% Gage R&R Calculation - 100x(GRR/ (Spec/6)) or 100x(GRR/TV)

T

tenaiwa

#1
Good day,
In regard to %GRR calculation:
when should I apply " 100x(GRR/ (Spec/6)) " --> for product control ?
when should I apply " 100x(GRR/TV)" ---->for SPC control ?
I don't get the point of MSA remark.

Would you pls clarify it for me ?
Tks
:bonk:
 
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Miner

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Staff member
Admin
#2
when should I apply " 100x(GRR/ (Spec/6)) " --> for product control ?
Use this formula when you use the gage for inspection. That is anywhere within the specification is acceptable.


when should I apply " 100x(GRR/TV)" ---->for SPC control?
Use this formula when you want to use the gage for Statistical Process Control to control the process to a target value.
 
T

tenaiwa

#3
Miner:
Thank you for your replying
Furtherly,I want to consult about sampling method under actual condition that
<1>Product spec is 0 ~ 10 (g)
<2>In fact, actual performance is indicated as pic attach (arond 90% locate at 0 ~3 g)

Should I take 10 samples randomly which means most of them located at 0 ~ 3 g ?
Or
Should I take 10 samples to cover 0 ~ 10 g as much as possible ?
:confused:

Tks and regards
 

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bobdoering

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#4
If you are going to use the gage for SPC, I recommend calculating the ndc as a function of the control limits:

ndc (cl)= (UCL-LCL)/GRR

And it should be>10

:cool:
 

bobdoering

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#5
Miner:
Thank you for your replying
Furtherly,I want to consult about sampling method under actual condition that
<1>Product spec is 0 ~ 10 (g)
<2>In fact, actual performance is indicated as pic attach (arond 90% locate at 0 ~3 g)

Should I take 10 samples randomly which means most of them located at 0 ~ 3 g ?
Or
Should I take 10 samples to cover 0 ~ 10 g as much as possible ?
:confused:

Tks and regards
Take 10 samples to cover 0 ~ 10 g as much as possible - over 10 if it is possible to occur. The point of the exercise is to see if the gage responds acceptably over the range it is going to be used.
 

Miner

Forum Moderator
Staff member
Admin
#6
Take 10 samples to cover 0 ~ 10 g as much as possible - over 10 if it is possible to occur. The point of the exercise is to see if the gage responds acceptably over the range it is going to be used.
The answer is "It depends."

If you will be using the gage for inspection purposes, bobdoering's suggestion is appropriate. However, if you plan to use the gage for process control (i.e., SPC), your samples need to represent the non-normal distribution that you show. Hopefully random samples will provide this representation. It is important that the variation of your samples is similar to the variation of your process.
 
T

tenaiwa

#7
Tks for replying from both of you.I see.
B.T.W
Does this sampling rules also apply during Cgk calculation ?
"For inspection-->sampling should cover spec.range"
"For SPC study-->sampling should present situation of actual process "


Auditor from Volkswagen raised Cgk study as a mondatory which I never met before.
Is there any thread where Cgk is described ?

Many thanks
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#8
Tks for replying from both of you.I see.
B.T.W
Does this sampling rules also apply during Cgk calculation ?
"For inspection-->sampling should cover spec.range"
"For SPC study-->sampling should present situation of actual process "


Auditor from Volkswagen raised Cgk study as a mondatory which I never met before.
Is there any thread where Cgk is described ?

Many thanks
There are, but you might also want to check this out:

Minitab: Rationale and Methodology for the Type 1 Gage Study
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#9
If you are going to use the gage for SPC, I recommend calculating the ndc as a function of the control limits:

ndc (cl)= (UCL-LCL)/GRR

And it should be>10

:cool:
Bob are you sure?
That isn't the formula for NDC. plus control limits are - typically - limits on subgroup averages and measurement error is calcualted in terms of individual values - the two standard deviations are very different in range as well as meaning. (not to mention the entire math on standard deviations thing...)
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#10
Bob are you sure?
That isn't the formula for NDC. plus control limits are - typically - limits on subgroup averages and measurement error is calculated in terms of individual values - the two standard deviations are very different in range as well as meaning. (not to mention the entire math on standard deviations thing...)
True, that isn't the regular formula for NDC. And, you might have caught that I did neglect to put in the factor 1.41. My bad. It was early...

So, we should have ndc (cl)= [1.41(UCL-LCL)]/GRR

That is why I put the (cl) next to it - to delineate it from the standard ndc formula (it would have been a subscript).

Might be thinking this thing through a little too hard, as far as the standard deviations and such. NDC is the number of non-overlapping 97% confidence intervals that will span the expected product variation. But, I do not care about the range of product variation in SPC, I am concerned about the number of non-overlapping 97% confidence intervals that will span the expected product variation in control - which is between the control limits. If you can get 10 categories to fit within the control limits, then you have enough statistically significant divisions to adequately detect variation in that range. The calculation of control limits itself is not the issue - the fact that they have become a physical target range, is.

:cool:
 
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