Properly modeling internal pipe thread 1

[takedownca]

I need to model a 2"-11.5 NPT internal thread. I believe I have all the dimensions I need, but I have a question about the nominal diameter. Since pipe thread is conical, it doesn't really have a set diameter. So if the above thread is spec'd to have a 2.375" nominal diameter, what does this number refer to? For regular straight thread, the nominal diameter is slightly larger than the major diameter. If this applies to pipe thread then the initial major diameter is something slightly less than 2.357", but I'm not sure of this. I know if I'm off by a little bit the thread will still be functional, but I'd like to be as close to the ASME B1.20.1 standard as possible.

 

[hydtools]

See this ref for thread dimension detail:

http://www.webfootcustomcalls.com/temp/NPTThreadSpecstable3.pdf

2.375 is not the thread nominal diameter. It is the pipe OD for 2" pipe.

Ted

 

[takedownca]

Thanks hydtools, but that Machinery's Handbook page is what I used to get all the dimensions. I misquoted the 2.375" dimension because of another page I found while googling. I guess a better question would be, which diameter on the handbook page should I use as the starting diameter for my internal thread?

I would guess it to be E1, the pitch diameter corresponding to hand tightened. However, from the diagram it looks like what they call pitch diameter is really the average of the major/minor diameter. So if I were to model the thread pre tapping, I'd create a tapered cone with a start diameter equal to E1 - h, where h is the thread height. I'm sure this is fine, but I just want to someone to corroborate in case I'm missing something.

 

[hydtools]

I think your approach is fine for modelling the thread.

Ted

 

[hydtools]

If it is helpful here are dimensions for taper reamers used to create the inside taper prior to tapping the thread.

http://www.formrelief.com/Form Relief Catalog 26.pdf

Ted