cl05_Quality_Management_II

Specification limits control limits vs specification

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Unformatted text preview: Limits vs. Specification Limits Control Limits vs. Specification Limits USL = 1.6 UCL = 1.526 1.6 1.5 1.4 1.3 1.2 Center = 1.17 1.1 1 0.9 0.8 0.7 0 5 10 15 20 25 LCL = 0.821 LSL = 0.7 Specification Limits Control Limits Purpose Quality tolerance Process control Defined by Customers, Designers Producers, Quality Controllers Type of Quality Quality Design quality Conformance quality quality What happens beyond limits Defects 25 ProcessStephen Shum out of control Spring 2009 Control Limits vs. Specification Limits Control Limits vs. Specification Limits USL = 1.6 UCL = 1.526 1.6 1.5 1.4 1.3 1.2 Center = 1.17 1.1 1 0.9 0.8 0.7 0 5 10 15 20 25 LCL = 0.821 LSL = 0.7 Ideally, control limits should fall within the Ideall fall the specification limits. Why? – Avoid false alarm But we are not always lucky… 26 Process Capability Index Process Capability Index Process capability index measures the deviation from process mean to the closest specification limit C pk k x LSL USL x min , 3 3 Cpk> 1 Cpk= 1 LSL USL LCL UCL Process variability variability matches specifications LSL USL UCL LCL Process variability is variability is well within specifications 27 LSL Cpk<1 USL LCL UCL Process variability variability exceeds specifications Stephen Shum Spring 2009 Process Capability and Sigma Capability Process Capability and Sigma Capability Assumes a centered process (X-bar midway between LTL & UTL) centered process (X midway between LTL UTL) Sigma Capability Prob. of meeting specs z 2*Normsdist(z)–1 1 0.682689480482 2 0.954499875928 3 0.997300065554 4 # Defects/million Cpk 2–2*Normsdist(z) z /3 317310.519518 45500.124072 0.33 0.67 1.00 0.999936627931 2699.934446 63.372069 5 0.999999425790 0.574210 1.67 6 0.999999998020 0.001980 2.00 7 0.999999999997 0.000003 2.33 28 1.33 Man Yu - Spring 2009 Process Capability Example Process Capability Example At McDonald restaurant it is specified that any regular At a McDonald’s restaurant, it is specified that any regular order order must be “assembled” within 30 10 sec. A quality inspector takes several sample measurements over time and determines the sample average assembly time is 32 an 32 sec. sec. with a standard deviation of 3 sec. a) Calculate the process capability index Calculate the process capability index b) If McDonald’s wants to achieve 3-sigma service, is this 3process capable of delivering desired service quality? How can the process be improved? can the process be improved? Improving Process Capability Improving Process Capability σ=5 σ =10 LTL LTL UTL σ =10 LTL UTL σ=5 LTL UTL 30 UTL Man Yu - Spring 2009 From Control to Improvement From Control to Improvement Quality = Process control + Kaizen Process control Kaizen Weight Out of Control of Control In Control Control Improved UCL LCL Time 31 Stephen Shum Spring 2009 Summary Summary Control charts – Concepts – Calculation of control limits Process capability index capability index – Concepts and calculation – Relationship to six sigma quality to six sigma quality 32 Man Yu - Spring 2009 Next Class Next Class Capacity Planning I 33 Man Yu - Spring 2009...
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