metric the other in different colours.

Yup its old fashioned but works well and you can see exactly what's going on and what the variation is. Tables and calculators give only one value without that instant view of where the result fits which is, mostly, a very bad thing.

Sorting and printing a nomogram / nomograph should be a piece of cake for a computer but I don't know of any suitable programs. Perhaps someone else here does.

I've done them by hand a couple or three decades back, tedious but not particularly difficult once in practice with a good text book to hand. Now lacking both I'm sorry I can't knock one out for you.

Clive

[Kyle Smith]

Seems like maybe I'm missing something.

Add the desired pitch to the minor diameter derived from the tube I.D.

0.745 + 1/24 = 0.787 Your spigot will be a 0.787"-24 thread. For the metric pitch;
0.745 + 1/25.4 = 0.784. Spigot will have a 0.784" X 1mm thread.

[Gordon B. Clarke]

Hello everyone

I have a question that should be a snap to answer but for some reason I struggle with it.

I do a lot of work threading the ID of CrMoly tube (various OD and wall thickness) then machining a matching spigot to thread into the end of the tube.

Starting from the ID of the tube is there a handy calculator or rule of thumb for figuring out the OD of my spigot.

For example today's tubes are 0.745 ID and the thread pitch can be either 1.00mm or 24tpi

whats the OD of my matching spigot?

There's something I'm not getting. The ID is 3/4". The pipe ODand ID are easily measured. Why not make a standard thread with the pitch you want on the closest standard diameter relevant to the pipe diameter? Are you trying to make a thread based on what your pipe ID is?

To hell with the ID What's the spigot OD? Does it even have a thread? They're usually made to a standard.

[RifledAir]

Seems like maybe I'm missing something.

Add the desired pitch to the minor diameter derived from the tube I.D.

0.745 + 1/24 = 0.787 Your spigot will be a 0.787"-24 thread. For the metric pitch;
0.745 + 1/25.4 = 0.784. Spigot will have a 0.784" X 1mm thread.

Yes that is what I thought originally as well but then I ran across this Lindow Machine Works - Makers of Modern Rose Engine Lathes
And of course I started second guessing what I was doing because the numbers are 0.011" apart. If I work it out long hand from Machinery's then I get a number in between these two.

There's something I'm not getting. The ID is 3/4". The pipe ODand ID are easily measured. Why not make a standard thread with the pitch you want on the closest standard diameter relevant to the pipe diameter? Are you trying to make a thread based on what your pipe ID is?

To hell with the ID What's the spigot OD? Does it even have a thread? They're usually made to a standard.

I can't use a standard OD spigot because the wall thickness changes with the application. Essentially the spigot is threaded on one end to suit the exact ID of the tube and the opposite end is a standardized high pressure connection.

When assembled these tubes are proofed at approx 15,000psi + depending on the application. The working pressures range from 1000 to 4500psi

[henrya]

I fit the male to female after getting most of the internal thread cut, or the other way 'round as the case may be, or most often, both internal and external in one part. Most fun are jobs joining one metric part made in 1960 to an American threaded assembly cut in 1880.

I figure from the "sharp" dimension given in Machinery's Handbook. If thread depth calculates to .036" I start trying the fit after .032" cut or so. Pretty soon, a couple of passes, it fits just right - almost goes in hard by hand and then I give it a spring cut to hit it dead on. I think when you're working one-at-a-time and with non-standard diameters, that fitting the parts together is the best way to get great threaded joints every time. Assuming proper thread form, the standard is the fit of the two parts and not the separate measured dimensions of the two pieces.

[Nadroj]

For example today's tubes are 0.745 ID and the thread pitch can be either 1.00mm or 24tpi

You should be able to fit a thread pitch gauge into that. You'd have to rely on feel. When you try to rock it, does it tend to remain nice and stable, or is the fit wobbly and uncertain?
Another way to get an idea of pitch is to use a wood dowel or similar. Turn one up until it's just a little tight, and thread it into your hole, then remove and check it. There should be an imprint of the pitch that you can check with your gauge. I sometimes do a quick check with the end of a pencil, and just rub it sideways.

[mjk]

"When assembled these tubes are proofed at approx 15,000psi + depending on the application. The working pressures range from 1000 to 4500psi"

And someone with an engineering title isn't specing out both pieces?

[Philabuster]

"When assembled these tubes are proofed at approx 15,000psi + depending on the application. The working pressures range from 1000 to 4500psi"

And someone with an engineering title isn't specing out both pieces?

My guess is this is a homebrew air rifle based on the OP's screen name.

[willbird]

I use Excel (or the openoffice version) a lot, if you write a simple excel spreadsheet to do the math for you, then save it, and add a tab for each dia, or copy and past the formula, soon you will have a collection of things that have worked for you in the past, and you can make it as simple or complex as you desire.

[RifledAir]

My guess is this is a homebrew air rifle based on the OP's screen name.

Well I guess in a sense you could call all of the high end target/match/hunting air rifles "homebrew" Because most of them are custom made. I just happen to be the guy who makes some of the pressure tubes that convert FT to Hunters to ISSF etc.

And most if not all of the designers and builders in this field are engineers, myself included

But that has no bearing on the issue that I can use three different "tools" for calculating thread and come up with three different answers. And as you can see the stakes are high enough that's it's worth posing the question to people who specialize.