statistical quality control
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Lecture5.pdf
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Lecture 5: Six Sigma
Chapter 3
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Outline
Historical Review
Statistical Aspects
DMAIC
Benefits
Note that organizational structure is not
discussed in this textbook.
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Learning Objectives
When you have completed this chapter
you should be able to:
Understand the concept of six sigma statistics.
Describe DMAIC project methodology.
Know the advantages of the methodology.
Historical Review
1980’s at Motorola, which developed and implemented Six Sigma in 1988
Significant improvement in quality.
Mid 1990’s other companies such as General Electric and Allied Signal obtained similar results.
Six Sigma is both a quality management philosophy and a methodology that focuses on reducing variation, measuring defects, and improving quality of products, processes and services.
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Statistical Aspects
Sigma, σ, is the Greek symbol for population standard deviation, which is the best measure of variation. If we can reduce variation to the point that the specifications are at ±6σ, then 99.9999998% of the items are satisfactory. The nonconformance rate is .002 ppm. See Fig.3-1 and Table 3-1
According to the philosophy processes shift ±1.5σ, which gives a conformance rate of 99.9996600% or a nonconformance rate of 3.4 ppm. See Fig. 3-2 and Table 3-2.
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Figure 3-1 Non-conformance rate when process is centered
TABLE 3-1 Nonconformance Rate When the Process Is Centered
SPECIFICATION PERCENT NONCONFORMAN CE
PROCESS
LIMIT CONFORMANCE RATE CAPABILITY ±1s 68.7 317,300 0.33 ±2s 95.45 485,500 0.67 ±3s 99.73 2,700 1.00 ±4s 99.9937 63 1.33 ±5s 99.999943 0.57 1.67
±6s 99.9999998 0.002 2.00
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Figure 3-2 Non-conformance rate when process is off center ±1.5σ
SPECIFICATION PERCENT NONCONFORMAN CE
PROCESS
LIMIT CONFORMANCE RATE (PPM) CAPABILITY (CPK)
±1s 30.23 697,700 –0.167 ±2s 69.13 308,700 0.167 ±3s 93.32 66,810 0.500 ±4s 99.3790 6,210 0.834 ±5s 99.97670 2,330 1.167
±6s
99.9996600
3.4
1.500
Table 3-2 Non-conformance rate when process is off center ±1.5σ
Statistical Aspects (Continued)
Actually the nonconformance rate is much closer to .002 ppm, because:
Process shift of 1.5 was envisioned in 1990.
Shift will not always be at 1.5. It will move back and forth.
Control charts will correct, so shift will only be at 1.5 about 5% of the time.
Use of improved technology will keep the
process centered.
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Improvement Methodology
DMAIC stand for Define, Measure, Analyze, Improve, Control.
Not a new concept, but no other methodology uses tools and techniques as effectively as Six Sigma.
Each phase requires a progress report to management.
Motorola developed MAIC and GE added the D
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Define Consists of project charter, process map, and the voice
of the customer.
Project Charter
Documents problem statement, project management, and progress toward goals.
Problem Statement describes the current state.
Important and why.
Contribute to attaining goals.
Defined clearly using objective measures.
May use Affinity Diagram and Pareto analysis.
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Define – Project Charter (Continued)
Project Management vested in quality council or other management authority authorizes the project, available resources, and operational guidelines.
Team may consist of a natural work group or a multifunction group. An upstream and a downstream representative is a good idea.
Goals and Progress can be stated in terms of quality, safety, satisfaction, financial, and environment for internal and external customers and suppliers. As well as a timeline for progress.
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Define Process Map
Process Map can be a VSM or an SIPOC process model as shown in Fig. 3-3.
Discuss inputs, process, and output.
Outcomes are the goals.
Conditions can be policies, constraints, or regulations.
Process should have at least one owner.
All functional areas have processes.
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Define – Process Map (Continued)
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FIGURE 3-3 SIPOC Process Model
Define Voice of the Customer
Provides info. that leads to those problems that have the greatest
impact.
Problems identified from many inputs:
Field failures, complaints, returns, etc.
Scrap, rework, sorting, 100% test.
Suggestions.
Study of user needs.
Performance of competitors.
Comments of key people and organizations.
Surveys and focus groups.
Brainstorming by work groups.
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Measure Measure consists of understanding the process,
validating the data accuracy, and determining the process capability.
This information is used to review the define phase, establish a baseline, and obtain a better knowledge of the process.
Understand the Process
VSM provides info. on waste or SIPOC to provide a graphic description of the process.
It is rare for all members of the team to understand the process.
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Measure – Understand the Process (Continued)
Target performance measures for inputs and outputs are determined.
