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In this chapter we will discuss: Design of quality control systems, process quality control, attribute control, variables control, using control charts, process capability, continuous improvement, six sigma, lean and six sigma, quality control and improvement in industry.
Operations Management Contemporary Concepts and Cases Chapter Nine Quality Control and Improvement McGrawHill/Irwin Copyright © 2011 by The McGrawHill Companies, Inc. All rights reserved Chapter Nine Outline Design of Quality Control Systems Process Quality Control Attribute Control Variables Control Using Control Charts Process Capability Continuous Improvement Six Sigma Lean and Six Sigma Quality Control and Improvement in Industry 92 Chapter Emphasis Process definition Statistical quality control Continuous improvement 93 Design of Quality Control Systems Break down production process into subprocesses and “internal customers” Identify “critical points” where inspection or measurement should take place Four steps in designing QC systems 94 Steps in Designing QC Systems Identify critical points • Incoming materials & services • Work in process • Finished product or service Decide on the type of measurement • Variables • Attribute Decide on the amount of inspection to be used Decide who should do the inspection 95 Types Of Measurement Attribute measurement –Product /service characteristic evaluated with a discrete choice: Good/bad, yes/no Variables measurement –Product/service characteristic that can be measured on a continuous scale: Length, size, weight, height, time, velocity 96 When the Inspector Finds a Defect… Containment: Keep the defective items from getting to the customer Correction: Find the cause of the defect and correct it Prevention: Prevent the cause from happening again Continuously improve the system 97 When the Inspector Finds a Defect e.g Strap on backpack comes loose Containment: pull the bad backpacks from the production line Correction: sewing machine misaligned; fix it Prevention: why was it misaligned? Find out and change system to prevent it happening again; may require additional training Continuously monitor and improve system 98 Process Quality Control Basic assumptions (tenets) of Process Quality Control: – Every process has random variation – Production processes are not usually in a state of control “State of Control” what does it mean? – Unnecessary variation is eliminated – Remaining variation is because of random causes 99 “It’s our job to drive the randomness out.” Bill James, statistician for the Boston Red Sox Wall Street Journal, 20 June 2007, p. D7 910 frequency Process Capability Index Examples (Figure 9.3) process measure process measure 917 frequency Computation of Cpk (Figure 9.4) process measure process measure 918 Continuous Improvement Aim of continuous improvement is to reduce the variability of the product or process Processes with strategic importance and low process capability should be selected first for improvement Use seven tools of quality control 919 Seven Tools of Quality Control (See Figure 9.5) Flowchart Check Sheet Histogram Pareto Chart Causeand Effect (fishbone or Ishikawa) Diagram Scatter (xy) Diagram Control Chart 920 Seven Tools of Quality Control Battery Manufacturer in NW Ohio in 6 weeks in 2004, using only the 7 tools of quality, decreased defectives from 7.2 per 100 to 2.6 per 100 921 Pareto Analysis Table 9.4 Defect Items Loose connections Cracked connectors Fitting burrs Improper torque O-rings missing Total #of Precent Cumulative Defectives Defective Percentage 193 46.8% 46.8% 131 31.8% 78.6% 47 11.4% 90.0% 25 6.1% 96.1% 16 3.9% 100.0% 412 100.0% Note: 40 percent of the items cause 78.6 percent of the defects 922 Pareto Diagram (Figure 9.6) 250 120.0% # of Defectives 80.0% 150 60.0% 100 40.0% 50 Percentage 100.0% 200 20.0% 0.0% Loose connections Cracked connectors Fitting burrs Improper torque O-rings missing 923 Cause-and-Effect (fishbone, Ishikawa) Diagram (Figure 9.7) M a t e r ia l c o n n e c to rs W o rk e rs S m a ll S iz e L a rg e C o n te n t N u ts T r a in in g S iz e F a tig u e K n o w le d g e Hose Loose c o n n e c t io n s S u rfa c e d e fe c t M e a s u re m e n t M e a s u r in g to o ls E x p e r ie n c e E rro rs Judgm ent In s p e c to r W ear A d ju s tm e n t T r a in in g In s p e c t io n T o rq u e A ir p r e s s u r e T o o ls 924 Six-Sigma Quality Pioneered by Motorola in 1988 (Juran claimed credit for the idea) 3.4 defects per million Sample size rules become unusable Most process are 4 sigma, e.g., payroll, prescriptions, baggage handling, journal vouchers, restaurant bills Airline fatalities are 6.4 sigma IRS tax advice is less than 2 sigma Criticism: accepts 3.4 defects/million. Is not zero defects! 925 Six Sigma Quality Process Improvement steps of Six Sigma (DMAIC): Define Measure Analyze Improve Control 926 Six Sigma Quality Six Sigma uses a project/team approach A process is selected for improvement A crossfunctional team is formed A six sigma ‘black belt’ is chosen to head the team The team uses the DMAIC method for finding root causes and improving the process 927 Lean and Six Sigma Are complementary approaches to improvement: – – Lean seeks to eliminate waste Six sigma seeks to eliminate defects Six sigma organization is more formal and training intensive Six sigma is project focused; lean is more broad based 928 Quality Control and Improvement in Industry 75% use process control charts More use of variable (xbar and R) charts than attribute (p) charts because of sample size requirements Six Sigma rapidly gaining acceptance Quality control in the service industry (SERVQUAL) 929 Summary Design of Quality Control Systems Process Quality Control Attribute Control Variables Control Using Control Charts Process Capability Continuous Improvement Six Sigma Lean and Six Sigma Quality Control and Improvement in Industry 930 End of Chapter Nine 931 ... Six Sigma Lean? ?and? ?Six Sigma Quality? ?Control? ?and? ?Improvement? ?in Industry 92 Chapter Emphasis Process definition Statistical? ?quality? ?control Continuous? ?improvement 93 Design of Quality Control Systems... Design of? ?Quality? ?Control? ?Systems Process? ?Quality? ?Control Attribute? ?Control Variables? ?Control Using? ?Control? ?Charts Process Capability Continuous? ?Improvement Six Sigma Lean? ?and? ?Six Sigma Quality? ?Control? ?and? ?Improvement? ?in Industry... process capability should be selected first for improvement Use seven tools of? ?quality? ?control 919 Seven Tools of Quality Control (See Figure 9.5) Flowchart Check Sheet Histogram Pareto Chart Cause? ?and? ? Effect (fishbone or Ishikawa)