Screw Thread Major and Minor Diameters - Parts vs. Gages

Wayne

Gage Crib Worldwide
The minor diameter of a screw thread ring gage is easily measurable as is the major diameter of a screw thread work plug gage. People frequently check these diameters and then they have questions as to apparent discrepancies in the gages. The three questions raised are:

1. Given that on a screw thread ring gage the NOGO pitch diameter is smaller than the GO pitch diameter, why is the NOGO minor diameter larger than the GO minor diameter?

2. Given that on a screw thread plug gage the NOGO pitch diameter is larger than the GO pitch diameter, why is the NOGO major diameter of a screw thread work plug gage smaller than the GO minor diameter?

3. Given that the screw thread pitch diameters are the same as the screw thread gage pitch diameters, why are the screw thread gage minor and major diameters so much different than the product screw thread minor and major diameters?

To totally discuss the major and minor diameters of thread gages, and the related inspection issues, let me state the following:

1. The screw thread gage major and minor diameters are different than the part major and minor diameters for specific reasons based on one of Taylor’s Principles: The GO should gauge all features simultaneously and the NOGO should gage each feature independently.

2. The GO ring gage major diameter is made as large as possible and is tested to assure that it clears a specific diameter. By design, the GO ring gage major diameter has no measurement function. The part thread major diameter is to be measured via alternate measurement methods.

3. The GO ring gage minor diameter is set at the maximum material condition. This tests the part to assure that the minor diameter has not grown to a point that it would interfere with the mating part.

4. The GO plug gage major diameter is set at the maximum material condition. This tests the part to assure that the major diameter has not shrunk to a point that it would interfere with the mating part. The part thread minor diameter is to be measured via alternate measurement methods.

5. The GO plug gage minor diameter is made as small as possible and is tested to assure that it clears a specific diameter. By design, the GO plug gage minor diameter has no measurement function.

6. The NOGO ring gage major diameter is made as large as possible and is tested to assure that it clears a specific diameter. By design, the NOGO ring gage major diameter has no measurement function.

7. The NOGO ring gage minor diameter is set at or just below the gage pitch diameter. This is to allow the NOGO ring gage to properly measure the pitch diameter and exclude data from the minor diameter. Once the minor diameter passes the GO gage it is considered good.

8. The NOGO plug gage major diameter is set at or just above the gage pitch diameter. This is to allow the NOGO plug gage to properly measure the pitch diameter and exclude data from the major diameter. Once the major diameter passes the GO gage it is considered good.

9. The NOGO plug gage minor diameter is made as small as possible and is tested to assure that it clears a specific diameter. By design, the NOGO plug gage minor diameter has no measurement function.

I hope that you will find this tidbit of information helpful.
 

Coury Ferguson

Moderator here to help
Trusted Information Resource
Very interesting information; so, I thought I would resurrect this thread for any Covers that may find it useful and valuable.
 
M

merrick65

Is there a formula to calculate the major/minor diameter? I see that the thread gages specify thread per inch and go & no-go pitch diameter but I do not know how to find or calculate the major/minor diameter.
 

Wayne

Gage Crib Worldwide
Is there a formula to calculate the major/minor diameter? I see that the thread gages specify thread per inch and go & no-go pitch diameter but I do not know how to find or calculate the major/minor diameter.
There certainly is a formula, but it will vary depending on the style of thread: ACME; Buttress; Unified; Metric; Whitworth; Centralizing ACME; Stub ACME; Trapezoidal; British Pipe; API and others.

The formula will be found in the standard that defines the thread. Alternatively, you could use thread engineering software.

For more information on Screw Thread Software I suggest you read Elsmar Cove post: Thread Gage Calibration Software to look up required/specs for thread gages.

If you have a one time need, there is a free 10-day trial of ThreadTech software.
 
G

Gordon Clarke

I've thought of a simple (and inexpensive) way to measure the minor diameter on an external thread and the major diameter on an internal thread. if anyone is interested I'll make a sketch as to how it can be done.

It (my method) is definately is a lot easier than making moulds for internal threads and/or using expensive optical appliances.

The good thing about the tolerances on those two diameters is that they are normally about twice as much as the pitch diameter tolerance.

Who knows, maybe my sketch (if asked for) will give someone the "kick" to come up with perhaps an even easier method.

The following remark is mainly for Wayne :)
I can't really imagine why anyone would want to measure those two diameters unless there was a problem and it had to be determined what exactly was wrong and by how much. ;)
 

Wayne

Gage Crib Worldwide
Who knows, maybe my sketch (if asked for) will give someone the "kick" to come up with perhaps an even easier method.

