Product Impact When Thread Rings Require Adjustment



Question: When a thread ring gage is adjusted by an outside calibration lab, with a cert that shows before and after PD measurements, how do I assess whether or not product gaged with this ring was impacted by the "before" setting? This is asked in a 'covered for an audit' context.

I know that direct measurement is not the "correct," or ASME B1.3 accepted, method, and even though actuals are given, the lab lists set plugs as their masters on their ring gage certs. I don't know if the lab uses these measurements or set plug drag to determine that the gages require adjustment. I assume the actuals are found with a CMM.

It'd be nice if they would state a judgement based on drag, as I do when I calibrate thread ring gages in house on truncated set plugs. If I find a ring a bit looser than I desire, I note whether the ring is free spinning, or still has some drag. Or if I find a ring a bit tighter than I desire, I note whether the ring binds on the plug, or is able to thread the full form section. In my opinion this fulfills the intention of calibrating and setting an adjustable ring gage to a master.

Seeing as how direct measurement is not an acceptable calibration method for thread rings, how do I interpret the direct measurement results? I would like a cut-off point, a % of tolerance amount, where a reinspection of stock will be required. But I know that line of thinking has problems, because variation in pitch, lead and flank may make a ring that is just a little loose seem really loose when only PD actuals are examined. Should I use the labs cert stated uncertainty for PD as a cut-off point?

I need a line in the sand. If I don't have this, I feel as though all outside labs ring gage adjustments should be treated as gage failures requiring reinspection and/or customer notification. The other option is to act like these adjustments do not matter, and assume that none of these adjustments were critical (spinning loose, binding tight), which is not an assumption I can make in good faith.

I've scoured online resources and cannot find this topic answered. I don't really understand why not when I know others must sometimes have thread rings adjusted at outside labs.

--Sorry for the lengthy post, I hope someone can help me on this. This topic has been bothering me a long time and is of real importance to me. Thank you for your time.--
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Does the cal cert show an out of tolerance condition? Or are they just adjusting it for fun, say to bring it to nominal in a range?

If it's not out of tolerance then there is not a problem. If it's out of tolerance, possibly there is. That depends on your product line and application.

Rings being looser and tighter than you desire sounds wholly subjective to me. Those things should be pretty stable unless they're leading a weird life.


I can say that the ones I calibrate are very stable. It's extremely rare for me to calibrate a gage and find it any different than I did during its last calibration. I can't much speak to the ones that I send out for calibration, because I don't really know what their process is (I plan to ask very soon). I'm actually considering going to a different lab, because their measurements for non-adjustable gages, even in 6 month intervals with no recorded use between calibrations, commonly vary significantly more than their stated measurement uncertainty...but that's another story.

To answer your question: It seems to me they leave it to me to decide whether the gage is out of calibration, while showing it's out of tolerance. I say that quite facetiously, as I know that out of tolerance is out of calibration. But because thread rings are meant to have their pitch diameter set via adjustment to a set plug, I'm not sure how useful, telling, proper, or misleading these out-of-tolerance actuals are. Pitch diameter actuals tell me nothing about this fit (not that a description would always prove useful). Are they a good representation of this fit? I think I obscured the topic in my initial post, but what I'm looking to discover is what these PD actuals can tell me, given that this is a functional gage meant to be set to a set plug. Or if I should just stock it up to this lab's or any cal lab's way of telling me that the ring felt too tight or loose on the set plug. Is it possible, probably for a thread ring's PD to measure out of tolerance, but be properly set to an in-tolerance set plug? From what I've read and understand, it seems like that possibility does exist. Can anyone chime in? The more I think about it the more I realize two things:

1) I need to have this conversation with the lab
2) I need to purchase set plugs for every ring gage we own and stop sending thread rings to an outside lab for calibration.
Because for one--what happens when the lab gets a new set plug? My ring may not have a good fit on a new plug, but still be in calibration (this is a bit mind boggling, but is fairly common knowledge, has to do with allowable variation or tolerances, and once accepted makes a great argument for mandatory system 22 gaging). Also the thread rings that are calibrated at an outside lab require adjustment shockingly more often than those calibrated in-house, so although I am curious why, I am more motivated to address the issue.

Allow me to attempt to address the "wholly subjective" comment. Anyone please anyone chime in when I get something wrong, I'm here to learn.

