G
Gordon Clarke
I have a chart (who knows where from) for tapping drill sizes for minor diameter of NPT threads:
NPT–NPTF (Taper) DRYSEAL
Tap Size-------------T.P.I.----------Tapping Drill mm
1/8---------- ----------27----------8.4
1/4--------------------18----------11.0
3/8--------------------18----------14.5
1/2--------------------14----------17.5
3/4--------------------14--------- 23.0
1"-------------------- 11-1/2 ------29.0
1-1/4 ------ ----------11-1/2------37.5
It must have come from somewhere - machinery supplier? Machinery's Handbook? tap manufacturer? gage manufacturer?
Any suspects? If we find one of them, we can find where THEY got it and so on (just a routine "root cause")
Thanks Wes. I'm certain most people will find that very helpful. However, the guy I was trying to help wanted to machine a tapered hole so that his thread cutting tip would only have to remove as little material as possible. A "straight hole" would mean that he had to use the thread tip tool as an ordinary cutting tip just to remove material. The actual thread was a 1 1/4" NPT with a 11½ TPI thread. And to top it off. it's a company that mass produces most of its products.
What I actualy did (and it was an internal thread) was to look up the pitch diameter (ANSI/ASME B1.20.1 pages 6 and 7) at diameter E1 and subtract the value for H. This let the thread cutting tool form a radius on the "sharp" V.
I calculated the hole diameter at the beginning of thread to be 38.31 mm which looks about right from your drill value of 37,5.
I've now done the same calculation for tapered threads NPT and the more common (at least in the rest of the world
) Whitworth profile pipe thread. BSPT. I don't mind sharing this information if someone could tell me that "my" 38.31 is correct as that would mean the rest of what I've done is correct too. It's the old "two heads are better than one" or "Doctor, I'd like a second opinion" 
If anyone is interested it only took the machine operator a few seconds to actually cut the thread once the machine had been programmed. The fact that we could measure the pitch diameter (instead of just using the usual thread plug gauge) and set the programme to run in the middle of the tolerance helped too
If the next bit sounds like a "sales plug" then just remove it
, but I have a single pair of thread measurement inserts that can measure all internal tapered threads (both NPT and BSPT) from 1/4" and up. I also have a single pair of thread measurement inserts that can do the same with both types of external tapered threads.
) be appreciated.