threading question

[3strucking]

I have a part that has a id of .880" and I want to thread it internally. Is the correct procedure to thread it first then make my male threaded part to fit?

 

[4ssss]

If your going to make your own thread size, I'd make the OD thread 1st, only because the outside is the easier of the 2 to make, and there's less chance of you moving something and not missing picking up the thread

 

[3strucking]

I guess I have gotten myself confused. I have a 1.00” od tube that has a id of .879”. Do I need to find the closest minor diameter in the thread chart and use it?

 

[Bob Korves]

There is not enough material there for common threads to fit and still have adequate supporting material. You will have to use a fine thread "special" size, either imperial or metric.

 

[mksj]

The closest size would be a 15/16-20 UNEF which has a minor of 0.8762 and a major of 0.9375. Not enough material in my mind, and I do not think it would work. I usually do the ID first (and use a known good thread to check it dimensionally) and then sut the OD. I had a previous job that I used this size, it had a bad 1" thread that I turned down and threaded to 15/16th. Cut most of the internal thread on the lathe and finished with a tap for accurate sizing over the 2".

 

[3strucking]

i had planned on using 28 or 32 TPI. I thought that i could plug my id measurement into the thread builder calculator on TM and it would give me the rest of the info.

 

[Bob Korves]

i had planned on using 28 or 32 TPI. I thought that i could plug my id measurement into the thread builder calculator on TM and it would give me the rest of the info.

That approach might help you find something that might fit in the small space. The real problems will come with workholding and keeping the cutting tool from deforming the work. It will not be an easy job, and I am glad I am not contemplating doing it for myself... Please let us know how the project proceeds and turns out.

 

[mikey]

In this instance, I would cut the female thread first. You have a 1" OD X 0.879" ID tube, leaving a wall thickness of only 0.060". That's pretty thin but if you don't need a lot of strength then it can be done. You do not need to have an established nominal thread in there. You can cut whatever thread pitch you want. For example, you can have a 0.0879" X 32 tpi thread. Estimated thread depth will be 0.023" deep, leaving a wall thickness of only 0.030" but if that is enough then you can cut it. Then cut the male thread to fit.

Something like a threaded male cap to cover the end of a tube would not require a lot of strength and this might work. However, if the thread must sustain a load then no, I wouldn't do it.

 

[3strucking]

That’s what I’m going to do. I’m sure it will be ok. Thank you all for the help.

 

[MarkM]

You ll still have some strength due to the v form of the thread and also don t undercut the internal thread. Reatract your tool in motion past the point of engagement you desire. It will keep your threaded portion stronger without the undercut.