NPT Thread callout 2

[MrGearhead]

New to the use of NPT. The key problem is the taper which introduces a question in the context of the design:

Threaded hole goes directly into an aluminum enclosure that has numerous parts installed, and a flat gasket at the parting line of the enclosure. The threaded holes provide the interface to run Helium in and out of the enclosure. After the enclosure is assembled, a Helium leak acceptance "manufacturing check" is done to ensure the box can pass a MIL-STD-810 rain test.

After the He Leak mfg check is complete with a pass, The inlet valves that allow Helium to flow into the enclosure are removed and then replaced with metal "plugs" (NPT bolts) that are covered over with Epibond sealing compound to make the box airtight, even around the seal.

All this is to say that the thread spec on the enclosure was machined "per the mfg dwg", and the finish plugs won't seat all the way down against their heads because the top dia of the thread where it contacts the underside of the bolt head is not large enough to tighten the plug sufficiently.

I'd assume then that part of a proper NPT thread callout on a mfg print is to specify the NPT thread OD in the plane the thread is cut into?

 

[hydtools]

NPT threads are inspected by plug and ring gages.
See here:

http://www.alphathreadgage.com/refsheets/REFnptpipe.htm

If you must use NPT thread, use a hex socket flush plug with Loctite PST to seal after test. Not a plug with a head.

Ted

 

[btrueblood]

" the finish plugs won't seat all the way down against their heads"

NPT plugs are supposed to seal by interference between the threads; i.e. if the headed plugs ever DID fully seat, they likely won't seal.

Hydtools' suggestion is a good one.

 

[HDS]

It is working as it should. Tapered threads can not be used with face seals. Use hydtools suggestion or switch to straight threads.

 

[cncek]

"I'd assume then that part of a proper NPT thread callout on a mfg print is to specify the NPT thread OD in the plane the thread is cut into?"

Hydtools, btrublood, and HDS are correct. The head of an NPT hex plug should never meet up with the mating surface. If it does it was machined incorrectly and wasn't checked with an L1 / L3 plug gage. As a machinist and Mfg Engineer I can tell you that most machinist will pay little attention to the size hole you dimension on the print. The L1 and L3 step gage are the NIST standards they will go by. The hole size is usually stated as the tap drill size. I would suggest looking in Machinery's Handbook on this one. However I wouldn't get too concerned about the hole size. I would however be specific on whether you want the hole drilled and taped or if you required a very specific seal I would suggest that you request the hole to be taper reamed prior to threading for the best fit and size control. You also need to take into consideration the thickness of the material you are putting an NPT thread into. If you do not have enough wall thickness the plugs will not seal well due to the lack of threads in the hole. I would probably suggest that you go with an SAE o-ring port - these machine much easier and will seal much better than NPT threads.



Eric K.
"Strive for perfection in everything you do. Take the best that exists and make it better. When it does not exist, design it." Sir Henry Royce

 

[MrGearhead]

I'm learning. Thanks folks.

But... after reading responses, I realized I described my issue incompletely.

The thread is cut through the bottom of an alum enclosure to allow a temporary test port with NPT thread to be screwed in to run the Helium Leak test. The thread starts down in a counterbore about .05 deeper than the head of a plug that will be installed later...

When the port is removed, a plug with a head is inserted and then tightened until the head of the bolt goes sinks into the C-bore deep enough for the head of the bolt to clear the bottom of the enclosure. At this point some sealing compound (epibond) can be used to seal out the entire bolt head... The epibond is the seal, not the NPT... I just need the original NPT thread to be big enough to allow the bolt head to be recessed far enough...

 

[hydtools]

MrGearhead, then you need to specifiy the npt thread to be gaged from the bottom of the counterbore. Not from the outermost surface of the boss.
I'd still use a hexsocket flush plug and PST. In the alternative you could use a flush plug and then fill it and the counterbore with epibond.

Ted

 

[MrGearhead]

What about NPT vs NPTF vs NPT dryseal.

My understanding is that NPTF and NPT dryseal are the same thread. They have a deeper "V" on the thread vs a standard NPT which is more blunt.

Yes?

If so, I'd assume an external NPTF (or dryseal) thread can accept a standard NPT fine?

It seems difficult (impossible) to find a Mil-Std Hex seal flush plug in the NPTF (dryseal) type... But, I'd assume a standard NPTF plug can be used to seal a standard NPT external hole...

 

[MrGearhead]

Did not see a way to edit post so let me clarify above...

External thread is NPTF (dryseal). Male thread going into that external thread is NPT (standard). Loctite PST thread sealant will be used.

Assumption is the above plan is acceptable?

 

[hydtools]

NPTF and dryseal are the same.
You can use NPTF plug in the NPT hole.
Your plan will work.
See this Holochrome reference for standard plugs and flush plugs.

http://www.mjvail.com/holokrome/dd_pressureplugs.html

Ted

 

[MrGearhead]

Was in a hurry... Internal thread is NPTF. The External Thread on the hex socket flush plug is NPT. Assumption is this will work...

Sounds like I'll get sealing interference on the crest of the NPT plug bottoming out on the flat of the NPTF internal thread root...

And that I'll have a seal gap on the NPT plug external thread root where it meets the crest of the NPTF internal thread. So... Use Loctite PST for this...

Still sound like a plan? Thanks for link.

 

[hydtools]

Still a plan.

Ted