Tolerance vs Total Variation in Gage R&R report - MSA 3rd edition pg. 116

[Geese]

In the GRR report, TV (total variation) is calculated using the calculated GRR and PV. However, in the analysis section of the MSA 3rd edition pg. 116, it states that the TV may also be calculated using process variation / 6.00 if the process variation is known.

The reason I'm presenting this is that we have 2 separate report forms that calculate TV using these 2 different methods, only they call TV Tol in the 2nd report. And in the 2nd report, Tol is calculated using upperlimit - lowerlimit / 6.00 instead of process variation / 6.00. Is process variation = upperlimit - lowerlimit?

To verify the authenticity of this method or our report rather, I need to know what the upperlimit and lowerlimit numbers refer to. Do they refer to the tolerance of the actual part being measured, or are they a calculated number. Our report thinks they are the actual tolerances of the part and puts them in as such, but I want to make sure that is what they actually are, but process variation does not sound like it would be the actual tolerance on a part.

[Marc]

Can anyone shed any light on this?

[Atul Khandekar]

The "standard" way of representing %GRR is (GRR/TV) where TV is the Total Variation (stDev) observed during the study. PV in the GRR form is the observed Part Variation. These numbers depend on the way you select parts for the study - whether they truly represent the process or tolerance spread etc.

The two other methos are:
1. GRR as % of Tol: Where Tol= Upperlimit-LowerLimt. You can substitute (Tol/6) in place of TV above to get GRR as % of Tol.
2. GRR as %of Process Variation: This process variation (sigma) comes from your routine SPC studies on the characteristic under study.

Hope this helps.

[canhchi]

Hello, nice to join this forum.

%GRR can be calculated by (GRR/TV) or (GRR/(Tol/6). But I wonder that in what case TV (Total variation) is used and what case use (Tol/6) to calculate as TV is not equivalent to (Tol/6).

Please explain to me.

Thanks and best regards,

Chi

[Howard Atkins]

Welcome to the cove
Please see this post MSA using tolerances - Page 116 of the AIAG MSA manual for more information

[canhchi]

Thanks for your reply!

I think I should ask more detail.

The page 74 of MSA book issue 3, refer to selection samples. If the sample parts DO NOT represent the production process, TV must be ignored and use Tolerance in assessment.
How are samples taken to represent the production process? And in what case, samples DO NOT represent? Please teach me.

I separated measuring devices into two groups: 1st group includes devices used in production line, and 2nd group includes devices used for final inspecting before shipment in Quality Inspection Division. With the 1st group, I taking one sample from process for several days and then calculate %GRR by 100*(GRR/TV). With the 2nd group, I don't take samples from process but taking only from a production lot (any lot) and calculate by 100*(GRR/(Tol/6). My thought is wrong?
Give me advice.

Thanks and best regards,
Chi

[boobab]

hi,

i just joined this forum. im not sure if you're still interested but i would just like to share something. please disregard my input if somebody has already mentioned this in the previous threads.

i also take care of MSA studies in our company and this is how we consider tolerance and total variation.

tolerance is considered when the gauge being studied is used to measure product characteristics.

total variation is used when the gauge being studied is used to measure process characteristics.

[gszekely]

Hello Chi !
I don't know if I1m right, but please have my interpretation:
1) Usually MSA is done in the early phase of product creation. At this stage you don't know your real process variation, even if you already have done a preliminary capability analyses. (ex. raw material, lot to lot variation)
2) In my opinion there is no process, which will not produce out of product tolerance parts.
So if at the stage, you conduct the MSA study, you can not include samples out of tolerance, then you should report to product tolerance.
If you can have parts out of tolerance, then you should report to the real process tolerance, which is larger, than product tolerance. Of course within a certain range.
There is not stated in the standard that you have to pick up your parts randomly from process. I usually try to collect samples, which covers the full production range:
EX: if my parts is 10mm +/- 0,1, then I would search for a parts something like this:
1) 9,88 out of tolerance
2) 9,93
3) 9,95
4) 9,97
5) 10.00
6) 10,03
7) 10,05
8) 10,08
9) 10.10
10) 10,12
Just my approach, how I interpret. Am I wrong ?
György