How to Measure the Minor Diameter of Internal Threads

[dunn2k]

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

This is a question for Gordon.....

An MJ22 x 1.5 4g6g external thread is being measured using a ring gauage. Using information you have provided to previous questions I can see that ring gauges are not designed to check the minor diameter of threads.

We had an issue where the mating part (a nut) wouldnt assemble due to the minor diamter being oversize, this was not picked up using the ring gauge.... What is the best way to inspect / measure the minor diamter so we catch the issue at the machining stage?

Many thanks for your help

Phil

 

[Gordon Clarke]

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

An MJ22 x 1.5 4g6g external thread is being measured using a ring gauage. Using information you have provided to previous questions I can see that ring gauges are not designed to check the minor diameter of threads.

We had an issue where the mating part (a nut) wouldnt assemble due to the minor diamter being oversize, this was not picked up using the ring gauge.... What is the best way to inspect / measure the minor diamter so we catch the issue at the machining stage?


Hi Phil,
There may be other possibilities, but (without it costing you a fortune) to do as I suggest you'll need a standard 6" digital caliper, a caliper pressure device and a pair of special thread inserts designed/manufactured with a radius slightly less than that of the permissable radius for this particular thread. The tolerance on the minor diameter would be well within the capacity of the digital caliper.

I have made this for a customer not too long ago, although with another thread dimension.
If you are interested in further details and information (all help and advice is of course free ) please contact me at gc(at)f-m-s.dk. Please note that I'll be on vacation weeks 6 and 7 of 2010.

 

[Wayne]

Measuring minor diameter of internal threads - How To?

Gordon... Using information you have provided to previous questions I can see that ring gauges are not designed to check the minor diameter of threads.

Phil;

I am sorry to report that you have made an incorrect extrapolation of the data provided. Gordon stated: A thread plug gage will not register if the minor diameter is too large....

Just because a thread plug gage does or does not do something; that cannot be transferred over to a thread ring gage.

The correct correlative statement for a thread ring gage would be: A thread ring gage will not register if the major diameter is too small....

An MJ22 x 1.5 4g6g external thread is being measured using a ring gauge.... We had an issue where the mating part (a nut) wouldn't assemble due to the minor diameter being oversize; this was not picked up using the ring gauge.... What is the best way to inspect / measure the minor diameter so we catch the issue at the machining stage?

The cause of your failed inspection was a worn minor diameter on the GO thread ring gage.

Beware: Thread ring gage calibration by set plug alone is insufficient. After a thread ring gage has been set using a set plug gage; the minor diameter of the thread ring gage must be checked with GO/NOGO plain plug gages.

NOTE: The minor diameter of a GO thread ring gage may be the first place to wear because the tip of the cutting tool is the first place to wear. When the tip of the cutting tool wears the thread root grows which wears the ring gage minor diameter.

NOTE: The NOGO thread ring gage minor diameter and the NOGO thread plug gage major diameter are basically nonfunctional dimensions.

MORE: The confusing world of screw thread minor and major diameters.

I hope this information is helpful.

 

[dunn2k]

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

Gordon, thanks for your reply, i have emailed you directly to the address provided. Please post a response if you dont get my email. Many thanks, Phil

 

[Gordon Clarke]

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

Wayne you're right (as usual )

I noticed that the thread in question (MJ22x1.5) was an external thread so I just assumed (OK I know - not always the wisest thing to do ) that it was the component minor diameter that was the problem as the OD (major diameter) on any external thread presents no problem to measure. The "largest" diameter on a thread ring gauge is of course to inspect the minor dameter of the external thread.

The only disadvantage I can think of with gauges is that they don't "measure" anything - they inspect and "say" good/bad. A good thread is within the tolerance - a perfect thread is in the middle of all the tolerances. Perfection requires measurement - at least where threads are concerned

I suppose that just about everyone, the first time they get a matching set of thread plug and ring gauges, try and screw the plug gauge into the ring gauge and wonder why they don't "fit".
My standard answer to that is that a plug gauge is a nut - but inside out and that the ring gauge is a screw - inside out. This usually "rings a bell"

The minor diameter on an external thread and the major diameter on an internal thread can easily be measured on components (not on gauges!) - it just requires special thread measurement inserts. They can only be used for the specific pitch they are made for. The good news is that they can measure all thread diameters having the same pitch. A PM will get more information

 

[Wayne]

Measuring minor diameter of internal threads - How To?

… I just assumed … that it was the component minor diameter that was the problem …

Well I believe that you were right in your assumption. The component minor diameter, the thread root diameter, was the problem. And the problem which was not noticed because the GO Ring Gage minor/crest diameter was worn beyond acceptable limits.

The only disadvantage I can think of with gauges is that they don't "measure" anything - they inspect and "say" good/bad. A good thread is within the tolerance - a perfect thread is in the middle of all the tolerances. Perfection requires measurement - at least where threads are concerned.

