My Personal Crusade

Mark S. RuscoInnovative Corporate Trainingmarkrusco@sbcglobal.net

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

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0 20 40 60 80 100

XX XX

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X

0 20 40 60 80 100

XX XX

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X

0 20 40 60 80 100

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X

0 20 40 60 80 1007

8

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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.