NPT Thread Calibration Method - NPT Taper Thread Plug Gages

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soosai

I have the 3 wire gauge set, to calibrate the parallel screw thread gauge. anyone suggest me, How can I calibrate the NPT taper thread plug gages.
 
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Hershal

Metrologist-Auditor
Trusted Information Resource
Someone with more experience in threads will likely reply soon.....
 

DietCokeofEvil

Trusted Information Resource
You need a taper sine block to get a really good reading on NPTs using a three wire method. There is a two wire method for calibrating the P.D., but I've never thought it was very good.

The ANSI B1.20.5 standard describes both procedures.
 

SpinDr99

Involved In Discussions
On a similar note regarding NPT plug gages, I have a 1/8-27 MPT L1 gage that was rejected for "Basic Length". The cal cert shows Nominal at 0.16150" and upper tolerance at 0.16150" with an "As Found" value of 0.17807". Clearly this gage exceeds the upper tolerance, and IF the hole had a bottom, I could see how this would be a problem IF the plug wouldn't go in smoothly. The Pitch Diameter is at the bottom end of the tolerance, so the gage will still go in the part being inspected. Can someone help me understand why, even if there was a bottom to a threaded hole, and the gage went all the way in smoothly, this is a problem?

I now have a Cal. Cert. which shows the gage failed calibration, but for our purposes, the gage will tell us whether the threads of the parts being inspected are acceptable. Is there a way "out" of this canumdrum?

Thanks for all your advice and input!
 

SpinDr99

Involved In Discussions
Unfortunately, that information was not provided by the calibration lab. Only that the PD nominal is 0.37360" the "Tolerance -" is 037340 and the "Tolerance +" is 037380". The "As Found dimension is 0.37340"
 

DietCokeofEvil

Trusted Information Resource
That's the small end, which is not really the place to have a reading taken- the notch P.D. is more important since that is where the contact will be made. Also the tolerance is wrong- you get ±0.0002 on the P.D.- that puts your small end P.D. exactly in the middle, which I find a little hard to stomach- nothing is exactly on the money.

In the end- if the gage is usable, it is up to you to determine. I can't speak for other cal houses, but as long as I received a request in writing from a customer asking to change a tolerance or remove a parameter, I'll do it.

Also- check the notch for and the end of the gage for any burrs. It's not often that a basic length is out- and it's usually because there is surface damage that can be stoned out.
 

SpinDr99

Involved In Discussions
Thanks DCoE. Sounds like you're well into this end of calibration and in a lab yourself. I'm pretty new to the world of threads and gages. Our customers are pretty loose with their product requirements and MAYBE have 3-4 pieces returned per year. I am using a new calibration lab since they seem to have better capabilities than our last lab, but rejecting this gage which has never created any customer issues for us, seems overkill.

Can I use that to justify using this gage for product acceptance, even though the cert says "Fail"? I've been an auditor for years and can't see an auditor accepting that, but stranger things have happened in the ISO world. We run a pretty lean ship here, and replacing the gage because of this cert will be a hard sell to management.

Do you know of any other reason where we might be able to justify us to continue using this gage for product release?
 
G

Gkokal

You can use the taper measurement, PD at small end and step height to calculate the PD at gage point. Many cal labs measure small end and calculate up to the step height. If this still comes out of tolerance at gage point, then i would replace the gage.
 

DietCokeofEvil

Trusted Information Resource
Thanks DCoE. Sounds like you're well into this end of calibration and in a lab yourself. I'm pretty new to the world of threads and gages. Our customers are pretty loose with their product requirements and MAYBE have 3-4 pieces returned per year. I am using a new calibration lab since they seem to have better capabilities than our last lab, but rejecting this gage which has never created any customer issues for us, seems overkill.

Can I use that to justify using this gage for product acceptance, even though the cert says "Fail"? I've been an auditor for years and can't see an auditor accepting that, but stranger things have happened in the ISO world. We run a pretty lean ship here, and replacing the gage because of this cert will be a hard sell to management.

Do you know of any other reason where we might be able to justify us to continue using this gage for product release?


If the notch measurement is legit, I would not be comfortable using this gage. You get ±1 turn from the notch, and if it's already out by over a quarter turn, you could get a false pass.
 
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