How to Measure the Minor Diameter of Internal Threads

[leetimothyj]

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!

 

[Jim Wynne]

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!

Your purchase order should specify the standard you expect the nuts to meet. If the supplier accepts the order and the nuts don't meet the standard, you're within your rights to reject them, regardless of the particular reason. You shouldn't specify "Class 2B" without also specifying "UNC" or "UNF" (e.g.), and the national standard. Whether or not the supplier has a case depends on what you ordered from them.

 

[leetimothyj]

Clarification; The nut was specified as "9/16"-18, UNF, 2B" and stated standard as ANSI B18.2.2. Thanks, Tim

 

[Jim Wynne]

Clarification; The nut was specified as "9/16"-18, UNF, 2B" and stated standard as ANSI B18.2.2. Thanks, Tim

You're welcome. If you specified the standard, you're on steady ground.

 

[Wayne]

Proper Thread Measurement

We are purchasing slotted hex nuts (9/16"-18 UNF-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.... Our supplier says that this is not a valid measurement, that you only use the pitch diameter....

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.

 

[Gordon Clarke]

Re: Proper Thread Measurement

This isn't exactly a "quick" reply as I've only recently found this forum. Still, better late than never
I agree with Wayne 100% in that gauges don't "measure" - they inspect for right or wrong. By measuring you always know where you are with relation to the tolerance.

I have invented and patented (and sell) a method for (among other things) measuring thread pitch diameter and can in fact also measure internal threads from M6 - 1/4".
I have on two occasions supplied thread measurement inserts for measuring diameter D on an internal thread. It is true that the tolerance for this is the only diameter (major diameter on an internal thread) that doesn't have a specified tolerance and is only given as "min." This applies to both Unified Inch Screw Threads (UN) and Metric Screw Threads. Of course this doesn't mean that it can't or musn't be measured.

My personal opinion on making the "perfect" thread is to use a combination of solid thread plug and7or ring gauges and pitch diameter measurement. A good thread is in the middle of the pitch diameter tolerance. Measurement also reveals how much of the tolerance is being used and whether or not the process is being controlled. A gauge doesn't reveal this. Inspection by solid gauges usually requires a much larger sample size than measurement. Another advantage is that when setting a machine up, if the thread cutting tool is set for the middle of the tolerance process control can be reduced and focus moved to viual inpection of the cutting tool. Keeping away from the tolerance limits also means less wear on the solid gauge.

Another Rule of Thumb for UN and Metric threads is that, if the pitch diameter tolerance for an external screw thread = 1, then the pitch diameter tolerance for an internal thread is 1.3
The pitch diameter tolerance for threads is invariably the smallest tolerance on the three diameters. If the cutting tool isn't damaged or worn then measuring the pitch diameter is usually enough to ensure that the overall profile is OK and the thread within tolerance.

I have also made a simple observation with UN threads:
if tolerance 2A is known then all other tolerances (1A, 3A, !B, 2B and 3B) follow this one.

If anyone has questions then I'd be happy to answer - assuming I can

Gordon

 

[MACIEK]

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

Hello.
Does any one have an idea where i can obtain threaded ring gauge minor diameter tolerances?

 

[Gordon Clarke]

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

Hello.
Does any one have an idea where i can obtain threaded ring gauge minor diameter tolerances?

It will depend on the type of thread you want this information for. There isn't a universal answer for "thread ring gauges" I notice by your spelling of gauge that you're probably not from the USA If it's a metric thread then ISO 1502 will probably be your best bet.
If you give more information i'm certain you can get the correct answer. Wayne (gageguy) is certain to know the answer if he reads this I can probably help too, but only if I get all the information you have on the thread type and size.

 

[Wayne]

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

Does any one have an idea where I can obtain threaded ring gauge minor diameter tolerances?

The best place is from the standard. Which standard depends on which type of thread you are working with and which country you are working in.

Type of thread could be: Unified; Metric; Whitworth; NPT; ANPT; NPTF; ACME; Stub ACME; Buttress; Tr; G-Series; R-Series; Pg; Rd; Various forms of API, and then there are proprietary threads. The minor diameter requirements may vary for each type.

Which country are you working in. In USA we use ANSI or ASME standards. In EC they use ISO standards. Of course the choice of standard may be decided by where the product is being shipped to, or by decree of design engineer regardless of country affiliation.The minor diameter requirements may vary for each standard convention.

There are multiple software programs which give you a good selection of standard types of thread to choose from. I am a seller of thread measurement tools and I can sell you: Thread Disc; ThreadTech or STETFS screw thread engineering software. I use ThreadTech most and there is a 10-day Free Trial available on my website. Stefanhg also has a shareware software available. For more detailed discussion on software read: Thread Gage Calibration Software to look up requireds/specs for thread gages.

Of course before you go out and buy software you should be sure to identify the answers to the first two questions.

Now for the most simple way to find the minor diameters of your ring gages: Look on the original manufacturer's Long Form Certification that came with the gages. The minor diameter with its tolerance will be listed there. If you have not been ordering Long Form Certifications with your gages, it may be a good time to start. The standard free certification usually does not list this type of data.

I hope this helps you in your search.

 

[Gordon Clarke]

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?

This is probably too late to be of help to you but it might help anyone else from getting into a similar predicament.

The correct specification for your thread would be 9/16-18UNF-2B

The nut tolerances are then:

Minor diameter min. 0.502 max. 0.515
Pitch diameter min. 0.5264 max. 0.5323
Major diameter 0.5625

This (and much more) can be found in the standard ASME B1.1

If any of these diameters are not within the tolerance then they can be rejected – regardless of what the thread plug gage “says”. A thread plug gage will not register if the minor diameter is too large if the pitch diameter is OK. I’ve found the easiest way to measure minor diameter on an internal thread is by using a digital caliper. It’s easy to spot potential faulty nuts, as the “bottom” of the thread (minor diameter) will have larger than normal “flats”.

Personally, I’d find another supplier simply because of the BS answers he gave to your legitimate complaint. His attitude to his products is a disaster waiting to happen.