Correct use of NPT (National Pipe Thread) Thread Gages

Wayne

Gage Crib Worldwide
#11
I'm not familiar with the term convolution. But what does it mean in the thread gage community?
Another term for convolution is Higby Cut.

And a bigger question: Where can I find a stated requirement on convolution with tolerance? That word doesn't appear in B1.20.1.
It may be found in ANSI B1.20.5; which is the gages for NPTF. If it is the case that it is not in ANSI B1.20.1 then a case may be made to allow convolution or chamfer. I am still searching for this data. What I have learned is that gage convolution was added to the pipe gages in 1968 (but I am still trying to learn where in the standards it was added) I have heard that some gage makers use chamfer on NPT and others convolute. I have also discovered some disagreement between how the standard tolerances are applied to the gages (I do not want to get into detail here) which will affect the size of the gage and broaden the gage tolerance.

I do have access to ANSI B1.2:1983 (R2001) and in paragraph it states: B1.2%20ppg%204.1.10%20convolution.jpg
B1.2%20Fig%202%20convolution.jpg
 

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Wayne

Gage Crib Worldwide
#12
And a bigger question: Where can I find a stated requirement on convolution with tolerance? That word doesn't appear in B1.20.1.
I am told that on page 16, in the last paragraph of paragraph 7.1.1 the convolution of NPT gages is addressed. I do not have a copy of this page, so I cannot confirm, but the person who told me should know.
 
Y

YankeePapa

#13
Wayne,
Thank you for all the excellent information.

The last paragraph of 7.1.1. reads "Partial end threads shall be removed on both ends of the ring gage and on the small end of the plug gage to full-form profile in order to avoid possible seating error from bent or malformed feathered edge."

As you can see, it's not as detailed as NPTF. PM me for a copy.

Have you ever seen a survey of measured L1 numbers for female threads across different manufacturers? I would think that the more expensive fittings mfrs would use this to show how their products meet spec and those of competitors often don't.

A general question, agree, disagree, or comment:

Threads on pipe nipples tend to be correct dimension because those are governed by taper angle and number of threads. The OD of the pipe is fairly tightly controlled, the taper angle doesn't change that much with tool wear, and the number of threads is plainly visible, so if it's wrong the product tends to get rejected with no measurement needed.

In contrast, female threads tend to be undersized (i.e. they have short L1's) because they have no reference dimension equivalent to pipe OD, and as long as they have all their threads, plus short hand-tight lengths in the field can be easily chalked up to dings and burrs. It pays for manufacturers to cut corners here because it's cheaper to leave the hole smaller and no one notices.

Thanks again, Wayne!
 

Wayne

Gage Crib Worldwide
#15
A general question, agree, disagree, or comment: <snip>
No comment. This is outside of my knowledge zone. :notme: I get involved in gages, but never use them on parts.

I am sure that others on the forum will have better input than me on this question. :D I will let them answer this from their expierence.
 
Last edited:

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#16
<snip> A general question, agree, disagree, or comment:

Threads on pipe nipples tend to be correct dimension because those are governed by taper angle and number of threads. The OD of the pipe is fairly tightly controlled, the taper angle doesn't change that much with tool wear, and the number of threads is plainly visible, so if it's wrong the product tends to get rejected with no measurement needed.

In contrast, female threads tend to be undersized (i.e. they have short L1's) because they have no reference dimension equivalent to pipe OD, and as long as they have all their threads, plus short hand-tight lengths in the field can be easily chalked up to dings and burrs. It pays for manufacturers to cut corners here because it's cheaper to leave the hole smaller and no one notices.
Anyone here who can shed some light on this one?
 
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