Thread Plug Gage is "growing" - why?

[Eric M]

I just received some gages back from our offsite calibration provider. One particular thread plug gage was reported as: "'Go' Major Diameter is out-of tolerance, see data for values." When I looked at the report, it showed the Major Diameter as being .0012" oversized. Last year's calibration report (from the same facility) showed this same feature on this same gage measuring .0015" SMALLER. My question is this: Since a thread plug gage (in my experience) gets smaller as it wears, what would (realistically) make it get BIGGER? Just wondering...

Additional info - we have used this calibration house for several years, and I've never doubted their findings.

 

[TDegenhart]

I would ask the manufacturer of the thread gage and ask them to duplicate your findings.

Tom

 

[Krutch]

No dings in the threads? Did someone use it for other than gage use? Was it certified at the same temp as last cert?
Do as Tom suggests.

 

[Milland]

I just received some gages back from our offsite calibration provider. One particular thread plug gage was reported as: "'Go' Major Diameter is out-of tolerance, see data for values." When I looked at the report, it showed the Major Diameter as being .0012" oversized. Last year's calibration report (from the same facility) showed this same feature on this same gage measuring .0015" SMALLER. My question is this: Since a thread plug gage (in my experience) gets smaller as it wears, what would (realistically) make it get BIGGER? Just wondering...

Additional info - we have used this calibration house for several years, and I've never doubted their findings.

An article I read previously in "Quality Magazine" specifically mentioned issues with QA of thread gages. Here's one reference, they likely have more: Choosing and Using the Right Thread Gage | 2013-03-01 | Quality Magazine

Improperly selected or handled steels can change shape/size over time, not critical for a most items but very much so for any metrology parts. That said, a swing of .0027 over two calibration cycles on a reasonable sized thread gage (under 3" OD) is a lot. There may be trace materials that transfered to the gage that haven't been cleaned off properly, maybe the person doing the work had a bad day, etc. I'd go back to that lab with the two test reports and politely ask for a free re-do, and a full explanation of process and control. It wouldn't be a bad idea to get another credible lab to do an independent check.

 

[Eric M]

Thanks for all the feedback. Here's what I know so far:

Kutch - there were no signs of damage to the gage before it left. Whether or not this was used for something other than gaging (such as thread chasing :0 ), I'll never know. As for conditions during calibration; according to the certs, both times were @ 46% RH, 70 deg. F this time; 68 deg. F last time.
Milland - the gage was made by Hemco, so I'm fairly confident the steel itself is correct. And I agree with you about .0027 swing in 1 year. This particular gage is not used daily. I've already contacted our sales rep at the lab, and am waiting a response. I'll also be digging in to the article you linked - thanks!
Bill - this particular gage is the only one of it's size we have. Our ID number is engraved on it, so I'm confident it is what it claims to be! I know it hasn't been replaced; it would have been me doing it.

Thanks AGAIN for all the help / suggestions.

 

[andrewmawson]

When was the calibration house last calibrated !

Andrew

 

[Gordon B. Clarke]

Two "explanations. There might be a slight "out of roundness" and the calibration isn't done on exactly the same thread spot each time.

A good certificate should have a specified "Measurement uncertainty" as there is no such thing as a truly accurate measurement. Does the difference between the two measurements exceed the measurement uncertainty?

The thing that does surprise me is that "your" calibration facility doesn't seem to have picked up on the "growing" difference as I would have expected them to offer an explanation. Personally I'd phone them and ask what they think could be the reason. it's a perfectly legitimate question and you are after all the customer.

Gordon

 

[extropic]

What? When you can go into a hospital and they replace the wrong knee, you shouldn't be at all surprised that Buba got a job at the cal. lab.

You're doing the right thing by bringing it to their attention and requesting an explanation.

Did you check it w/wires yourself?

 

[Red James]

Guys, the report cited the MAJOR diameter of a plug gage. Just mic it with one of the micrometers you just had certified and compare results.

 

[Gordon B. Clarke]

Guys, the report cited the MAJOR diameter of a plug gage. Just mic it with one of the micrometers you just had certified and compare results.

Red, some of us did note that and it's why I wrote as I did in post #8 without mentioning pitch diameter. I will admit that 0.0015" is quite a bit for a gauge
regardless of the thread diameter, and that bit of info was left out.

if the OP really meant OD and not PD then as you write, it can easilly be checked with a micrometer. OTOH nobody really knows what it really measured at the last calibration so all that can be achieved with a micrometer now is to see if the most recent calibration certificate is reasonably accurate or not and I'm assuming their micrometers are calibrated regularly too.

Gordon

 

[The real Leigh]

showed the Major Diameter as being .0012" oversized.
showed this same feature on this same gage measuring .0015" SMALLER.

I'm wondering if this is just a grammatical error, given how close the two magnitudes are.

For example "A is bigger than B" and "B is smaller than A" mean the same thing.
Perhaps the references were somehow reversed.

- Leigh

 

[drgnrider]

"2" and "5" are below/above each other on a 10-digit keypad, could be a simple "wrong keystroke". I had one a few months ago where the CAL lab reversed the last two digits of my "administrative number" on my thread wires. Simple fix, contacted the lab and they sent a revise cert.

Eric, in your case, as others have said, contact the lab with your concerns and evidence. They might have a reasonable answer and be willing to put it in writing, they might even offer a re-check. Their credibility (to you and other customers) and certifications, (A2LA, ISO, etc.), ride on these issues.

As an ISO-Auditor, if I were to find this in any lab I would be asking why and expect them to be able to provide a documented answer. A documented answer might even involve customer contact. In this instance, it is also in their best interest to have some sort of an answer as well, it's better to have answers before the auditor asks.

 

[PeteM]

Was Viagra involved? I thought it was only good for wood.

As others suggest, it's most likely human error. What would bother me is that the cert. shop didn't bother to compare it's own readings.

 

[Eric M]

Follow-up: The thread gage in question is going back for re-verification. In my communications with the lab, I included copies of their own certs, so they didn't question me too much about it. I'll post their findings when I receive them.
PeteM - as for pharmaceuticals involved: I didn't wanna go there!

 

[Eric M]

Following-up my follow-up: The gage went back to the calibration house, and they confirmed their most recent calibration. According to the person I talked to, they used 2 different P&W Super-mikes, and 2 different technicians, with comparable results.
While doing a little research on a totally unrelated topic, I stumbled across this: Why Gages Grow

You learn something new every day....