Can‘t improve it, if you can’t measure it.
Team will:
Determine measures with respect to customer requirements.
Determine data to manage the process.
Establish feedback with customers and suppliers.
Establish measures for quality, cost, waste, and timeliness.
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Measure – Understand the Process (Continued)
Gathering data helps confirm that a problem exists, enables working with facts, establishes a baseline, and determines the effectiveness of a solution.
Develop a plan to learn more about the problem, uses for the data, amount of data needed, possible conclusions, and resulting
actions.
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Measure – Understand the Process (Continued)
Data and information include:
Customer – complaints, surveys, competitors
Design – function, drawings, specifications, costs, reviews, maintainability, field data
Process – routings, equipment, operators, materials, supplies, components
Statistical – average, median, range, standard deviation, skewness, kurtosis, distribution
Quality – SPC, process capability, acceptance
sampling, life testing, inspection,
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Measure – Understand the Process (Continued) Supplier – on-time delivery, process variation,
technical competence
Data mining – use computer to search large amounts of data
Validate the Data Accuracy
All devices calibrated, GR&R – Chapter 7
Determine Process Capability
Compares process variation to specifications – Chapter 6
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Analyze
Phase consists of process analysis, cause investigation, charter updating.
Pinpoint and verify causes affecting problem.
Process Analysis
Review VSM, calculate takt time, identify non-value added, determine bottlenecks.
Review measure phase data.
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Analyze (Continued)
Cause Investigation
Identify all potential causes.
Tools are brainstorming, cause and effect diagram, why why, tree diagram, and interrelationship diagram – Ch. 12
Seek causes not solutions.
Reduce list by multivoting, Pareto analysis, and stratification.
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Analyze – Cause Investigation (Continued)
Verify most likely or root cause(s)
Examine against problem statement
Recheck all data
Use scatter diagram, experimental design, Taguchi’s Quality Engineering
Calculations that show reduction in non-value added activities
Charter Review
Review problem statement, team membership, schedule, resources needed, and goals.
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Improve This phase selects optimal solution, tests a pilot,
and implements solution.
Objective – improved process to meet goals
Optimal Solution
Team uses brainstorming to be creative and innovative in selecting possible solution.
Three types of creativity: create new process – highest type; combine processes; modify existing process.
Select optimal solution
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Improve – Optimal Solution (Continued)
Evaluate to determine, which has greatest chance for success and pros & cons.
Criteria – costs, feasibility, effect, resistance to change, consequences, and training. Also short and long term solutions considered.
VSM is revised to reflect optimal solution.
Pilot Testing
Train participants. Verify that goals are met.
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Improve
Implementation
This phase -- develops the plan, obtains approval, and implements.
Plan must describe the why, how, when, who, and where it will be done.
Depending on the improvement complexity an oral or written report may be required. Approval by the quality council may also be required.
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Improve – Implementation (Continued)
If not already involved advise and consent of upstream and downstream activities should be obtained.
Measuring monitoring activity determines what measurements and resources needed; who is responsible; and where, how, and when taken.
Table 3-3 in text provides and action plan to help
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Control
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This phase consists of evaluating the process, standardizing the procedures, and final actions.
It’s objective is to evaluate the effectiveness of the improvement.
Evaluating the Process
Team should meet periodically to evaluate the improvement. May need to repeat some phases.
Tools--SPC, capability, & combination map
Control
Standardize the Procedure
Requires process control, process certification, and operator certification.
Process control is an updating of the monitoring activity. Prevents backsliding.
Process certification or peripherals include the system, environment, and supervision.
The system includes shutdown authority, TPM, alarm signals, self & foolproof inspection
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Control – Standardize the Procedure (Continued)
Environment includes controls on water/air, dust/chemical, temp/humidity, storage/inventory, and electrostatic.
Supervision includes coach—not boss, clear instructions, suggestions, feedback.
Operator certification includes competency, cross training, process improvement ideas.
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Control -- Final Actions
Celebrate success.
Process owner
Lessons learned are reported to the appropriate authority.
Appropriate authority will review for application of lessons learned to other processes within the organization.
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Additional Comments
Modifications to DMAIC
Recognize at beginning
Standardize and Integrate at the end
Replicate for multiple facilities
Six Sigma works because it gives bottom line results; trains leaders; reduces variation, improves quality, increases customer satisfaction, and uses statistical techniques.
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Additional Comments – (Continued)
Continuous improvement requires everyone to continually seek ways to improve the processes.
Establish the system to continually review the continuous improvement culture.
Track changing customer requirements
Orderly approach yields the greatest results.
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Assignment
InClass 3: Class survey
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