Process Capability BA 339 Mellie Pullman
Process Capability • Can the process provide acceptable quality consistently?
Process Capability Ratio (Cp) Upper Tolerance Limit – Lower Tolerance Limit 6σ • σ is the estimated standard deviation for the individual process observations (SPC) • Tolerance or specifications are dictated by the customer, part drawings, laws, or other non-process related entities.
Shown Graphically: Process Capability ratio of 1 (99.7% coverage)
Climbing Carabineer • In the lab, average breaking strength is: m = 6500 lbs and s = 150 lbs Industry Safety Specification = 6000 lbs lower limit and 7000 lbs upper limit. Can the process make a product to this specification 99.7 % of time ?
Process Capability Ratio (Cp) Upper Tolerance Limit – Lower Tolerance Limit 6σ (7000-6000)/(6*150)= 1.11 Capable?
Process Capability Index (Cpk) • Used when the process is not precisely centered • Assume process mean is actually m= 6300 • Cpk = min [(6300-6000)/(3*150), (7000-6300)/(3*150)]
“Six Sigma Quality” When a process operates with 6σ variation inside the tolerance limits, only 2 parts out of a million will be unacceptable.
3 Lines of Defense • PREVENTdefects from occurring • TQM and continuous improvement • DISCOVER problems early • Process control charts • CATCH DEFECTS before used or shipped • inspection