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Each is just a Standard Deviation. From Page 115 of the MSA (2nd
Paragraph)
And from Page vi
Page 55 reminds us to add variances, not standard deviations.
X
0 20 40 60 80 100
XX
0 20 40 60 80 100
XXX
0 20 40 60 80 100
X
XXX
0 20 40 60 80 100
X
X XX
XX XXX
X
0 20 40 60 80 100
X
X XX
XX XXX
X
0 20 40 60 80 100
X
X XX
XX XXX
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0 20 40 60 80 100
78
X
X XX
XX XXX
X
0 20 40 60 80 100
78
This Standard Deviation defines the Error of Width
This distance defines Error of Location
Bias Linearity Stability Averaging several readings does not
help. Found by measuring known standards. Eliminate/minimize by calibration.
Repeatability Reproducibility Averaging several readings reduces
error. Found by repeated measurements of
the same parts. Minimize by operator training, gauge
improvement, etc.
R & R are both Standard Deviations We combine them as Variances to get
GRR There’s a major difference between
Errors of Location and Errors of Width.
Pg 74, in bold letter, tells us how to sample
“selected from the process” is not: Consecutive parts Random Parts
At least one part should be as small as normally expected, and one part should be as large as normally expected. All the other “in-between” parts don’t really matter.
Start with Equation for %GRRtv
Substitute in EV and AV for RR. Substitute in RR and PV for TV
Substitute in EV and AV for RR on the bottom
%GRRtv is driven by PV.
What drives PV?
PV = Rp x K3
Rp = Biggest Part – Smallest Part
You want Rp to be as big as possible, so %GRRtv is as small as possible.
Sort through your parts to find the biggest and smallest part you can find.
This makes Rp big, which makes PV big, which makes %GRRtv small.
Small %GRRtv makes your life easier.
You know the Standard Deviation of your Gauge System. Is it a good gauge?
Can the gauge discriminate between Good/Bad Parts?
Can the gauge detect process variation?
Can the gauge discriminate between Good/Bad Parts?
Answered by %GRRtot tol
..
*6*100% .. TolTot
GRRGRR toltot
Can the gauge detect process variation?
Answered by %GRRtv
Just because %GRRtv <10% and %GRRtot
tol <10% doesn’t mean the situation is good.
Which situation is better for your company? %GRRtv = 6% and %GRRtt = 9% OR %GRRtv = 15% and %GRRtt = 9%
Understand the difference between %GRRtot tol and %GRRtv
Don’t plug your data into software and blindly accept the %GRR values.
