chapter 12
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Chapter 12Chapter 12Chapter 12Chapter 12
Design forDesign for
Six SigmaSix Sigma
DFSS ActivitiesDFSS ActivitiesFour Principal ActivitiesFour Principal Activities
Concept development, determining product functionality based upon customer requirements, technological capabilities, and economic realities
Design development, focusing on product and process performance issues necessary to fulfill the product and service requirements in manufacturing or delivery
Design optimization, seeking to minimize the impact of variation in production and use, creating a “robust” design
Design verification, ensuring that the capability of the production system meets the appropriate sigma level
Key IdeaKey Idea
Like Six Sigma itself, most tools for DFSS have been around for some time; its uniqueness lies in the manner in which they are integrated into a formal methodology, driven by the Six Sigma philosophy, with clear business objectives in mind.
Tools for Concept Tools for Concept DevelopmentDevelopment
Concept development – the process of applying scientific, engineering, and business knowledge to produce a basic functional design that meets both customer needs and manufacturing or service delivery requirements. – Quality function deployment (QFD)– Concept engineering
Key IdeaKey IdeaConcept DevelopmentConcept Development
Developing a basic functional design involves translating customer requirements into measurable technical requirements and, subsequently, into detailed design specifications.
Key IdeaKey IdeaQFDQFD
QFD benefits companies through improved communication and teamwork between all constituencies in the value chain, such as between marketing and design, between design and manufacturing, and between purchasing and suppliers.
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House of QualityHouse of Quality
Technical requirements
Voice of the customer
Relationship matrix
Technical requirement priorities
Customerrequirement priorities
Competitive evaluation
Interrelationships
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Quality Function Quality Function DeploymentDeployment
technicalrequirements
componentcharacteristics
processoperations quality plan
Building the House of Building the House of QualityQuality
1. Identify customer requirements.2. Identify technical requirements.3. Relate the customer requirements to the
technical requirements.4. Conduct an evaluation of competing
products or services.5. Evaluate technical requirements and
develop targets.6. Determine which technical requirements
to deploy in the remainder of the production/delivery process.
Tools for Design Tools for Design DevelopmentDevelopment
Tolerance designTolerance design Design failure mode and effects Design failure mode and effects
analysisanalysis Reliability predictionReliability prediction
Key IdeaKey IdeaTools for Design DevelopmentTools for Design Development
Manufacturing specifications consist of nominal dimensions and tolerances. Nominal refers to the ideal dimension or the target value that manufacturing seeks to meet; tolerance is the permissible variation, recognizing the difficulty of meeting a target consistently.
Tolerance DesignTolerance Design
Determining permissible variation Determining permissible variation in a dimensionin a dimension
Understand tradeoffs between Understand tradeoffs between costs and performancecosts and performance
Key IdeaKey IdeaTolerance DesignTolerance Design
Tolerances are necessary because not all parts can be produced exactly to nominal specifications because of natural variations (common causes) in production processes due to the “5 Ms”: men and women, materials, machines, methods, and measurement.
DFMEADFMEA
Design failure mode and effects analysis (DFMEA) – identification of all the ways in which a failure can occur, to estimate the effect and seriousness of the failure, and to recommend corrective design actions.
DFMEADFMEA
Failure modesFailure modes Effect of the failure on the Effect of the failure on the
customercustomer Severity, likelihood of occurrence, Severity, likelihood of occurrence,
and detection ratingand detection rating Potential causes of failurePotential causes of failure Corrective actions or controlsCorrective actions or controls
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Reliability PredictionReliability PredictionReliability PredictionReliability Prediction
ReliabilityReliability – Generally defined as the ability of a Generally defined as the ability of a
product to perform as expected product to perform as expected over timeover time
– Formally defined as the Formally defined as the probabilityprobability that a product, piece of equipment, that a product, piece of equipment, or system or system performsperforms its intended its intended function for a stated period of function for a stated period of timetime under specified under specified operating conditionsoperating conditions
Types of FailuresTypes of Failures
Functional failureFunctional failure – failure – failure that occurs at the start of that occurs at the start of product life due to product life due to manufacturing or material manufacturing or material detectsdetects
Reliability failureReliability failure – failure – failure after some period of useafter some period of use
Types of ReliabilityTypes of Reliability
Inherent reliabilityInherent reliability – predicted – predicted by product designby product design
Achieved reliabilityAchieved reliability – observed – observed during useduring use
Reliability Measurement Reliability Measurement
Failure rate (Failure rate ()) – number of – number of failures per unit timefailures per unit time
Alternative measuresAlternative measures– Mean time to failure (MTTF)Mean time to failure (MTTF)– Mean time between failures Mean time between failures
(MTBF)(MTBF)
Cumulative Failure Rate Cumulative Failure Rate CurveCurve
Failure Rate CurveFailure Rate Curve
“Infant mortality period”
Average Failure RateAverage Failure Rate
Key IdeaKey IdeaReliability PredictionReliability Prediction
Many electronic components commonly exhibit a high, but decreasing, failure rate early in their lives (as evidenced by the steep slope of the curve), followed by a period of a relatively constant failure rate, and ending with an increasing failure rate.
