Calibration Due Date - How can I word my procedure?

Atul Khandekar

Quite Involved in Discussions
#11
Re: Frequency of Gage R&R

Originally posted by Yasupawoman

does QS9000 via MSA set a guideline on how often something needs to have Gage R&R studies done?
I don't think there are any definitive guidelines for R&R frequency.

You should do R&R :

1. Whenever there is a significant change in the measurement system characteristcs ... instruments, methods, operators, etc

2. (according to some) after every calibration !!

3. Whenever your customer requires it.

-Atul
 
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Jerry Eldred

Forum Moderator
Super Moderator
#12
I think what you would have to convince an auditor is that you have a defined confidence that the measurement equipment will perform measurements within some acceptable limits of error/confidence (either tolerance limits or statistically based (control charts, X-Bar - R, etc..)) for some definable length of time.

The standard method (in the calibration world) is, based on historical data, to set an appropriate interval based on the equipments historical ability to remain within a range of tolerance limits.

In the semiconductor world it is quite common to have golden units which check upper/lower control limit confidence, etc. The golden unit has to be well characterized, and meet the stability requirements of the MSA, etc.. Then the MSA is performed at some defined interval.

One of the challenges of replacing a defined calibration interval method with an MSA method is that you still need to have a defined confidence of how long the equipment remains within acceptable CPk or whatever method you use.

Whether this is a better method or not will depend on the specifics of your application. There are many circumstances where a calibration at defined interval is simpler.

I do not believe in fixed frequency calibrations (in general), as they tend to overlook variables of use, environment, equipment degradation. Having a "fixed frequency" for calibration will over the long term, be likely to do one of two things. It will either overuse resources (cal interval too short) or inflict product risk due to unit drifting out of tolerance too often (cal interval too long). For critical applications, it is more reliable to either define a confidence level and adjust intervals as needed to the confidence level, or use MSA (a lot of work in places where it is not required).

Using the stability test in MSA would still need to be done at some defined intervals. The defined interval may well end up being the same as the calibration interval you would arrive at through regularly scheduled calibrations, and may not buy you any resource savings. If you could tell me a little more about what test equipment you are referring to, I might (or might not) be able to give a little further comments.
 

Atul Khandekar

Quite Involved in Discussions
#13
Stability Test does involve obtaining measurements at a regular frequency. But the period here is much shorter, something like one reading per day or shift. Then you plot these readings on a chart (a simple XBar-R or X-MR chart) whose control limits are previously determined through a benchmark study. You can catch out-of-control and trend situations and decide whether an instrument is drifting. This way statistical stability over a period of time can be monitored. When you do calibrate an instrument, conduct a benchmark study again to determine new control limits and start over...

What I wanted to say is that the QS certified people must already be doing stability tests as part of QS compliance. It should be possible to carry out condition based calibration and justify it.

The question still remains unanswered for

1. Instruments not covered in Control Plan, (and hence not coming under MSA but still need calibration) and
2. Non-QS companies

??

- Atul

PS: er.. yes, Jerry. I am referring to instuments such as verniers, micrometer, dials, etc, typically in an auto/engg industry. I have no knowledge of calibration requirements in chemical and other indutry. There as you say, periodic calibration may be a simpler option.
 
S

sgiorgi

#14
Just a few comments which you may or may not agree with.

1) As long as your procedures state that all calibrations are valid until the end of the month I cannot see a problem. (Say what you do and do what you say).

2) If you always have 'as found' results on items that require calibration if you calibrate late (it happens) and these 'as found' results are in spec you have no problem. A instrument does not magically go out of spec just beacuse you have passed the date specified on its calibration label.

You may get pulled up because you have not kept to your schedule but if the 'as found' results are in spec you do not have to worry about a potential product recall.
 
A

Al Dyer

#15
How about this train of thought?

Do an initial R&R and use ongoing Bias, Linearity, and Stability as the drivers for determining the frequency of R&R studies.

Just my opinion, but bias, linearity, and stability give me better confidence in a gage than R&R.
 
E

energy

#16
The Calibration due date

The train has left the station:bigwave:
Does R&R using ongoing bias, linearity and stability and better confidence in a gage than R&R determine the end of the month calibration date on my caibration? Just asking
:smokin:
 

Atul Khandekar

Quite Involved in Discussions
#17
Frequencies

Bias test is done at a fixed 'master' value, linearity is bias study over the range of the instrument. In essence these are 'static' tests, that help you determine 'limitations for use', if any.

Stability test confirms statistical stability over a period of time. I would trust stability test to determine if instrument is drifting and hence due for calibration.

R&R is a different ball game. The idea is to determine errors produced due to factors like instrument, measurement methods, operators, environment to decide if the instrument is indeed good enough to control the process it measures. As such, any time any of these factors change, you should do an R&R test, like if you change the fixtures, introduce a new instrument / operator, repair/calibrate existing one, use the instrument to measure some other part,..... Also you can find out if a certain operator needs to be trained.

Hope the train hasn't left the tracks! ;)
- Atul.
 
E

energy

#19
Re: CAL DUE DATE

Originally posted by SCOTT SNYDER
THIS IS A 2 PART QUESTION.
1) I am have a dissagrement with my boss over my cal due date. My computer software gives a due date as mm/dd/yy, however I would like to state in the procedure that the gauge is not past due until the last day of the month. Everyone outside of my plant says this is how they do it. He insists we would not pass our qs audit with the procedure written this way. He claims that a gauge due on the 1st of the month would still be overdue by 30 days?
This is what my question was about, boys. Let's not miss the forest because of the trees:smokin:
However, I defer to the previous posts regarding this issue to answer my question. There are some good ones. So in keeping in line with question 2, would the following apply to my calibration frequency for a 1" Micrometer with a +/-.002 tolerance on my bar stock parts?
Process control for positional specifications can be separated into the X,Y,&Z components of the measurement. It can be very beneficial to the process owner to identify the source of variation. Especially if the ratio of tolerance consumed to tolerance specified is high. Monitoring the resultant diameter of position deviation adds little value to the process owner when one process delivers a position deviation that is scattered in all four quadrants and another with an equivalent magnitude of deviation emanates from a single point in one quadrant. If the ratio of tolerance consumed to tolerance available is low it may not be worth the effort to monitor the coordinates separately.
To measure process capability of geometric specifications with variable tolerances (MMC-bonus) one can monitor the residual or remaining unused value of tolerance rather than the amount consumed. For each measured feature just add the bonus to the tolerance specified and subtract the amount of positional tolerance consumed. Rather than comparing the mean and standard deviation to a "variable USL" the mean and standard dev of the residual is compared to zero. Cpk of the residual is derived from the Cpl. The variation in feature size when merged with that of position changes the distribution. The standard deviation typically increases but so does the separation of the mean of the residual from zero (consistent with the magnitude of the bonus). It is important that feature size is "in-control" as well as the coordinates of feature position when predicting capability from the residual.
One enormous benefit of doing process capability from the residual is the realization the dynamic relationship between size and position. One should not necessarily target feature size at its specification midpoint! A positional tolerance that is specified as "Zero @ MMC" where the amount of tolerance is fully dependent upon size probably demonstrates that relationship the best:biglaugh:
:smokin:
 
Last edited by a moderator:
K

Ken K

#20
Marc, I remember that thread well. I was the one that started it, but I can't find it either.

I do remember I wanted to put a +/- 2 days tolerance on the calibration due date to compensate for weekends. The majority of the replies gave me grief for that, but it seems times sure have changed after reading the current replies.

My only advice is if the calibration is due, do it. There shouldn't be any reason not to.
 
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