How to mill NPT thread using a threadmill

[koenbro]

I don't have a 7/16" drill or a NPT tap. So my workaorund was to spot drill, then 1/4" endmill, then 7/16" endmill, and then a boring head/ boring bar. I targeted 0.4774" but oveshot by 10 thou as this was my time using the boring head, and wasn't sure how to adjust the wormgear. Then threadmilled.

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[Firebrick43]

Something is wrong with those, I "assume" the hole is drilled oversize. NPT depends on thread crest to seal and those have quite the "flat top" on them which will be a leak path.

 

[koenbro]

Yeah, I haven't tested them yet, but have no experience with thread milling and there are a few adjustments that look critical. Will let you know how thhey work.

 

[aliva]

As Firebrick43 said those threads are flat, that's not likely seal. Pipe threads rely on a good crest on the threads to tighten into the taper . The taper creates the seal.

 

[koenbro]

Yes the threads look funny and there is indeed an airleak. I think I programmed the threadmilling cycle wrong. Will try again and leave the hole at 7/16" or 0.4375" (instead of enlarging it to 0.4774). What PDO (pitch diameter offset do you recommend)?

 

[bretthl]

NPT threads also rely on pipe dope to seal properly. The only threaded connections that seal without pipe dope are metal to metal connections such as PH-6, Vam Top, etc. API LTC and STC are similar to NPT (tapered V thread) and rely on pipe dope to seal.

 

[WobblyHand]

NPT threads also rely on pipe dope to seal properly. The only threaded connections that seal without pipe dope are metal to metal connections such as PH-6, Vam Top, etc. API LTC and STC are similar to NPT (tapered V thread) and rely on pipe dope to seal.

What you say is true. However, no amount of pipe dope will make the OP's threads seal. They are way off.

I had an HF pipe threader, whose dies were wrong, they cut too much metal. No matter what I did, teflon, pipe dope, those threads wouldn't hold 100 psi, nevermind 200. Yes, they threaded in easy, too easy. My friends at HF didn't execute the NPT taper correctly. Replaced the dies with a Ridgid, and only a tiny amount of pipe dope was required to seal. The thread dimensions matter a lot for NPT.

 

[Jason812]

NPT threads also rely on pipe dope to seal properly. The only threaded connections that seal without pipe dope are metal to metal connections such as PH-6, Vam Top, etc. API LTC and STC are similar to NPT (tapered V thread) and rely on pipe dope to seal.

NPTF will seal without dope or tape.

 

[ropedrag]

NYC CNC has great youtube content for thread milling and yes NPT as well. Watched their video and went to work, nailed 10 1/4 NPT in one
part no issues. It's black arts!

 

[koenbro]

Thank you for the recommendation to the videos by NYC CNC. I followed their method of making a conical bore and then tapping it and it works great. Still used Teflon tape and have no perceptible air leak.

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