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A way of calculating the process capability
of data that do not conform to a normal
distribution. The data are transformed using
a transformation based on yl
where l
is selected to minimize the standard deviation
of the standardized transformed variable.
The value of l
is found by trying all reasonable values
and selecting the best.
The reciprocal of Cp.
Where:
| USL, LSL |
Upper and Lower Specification Limits
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| sR |
The process standard deviation. It
must be estimated by the range method.
See Process Performance |
A minimum Cp value of 1.67
is usually required.
A measure used to predict
actual process performance:
Where:
The Taguchi Capability Index combines variability
and distance from the target into one measure
(related to the 'Signal to Noise Ratio'
concept):
Where:
The Process Performance Indices Pp and
Ppk are often referred to as 'long term'
process capability. This is because the
standard deviation used in their calculation
is calculated using the 'Root Mean Square
Error' (RMSE) method which is affected (inflated)
by shifts in the process mean. The standard
deviation used in the Process Capability
calculation is calculated using the Range
Method which is not affected by changes
to the process mean.
The Process Performance is generally lower
than the Process Capability. However there
is much confusion and debate about the meaning
of short and long term capability, so use
these terms with care and be sure that everybody
has the same understanding.
The 'Natural Process Limits' are at plus
and minus three standard deviations from
the target. If the natural tolerance limits
are within the specification limits, the
Cp value is greater than 1.
Process Capability is a method used to
demonstrate that a process is 'capable'
of achieving the required tolerances (specifications).
It is a routine requirement for all processes
in the automotive industry, among others.
Process Capability is measured using the
indices Cp and Cpk.
See Process Performance Index
See Process Performance Index
| Process
Performance Index |
|
The Process Performance Indices Pp and
Ppk are calculated in the same way as Cp
and Cpk, except that the standard deviation
is calculated using the RMSE method, not
the Range Method
Pp and Ppk are measures of the long term
performance of a process, whereas Cp and
Cpk measure the short term performance.
See Long Term Capability
The limits imposed by the designer, or
the customer. The onus is on the supplier
to ensure that the process can achieve the
specifications to the required process capability
value.
See Specification Limits
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