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Inspecting NPT Threads

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.
1", 1-1/4", 1-1/2", and 2" NPT all have the same thread pitch (11.5 tpi) and taper angle.

If you're in a bind and have access to ring gages in any of those other sizes, machine a test piece in that size and dial in the tool offsets.

Then hit the 2" and it should be pretty dead on.
 
Thanks for all the replies.

I ended up quoting a longer lead with a gauge and a shorter "make it fit" lead with the mating part. Unfortunately I'm in the UK so McMaster isn't a viable option although prices for the gauge are similar over here. I'm reluctant to buy used gauges. Did that when I was first starting out - bought a job lot of about 20 gauges. Sent them out for calibration and they all failed by quite a significant amount. Probably cost almost as much in calibration as it would to just buy new (although now I would at least check over wires myself before sending away just to be sure).

I do prefer to have the gauges on hand - I've got plug gauges up to 1.5" and a few smaller ring gauges. Just seems that every time an NPT job comes by it's a screamer.

I've got a few fittings around the shop so I'll have a go at measuring over threads just to see if it works well. I wasn't sure given the mic wouldn't want to sit flush but that makes sense to account for the taper. I also wasn't sure about exactly where to measure from to get an accurate result - am I in tolerance or is it just rotated 1/8 a turn from where I should be?
 
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 have done this with success and failure.

Sometimes those off the shelf fittings are not even close…

I’ve both had parts sent back, and a customer ask me to start using unbrako npt set screws as a gauge because they didn’t like where the gauge would land the screw.

Both extremes equally frustrating.
 
NPT is probably best to get the exact part they are using, just because NPT fittings aftermarket can vary quite a bit either way if they are particular, unless another shop is making it to the exact specs they need or just a gauge. which all depends on the thread fit accuracy it has, as mass produced parts are always all over the place.
 
Thanks for all the replies.

I ended up quoting a longer lead with a gauge and a shorter "make it fit" lead with the mating part. Unfortunately I'm in the UK so McMaster isn't a viable option although prices for the gauge are similar over here. I'm reluctant to buy used gauges. Did that when I was first starting out - bought a job lot of about 20 gauges. Sent them out for calibration and they all failed by quite a significant amount. Probably cost almost as much in calibration as it would to just buy new (although now I would at least check over wires myself before sending away just to be sure).

I do prefer to have the gauges on hand - I've got plug gauges up to 1.5" and a few smaller ring gauges. Just seems that every time an NPT job comes by it's a screamer.

I've got a few fittings around the shop so I'll have a go at measuring over threads just to see if it works well. I wasn't sure given the mic wouldn't want to sit flush but that makes sense to account for the taper. I also wasn't sure about exactly where to measure from to get an accurate result - am I in tolerance or is it just rotated 1/8 a turn from where I should be?

A worn straight thread gauge is not the same as a worn taper. (or maybe should be worded in vs/vs?)

If you are working in dry seal, then that may be a horse of a 'nother colour, but for basic NPT, BAH!



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Think Snow Eh!
Ox
 
I checked a bunch of Harbor Freight pnuematic fittings with female threads (they were leakers so got replaced) with a certified thread gage, and low and behold, right on the money, the L1 land was Exactly flush with the start of the threads on the fitting. Quite suprised was I.

Better then my customers get, I aim for +/- 1/2 turn as a tolerance.
 
I checked a bunch of Harbor Freight pnuematic fittings with female threads (they were leakers so got replaced) with a certified thread gage, and low and behold, right on the money, the L1 land was Exactly flush with the start of the threads on the fitting. Quite suprised was I.

Better then my customers get, I aim for +/- 1/2 turn as a tolerance.
I've had Rapidair fittings that leaked before. L1 was ok but crest truncation was way too much, creating a large leak path.
 
Well either way, the crests is a Dryseal (NPTF) spec.
NPT by it'self is (can be) a rather crude application.


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Think Snow Eh!
Ox
 
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)
Cut the thread on a pipe machine with sharp dies, I like to "season" the threads to its mate with oil and tighten/loosen a couple times. Should be good to go. Forget the gage on a one off, its custom anyway.
 
Cut the thread on a pipe machine with sharp dies, I like to "season" the threads to its mate with oil and tighten/loosen a couple times. Should be good to go. Forget the gage on a one off, its custom anyway.
Whether it just needs to work or needs to be to spec depends on your customers.

If I forgot the gauge, even on a one-off, I would get it back with a nice nastygram from the customer.

Then I'd have to buy the gauge anyway, make another part, and provide a written reason the part was non-conforming in the first place, and provide details on preventing further non-conforming parts. And have egg on my face.

As for the pipe machine, most of us aren't threading nipples. About impossible to thread special fittings on a pipe machine.
 








 
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