Product Life Product Life Characteristic Curve Characteristic Curve
Three distinct time periodThree distinct time period– Early failureEarly failure– Useful lifeUseful life– Wearout periodWearout period
Predicting System Predicting System ReliabilityReliability
Series systemSeries system Parallel systemParallel system Combination systemCombination system
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Series SystemsSeries Systems
RS = R1 R2 ... Rn
1 2 n
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Parallel SystemsParallel Systems
RS = 1 - (1 - R1) (1 - R2)... (1 - Rn)
1
2
n
Series-Parallel SystemsSeries-Parallel Systems
Convert to equivalent series Convert to equivalent series system system
AA BB
CC
CCDD
RRAA RRBB RRCCRRDD
RRCC
AA BB C’C’ DD
RRAA RRBB RRDD
RRC’C’ = 1 – (1-R = 1 – (1-RCC)(1-R)(1-RCC))
Tools for Design Tools for Design OptimizationOptimization
Taguchi loss function Optimizing reliability
Key IdeaKey IdeaTools for Design OptimizationTools for Design Optimization
Design optimization includes setting proper tolerances to ensure maximum product performance and making designs robust, that is, insensitive to variations in manufacturing or the use environment.
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Loss FunctionsLoss Functions
loss lossno loss
nominaltolerance
loss loss
Traditional View
Taguchi’s View
Loss functionLoss function
Taguchi Loss FunctionTaguchi Loss Function
No strict cut-off point divides good No strict cut-off point divides good quality from poor quality. Rather, quality from poor quality. Rather, losses can be approximated by a losses can be approximated by a quadratic function so that larger quadratic function so that larger deviations from target correspond deviations from target correspond to increasingly larger losses.to increasingly larger losses.
Optimizing ReliabilityOptimizing Reliability
StandardizationStandardization—use components —use components with proven track recordswith proven track records
RedundancyRedundancy—provide backup —provide backup componentscomponents
Physics of failurePhysics of failure—understand —understand physical properties of materialsphysical properties of materials
Tools for Design Tools for Design VerificationVerification
Reliability testingReliability testing Measurement systems evaluationMeasurement systems evaluation Process capability evaluationProcess capability evaluation
Key IdeaKey IdeaTools for Design VerificationTools for Design Verification
Design verification is necessary to ensure that designs will meet customer requirements and can be produced to specifications.
Reliability testingReliability testing
Life testingLife testing Accelerated life testingAccelerated life testing Environmental testingEnvironmental testing Vibration and shock testingVibration and shock testing Burn-in (component stress Burn-in (component stress
testing)testing)
Measurement System Measurement System EvaluationEvaluation
Whenever variation is observed in measurements, some portion is due to measurement system error. Some errors are systematic (called bias); others are random. The size of the errors relative to the measurement value can significantly affect the quality of the data and resulting decisions.
Metrology - Science of Metrology - Science of MeasurementMeasurement
Accuracy - closeness of agreement between an observed value and a standard – can lead to systematic bias.
Precision - closeness of agreement between randomly selected individual measurements – can lead to random variation.
Accuracy vs. PrecisionAccuracy vs. Precision
Repeatability and Repeatability and ReproducibilityReproducibility
Repeatability (equipment Repeatability (equipment variation)variation) – variation in multiple – variation in multiple measurements by an individual measurements by an individual using the same instrument. using the same instrument.
Reproducibility (operator Reproducibility (operator variation)variation) - variation in the same - variation in the same measuring instrument used by measuring instrument used by different individualsdifferent individuals
Key IdeaKey IdeaCalibrationCalibration
One of the most important functions of metrology is calibration—the comparison of a measurement device or system having a known relationship to national standards against another device or system whose relationship to national standards is unknown.
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Process CapabilityProcess Capability
The range over which the natural The range over which the natural variation of a process occurs as variation of a process occurs as determined by the system of determined by the system of common causescommon causes
Measured by the proportion of Measured by the proportion of output that can be produced output that can be produced within design specificationswithin design specifications
Process Capability StudyProcess Capability StudyTypical Questions AskedTypical Questions Asked
Where is the process centered?Where is the process centered? How much variability exists in the How much variability exists in the
process?process? Is the performance relative to Is the performance relative to
specs acceptable?specs acceptable? What proportion of output will be What proportion of output will be
expected to meet the specs?expected to meet the specs? What factors contribute to What factors contribute to
variability?variability?
Types of Capability Types of Capability StudiesStudies
Peak performance studyPeak performance study - how a - how a process performs under ideal conditionsprocess performs under ideal conditions
Process characterization studyProcess characterization study - how a - how a process performs under actual process performs under actual operating conditionsoperating conditions
Component variability studyComponent variability study - relative - relative contribution of different sources of contribution of different sources of variation (e.g., process factors, variation (e.g., process factors, measurement system)measurement system)
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Process CapabilityProcess Capability
specification specification
specification specification
natural variation natural variation
(a) (b)
natural variation natural variation
(c) (d)
2020 2525 3030 MinutesMinutes
UpperUpperspecification specification
LowerLowerspecificationspecification
NominalNominalvalue value
Process CapabilityProcess Capability
Process is capable
Process distributionProcess distribution
Process is not capableProcess is not capable
2020 2525 3030 MinutesMinutes
UpperUpperspecification specification
LowerLowerspecificationspecification
NominalNominalvalue value
Process distributionProcess distribution
Process CapabilityProcess Capability
LowerLowerspecificationspecification
MeanMean
UpperUpperspecification specification
Nominal valueNominal value
Six sigmaSix sigma
Four sigmaFour sigma
Two sigmaTwo sigma
Effects of Reducing Effects of Reducing Variability on Process Variability on Process CapabilityCapability
Key IdeaKey IdeaProcess CapabilityProcess Capability
The process capability index, Cp (sometimes called the process potential index), is defined as the ratio of the specification width to the natural tolerance of the process. Cp relates the natural variation of the process with the design specifications in a single, quantitative measure.
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Process Capability IndexProcess Capability Index
Cp = UTL - LTL 6
Cpl, Cpu }
UTL - 3
Cpl = - LTL 3
Cpk = min{
Cpu =