The root diameter of a thread is not a hard target to hit. Unless there is a reason to fear cutting too deep; who cares how deep it is, or if it has a flat or radius (excepting controlled root radius threads) as long as the root diameter clears the mating part.

The minor diameter on an external thread and the major diameter on an internal thread can easily be measured on components … it just requires special thread measurement inserts. They can only be used for the specific pitch they are made for. The good news is that they can measure all thread diameters having the same pitch.

Yes, these two diameters are the fault of too many thread assembly failures. :mg: It is important then that they be monitored closely during production. The tool you are talking about is a good tool, and one I would recommend for product thread evaluation, as an assist to help diagnose a thread which the GO gage is rejecting. I do think that it may be too labor intensive, and not necessary, to use on the machine with the same frequency as a thread ring gage. If desired to be used on the machine I might suggest a frequency of 10x the frequency of the GO gage.

An alternative method of closely monitoring the root diameter would be to monitor the wear on the GO gage thread crest diameter. Because the minor/crest diameter of a GO ring gage and the major/crest diameter of a GO work plug gage have an increased potential for wear, monitoring them while the gage is in use is advisable. The GO work plug gage major/crest diameter can easily be monitored with a micrometer. The GO ring gage minor/crest diameter is easy to evaluate with GO/NOGO Plain Plug Gages. Both these checks can be scheduled into the job process sheet, and are well within the skill set of your common machine operator.

Ah ... Yes ... The devil is in the details, isn’t it? Buy a nice gage and assume that all will be well. But remember that a gage is a precision tool, and precision tools need to be monitored for function during use to assure that they are still precise.

 

[Gordon Clarke]

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

I'll just try and sum up part of Wayne's explanation as I understand it. He is after all, the expert in gauges and has probably forgotten more about gauges than I could ever hope to know about them I'm not saying I'm completely ignorannt though LOL

A thread plug gauge is of course for an internal thread and cannot register if an internal thread diameter (D) is above tolerance. This situation would normally occur if the thread cutting tool had a too small or no nose radius.
A thread ring gauge is for an external thread and cannot register if the minor diameter (d1 or d3) is undersize. Again due to a too small or no nose radius on the cutting tool.

The biggest problem with an undersize minor diameter (if everything else is within tolerance) on an external thread (bolt) is that shearing could present a serious risk hazard as a radius gives more strength.

Wayne, correct me if I'm wrong, but I believe the only real difference between a MJ thread and a M thread is that MJ threads have a slightly greater radius than M threads.

Probably the best thing to readers about Wayne's and my view points is that we each look at threads through slightly different glasses. He is "The Gageguy" and I'm "the measurement guy". Measurement will never replace gauges, but using both is definately a plus. The biggest advantage (as I see it) with measuring pitch diameter, is that very few companies buy gauges for one off items and tend to take a chance, so a measurement is definately infinately better than even the best guess. It's also best to measure when setting up a machine for large series or mass production. Not running in the middle of a tolerance when possible is stupidity. If pitch diameter is measured then a suitable threaded component (nut or bolt) can be used to establish correct pitch and profile with a small number or one off. Not the perfect solution, but not bad either. Better than taking a chance.

Probably not too many people realise how big thread pitch diameter tolerances are, and the pitch diameter tolerance is almost always the diameter with the tightest tolerance. A standard 1/4"/6mm external thread has usually at least 0.004"/0.1mm tolerance on the pitch diameter and the internal thread (nut) approx.30% more. The larger the diameter and/or pitch the greater the tolerance.

I'm now off on a two week vacation and come Monday, I'll be complaining about the heat while I enjoy a cold beer.

 

[Wayne]

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

A thread plug gauge is of course for an internal thread and cannot register if an internal thread diameter (D) is above tolerance. This situation would normally occur if the thread cutting tool had a too small or no nose radius.
A thread ring gauge is for an external thread and cannot register if the minor diameter (d1 or d3) is undersize. Again due to a too small or no nose radius on the cutting tool.

Nicely worded.

The biggest problem with an undersize minor diameter (if everything else is within tolerance) on an external thread (bolt) is that shearing could present a serious risk hazard as a radius gives more strength.

If the J-Series, or R-Series has not been specified, the engineer should have already determined that shearing would not be a problem. If the risk of shearing is present, the design engineer should have specified the J-Series, or an R-Series.

Wayne, correct me if I'm wrong, but I believe the only real difference between a MJ thread and an M thread is that MJ threads have a slightly greater radius than M threads.

Not wrong, just comment worthy. The J-series has a controlled root radius on the external thread. The non-J-Series thread root, at least as per ANSI B1.1, the UN-series, may have a flat root or a radius root or anything between depending on tool wear.

Measurement will never replace gauges, but using both is definitely a plus.

I fully agree.

The biggest advantage (as I see it) with measuring pitch diameter, is that very few companies buy gauges for one off items and tend to take a chance, so a measurement is definitely infinitely better than even the best guess.

And many times better than using a mating part – shudder -.

Good discussion; and all from one small question. Nice.

