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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"
)
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"
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
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
) be appreciated.