Roughness standards for roughness comparators calibration interval?

Quality Josh · May 21, 2015

Just wondering if there is any suggested calibration interval for roughness standards that are used to calibrate and set roughness comparators? Seeing as we have one for set-up and check that is used once every month or so and one for calibration that is used twice a year is there really any need to send them back for calibration on a once a year interval? It just seems excessive...

Gordon B. Clarke · May 21, 2015

Quality Josh said:
Just wondering if there is any suggested calibration interval for roughness standards that are used to calibrate and set roughness comparators? Seeing as we have one for set-up and check that is used once every month or so and one for calibration that is used twice a year is there really any need to send them back for calibration on a once a year interval? It just seems excessive...

Who or what says they have to be calibrated? Did the one you have even come with a calibration certificate?

You might just be making a mountain out of a molehill.

Even if you got it calibrated do you know what the limits are on the steps or otherwise you're just documenting what it is "right now".

Clive603 · May 21, 2015

Any proper quality system will require such standards to be calibrated if they are used to generate workpiece specifications.

I'd guess that mostly they aren't used in that manner being just an in process check with proper sampling and statistical process control applied by testing a suitable proportion of parts with a properly calibrated instrument such as a Talysurf used to prove parts are in spec.

That said Rubert claim the capability UKAS Calibration Laboratory - Rubert & Co Ltd to calibrate so presumably its possible to use them as a local reference standard once such calibration has been done. Which presumably is what the OP wants to do.

That said "my" laser Talysurf was somewhat underwhelmed when I ran it over one of the common finger test specimens around 20 years back.

Clive

Quality Josh · May 21, 2015

Yeah we are definitely not using the comparator in the real stastical sense, just to spot check the roughness against the drawing if it is in question or if it is a really smooth surface (for our shop) like 16 microns or less. The tracable one that im using for our in house calibration once a year did come with a calibration slip. Its a mitutoyo branded plate if it matters. I was thinking that to send the tracable one at the same interval as our gauge blocks (every 3 years) and just mark on the daily use one that it is just for referance, could you see a auditor having a problem with that as long as it was built into the calibration procedure for the comparator ?

Quality Josh · May 21, 2015

Gordon B. Clarke said:
Who or what says they have to be calibrated? Did the one you have even come with a calibration certificate?

You might just be making a mountain out of a molehill.

Even if you got it calibrated do you know what the limits are on the steps or otherwise you're just documenting what it is "right now".

I think its just to ensure that the marked roughness on the plate doesnt deviate from the actual roughness due to misuse or wear or damage etc. I do feel though that once a year is excessive. Unforunately on the mitutoyo calibration certificate it doesnt have a date of re-cal just a date of issue.

The real Leigh · May 22, 2015

Just a general comment...

The QA SOPs that I've seen restrict the gages that can be present in the QA facility to:
1) those that have current calibration stickers, or
2) those that are marked REFERENCE, INACTIVE, or similar wording.

Items that don't conform to one or the other cannot be in the room at all.
The presence of even one such item is grounds for failing a QA audit.

That in itself is sufficient reason for having the roughness standards certified.
It obviously doesn't define the calibration interval, which could be quite long.

- Leigh

Gordon B. Clarke · May 22, 2015

Quality Josh said:
I think its just to ensure that the marked roughness on the plate doesnt deviate from the actual roughness due to misuse or wear or damage etc. I do feel though that once a year is excessive. Unforunately on the mitutoyo calibration certificate it doesnt have a date of re-cal just a date of issue.

I'll start with a partial quote from Leigh´s post that I agree with.

"That in itself is sufficient reason for having the roughness standards certified.
It obviously doesn't define the calibration interval, which could be quite long."

Once a year doesn't just sound obsessive, to me just plain ridiculous given how often you use it. I'd go with once every 5th year unless you suspect that something has happened to it. There's no law, as far as I know, that states that you can't calibrate more often than stated.

As to the Mitutoyo "calibration certificate" then I assume you are not referring to a small slip of paper included in the box saying it conforms to "standards". Many measuring equipment manufacturers do the same and it is NOT a bone fide acceptable certificate.

If you want wear both a belt and braces then you could buy one more and use that (after certification) as your own calibration master. Might be cheaper in the long run.

Quality Josh · May 22, 2015

I am definitely talking about a real calibration slip, not just the little piece of paper you get with most measuring tools. You guys both answered my question thou with the duration suggested by Leigh and the once a year comment with Gordon. This is exactly what i though, that once a year was excessive. Whoever set up this calibration system before was out to lunch. Thanks again for your help guys.

The real Leigh · May 22, 2015

Any gage manufacturer should provide recommended calibration intervals for all their products.

There may be several suggestions depending on application and frequency of usage.

I suggest sending them an email, or calling them.

- Leigh

CarbideBob · May 22, 2015

Don't know the numbers that you are working at but be aware that you will most likely kill the the probe radius before your calib. standard goes bad.
Bob

Gordon B. Clarke · May 22, 2015

The real Leigh said:
Any gage manufacturer should provide recommended calibration intervals for all their products.

There may be several suggestions depending on application and frequency of usage.

I suggest sending them an email, or calling them.

- Leigh

I agree with you in theory but how is a manufacturer going to know the conditions their equipment will kept and used in? If they did as you suggest they'd either suggest calibration intervals for "the worst case scenario" environment or "laboratory and cotton gloves scenario" and we haven't even touched on frequency of use.

One thing that has always surprised me is that companies that have "fixed" intervals don't seem to evaluate calibration results to determine if calibration intervals should be shortened or extended.

The "It's always been done like that" utterance gives me nightmares.

Gordon B. Clarke · May 22, 2015

CarbideBob said:
Don't know the numbers that you are working at but be aware that you will most likely kill the the probe radius before your calib. standard goes bad.
Bob

Good point :bowdown:

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