Calculation diameter beginning for NPT thread.

[oilgasunity]

Hi,Can anybody know how to calculate this,the diameter beginning for 3/4-14 NPT (for box & pin thread),Thanks for any helping.

 

[JohnRBaker]

I'm not sure that there even IS a way to 'calculate' these values based only on the nominal pipe size. This is the sort of thing one would use the 'Machinery's Handbook' for.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[oilgasunity]

Hi John, Is that possible to get the value by reading/calculate from the NPT table something like below:

http://thread-chart.ueuo.com/NPT-Thread-Specification.html

 

[racookpe1978]

You're trying to insert a solid "pin" (a tapered dowel ?) INTO a NPT threaded hole?

By definition, the two NPT threads making up the "threaded joint" WILL interfere with each other, and so CANNOT be represented by your diagram. (Now, if you "drew" the bottom of the threads in both sides, then "slid" the two together until the threads overlapped, you'd start to see the effect of threading the two.)

BUT - each tapered NPT type joint will vary depending on tolerances of the threads and amount of deformation when they are screwed together.

 

[dgallup]

The thread pitch diameters are 0.98887 and 0.96768 per Machinery's Handbook.


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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Tmoose]

the final axial position of the internal/box external/pin components will depend on how tightly they are assembled, making predicting locations kind of futile. Similarly thread runout or coaxiality is likely to be poor or worse, especially with hand tapping.

 

[Cockroach]

You need to figure it out. It is a matter of simple geometry, half the taper angle over thread run-out.

What buddy is after is the ingress point for roughing operation on the Box minor diameter.

Regards,
Cockroach

 

[tyger3827]

First of all the angle of a NPT is 1⁰ and 47 minutes, or 1 and 47/60, or 1.78333333⁰. Just wanted you to know that your drawing is wrong at 1.47 degrees.

 

[JohnRBaker]

I thought that 'taper' looked a little anemic ;-)

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[FACS]

The effective thread length on that thread is .546-inches (this is the adjacent)
The angle is 1.78333333 degrees
Calculate the "opposite" length, using that basic trig formula TOA

The opposite length is subtracted from the outside diameter of a 3/4-inch NPT.
the OD of a 3/4 NPT is 1.050

1.050-[Tan 1.78333333 (.546)]2 = 1.016 diameter
That's the small diameter on the pin

Of course, the opening for the 3/4-NPT is 1.050; which is the nominal size of 3/4-inch pipe. (your left hand view)

All assuming, of course, you are only using the effective thread length.

For lack of any other explanation I can give.




Charlie

http://www.facsco.com

 

[racookpe1978]

No. Don't do this.

That is, don't try to "calculate" some arbitrary hole dia based on assumed thread pitches, thread engagements, and tapered hole diamter. Just look it up:


Standard NPT Tap Drill Sizes
NPT Size

Tap Drill Size (in.)

Decimal Equivalent (in.)
1/16 - 27

"C"

0.242
========
1/8 - 27

"Q"

0.332
========
1/4 - 18

7/16

0.438
========
3/8 - 18

9/16

0.562
========
1/2 - 14

45/64

0.703
========
3/4 - 14

29/32

0.906
=========
1.0 - 11•1/2

1•9/64

1.141
=========
1•1/4 - 11•1/2

1•31/64

1.484
=========
1•1/2 - 11•1/2

1•23/32

1.719
=========
2.0 - 11•1/2

2•3/16

2.188
=========
2•1/2 - 8

2•39/64

2.609
=========
3.0 - 8

3•15/64

3.234

Ref:

http://www.engineershandbook.com/Tables/npttap.htm