 

[Gordon Clarke]

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

I got a huge smile on my face at your compliments Wayne. Thanks . As usual I learn a little each time

I’m writing this to pass some time before leaving home to get the first of three flights to the first destination of my vacation – Chiang Mai in north west Thailand.

OK, now to the serious stuff, and possibly not entirely in keeping with the original theme of minor diameter, but not way off target either

Standard Metric threads are as good as identical to American threads (UNC etc.) in their profile tolerance systems. I’ll write covering Metric threads but more or less the same applies to American threads.

Let’s forget about threads just for a moment. Anyone working with identical diameter axles and holes having h (external) and H (internal) tolerances will know that an they have a common nominal diameter and that an axle at max. (+0 / - something) and a hole on min. size (-0 / + something) will not fit together. Neither are out of tolerance, but they just won’t fit!

Threads are the same, and a standard external Metric thread (e.g. 6g) has an pitch diameter allowance below nominal pitch diameter of about 30% of the pitch diameter tolerance. A standard nut is 6H and thus has a min. diameter equal to nominal pitch diameter. The allowance means that even a screw with the largest permissible max. pitch diameter can be screwed into a nut with the smallest permissable pitch diameter. Here I should add that the normal pitch diameter tolerance for a nut is about 30% more than that for an external thread with the same diameter and pitch. A screw with an h tolerance at max. and a nut with an H tolerance at min. is certain to give problems as the slightest variation in either pitch or flank angle will result in it being impossible to screw the two parts together. This is why it is important to measure when possible to ensure that max/min situations are avoided.

Threads have three diameters. Major, pitch and minor. On normal, standard Metric and American threads the pitch diameter tolerance has the smallest tolerance followed by major diameter and finally minor diameter. Major diameter tolerance is usually about 30% more than that of pitch diameter tolerance and the minor diameter tolerance about twice that of pitch diameter tolerance.

A MJ thread is a Metric thread with inevitably finer tolerances than M and is primarily used for the aerospace industry and other situations where a better than normal fit is essential.

I could go on but risk losing what is left of the readers, so if there are any questions I’ll be happy to answer them in two weeks plus! If it’s urgent I’m sure Wayne can answer too.

I’m also very curious as to what Wayne comments on

 

[Wayne]

Measuring minor diameter of internal threads - How To?

Let’s forget about threads just for a moment. Anyone working with identical diameter axles and holes having h (external) and H (internal) tolerances will know that an they have a common nominal diameter and that an axle at max. (+0 / - something) and a hole on min. size (-0 / + something) will not fit together. Neither is out of tolerance, but they just won’t fit!

Threads are the same, and a standard external Metric thread (e.g. 6g) has a pitch diameter allowance below nominal pitch diameter of about 30% of the pitch diameter tolerance. A standard nut is 6H and thus has a min. diameter equal to nominal pitch diameter. The allowance means that even a screw with the largest permissible maximum pitch diameter can be screwed into a nut with the smallest permissible pitch diameter. Here I should add that the normal pitch diameter tolerance for a nut is about 30% more than that for an external thread with the same diameter and pitch. A screw with an h tolerance at maximum and a nut with an H tolerance at min. is certain to give problems as the slightest variation in either pitch or flank angle will result in it being impossible to screw the two parts together. This is why it is important to measure when possible to ensure that max/min situations are avoided.

Gordon, why do you open such a cans of worms?

My quick comment is that if you are making threads, and you are using properly calibrated and maintained thread gages, and those gages say your product is acceptable, you will not have an assembly problem.

Now for the reader that wants to be put to sleep…

Because the major and minor diameters of the thread have larger tolerances than does the pitch diameter, and because all thread dimensions are basically driven from the pitch diameter, I will limit this discussion to the pitch diameters.

The M-series threads, in what is considered the medium fit, has a class-of-fit combination of 6g for the male and 6H for the female. This combination should always allow for hand assembly because of the thread allowance designed into the 6g male class-of-fit. Similarly the in the UN-series the class-of-fit 2A for the male and 2B for the female should always allow for hand assembly because of the thread allowance designed into the 2A male class-of-fit.

When we are talking about the more precision threads used in precision applications, the UN series class-of-fit 3A male and 3B female is used. In the M-series the class-of-fit 4h male and 4H female is used. Both of these thread combinations have the same maximum material condition pitch diameter for the male and the female thread. Yes it is theoretically possible that a male and female thread may both be made at the maximum material condition, but it is also statistically improbable that those maximum material condition pieces will be presented to each other for assembly.

This is different than the condition you would find in a pure cylindrical assembly. In a threaded assembly there are so many places on a thread where a deviation from the nominal/basic thread design will occur, within the allowable thread tolerances. Know also that all the thread tolerances are specified in a direction away from the basic dimension so that any deviation makes for easier assembly. Considering these deviations, even if a male and female were measured at the same pitch diameter, a slight wrench pressure would accomplish assembly. If this situation was untenable, a precision thread should not have been selected.

I hope that this has re-capped the can of worms.