%Total Variation & %Tolerance - Differences in Gage R&R Forms and Templates

A

Alex Silinciario

#1
Hi everybody, i'm newbie on forums and I have a question I'd like you to support me to answer, i've been looking trough gage R&R forms and templates and some of them differ in %Total Variation and %Tolerance as main factor for calculations, and I don't quite understand the reason. Could you help me understand it??

Thanks in advance...
 
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S

Sturmkind

#3
Hi, Alex!

The two calculations answer two very different questions. The percent tolerance analysis compares the consumption of tolerance by both the appraisers and the equipment.

The percent variation analysis breaks out the percentage of the study used by the appraisers, the equipment, and their potential interaction. Percentage of study variation is frequently used to assess where within the measurement system improvement opportunities exist. This is particularly important when one is interested in the percentage of Process Variation being consumed by the gaging system and is an important element when using SPC control limits.
-Reason: Gage may pass customer specification tolerance consumption but be unable to distinguish one part from another for SPC purposes. As processes have improved the need for adequate measurement discrimination (Number of Distinct Categories) has become increasingly challenging.

I hope this helps you out!
 
A

Alex Silinciario

#4
Hi Sturmkind

Thanks for your answer :agree:, now i have a wider view on this topic, so, if I understood, and based on several sources, we may say utility for percentange tolerance and total variation is:

%Tolerance: product characteristics, e.g. inspections, determine OK and NO OK.
%Total Variation: Process characteristics

Thanks a lot once again...
 

bobdoering

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#5
.... As processes have improved the need for adequate measurement discrimination (Number of Distinct Categories) has become increasingly challenging.
Yes, an that is why when using a gage for SPC, you should calculate the ndc as a function of the control limits - not the process variation. This is because the control limits were likely developed over a larger sample of the process than the GR&R - and are more meaningful. You should have an ndc of 10 or greater, calculating the ndc as a function of the control limits for the gage to be of any value, and to avoid "chunky" data.
 
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