How to Measure the Minor Diameter of Internal Threads

[dickgent]

Re: Measuring minor diameter of internal threads - How To?

Sorry if this answer was already posted. After working for a major fastener manufacturer for more than 10 years, Our only method of accurately checking the thread geometry of internal threads was using specific gages manufactured for this purpose. Johnson gage has the patent and manufactures the gage for sale (very expensive). However there are gage houses and labs than can perform this type of inspection for you.

the link is
http :// www .johnsongage.com/pr_insSystems.shtml - OBSOLETE BROKEN 404 LINK(s) UNLINKED - PLEASE HELP - REPORT POSTS WITH BROKEN LINKS

This is the only way to accurately measure internal thread dimensions to my knowledge.

Dick Gent

 

[TMSQAM]

Re: Measuring minor diameter of internal threads - How To?

Wayne the Gage Guy sells a great thread program that provides just about any information you need for threads. This includes standard/metric threads, gages, Acme, Buttress etc. It is not that expensive and well worth the price if you deal with a bunch of different threads. Very powerful tool. Go to www.gagecrib.com for info.

The program does gages as well.

 

[jane-hu]

Re: Measuring minor diameter of internal threads - How To?

We are purchasing slotted hex nuts (9/16"-18, class 2B) for a load supporting application for overhead material handling equipment. We have been measuring the nuts with both a thread pitch (go / no-go) gage and a cylindrical plug gage to measure the maximum minor diameter. According to ANSI/ASME B1.1, the minor diameter for this internal thread can be a maximum of 0.515". The nuts conform to the thread pitch gage, but we are rejecting them based on an oversize condition on the minor diameter, e.g. 0.520" - 0.525". Our supplier says that this is not a valid measurement, that you only use the pitch diameter. I am not that well versed in fasteners, but my thought is that the less material (i.e., the larger the minor diameter), the less support and greater potential for the nut to strip out. If this dimension is not 'valid' then what determines the minimum amount of thread surface? Thanks in advance for your input and any direction to an 'expert' source (not that you aren't experts!) I did once hear the definition of an expert as "ex" = has been + "spurt" = a drip under pressure. I am an ex-spurt in many fields!

if the Special Designed Calipers==>Screw Thread Digital Calipers can be help?

 

[Wayne]

Re: Measuring minor diameter of internal threads - How To?

We are purchasing slotted hex nuts (9/16"-18, class 2B) …. We have been measuring the nuts with both a thread pitch (go/nogo) gage and a cylindrical plug gage to measure the maximum minor diameter. According to ANSI/ASME B1.1, the minor diameter for this internal thread can be a maximum of 0.515". The nuts conform to the thread pitch gage, but we are rejecting them based on an oversize condition on the minor diameter, e.g. 0.520" - 0.525". Our supplier says that this is not a valid measurement, that you only use the pitch diameter. … Thanks in advance for your input and any direction to an expert source…

I know I replied to this ages ago, but have thought of some additional data that needed to be added…

The only official expert source is the ANSI/ASME standards. ANSI/AMSI B1.1 contains the full definition of the Unified National Screw Thread. ANSI/ASME B1.2 defines the methods of measuring the afore mentioned screw thread.

According to ANSI/ASME B1.2 Chapter 4.8 (pages 24 to 27): The minor diameter is required to be measured and the methods of such measurement are defined; which include smooth cylindrical GO/NOGO plug gages. There are even nice drawings. Pick up a copy for 198 pages of reading pleasure.

 

[jane-hu]

Re: Measuring minor diameter of internal threads - How To?

maybe can think about special calipers

 

[Gordon Clarke]

Re: Measuring minor diameter of internal threads - How To?

maybe can think about special calipers

The minor diameter of any internal thread (excluding hindrances such as shoulders etc.) can easilly be measured with any standard caliper. Personally I'd use a digital caliper

The tolerance on the minor diameter for both Metric and ISO Inch threads is about twice that of the pitch diameter, so it shouldn't be a problem for a digital caliper.

