Thanks Seymour, I'm looking at the drawing in machinist handbook, and understanding a bunch of it, but still not seeing how that major at the front of the thread got determined. Can you explain that?
Dave, sorry, but no! Not anymore!
I once spent about 3 hours drawing up a true 3/4NPT thread in CAD.
I've used the Pitch diameters at various lengths ( which are given ), then drew in the 60deg thread angles at the given pitch diameters, then added the basic
flats and the theoretical thread heights to come up with a very exact number.
And that was for a single external thread!
I would have had to go through the very same routine to draw the internal equivalent.
NO FINE WAY!!! Not Again!!!
Literally the day after I've contacted MRainey and purchased ThreadPal.
Frankly, that was one of three best decisions I have ever made when it comes to software! ( NcPlot is the second, I will not dare mention the third )
On a serious note though, damm shame on the publishers of Machinery Handbook ( and any and all other publications ) that makes you go through
a fucking set of calculations ( rivaling that of a moon-launch ) just to come up with a diameter that you need to hold for an NPT thread.
They give a shit ton of calculations to figure out the pitch here, there and everywhere, when at the end of the day there is no fine way to realistically and accurately
measure pitch on an NPT thread without a top-notch CMM ( and perhaps a profilometer added )
How'bout an accurate MAJOR diameter at the beginning of the thread ( perhaps with tolerances ) and the angle of the thread ( and do us all a favor and forget deg/minute angles will'ya )
After that we can now either CAD it or trig-it the rest of the way!
Gordon:
I am not in any way bashing you, but perhaps you can chime in and let us know how one could use the information available from the idiotic Machinery Handbook
and apply a formula to get the exact values for Major and Minor diameters ( since no CNC control that I know of has any setting where a Pitch is to be defined in it's cycle )
Otherwise, I recommend to all to just go out and buy
ThreadPal!