3/8" NPT thread question 1

[adchampion]

Cross Posted from Valve Engineering-

Does anyone have a definitive reference for the required tap depth for a 3/8" NPT port. I understand from reading on here the tap depth is not usually called out, but if anybody has a standard or a guide that I could check our test parts against, I would much appreciate it.

I have a bonnet with 4 measurable ports. On three of them the depth of full thredas range from .330 to .350 and the 4th one is .470. This variation of almost 3 threads seem excessive. All the ports gaged within 1/2 turn of each other and the drill diameters are all within .003, so i ar struggling to figure out how these port differ so much in the threading.

Thanks,

Ben

 

[diamondjim]

The extra tap thread depth is not
necessary but probably just a precaution
by the machine operator.

 

[adchampion]

Thank, but I am not convinced this is the case becuase of the following:

ANSI/ASME B1.20.1-1983 states that the nominal engagement for wrench tight is 4.32+3=7.32 turns.

At 18 threads per inch, that is .407" engagement. The depths measured on the first 3 ports are more than 1 turn less than this:
.330 = 5.94 turns
.350 = 6.30 turns
.470 = 8.46 turns

However nowhere can I find documentation of this required depth. Any ideas?

~Ben

 

[mrainey]

Looking at my copy of Guide to Threads, Threading, and Threaded Fasteners (Modern Machine Shop, 2004)


On page 262 there's a drawing of a tap screwed into a ring gage, which is .240 thick for 3/8 NPT (labeled as L1 dimension)

Projection of the tap through the ring is shown as .392 - .516, which would be added to the .240 ring thickness to get a tapping depth. I normally program pipe taps depths to the middle of this very wide tolerance (.694 in this case.) I've never had a problem with going too deep, unless the hole wasn't deep enough for a standard tap - at which point, we go to a short-projection style.


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[diamondjim]

Machineries Handbook shows the L1 and
L2 depths of .240 and .4078 where L2
is at least 2p longer than any thread
contact. An internal thread is not
tapered so the depth looks reasonable
at the .320 depth. If the guage only
goes that deep it should be assumed
that the internal part would not exceed
the guage depth or the guage depth
should be increased to be meaningful.

 

[mrainey]

"An internal thread is not
tapered"


???


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[kenvlach]

This is not so difficult. To properly tighten (seal) taper threads, the external thread length must be equal to or greater than the internal thread length.

Info for 3/8” NPT from ANSI/ASME B1.20.1-1983, R1992 in Machinery's Handbook, 25th Edn, pp. 1774-1775 (1996):

Internal thread length
L₁ (handtight engagement) + L₃ (wrench makeup length)
= 0.240 + 0.1667 = 0.4067”

Effective Thread, External
L₂ (also length of plug gage) = 0.4078”

Overall Length, External Threads
L₄ = L₂ + V (Imperfect Threads due to Chamfer on Die)
0.6006 = 0.4078 + V, so V = 0.1928

Your internal threads should be 0.4067” long, and external threads at least 0.4078” long.
Shorter threads will not provide maximum strength, although sealing is not a problem per se.
The one port with 0.470” thread depth is out-of-spec. You must have correspondingly external threads in order to seal. The only apparent harm of longer threads is wasted material, as the thinnest sections of the coupling do not contribute strength. [Taken further, with enough wall thickness on the port, you could cut the threads long enough that the pipe 0.675” OD fits within the port.]

 

[kenvlach]

Hmmm, maybe not so simple (haven't threaded any pipe since 1999).
The overall external thread length is only longer due to the vanishing thread length V (from the practical need for chamfer on the die). Shorter male threads will seal (don't want to bottom out using overly long male threads).

Either use the given values from Machinery's Handbook or pipe thread gauges.

 

[icrman]

As far as I know, its not the designers or engineers place to worry about the depth of an NPT thread. That is for the machinist to worry about. And he will either use the proper thread gage, or a fitting itself. Been there done that as machinist. So not to worry us machinist can get the job done for ya.

 

[adchampion]

Thanks to everybody for the insight.

 

[Cockroach]

Good work, Kenvlach! You are bang on the money. This obviously deserves the "star" award.

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