As this fact may have got lost along the way, the minor diameter on an internal thread is the bore diameter

The major diameter (D) on an internal thread is the difficult one to measure, but needn't be expensive to do. The same applies to the minor diameter (d1) on an external thread.
There's quite a bit of free screw thread information on www.f-m-s.dk

Guilin = Anyi???

 

[Gordon Clarke]

Re: Measuring minor diameter of internal threads - How To?

"After working for a major fastener manufacturer for more than 10 years, Our only method of accurately checking the thread geometry of internal threads was using specific gages manufactured for this purpose. Johnson gage has the patent and manufactures the gage for sale (very expensive). However there are gage houses and labs than can perform this type of inspection for you.
the link is
http :// www .johnsongage.com/pr_insSystems.shtml - OBSOLETE BROKEN 404 LINK(s) UNLINKED - PLEASE HELP - REPORT POSTS WITH BROKEN LINKS
This is the only way to accurately measure internal thread dimensions to my knowledge.
Dick Gent "

For those that can afford it there are now thread measuring machines on the market that can measure all dimensions (very accurately) on internal (and of course external) threads. All three diameters, as well as flank angle and pitch are measured. These machines can also print out a "drawing" showing all measurements. As documentation - excellent.

 

[Gordon Clarke]

Re: Measuring minor diameter of internal threads - How To?

Sorry if this answer was already posted. After working for a major fastener manufacturer for more than 10 years, Our only method of accurately checking the thread geometry of internal threads was using specific gages manufactured for this purpose. Johnson gage has the patent and manufactures the gage for sale (very expensive). However there are gage houses and labs than can perform this type of inspection for you.

This is the only way to accurately measure internal thread dimensions to my knowledge.

Dick Gent

Thinking a bit more over your reply I tend to disagree that it "is the only way to accurately measure internal thread dimensions". The gauges you refer to are good when the thread cutting tool is good, but it is debatable as to what they really measure. As they measure over several pitches it is actually the thread geometry that is being "checked" more than measured, although it is possible to see if the overall thread geometry is acceptable if the thread cutting tool is OK. One "problem" with this type of gauge is that if something is wrong then the gauge doesn't "say" which dimension is "off". In fact if I chose to be nit-picking, then a wrong flank angle wouldn't necessarilly be picked up by this type of gauge and neither would a major or minor diameter out of tolerance.
There are other methods and not as expensive, I'd prefer to use, but that is my personal preferance and opinion

 

[Gordon Clarke]

Re: Measuring minor diameter of internal threads - How To?

Because Dick Gent (on page 4) added Thread Geometry Measurement Fixtures to the number of possibilties for thread inspection, this made me feel the urge to change (add to) something I wrote a few years ago on the adventages and disadvantages of various thread inspection methods and possibilities.

If anyone feels that I should add more methods, or that anything I've written is incorrect please get in touch and I'll give it serious thought. If anything I've written is incorrect then of course it'll be changed. In short, all suggestions are more than welcome

See the attached PDF file

A point to ponder, "The USA is slowly moving towards the metric system - inch by inch"

 

[Sathyanarayanan]

Re: Proper Thread Measurement

I sell thread plug gages for a living. Your supplier is wrong. The thread plug gage does not fully check the thread. The GO gage checks the thread maximum material condition of the pitch diameter and the major diameter but does no measurement of the minor diameter. The NOGO gage only checks to see if the pitch diameter has not been machined too large. It does not measure either the major or minor diameters. To fully gauge an internal thread, a Minor Diameter GO/NOGO Plain Pin Gage must be used.

BTW: For an external thread, the same is true of the major diameter. It must be measured by a tool other than the thread ring gage.

That was indeed a very succinct comment. The importance of the minor diameter of internal threads is often forgotten. I however wonder what benefit it has to measure or even specify the go condition of the internal threads provided the screw plug answers the thread to both Go and No go condition perfectly. An oversized minor diameter may lead to a weak thread which may strip under torque. But an undersized minor diameter would not affect if not enhance the thread strength. I understand this may do harm to the tap but the thread quality per se should not get affected. I am probably commenting on this thread which was dropped several years back. But would this bring a a different perspective to the issue?