I think it's interesting to note that while ASME B1.2 is the main standard in use, none of these standards really go into a calibration pass/fail analysis. They mainly focus on "setting the ring gage." As if the "as found" condition is unimportant. I think this may have something to do with an assumed consistency with these gages, but I also think it could be used to make a good argument for "as found" direct measurement. So is direct measurement "best practice" for calibrating a thread ring gage or will an experienced operator's opinion of the thread fit suffice?

According to all specifications and industry knowledge I am familiar with, a thread ring gage's PD is supposed to be transferred from the set plug. My research leads me to believe that these calibration lab PD measurements may be misleading, because the pitch diameter is meant to be transferred from a functional, multiple thread, fit. This fit is not just the result of the PD, but results from a combination of all of the characteristics of the set plug and the thread ring. Thread ring gages are size limit gages, but also functional gages. They are meant to check maximum and minimum PD, while checking that the combination of lead, flank angle, helix, roundness etcetera results in a functional, within standards, thread. And set plugs are meant to check the functionality of every aspect of the thread ring except minor diameter.

In calibration Go & NoGo thread rings operate the same way, so let's focus on Go rings a moment.

ASME B1.2 (1983) Section 5.1.1 says "Adjustable GO thread rings gages must be set to the applicable W tolerance-setting plugs" [by W tolerance they actually only mean W on everything but the PD, see Table 12 Note (7)]. ASME B1.2 Table 12 Section 1 says the "setting gages and standards" is "W thread-setting plug for GO; X tolerance plain plug for minor diameter for small thread sizes." While the calibration requirements for gages and measuring equipment is listed as "Pitch (also helical offset at split; see
A3.4), flank angles, minor diameter, pitch diameter, taper of pitch cylinder, straightness, roundness, clearance at root (Note 6)." Note 6 says "Pitch diameter is usually transferred from thread-setting gages."

FED-STD-H28/6A (1985) section states "[...] the truncated portion controls the pitch diameter, and the full-form portion assures that proper clearance is provided at the major diameter of the ring gage. Also, the use of the full-form portion in conjunction with the truncated portion checks, to some degree, the flank angle fo the thread gage." And "The size of adjustable limit or indicating thread gages is controlled by utilizing the applicable W tolerance thread setting plugs." This spec says that using a cast mold can be used in conjunction with a full form set plug as an alternative to using a truncated set plug.

MIL-STD-120 (1950, not active) section 8.12 covers "Inspection of Thread Ring Gages" and says "Ring gages should be adjusted to fit the basic form portion of the setting plug and then tried on the truncated portion. There should be only a slight difference in the fit. The presence of shake or play on the truncated portion indicates inadequate clearance at the root of the ring thread." And "The pitch diameter of adjustable thread ring gages is adjusted by fitting the ring to a setting plug of known dimensions." However this spec also goes into creating a cast of the internal threads for inspection, but I've never heard of anyone doing this when using a truncated set plug and this spec is inactive...

But the standard most people write about and work with is for thread ring gages to have some noticble drag when threaded on a truncated set plug, both on the full form and truncated sections, the more even the drag across both, the better. And so long as it has this, it passes. Once you've done this a while it's easy to tell if there's drag or not.

If nothing else is clear, it's clear that the specifications could use some updating and flushing out.


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Thank you for your detailed response. I only know a little bit about this stuff from my distant past, and it's nice to get a primer from somebody more knowledgeable.

It sounds like the reference to drag helps to at least somewhat quantify what I thought would be a wholly subjective call.


Just in case someone else comes upon this post. And I'm still waiting for the people experienced in this issue to chime in...

I discovered a very important clause in the Appendix of ASME B1.2 today.
Yes, this appendix says it's "for information purposes only," and "not part of ASME B1.2," but it would be foolish not to structure requirements around it.

ASME B1.2-1983 Section A3.1.2
“The measured pitch diameter on rings fitted to a setting plug may be 0.001 in to 0.002 in larger than the measured pitch diameter on the plug because the pitch diameter equivalents from permissible pitch, lead, and flank angle tolerances on matched plug and ring cause some unavoidable discrepancy.”

So there's that. That is the limit, straight from the spec (even if they didn't own up to it). The question left hanging is, is this truly only for "larger than," what about smaller than?

Also section A3.1.3
“Measurement of internal pitch diameter using “best size” thread ball is restricted to sizes from 3/16” and larger.”


