Calibration Determination decision

Jen Kirley

Quality and Auditing Expert
The OEM doesn’t have anymore idea on how frequently a device is used - they often recommend 12 months! You have to experiment and get data. A screw plug gauge used to check threads in aluminium will wear less than one used on stainless steel. Yes, it’s work, and you may not want to do what is necessary to generate the data on which to base a decision. However, this talk of time based recalls is only OK when you want to throw money at calibration.
Not only does the OEM not know how often the instrument is used, the OEM does not know the environment in which the instrument is stored and used. If we are to question their recommended period (for example, one year) we should be ready to say why we deviate from it.

To be clear, I am completely aligned with the suggestion that usage frequency should be a factor in determining calibration frequency. However, a gage that sits on a shelf with zero usage can become unreliable as is the gage that is used every day. So the sum of considerations should be applied to decide the calibration frequency.

Ron Rompen

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Funboi, i agree with your idea of calibration based on usage - but (for most of us) there are just too many gauges out there to track usage on. Easier (in some ways) to just schedule calibration based on time intervals. This is (or can be) based on associated costs/down time for instruments that have to be sent out for a 3rd party to calibrate.


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In the real world you speak of, at least in my real world, we calibrate or verify it in the first place to ensure we are starting off with a good tool and to comply with customer/standard demands.

There are a few items that are cheaper to replace than calibrate, so we consider the risk: how likely is it that one of the pins goes out of tolerance and if it does what is the likely impact? If the risk is low, we replace vs recalibrate.

So again my question is "Why even bother calibrating in the first place if it doesn't matter if the tool goes out of tolerance?"

John Predmore

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i agree with your idea of calibration based on usage - but (for most of us) there are just too many gauges out there to track usage on

I worked in one machine shop that kept less-frequently-used gages in a central gage crib. When I checked out one ring gage to measure one part, the attendant had me sign my name, department and phone #. I explained that I was going to use the gage right there, I didn't need to take the gage back to my workstation, I would return it as soon as I was finished. He still wanted my entry on the log. He said the usage record was entered into the computer, and cumulative days used was how they scheduled re-calibration, instead of a fixed number of months.

I was fascinated by this alternative, and I asked questions. It didn't matter whether I used the gage for 5 minutes or 5 hours, it was recorded as one day of use in the tracking log, they only tracked # days, not fractions of a day, for scheduling purposes. It didn't matter if I gaged 1 part in 1 day or 500, they had decided to use # days as their basis for this gage. Not every gage was treated on this basis, only certain less-frequently used gages. He showed me the computer record for that particular ring gage. It had been used 4 times in 6 years, so I could see why it was an economic decision not to recalibrate every 12 months. I don't remember any longer how many cumulative days use triggered a re-cal. He said they would re-calibrate it after 10 years regardless, just for good measure (I think that was a pun!). While this calibration protocol was different from anything I had previously encountered, the protocol was documented, the rationale was based on gage history and sound science, and presumably the protocol worked for them. The point of the story is there are valid alternatives to fixed timeframes as a basis.


On Holiday
It shouldn’t be too difficult to determine uses based on knowledge of the number of parts being made and things like the process control methods used. If low quantities, operators commonly gauge 100% or with higher volumes, to a control plan (spc sample or something less sophisticated) and that should inform the number of times its used. Not much of a task.

John Predmore

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I made this graphic today, and decided to share by posting it here. I found myself re-reading and parsing section 7.1.5 to make sense of what the quality standard says about calibration, trying to explain to others. My company operates in alignment with AS9100, so the grey text shows what is unique to AS9100. Fun with Venn Diagrams is tongue-in-check homage to Sheldon Cooper and his Fun with Flags.



  • Venn Diagram - AS9100 7-1-5.pptx.pdf
    547.8 KB · Views: 137


Starting to get Involved
I was fortunate enough that the previous company I worked for purchased a ROBOCRIB. The automated tracking made it easy to identify how often a gage is used, so adjustments to frequency based on cycles/usage were easily managed. Having first hand knowledge of the manufacturing environment and the conditions most gages are exposed to most often seemed paramount for determining the initial frequency, which may be more than the typical 1 year, Third party calibration certification coverage.

Purchasing gages with longform certs by an accredited Lab was the first step so no initial calibration was required - just get them in the registry and apply the standardized frequency per current practice.

The most reliable means for determining frequency was based in understanding how dirty the environment was and how often adjustments had to be made within gage families for specific work groups. For example, micrometers needed frequent maintenance when they were used constantly for inspecting Rare Earth Materials in the Grinding Cells. Frequent loading and unloading of parts on machines meant very frequent in-process use with a lot of handling in dirty conditions. In these cases, Micrometers were calibrated every 6 months with frequent functional checks on the shop floor. In the CNC cells handling alloys, annual calibration of Micrometers and Calipers was suitable, even with constant use. Same with Calipers and test indicators etc..
Another thing that helped was standardizing calibrations more easily by eliminating employee-owned gages, and issuing the same models across the board to individuals for Assembly, Manufacturing and Inspection. Not only for a more apples to apples inspection comparison, but more consistency in calibration frequencies and very predictable calibrations with only rare, minor out of tolerance conditions popping up.

This thread can go on and on and I love the read. I'll stop here.


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"The OEM doesn’t have anymore idea on how frequently a device is used - they often recommend 12 months!"

Yesterday I had an external 9001:2015 auditor inform me that we should be calibrating our tools (case in point a digital caliper) in accordance with manufacturers recommendations.

Serveral years ago while I was with this organization as a consultant for a brief time, we made the decision to look at our list of several thousand tools and decide whether calibrating the majority of them every year was necessary. We reviewed years of data regarding the frequency of out-of-calibration events (extremely rare) and the potential impact of using an out-of-calibration tool to inspect product that would go to a customer (surprisingly low risk due to multiple levels of verification along the way) and made the decision to take a lot of our tools from one year to two year intervals resulting in a savings of approximately 10K USD yearly.

I recall finding somebody here on the Cove who had done some math regarding this issue (way over my head, I can cipher and count on my fingers) and I contacted them and shared them with my boss.

Anyway, I don't expect the auditor to make a big deal of it, but now that I'm an RFT employee of said organization I may as well begin stockpiling evidence that would support this policy, since it's now my task to defend it.
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