Capability study through CP and CPk measurement.
and process performance is through the study of the statistical
measurements of Cp and Cpk.
of Cp and cpk
Cp = Represent Process Capability. An indicator to the capability
of a particular process.
Cpk = Represent the index of the Process Capability study.
A tool to measure the Cp value for the effect of non-centered distribution.
study of CP
A common Process
Capability measure, Cp indicates how well the process distribution
fits within its specification limits ( Upper and Lower limit ).
In the examples below, processes (a) has Cp greater than one, capable
for mass production (b) is equal to one and (c) is less than one.
Higher chances to fall out of tolerance.
problem with Cp is that it does not measures how well the process
distribution is, the process may have high Cp but distributed off
the center of its limits, which can result in many rejects. The
solution to this is the study of it Cpk.
study of Cpk
Cpk study is
to measures where the Cp value distributed between the upper and
lower limits. Cpk only considers the variation half that is closest
to the specification limits, as in the figure below.
Cp and Cpk
have to be studied together to give a measure of both the potential
and centering of the process distribution within the specification
The larger the index, the more stable the process is, the less likely
that any item will be outside the specification. CPK value of 1.33
is the industry standard of minimum acceptable level.
and action needed
|100 % inspection
may be needed for the production batch. Process improvement
is highly recommended.
1 to 3
sampling plan is needed. Process is stable enough for mass production.
is required, process is very stable.