Download - Understanding Gauge R&R
My Personal Crusade
Mark S. RuscoInnovative Corporate [email protected]
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.
X0 20 40 60 80 10
0
XX0 20 40 60 80 10
0
XXX
0 20 40 60 80 100
XXXX
0 20 40 60 80 100
XX XX
XX XXX
X
0 20 40 60 80 100
XX XX
XX XXX
X
0 20 40 60 80 100
XX XX
XX XXX
X
0 20 40 60 80 1007
8
XX XX
XX XXX
X
0 20 40 60 80 1007
8
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% .. TolTotGRRGRR 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.