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Process Capability

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  1. 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

  2. (1) (3) specification specification common variation common variation (2) (4) specification specification common variation common variation Process Capability

  3. Process Capability Index , where x-bar is the mean of the process, sigma is the standard deviation of the process, UTL is the customer’s upper tolerance limit (specification), and LTL is the customer’s lower tolerance limit.

  4. Interpreting the Process Capability Index Cpk < 1 Not Capable Cpk > 1 Capable at 3 Cpk > 1.33 Capable at 4 Cpk > 1.67 Capable at 5 Cpk > 2 Capable at 6

  5. 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 .

  6. 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.