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Inspecting NPT Threads

Sam I

Aluminum
Joined
Nov 6, 2020
Is there a way to inspect an external NPT thread without the use of a gauge?

I've got a customer who wants me to machine a 2" external thread on a part but it's a pretty urgent job and the lead time on a ring gauge is too long. Is it possible to accurately measure the threads without the use of a gauge with basic metrology equipment (micrometers, thread wires etc)
 
Is there a way to inspect an external NPT thread without the use of a gauge?

I've got a customer who wants me to machine a 2" external thread on a part but it's a pretty urgent job and the lead time on a ring gauge is too long. Is it possible to accurately measure the threads without the use of a gauge with basic metrology equipment (micrometers, thread wires etc)
Have you considered just using a pipe threader?
 
It's not something I have considered however I would still need to inspect the thread after to make sure it's within tolerance.
 
No not really unless you have an MRP gauge.

You can guess close with hand tight engagement in a mating part but it’s only a decent guess.
 
Or buy a gage from McMaster. You'll have it tomorrow.
Sure can..........for the low low price of $456 plus tax 'n shippin'...............ain't gonna do it for a one off unless the customer buys/provides it..............but.....if'n it's a rush/urgent job. Bill them for it(tell them up front of the extra cost 'o course)...................
 
Is it possible to accurately measure the threads without the use of a gauge with basic metrology equipment (micrometers, thread wires etc)

You should be able to get plenty close enough for pipe threads with thread wires, if you have a close selection, and a cad program. You could do it mathematically also but to me that'd be no fun at all.
 
Sure can..........for the low low price of $456 plus tax 'n shippin'...............ain't gonna do it for a one off unless the customer buys/provides it..............but.....if'n it's a rush/urgent job. Bill them for it(tell them up front of the extra cost 'o course)...................
And once they find that out, Ox's method will be fine.
 
A) Go to Hardware
B) Buy a 2" Close nip
C) Buy a 2" Coupling

Make the coupling fit your part the same as it does the nip.


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Think Snow Eh!
Ox
I've put on a print for a female NPT that McMaster such and such must fit flush.

Call the customer. If it isn't aerospace then they likely just want it to work with the mating part.
 
Look up the thread data in your reference of choice. It's in the MH if you don't have anything better. There you can find the pitch diameters at L1-L4 distances. You can then measure over wires at these distances from the front of the thread. You will have an error equal to the taper difference in one pitch, but that is going to be well with the P.D. tolerance. It's a bit tricky and is one of those jobs that would be easier with three hands, but you can get it done.
 
We have used Ox's method for 25+ years. Never had any issues.


OK, well I have maybe used that method once or twice.

I have his and hers guages for dry seal for many/most sizes up to 1".
And if I had time, and could find a screamin' deal, I would try to pick up a used ring off The Bay.

But if I had to knock out a 2" in a hurry, I would use the above method.


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Think Snow Eh!
Ox
 
OK, well I have maybe used that method once or twice.

I have his and hers guages for dry seal for many/most sizes up to 1".
And if I had time, and could find a screamin' deal, I would try to pick up a used ring off The Bay.

But if I had to knock out a 2" in a hurry, I would use the above method.


------------------

Think Snow Eh!
Ox
Yeah, we have lots of ring gauges, but like you said in a pinch it has worked fine. Even better make your own, have done that often also.
 
A) Go to Hardware
B) Buy a 2" Close nip
C) Buy a 2" Coupling

Make the coupling fit your part the same as it does the nip.


-----------------

Think Snow Eh!
Ox

Buy Home Depot or Ace fittings, check female threads and discover that invariably their way out of tolerance when checked with a Male thread gage.

I stopped using fittings as thread gages years ago. Buying thread gages seems expensive in the short term. Pays off in the long term.

Nothing better then having a customer complain about an NPT thread, then go over with an NPT thread gage and show them it's within tolerance. Especially when it's the cute Filipino female engineer.

I used to use Parker fittings as Gages, if your going to use a fitting as a gage, Parker are probably as good as any, and better then the rest.
 
Since +- 1 turn on this thread is just over +- .005 on pitch diameter I think you can get a pretty good idea with wires.

Contrive to find the L1 dimension conciding with the thread root. An optical comparator would work great, but in the machine maybe a caliper or scale. Mark this spot. Put one thread on the mark and of course two opposite. Sticky grease is your friend. Measure this E1 dimension and multiply it by cosine(1/32), .9995, to correct for the thread taper. Try it a few times to check repeatability.

Since a quarter turn makes about .0013 difference on PD and you've got a bit more than +- .005 total I think it will work.
 
When I had to cut a male npt that I didn't have the gage for I used a fitting and threaded it on until the hand tight engagement distance called out in the machinist bible said it was right. Never came back so it must've been close enough.
 








 
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