Lean six sigma process improvement tools and techniques by donna summers chapter 19

Process Capability Chapter 19 LeanSixSigma:ProcessImprovementToolsandTechniques DonnaC.Summers â2011PearsonHigherEducation, UpperSaddleRiver,NJ07458.AllRightsReserved Process Capability • She had the virtuous sense of one who has made an intuitive leap from the most fragile collection of evidence • Lean Six Sigma practitioners know that they can not make quick judgments without the data to support their conclusions They need evidence They need the answer to the question: How we know the process will generate products or services that meet the customer’s specifications? Lean Six Sigma: Process Improvement Tools and Techniques Donna C. Summers © 2011 Pearson Higher Education, Upper Saddle River, NJ 07458. • All Rights Reserved Process Capability • Process capability refers to the ability of a process to produce products or provide services capable of meeting the specifications set by the customer or designer LeanSixSigma:ProcessImprovementToolsandTechniques DonnaC.Summers â2011PearsonHigherEducation, UpperSaddleRiver,NJ07458.AllRightsReserved Process Capability The relationship between the population standard deviation  and the standard deviation of the subgroup averages is shown by the formula: X  LeanSixSigma:ProcessImprovementToolsandTechniques DonnaC.Summers n â2011PearsonHigherEducation, UpperSaddleRiver,NJ07458.AllRightsReserved Process Capability The population standard deviation can be estimated from either the standard deviation associated with the sample standard deviation (s) or the range (R): s ö  c4 Lean Six Sigma: Process Improvement Tools and Techniques Donna C. Summers R or d2 â2011PearsonHigherEducation, UpperSaddleRiver,NJ07458.AllRightsReserved Process Capability CONTROL LIMITS VERSUS SPECIFICATION LIMITS – Specifications communicate what the customers expect, want, or need from the process Specifications can be considered the voice of the customer – Control limits are the voice of the process The centerline on the X-bar chart represents process centering The R and s chart limits represent the amount of variation present in the process Control limits are a prediction of the variation that the process will exhibit in the near future LeanSixSigma:ProcessImprovementToolsandTechniques DonnaC.Summers â2011PearsonHigherEducation, UpperSaddleRiver,NJ07458.AllRightsReserved Process Capability Case I 6  USL  LSL – The 6 spread of the process individuals is shown by the solid line As expected, the spread of the individual values is greater than the spread of the averages; however, the values are still within the specification limits The 6 spread of the individuals is less than the spread of the specifications This allows for more room for process shifts while staying within the specifications Notice that even if the process drifts out of control, the change must be dramatic before the parts are considered out of specification Lean Six Sigma: Process Improvement Tools and Techniques Donna C. Summers © 2011 Pearson Higher Education, Upper Saddle River, NJ 07458. • All Rights Reserved Process Capability Case II 6 USL  LSL • In this situation, 6 is equal to the tolerance As long as the process remains in control and centered, with no change in process variation, the parts produced will be within specification However, a shift in the process mean will result in the production of parts that are out of specification An increase in the variation present in the process also creates an out-of-specification situation LeanSixSigma:ProcessImprovementToolsandTechniques DonnaC.Summers â2011PearsonHigherEducation, UpperSaddleRiver,NJ07458.AllRightsReserved Process Capability Case III 6  USL  LSL • Any time that the 6 spread is greater than the tolerance spread, an undesirable situation exists Even though the process is exhibiting only natural patterns of variation, it is incapable of meeting the specifications set by the customer Lean Six Sigma: Process Improvement Tools and Techniques Donna C. Summers © 2011 Pearson Higher Education, Upper Saddle River, NJ 07458. • All Rights Reserved Process Capability • CALCULATING PROCESS CAPABILITY INDICES • Capability Index Cp USL  LSL Cp  6ö Lean Six Sigma: Process Improvement Tools and Techniques Donna C. Summers © 2011 Pearson Higher Education, Upper Saddle River, NJ 07458. • All Rights Reserved Process Capability • CALCULATING PROCESS CAPABILITY INDICES • Capability Ratio Cr 6ö Cr  USL  LSL Lean Six Sigma: Process Improvement Tools and Techniques DonnaC.Summers â2011PearsonHigherEducation, UpperSaddleRiver,NJ07458.AllRightsReserved Process Capability CALCULATING PROCESS CAPABILITY INDICES Z • Cpk C pk    Z USL  USL  X ö Lean Six Sigma: Process Improvement Tools and Techniques Donna C. Summers     or Z LSL  X  LSL â2011PearsonHigherEducation, UpperSaddleRiver,NJ07458.AllRightsReserved Process Capability INTERPRETING PROCESS CAPABILITY INDICES When Cp has a value of 1.0 or greater, the process is producing product capable of meeting specifications The Cp value does not reflect process centering When the process is centered, Cp = Cpk Cpk s always less than or equal to Cp When Cpis greater than or equal to 1.0 and Cpk has a value of 1.00 or more, it indicates the process is producing product that conforms to specifications When Cpk has a value less than 1.00, it indicates the process is producing product that does not conform to specifications A Cp value of less than 1.00 indicates that the process is not capable A Cpk value of zero indicates the process average is equal to one of the specification limits A negative Cpk value indicates that the average is outside the specification limits LeanSixSigma:ProcessImprovementToolsandTechniques DonnaC.Summers â2011PearsonHigherEducation, UpperSaddleRiver,NJ07458.AllRightsReserved  ... the population standard deviation  and the standard deviation of the subgroup averages is shown by the formula: X LeanSixSigma:ProcessImprovementToolsandTechniques DonnaC .Summers n â2011PearsonHigherEducation,... present in the process Control limits are a prediction of the variation that the process will exhibit in the near future Lean Six Sigma: Process Improvement Tools and Techniques Donna C. Summers © 2011 Pearson Higher Education,... specifications set by the customer Lean Six Sigma: Process Improvement Tools and Techniques DonnaC .Summers â2011PearsonHigherEducation, UpperSaddleRiver,NJ07458.AllRightsReserved Process Capability