National Pipe Thread 2

[TheMotionGuy]

I am not completely familiar with NPT. How do you read an NPT callout. I know we don't need to call out depth, as that is per machining spec.

The callout is a 1/8-27 NPT. Is the 27 just referring to threads per inch? And since an NPT hole is tapered, is 1/8 the diameter of the inside part of the hole or the outside?

I just want to make sure of what I am getting made so I can purchase the proper fittings.

 

[JMirisola]

http://en.wikipedia.org/wiki/National_pipe_thread

Jeff Mirisola, CSWP
Design Manager/Senior Designer
M9 Defense
My Blog

 

[TheMotionGuy]

Got it.

 

[EngineerTex]

A part that was touched on in the Wikipedia article, but was not mentioned is that the NPTF (National Pipe Thread Fuel) when used, can only be counted on for a leakproof connection one single time. If the two parts are ever disassembled or backed off, they have to have sealing dope or tape applied to the threads to guarantee a liquid-tight seal.

Engineering is not the science behind building. It is the science behind not building.

 

[TheMotionGuy]

Thank you, Tex. During my research, I did read that somewhere as well (don't remember where). The parts shouldn't have to be disassembled--but in the even that they are, I will keep this in mind.

 

[Cockroach]

To answer your questions:

1) You just call out 1/8 NPT. I like to put on 1/8 MNPT, "M" for male or Pin, 1/8 FNPT, "F" for female or Box, but I find the machine shops prefer not to use such identification on the prints.

2)the "1/8" is a nominal size reference and has no physical meaning. The OD for this NPT geometry for example is 0.405 inches on the male.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[metalonis]

I've seen the MNPT/FNPT designation before, too. It occurs to me, though, that one should be able to tell whether a thread is internal or external just bt looking at it.

 

[SnTMan]

I had a spec called out "FNPT", and I mad a point to clarify that it did NOT mean "NPTF". I think it is generally unnecessary to state male or female, except perhaps in a wholly written description of the parts involved where no other context would be available.

Regards,

Mike

 

[Cockroach]

Metalonis has it right though, SnTMan, a casual glance at the print pretty much defines Box from Pin. "F" and "M" are redundant as far as labelling goes.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[SnTMan]

The spec referred to couplings, but it was that kind of a customer...