26
Following display :
-3s
= - 3
σ
= lower regulation or intervention limit.
+3s
= + 3
σ
= upper regulation or intervention limit.
-NG
= number of stored values smaller than lower tolerance.
+NG
= number of stored values greater than upper tolerance.
%Def
= percentage of defective stored values =
Cp
= process capability =
Cm
= machine = capability
s
given for a sample at time t.
A high
Cp
will indicate a small dispersion of measured values inside the tolerances :
High
Cp
:
mauvais
Cp
: Low
Cp
:
Cmk
= center capability= the smaller value of the 2 formulas or
Cpk
= process capability in relation to spec or
mean = the small value of
Cmk/Cpk
will formulate capability in a manner that indicates shifts (skewing) in the mean of distribution away from
the midpoint :
Good
Cmk/Cpk
Poor
Cmk/Cpk
:
Cp
>=
1.33
corresponds to a
capable
process.
Note: some users require a
Cp
> = 1.66 which corresponds to 10
σ
.
These indices of capability are certainly an efficient way of taking decisions but their importance must not be
exaggerated. Without process analysis, i.e. without knowing the dispersion, random factors and system influences,
the indices are only of limited value and may lead to erroneous decisions being taken.
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