Basic Gage R&R Sample Questions

M

mallen92705

#1
This might be a unsmart question but here we go...

Regarding performing a Measurement System Analysis Study (Gage R&R) for a PPAP... Does the sample used to be measured by at least two appraisers need to be the sample part in which the PPAP is being performed?

In other words, if I am performing a PPAP on a small widget, do I have to perform the Gage R&R study on the same widget? Or can it be another similar item that may contain more variation?

I am fairly new to performing PPAP's and have noticed on the few examples that I have reviewed that it is always a dimension from the item that the PPAP is being performed on... Is this a rule?

Any help would be greatly appreciated!

THANKS!

Mike Allen
 
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Q

Qclisa

#2
Re: Gage R&R Sample Question

You are actually performing a GAGE R&R so you want operators to measure same widget to see variation in your GAGE not part.
 
M

mallen92705

#3
Re: Gage R&R Sample Question

Thanks!

Does that mean that widget needs to be the same widget that the PPAP is being supplied for?

Thanks Again!
 

bobdoering

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#4
Re: Gage R&R Sample Question

Does that mean that widget needs to be the same widget that the PPAP is being supplied for?
Yes. You want to detect any effect the part has on the measurement (cantilevered load, etc.) Also, if the part process has less variation, you want to make sure you have resolution within that variation to detect changes in the process. Using data from a process with more variation will render the PV value invalid. You must also have the operators measure the same spot on the part, because variation within the part is not the issue with the gage.
 

Miner

Forum Moderator
Staff member
Admin
#5
Re: Gage R&R Sample Question

Please read my blog entries on MSA, particularly for parts 5a and 5b.

The answer depends on a number of factors. First, is the gauge to be used solely for inspection, for SPC, or for both.

The easier answer first. If the gauge will only be used for inspection, part variation is not important, but similarity to the actual product is important. In this situation, you may use the actual PPAP part, a non-PPAP part, or a surrogate part provided the features being measured are essentially the same.

If the gauge is to be used for SPC, the parts selected must represent the actual process variation. You have three options here. Presuming the PPAP samples were taken from a significant production run, select your parts from that run. You may select representative surrogate parts as stated above. Again, they must represent actual process variation. Your last option if you cannot find parts that represent actual process variation is to manually calculate the %Study Variation using historical process variation data.
 

Miner

Forum Moderator
Staff member
Admin
#6
Re: Gage R&R Sample Question

You must also have the operators measure the same spot on the part, because variation within the part is not the issue with the gage.
Bob,
Maybe not with the gauge, but with the measurement system, which does include the measurement procedure. Only restrict the measurement location IF you have a documented measurement procedure that specifies the measurement location. If you do not have such a procedure, operators will measure it as they think best, and it will show up as measurement variation.

Think of it in terms of your total variance equation. Within part variation can show up in both repeatability and reproducibility. If not controlled in some fashion through a measurement fixture or a procedure, it is legitimate measurement variation.
 
Last edited:
M

mallen92705

#7
Re: Gage R&R Sample Question

THANK YOU!

It now makes sense why you would want to measure the actual widget that you are providing the PPAP for!
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#8
Re: Gage R&R Sample Question

Bob,
Maybe not with the gauge, but with the measurement system, which does include the measurement procedure. Only restrict the measurement location IF you have a documented measurement procedure that specifies the measurement location.
I disagree that this is an appropriate way to determine if the gage repeatability and reproducibility is adequate for the measurement. If you include that variation in the GR&R study, you will mask the gage's ability to detect the variation within the part. Ignoring the within-part variation in you measurement technique by allowing the operator to measure anywhere is not gage error - that is measurement error. You will never be able to sort out gage error and measurement error if you mix them together. Better yet - fix the problem and document the measurement technique correctly.

Think of it in terms of your total variance equation. Within part variation can show up in both repeatability and reproducibility. If not controlled in some fashion through a measurement fixture or a procedure, it is legitimate measurement variation.
In the total variance equation I have gage error and measurement error as two separate factors specifically to deal with that error. True, you want within-part variation - but only as the gage is impacted by the specific location of the part - again, errors such as cantilevered loads, running up against corner radii, etc. Part roundness (for example) that you pick up by allowing measurement anywhere on the part is not meaningful for this - but it is great to pick up on a properly prepared process capability study.
 

Miner

Forum Moderator
Staff member
Admin
#9
Re: Gage R&R Sample Question

I am not disagreeing that you should understand the difference, but never forget it is called MEASUREMENT systems analysis, not gauge analysis.

You should purposely separate each of the components of the measurement system to fully understand them. The part that I strongly disagree about is limiting the effect of the within part variation in the situation where it is not ALSO controlled during normal measurement. If this is not made clear, the person will control variation for the purpose of the study, get an acceptable %SV, then promptly allow the operators to measure however they will. This is only kidding yourself.
 

Statistical Steven

Statistician
Staff member
Super Moderator
#10
Re: Gage R&R Sample Question

I disagree with that the parts have to be the actual parts that are being measured with the gage. You might want to have various different parts, with different sizes and dimensions to evaluate the range of the part and any Part x Operator interaction. Maybe I am naive, but that is how I see it :)
 
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