Chapter 10 Quality Control McGraw-Hill/Irwin Copyright © 2009 by The McGraw-Hill Companies, Inc All Rights Reserved Chapter 10: Learning Objectives • You should be able to: – – – – – – List and briefly explain the elements in the control proces Explain how control charts are used to monitor a process Explain the concepts underlying the use of control charts Use and interpret control charts Understand the use of run tests Assess process capability 10-2 Statistical Process Control (SPC) • Quality control seeks – Quality of Conformance • A product or service conforms to specifications • A tool used to help in this process: – SPC • Statistical evaluation of the output of a process • Helps us to decide if a process is “in control” or if corrective action is needed 10-3 Process Variability • All processes generate output that exhibits some degree of variability • The fundamental question: – Is the variation within acceptable boundaries? 10-4 Control Chart Abnormal variation due to assignable causes Out of control UCL Normal variation due to chance Mean LCL Abnormal variation Each point on the control chart represents a sample of n observations 10-5 Control Charts for Variables • Variables generate data that are measured – Mean control charts • Used to monitor the central tendency of a process – X bar charts – Range control charts • Used to monitor the process dispersion – R charts 10-6 Establishing Control Limits k k x  xi i 1 k where R i R  i 1 k where x Average of sample means R Average of sample ranges x i mean of sample i k number of samples Ri Range of sample i 10-7 X-Bar Chart: Control Limits • Used to monitor the central tendency of a process x  chart Control Limits UCLx  x  A2 R LCLx  x  A2 R where A2 a control chart factor based on sample size, n 10-8 Range Chart: Control Limits • Used to monitor process dispersion R Chart Control Limits UCLR D4 R LCLR D3 R where D3 a control chart factor based on sample size, n D4 a control chart factor based on sample size, n 10-9 Control Charts for Attributes • Attributes generate data that are counted – p-Chart • Control chart used to monitor the proportion of defectives in a process – c-Chart • Control chart used to monitor the number of defects per unit 10-10 Process Capability • Once a process has been determined to be stable, it is necessary to determine if the process is capable of producing output that is within an acceptable range – Tolerances or specifications • Range of acceptable values established by engineering design or customer requirements – Process variability • Natural or inherent variability in a process – Process capability • Process variability relative to specification 10-11 Cp : Process Capability Ratio UTL - LTL Cp  6 where UTL upper tolerance (specifica tion) limit LTL lower tolerance(specification) limit 10-12 Cpk : Process Capability Index • Used when a process is not centered at its target, or nominal, value C pk min C pu , C pl   UTL  x x  LTL  min  ,  3   3 10-13 Taguchi Loss Function 10-14 Operations Strategy • Quality is a primary consideration for nearly all customers – Achieving and maintaining quality standards is of strategic importance to all business organizations • Product and service design • Increase capability in order to move from extensive use of control charts and inspection to achieve desired quality outcomes 10-15 .. .Chapter 10: Learning Objectives • You should be able to: – – – – – – List and briefly explain the elements... is needed 10- 3 Process Variability • All processes generate output that exhibits some degree of variability • The fundamental question: – Is the variation within acceptable boundaries? 10- 4 Control... defectives in a process – c-Chart • Control chart used to monitor the number of defects per unit 10- 10 Process Capability • Once a process has been determined to be stable, it is necessary to determine