[amphib]

I have to cut a bunch of parts with an internal 3/4-in. NPT thread and will use a G76 cycle on a lathe with a Fanuc 0Tc control. I know that NPT threads taper 0.75" per foot. I need the diameter (preferably at the face of the part) for the tapered hole that I'll bore and the major thread diameter. I've been all through Machinery's Handbook and can't figure out how to derive these dimensions from their charts. Can anyone help?

[Jim Dawson]

Maybe this will help

Credit: Pipe Nipples from Robert-James Sales, Inc.

3.1.5 Diameter of Thread. The basic pitch diameters of the taper thread are determined by the following formulas' based on the outside diameter of the pipe and the pitch of the thread:
E0 = D - (0.05D + 1. 1) 1 In
= D - (0.05D + 1.1) p
E I = E0 + 0.062 5L

where

D = outside diameter of pipe
E0 = pitch diameter of thread at end of pipe or small end of external thread
E1 = pitch diameter of thread at the gaging notch or large end of internal thread
L I = normal engagement by hand between external and internal threads
n = threads per inch
2 For the 1/8-27 and 1/4-18 sizes, E1 approx.= D - (0.05D +0.8 27) P.

3.1.6 Length of Thread.
The basic length of the effective external taper thread L2, is determined by the following formula based on the outside diameter of the pipe and the pitch of the thread:
L2 (0.80D + 6.8) 1 In
(0.80D + 6.8) p

where
D = outside diameter of pipe n = threads per inch
This formula determines directly the length of effective thread which includes two usable threads slightly incomplete at the crest.

[amphib]

Thanks, Jim, but this is not quite what I need. I'm looking for the dimensions that I can actually program my tools to follow. If there was a formula that related thread height to pitch diameter I'd be good to go.