The following remark is mainly for Wayne :)
I can't really imagine why anyone would want to measure those two diameters unless there was a problem and it had to be determined what exactly was wrong and by how much. ;)
Gordon;

Typically the only measurement that need be taken related to these two diameters is one to assure that the maximum material condition has not been violated. Properly calibrated thread gages will assess that condition.

With the J-series threads, measurement of the external thread minor diameter can help assess the condition of the controlled root radius.

If measurements are desired, variable thread measurement systems have attachments which can be used to measure the external minor diameter and internal major diameter of screw threads.
 
G

Gordon Clarke

This could be an "apple pears" discussion but I'll try anyway :)

"Typically the only measurement that need be taken related to these two diameters is one to assure that the maximum material condition has not been violated. Properly calibrated thread gages will assess that condition".

It isn't that I disagree but "assessing" isn't quite the same as measuring. The biggest disadvantage with gauges is that "a miss is as good as a mile". In other words when the gauges say "fault" the user is left wondering by how much and what is wrong.

"If measurements are desired, variable thread measurement systems have attachments which can be used to measure the external minor diameter and internal major diameter of screw threads".

"If measurements are desired" is the whole point. If at an inspection (process or receiving) a gauges "says" defect then production can either be stopped or returned to supplier. It still doesn't help the machinist as he'll/she'll want to know what is wrong and by how much. Still, if it's a supplier it's easier to say, "your problem". :notme: If manufacturing and inspection each have their own gauges then a micron difference is enough to affect the result.

" ...... variable thread measurement systems have attachments which can be used to measure the external minor diameter and internal major diameter of screw threads".

Are any of these measurement systems you refer to inexpensive compared to for example, gauges?

I'll make a sketch (very soon) of what I think could be an inexpensive method of measuring the minor diameter on an external thread and the major diameter on an internal and you (and anyone else) can let me know what you think.

Wayne, one thing I think we are in complete agreement with is that making the perfect thread is a combination of gauging and measurement. The measuring for machine set up (especially pitch diameter) and gauging for a quick check during manufacturing.
 

Wayne

Gage Crib Worldwide
It isn't that I disagree but "assessing" isn't quite the same as measuring. The biggest disadvantage with gauges is that "a miss is as good as a mile". In other words when the gauges say "fault" the user is left wondering by how much and what is wrong.
Hopefully the measurement is taken at the machine during screw thread production. Analysis of the problem is more expensive than, and will usually lead to the same result: Replacing the cutting tool. So why bother.

If ... a gauge "says" defect then production can either be stopped.... It still doesn't help the machinist as he'll/she'll want to know what is wrong and by how much.
Again, the least expensive thing to do is replace the worn cutting tool. Why do we care what is worn on the cutting tool, and adjusting a worn cutting tool to bring the thread into agreement with the gage will only increase the thread form deviation in another area.

If manufacturing and inspection each have their own gauges then a micron difference is enough to affect the result.
Thus the stipulation in ANSI B1.2 paragraph 2.2.1 (see attachment): "...a product screw thread is considered acceptable when it passes a test by any ... gages,...provided the gages being used are within the tolerances specified..."

Are any of these measurement systems you refer to inexpensive compared to for example, gauges?
They are not inexpensive compared to gages, but if such measurement, for some reason becomes critical, they are not prohibitively expensive.

... making the perfect thread is a combination of gauging and measurement. The measuring for machine set up (especially pitch diameter) and gauging for a quick check during manufacturing.
Gordon, I agree with most of this statement. The measurement of the pitch diameter is important during machine set-up. Monitoring production with screw thread gages is vital during manufacturing. That said, the making of a "perfect" thread need not be attempted because it is virtually impossible.

Regardless of all that, tools to measure threads abound, and making a better one is always a good thing to accomplish.
 

Attachments

  • B1.2 ppg 2.2.1.pdf
    321.1 KB · Views: 225
G

Gordon Clarke

I've finally managed to write what I promised and it's in the enclosure/attachment. Wayne, I agree more or less with what you write. The "more or less" is due to the different ways we approach thread inspection.

From your point of view I can't imagine a company manufacturing threaded components without thread gauges ;)
 

Attachments

  • MinorDiameter.pdf
    153.4 KB · Views: 174
G

Gordon Clarke

Wayne (gageguy) gave me a few suggestions as to how i could improve my attachment so here is the updated, and hopefully improved, version.

Thanks Wayne :applause: :thanx:
 

Attachments

  • MinorDiameter.pdf
    160.6 KB · Views: 545
Top Bottom