NewSonic SonoDur2 - Page 70

NewSonic SonoDur2 Issue 04 09/2016 Page 70
The process parameters Cp and Cpk:
Both parameters essentially describe the process capability, mainly in automated test facilities and
when measuring large quantities.
Cp describes the dispersion of the measured values (Xi) around an average value Overline X within an
admissible tolerance range (upper and lower thresholds Tmax, Tmin). The formula reads as follows:
Cp =
__Tmax – Tmin__
6 σ
(10)
With σ = mean error of an individual measurement and 6σ = width of normal distribution curve.
As already mentioned, the application of this formula requires the presence of a large number of
measured values, which come close to a normal distribution.
The position of the measured value distribution is characterized by the second parameter Cpk. The
distance of the average value Overline X to the respective closer threshold value (Tmax – Overline X)
or (Overline X - Tmin) across half of the distribution width of the Gaussian distribution curve will be
correlated herewith.
Cpk =
__Tmax – Overline X__
3 σ
or
__ Overline X – Tmax__
3 σ
(11)
depending on which difference is smaller.
A negative sign indicates that the process has moved beyond the tolerance limits. For further
considerations it is referred to relevant literature.
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