Practical Screw Thread Information & Tolerances

G

Gordon Clarke

#71
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" :D

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 :bigwave:

If the next bit sounds like a "sales plug" then just remove it :eek:, 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.
 
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S

stefanhg

#72
Hi Gordon.

The minor diameter at E1 is:

Min. Minor diameter = Min.PD_E1 - H + 2*Min.trunc. = 1.57795-0.07531+2*0.0029 =
= 1.50844 in = 38.314 mm

Max. Minor diameter = Max.PD_E1 - H + 2*Max.trunc. = 1.58882-0.07531+2*0.0063 =
= 1.52611 in = 38.763 mm

Nominal Minor diameter = 1.517275 in = 38.538 mm

Minor diameter Tolerance = PD_E1_Tol + 2*Tolerance of crest truncation =
= 0.0109+0.0068 in = 0.1767 in = 0.448 mm
 
Last edited:
G

Gordon Clarke

#73
Hi Gordon.

The minor diameter at E1 is:

Min. Minor diameter = PD_E1 - H + Min.trunc. = 1.58338-0.07531+0.0029 =
= 1.51097 in = 38.379 mm

Max. Minor diameter = PD_E1 - H + Max.trunc. = 1.58338-0.07531+0.0063 =
= 1.51437 in = 38.465 mm

Nominal Minor diameter = 1.51267 in = 38.422 mm
Thank you too Stefan. I'm starting to think my calculations were OK and also allowing the thread cutting tip to do work on the entire profile.

One thing does puzzle me. You have the max/min tolerance on the minor diameter at 0.086 mm, whereas I believe the pitch diameter tolerance at the correct position is 0.28 mm. Quite a difference. Usually on a thread, the pitch diameter is the one with the smallest tolerance, followed by the major diameter and then the minor diameter. Are you sure the information you gave me wasn't meant for the thread plug gauge? It is the actual component thread diameter dimensions and tolerances I'm interested in.

I used 0.9605P for H with the 55 degree flank angle and 0.866P for the 60 degree flank angle.
 
G

Gordon Clarke

#77
OK, I'll try and post the file I've made. Any remarks or comments will (probably :tg:) be appreciated.
 

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G

Gordon Clarke

#79
I had a couple of language mistakes in my file. Danish where it should have been in English :eek:
I'll retry!
 

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G

Gordon Clarke

#80
In the previous post, I corrected the values in the same post.
Sorry Stefan, but I just don't believe the minor diameter tolerance for the minor diameter at E1 on an actual component is 38.422 +/- 0.043 for 1 1/4" NPT.
It seems much more like the tolerance for a gauge!
 
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