Do all gages need calibration or only those used for final inspection?

T

trainerbob

#21
I guess that subconsciously I wanted to make sure "internal" was there and so my hands must of done this of their own volition. Thanks for noticing. As for "The Voices", I think there are times when we all may have a touch of that. Maybe it has something to do with this business we are in.
 
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M

M Komarmy - 2012

#22
Another equation I like to consider is the one attached, total variation. When people measure an item, and expect it to represent the process, the forget that the result they get is not the measure of the process variation, but of the total variation. It includes the additive effects of all errors. So, if you have no idea what those other errors are, you really have no idea if the data you have comes anywhere near reflecting the process. I am not sure how that "shortcut" in analysis would make anyone comfortable with results of uncalibrated gages, as well as the other errors involved.
Of course you are spot on with this. All any gauge can give you is the total variation, it is up to you to determine the individual components of that variation. That is the primary purpose of gauge R&R, it will tell you whether the variation due to the gauge system (including operator and part influences) is overshadowing the variation of the process.
 

bobdoering

Stop X-bar/R Madness!!
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#23
We must also separate calibration from verification. In many cases, verification may be that is all required.
The definition of calibration is pretty straight forward....but I suppose we can conjure up many definitions of "verification." For devices that have been shown to have correct linearity, but have the option to either have the zero set, or set to a offset - then "verification" to me is ensuring that the zero is still correct, or the setting to the offset is still correct. It is handy to do each shift - just in case the person before you "didn't get it quite right." But, IMHO, I do not look a verification as an alternative to calibration.
 

Jim Wynne

Staff member
Admin
#24
The definition of calibration is pretty straight forward....but I suppose we can conjure up many definitions of "verification." For devices that have been shown to have correct linearity, but have the option to either have the zero set, or set to a offset - then "verification" to me is ensuring that the zero is still correct, or the setting to the offset is still correct. It is handy to do each shift - just in case the person before you "didn't get it quite right." But, IMHO, I do not look a verification as an alternative to calibration.
If a gage is compared to a standard, calibration has taken place. There are different forms or levels of calibration, but what's commonly referred to as "verification" isn't different from "calibration."
 

Sidney Vianna

Post Responsibly
Staff member
Admin
#25
I am still fairly new to the Cove and find this web-site so helpful and if I'm not in the correct thread please tell me.
Welcome to The Cove.
We are looking at AS9100 certification.
What makes me think you are in the supply chain of the aviation, space & defense sector. That is why I got really concerned when you stated
During one customer audit the auditor told us that we need to only calibrate those gages needed for final inspection of our final product.
It is mind boggling that, in this day and age, a 2[sup]nd[/sup] party auditor, (supposedly) quality professional would say something like this. So, while we have to deal with incompetence in the CB auditor community, incompetence of customer reps is also an issue.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#26
If a gage is compared to a standard, calibration has taken place. There are different forms or levels of calibration, but what's commonly referred to as "verification" isn't different from "calibration."
I suppose I can agree with that if zero or the setpoint is the range of measurement the gage will be used for...
 

Jim Wynne

Staff member
Admin
#27
I suppose I can agree with that if zero or the setpoint is the range of measurement the gage will be used for...
It depends on the measuring instrument. For example, XRF devices used for measuring plating thickness and metal assaying normally undergo a periodic "full-blown" calibration, but they're also calibrated before each use, using a special "master" standard. The same is true for other similar types of instruments, such as spectrophotometers.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#28
It depends on the measuring instrument. For example, XRF devices used for measuring plating thickness and metal assaying normally undergo a periodic "full-blown" calibration, but they're also calibrated before each use, using a special "master" standard. The same is true for other similar types of instruments, such as spectrophotometers.
Yes, people do like to casually refer to those as calibrations. But, still, IMHO they are verifications - even (or especially) if an adjustment is made - just like a zero or preset setting on a micrometer. And, when that occurs over time and not tracked, any evidence of drift that is occurring - and it does, especially with electronics and light-based detection - it is lost (unless the machine will track it.)

So, your point is well taken There is not even agreement on definition of the term calibration. I suppose because of that I will have to coin a new term - "jack calibration" (sometimes referred to as "full-blown calibration.") If you are not verifying the full range of measurement, then you are not doing "jack". :tg: I sense an opportunity to use that term here often.
 
M

M Komarmy - 2012

#29
I am enjoying all the great comments about this topic. If I may I will add my 2 cents worth...

There are many types of gauges. Some are mechanical and not adjustable for gain, such as dial indicators, calipers, micrometers but may be adjustable for bias.

Others use transducers such as LVDT's, strain gauges, etc. and their signals are processed with computers or other methods (A/D converters and amplifiers) and give digital outputs. These are usually adjustable for both bias and gain.

For the first type, you can use artifacts such as certified master parts, gauge blocks, gauge rings, etc. to verify that they are measuring to the correct value and that they haven't changed. To me, that is verification. Depending on the robustness of the results, this verification may need to happen frequently (once per shift) or it could be once per day or week. The artifacts or masters used for verification need to be certified by a traceable lab on a scheduled frequency.

For electronic gauges, the electronics and mechanical settings need to be verified. Probes need to be mechanically set to the proper travel, as their linearity can be valid only over a relatively small range. This is best done with artifacts or master parts that cover 120% or more of the range of parts that will be measured. It is necessary for the gauge to be linear over more than the specification, because it is important to know the actual values of the parts that lie outside the specifications. I have used max, mean, and min masters for such gauges where the max and min exceed the spec limits by 20%. On the floor, this is typically called "mastering" the gauge, and unless probes are changed it usually only involves verifying that the zero(bias) and gain are correct. Many of these types of gauges will correct bias and gain errors automatically because the certified master values are entered into their databases. They can even recognize when the masters are loaded into the gauge in the incorrect order. If a change is made to correct gain and/or bias, I consider this calibration.

For CMM's, we verify them by measuring ball bars or ring gauges, and probes can be calibrated by measuring the artifacts and comparing the results to the certified values. The entire CMM is calibrated yearly by lazer alignment and compensating for wear of the mechanical elements. There are also standards available for doing this, such as ISO 10360-1 to 6. CMM probes are required to be calibrated because every time they are changed their position relative to the reference axis is slightly different.
 
P

Pro CNC

#30
We are AS9100 certified and we place much more emphasis on in-process inspection than we do final inspection. After all, you want to catch non-conformance at the source, not at the end. So we have not allowed machinists to have their own equipment. Everything is company owned and calibrated on a schedule. We came up with our own web-based quality software which tracks the history, results, calibration process, and live alerts for when it is due in the future. It helps immensely.

I am certain and auditor wouldn't go for only final inspection equipment being calibrated.

my 2 cents.

Paul
procnc.com
 
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