Process
Capability study through CP and CPk measurement.
Process
capability and process performance is through the study of
the statistical measurements of Cp and Cpk.
Definitions
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 noncentered
distribution.
The
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.
The
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.
The
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 limits.
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.
CPK
Value

Capability

Inspection
and action needed

Less
than 1

Incapable

100
% inspection may be needed for the production batch. Process
improvement is highly recommended. 
Between
1 to 3

Capable

Normal
sampling plan is needed. Process is stable enough for
mass production. 
Greater
than 3

Very
capable

No
inspection is required, process is very stable. 
