Gage R & R to different Specifications on Departmental Measuring Equipment

Miner

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#21
I see where we went wrong. I misinterpreted your use of %PV as measurement variation as a % of Part Variation. The PV of 320% means that the variation of the parts in this study vary more than three times the tolerance. If this reflects your process, then it is not capable. %PV is not typically used to assess adequacy of the gauge. That is normally done using %Tolerance for an inspection gauge or %SV / ndc for a process control gauge.
 
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S

SPC_Newbie

#22
Thanks Miner.

So what does %PV (for product control) mean to us? It is definately a common output that I need to understand and explain when reporting GR&R results.

Part of why I ask is that the %PV (for product control) seems to be at least a little counter intuitive. If I take my Rp and edit my tolerance such that it is equal to Rp/.75, assuming this gives me Cpk of 1.33, I get %PV = 188.7. I guess this makes sense as we multiply Rp x 1.887 (K3 for 10 parts).

So is 188.7 % a good value to have here??

I think I still understand that this number does not really determine if my gauge is adequate or not but yet it is commonly reported and is shown on the Components of Variation chart so I do need to put it in it's proper context - whatever that may be :)
 
S

SPC_Newbie

#23
I keep thinking on this one so I'll share my thoughts. I'm still of the mindset that 188.7 % is a decent number so that notion, if wrong, needs to be corrected.

Moving along - A common scenario I've seen in my data and data for GR&R's others have shared is that all of the graphs (typical minitab output) look very good. The only indicator of something gone awry is a %PV (product) that is very high (I'll consider anything above 200% high per my above above assertions).

Near as I can tell the main impact of a high %PV will be our %GR&R values appearing to be better than they really are (part variation being a significant contributor to TV which is used in denominator of our % Study calculations).

I'd say we could then use NDC to guide us towards considering how decent our results REALLY are (we do each of our GR$R's to assess both product and process control)

Do these things sound right? Would anyone use %PV for any other assessments of a GR&R?
 
Last edited:

Miner

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#24
%PV is not really used when assessing gauge adequacy. I am not certain why it is even calculated at all. I can speculate that when they started calculating the percentages for Repeatability, Reproducibility and R&R, they got on a roll and kept on going.

The only thing you can really glean from it is when %PV is 75% of the tolerance or less, you MAY have a capable process, and if it is greater than 75%, you PROBABLY do not have a capable process.
 
S

SPC_Newbie

#25
So my math in #23 and approach to evaluating GR&R results above in #24 flawed or simply misses the mark?
 

Miner

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#26
%PV = 6s/Tol and Cp = Tol/6s
Therefore; %PV = 1/Cp

If Cp = 1.33, %PV = 1/1.33 = 0.75
If Cp = 1.67, %PV = 1/1.67 = 0.60
If Cp = 2.00, %PV = 1/2.00 = 0.50

However, %PV is based on the variation of only 10 parts, whereas Cp should be based on 100 parts.
 
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