Quite Involved in Discussions
I am by no means an expert. Your questions and research brought up a few good points and questions of my own to look into.

I too have some of my thread rings that are checked by outside lab. How I typically handle any adjustments (by me or them) is determine the amount of adjustment and what the tolerance of the parts are (our part tolerances are usually more liberal than the gage tolerance: industry -.0005?, part -.0052?). Then advise the QA department they need to look at the past part(s) and do their risk analysis.

Seeing as how direct measurement is not an acceptable calibration method for thread rings, how do I interpret the direct measurement results?

I need a line in the sand.
But because thread rings are meant to have their pitch diameter set via adjustment to a set plug, I'm not sure how useful, telling, proper, or misleading these out-of-tolerance actuals are. Pitch diameter actuals tell me nothing about this fit ?
I can?t give you the line in the sand, but you noted in your original post the cal cert says multiple plug gages, does this mean they are using GO & NOGO? What are the PD?s of these and what direction did they adjust? This, along with the part tolerances, should be a starting point for determination of what you need to look at.

[Are they a good representation of this fit? I think I obscured the topic in my initial post, but what I'm looking to discover is what these PD actuals can tell me, given that this is a functional gage meant to be set to a set plug.
As for quality of fit, not sure if they can tell you anything, but they can be helpful in quantifying to QA manager and/or the rest of management how far off they might be looking.

[ 2) I need to purchase set plugs for every ring gage we own and stop sending thread rings to an outside lab for calibration.
Because for one--what happens when the lab gets a new set plug? My ring may not have a good fit on a new plug, but still be in calibration
This also holds true if you replace the set plug you own and it doesn't fit the same? granted it is more on you for if fit was acceptable or not, to adjust the ring or not, etc.

As you noted, thread ring gages are ?wholly subjective? as to their fit. In calibration we typically look for a ?black/white?, ?GO/NOGO?, ?yes/no? solution, unfortunately, it doesn't work with these gages, as they work in the gray-area. So it is now on you and your judgment, as the calibration technician, as to where these will pass/fail? Yes, you having set masters for all your gages is helpful as you will have already determined your ?consistent feel? for the appropriate drag.

ISO/calibration coordinator


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You are not going to get a line in the sand.

If you are sending your thread rings into a calibration house, then there are several factors that come into play.

1. The setting plug being used by the Cal house- which limit is it at? Are they compensating if their set plug has any back taper? Are they using X Tolerance, W Tolerance, or hi-lo's? Etc.

2. The technician- their opinion and experience plays a large role in thread ring calibration.

3. How the thread ring fits on the setting plug. Too tight or too loose- there is no way to quantify this situation. You can't thread a ring on a set plug and say "gee, I think this is 0.004 too tight". Add in the two above questions, and you can see that there is no "answer" that is going to satisfy.

4. The technicians have no idea what the usage of the thread ring is, and will not remember from time to time what the previous fit was. At my company, we can estimate based on the past certs- especially if a gage was adjusted, but if we replaced a set plug in between calibrations, it makes it more difficult to keep track.

DRGNRIDER has it in one- thread rings are a giant gray area. I always advise my customers that the best course of action is to have the set plugs (which should get measured by an outside source, unless you have the equipment to do so) and set the rings yourself. You can then adapt your own system of fit that you can justify.


Involved In Discussions
I've actually gone one step further, in having the set master calibrated by the outside service and then having them set the threaded ring gage to that specific set master. This isn't my preferred method of measuring threads, but there are rare cases when there is no practical alternative. I haven't had to do this in a couple years now.

If I properly understand the last question, there are both "fixed" thread rings and adjustable thread rings. The disadvantage to a fixed thread ring is that eventually it will wear out. But, as detailed here, an adjustable thread ring is only an advantage if it's set properly.



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All this tweaking back and forth of thread gages seems improbable to me. Granted I no longer work in an environment where such things are required, so my viewpoint is of limited value.

But based on my experience ( back when we first invented threads carved out of stone using other stones ), it seems to me that in order to wear one of these things out or wear it sufficiently to require adjustment you'd have to either have three shifts going 24 / 7 doing 100% inspection, or have a final application of your product that was so sensitive that both the gages and products were delivered by temperature controlled armored car.

Is this one of the exercises in hair-splitting that come with the territory, or is the application of the product produced critically dependent on a threaded function?
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