Process Capability

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Process Capability

• Process limits (The Voice of the Process or The
  Voice of the Data) - based on natural (common
  cause) variation
• Tolerance limits (The Voice of the Customer)
  customer requirements
• Process Capability A measure of how capable
  the process is to meet customer requirements
  compares process limits to tolerance limits

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Process Capability

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Process Capability Index
,
where x-bar is the mean of the process, sigma is
the standard deviation of the process, UTL is the
customers upper tolerance limit (specification),
and LTL is the customers lower tolerance limit.
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Interpreting the Process Capability Index
Cpk lt 1 Not Capable Cpk gt 1 Capable at 3? Cpk gt
1.33 Capable at 4? Cpk gt 1.67 Capable at 5? Cpk
gt 2 Capable at 6?
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Process Capability Index, Cpk
A process has a mean of 45.5 and a standard
deviation of 0.9. The product has a
specification of 45.0 3.0. Find the Cpk .
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Process Capability Index, Cpk
Example problem solution

min (45.5 42.0)/3(0.9) or
(48.0-45.5)/3(0.9) min (3.5/2.7) or
(2.5/2.7) min 1.30 or 0.93 0.93
(Not capable!) However, by adjusting the mean,
the process can become capable.