Threading aluminum pipe

digadv · May 2, 2019

Hi,

I am wanting to cut and then thread two pieces of aluminum pipe. OD is 1", ID is 0.8". I would inside-thread one piece and outside-thread the other so that the two pieces will screw together, essentially recreating the pipe. There will be no load or stress on this junction - project is decorative in nature.

Question - is the 0.2" delta between OD and ID enough to allow adequate inside and outside threads?
Question - what TPI would you recommend?

Thanks in advance,
D

Milland · May 2, 2019

There's little wall to make thread in the size you mention, could some light press fit plastic dowels (even ones that have split ends and have been spread a bit for extra grip) work for this instead? If you need pass-through, how about a modest hole in the dowel?

Al's not known for liking thread fits that are used more than once, either...

Limy Sami · May 2, 2019

Milland said:
There's little wall to make thread in the size you mention, could some light press fit plastic dowels (even ones that have split ends and have been spread a bit for extra grip) work for this instead? If you need pass-through, how about a modest hole in the dowel?

Al's not known for liking thread fits that are used more than once, either...

What he said AND if the connection has to be screwed, make female threads in both tube ends and use a (say?) brass connector.

Guest · May 2, 2019

If you have to go with aluminum use as course of threads as possible, deburr the I.D. and O.D. well. I would also lube them before assembly. As Milland said aluminum threads want to be screwed together and left that way.

JCByrd24 · May 2, 2019

I think it can be done fairly easily. Dividing the 0.100" wall into 3 is roughly 0.033" for material to leave on the ID, OD, and then share in the thread area. 0.033" for threads translates to 16 TPI, which is not even that fine. Aluminum on aluminum does need to be treated carefully as has been mentioned.

DDoug · May 2, 2019

For-What-Purpose ?

BoxcarPete · May 2, 2019

digger doug said:
For-What-Purpose ?

Already mentioned - decorative, non load-bearing.

In addition to the above, one thing to consider is anodizing. If you want pretty colors for your decorative parts, keep in mind that the anodization layer will grow the parts slightly (amount depends on anodize type). Therefore, you need to cut threads deeper so they will still fit after anodizing. The other thing is that anodized aluminum is much more resistant to galling, so threaded connections will be fine in that case.

digadv · May 2, 2019

Thank you everyone for your replies. I have some things to think about.

As another option, I might consider a press-fit connection using a couple of grooves and O-rings on the female end to retain the two pieces. Any input on how aluminum will handle this sort of connection? Would probably require some silicon lube at least.

BoxcarPete · May 2, 2019

digadv said:
Thank you everyone for your replies. I have some things to think about.

As another option, I might consider a press-fit connection using a couple of grooves and O-rings on the female end to retain the two pieces. Any input on how aluminum will handle this sort of connection? Would probably require some silicon lube at least.

If you cut the parts to have proper clearances for o-rings and use assembly lube, it will assemble easily... and disassemble just as easily if you don't hold it with something else. You'd be pushing the boundaries a bit as the smallest standard oring cross section in that diameter is 0.070" which would give you somewhere between 0.045" and 0.042" left per side at your grooves.

Press-fit in aluminum would make the orings obsolete, and it's best done with a thermal shrink. Big side in the oven, little side in the freezer, assemble, and wait for the magic to happen. Actually pressing the pieces together will gall them and you might not be happy about the fact that it's permanently frozen together 1/16" away from being flush :wall:

digadv · May 2, 2019

Unfortunately the pieces will need to be separated, maybe once a quarter.

72bwhite · May 2, 2019

how important is the id ?

digadv · May 3, 2019

72bwhite said:
how important is the id ?

The ID is not that critical. Could be reduced to 1/2". What are you thinking?

e-fishin-c · May 3, 2019

Not sure if this will work in your setup but here’s something I do routinely.

I have a hyd hose crimper with 8 finger dies (Gates 707) in our shop. When connecting thin wall pipe or tubing I crimp one end to slip fit, sometimes turning the crimped end for better fit, welding is very rigid after connection too. Think tent poles.

In your case the crimped end could be crimped less than a slip fit but turn threads, then chase a tap through the other mating end.

Sometimes we have to get creative if the pipe or tubing is long like laying the crimper down horizontally etc.

The Gates 707 crimper has a digital readout to adjust crimp diameter real easy. So if you know of any hyd hose guys in your area this could work.

rons · May 4, 2019

The dimensions you wrote appear to leave enough material for the threads. Some anti-seize compound that is grey would go on the threads.

Thread the external thread piece and use it as a gauge when running the internal threaded piece. The dimensions leave little room for error.

digadv · May 4, 2019

rons said:
The dimensions you wrote appear to leave enough material for the threads. Some anti-seize compound that is grey would go on the threads.

JCByrd24 said:
I think it can be done fairly easily. Dividing the 0.100" wall into 3 is roughly 0.033" for material to leave on the ID, OD, and then share in the thread area. 0.033" for threads translates to 16 TPI, which is not even that fine. Aluminum on aluminum does need to be treated carefully as has been mentioned.

I don't think the o-ring concept is going to work so I am back to the option of threading. I've threading before but never joined two pieces of pipe with a wall of 0.2". JCByrd offered some geometry (thank you) and I just want to make sure I do it right. As he stated, the thread area (1/3 external thread, 1/3 internal thread, 1/3 shared/overlap) could be accomplished in 0.1". Does this imply that I should reduce the OD on the male end by 0.05" and increase the ID on the female end by 0.05" before I start threading?

Rob F. · May 4, 2019

Why not just thread the inside of both pieces to be joined and use a smaller piece that is threaded on the OD to join the two of them. Use loktite to lock the sleeve into one pipe, never-sieze on the other pipe. Taper the ID of the sleve so if you push wires or anything through it they don't snag there.

digadv · May 4, 2019

I think that's what I'm going to have to do. Thank you for your help.

ratbldr427 · May 4, 2019

I was going to suggest as RobF said except I would thread and tap only one end.Green Loctite Retaining compound on the unthreaded end could be as strong as the threaded end depending on the length. Just a little quicker than threading both ends.

Rob F. · May 4, 2019

ratbldr427 said:
I was going to suggest as RobF said except I would thread and tap only one end.Green Loctite Retaining compound on the unthreaded end could be as strong as the threaded end depending on the length. Just a little quicker than threading both ends.

I thought it might be easier to thread (or buy) fully threaded sleeves. Then he can internal thread the pipes and use the worst internal threads to be loctited to the sleeve. This avoids having to turn down 1/2 of each sleeve or boring half of the pipes.

72bwhite · May 4, 2019

Rob F. said:
Why not just thread the inside of both pieces to be joined and use a smaller piece that is threaded on the OD to join the two of them. Use loktite to lock the sleeve into one pipe, never-sieze on the other pipe. Taper the ID of the sleve so if you push wires or anything through it they don't snag there.

exactly threat the inside and make a nipple to connect it

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