Lecture The management and control of quality - Chapter 12: Design for six sigma

Chapter 12 - Design for six sigma. This chapter presents the following content: DFSS activities four principal activities, tools for concept development, house of quality, quality function deployment, building the house of quality, tolerance design,...

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Chapter 12

Design for

Six Sigma

Four 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

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.

– Quality function deployment (QFD)

– Concept engineering

Concept Development

Developing a basic functional design involves  translating customer requirements into 

measurable technical requirements and, 

subsequently, into detailed design 

specifications.

QFD

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.

Technical requirement priorities

Customer requirement priorities

Competitive evaluation Interrelationships

process operations quality plan

 Determining permissible variation in a dimension

 Understand tradeoffs between costs and performance

 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. 

 Failure modes

 Effect of the failure on the customer

 Severity, likelihood of occurrence, and detection rating

 Potential causes of failure

 Corrective actions or controls

operating conditions

occurs at the start of product life  due to manufacturing or material  detects

 Reliability failure – failure after 

some period of use

Cumulative Failure Rate  Curve

“Infant  mortality  period”

Average Failure Rate

with an increasing failure rate.

Product Life Characteristic  Curve 

 Three distinct time period

– Early failure

– Useful life

 Series system

 Parallel system

 Combination system

Series Systems

RS = R1 R2 Rn

1 2

n

 Taguchi loss function

 Optimizing reliability

Tools 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.

loss no loss loss

nominal tolerance

Traditional

View

Taguchi’s

Loss function

 Standardization—use components with proven track records

 Redundancy—provide backup 

components

 Physics of failure—understand 

physical properties of materials

 Reliability testing

 Measurement systems evaluation

 Process capability evaluation

Tools for Design Verification

Design verification is necessary to 

ensure that designs will meet customer  requirements and can be produced to  specifications.

random. The size of the errors relative to  the measurement value can significantly  affect the quality of the data and resulting  decisions.

Accuracy vs. Precision

Reproducibility

 Repeatability (equipment variation) – variation in multiple measurements by 

Calibration

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.

 The range over which the natural 

variation of a process occurs as 

determined by the system of common causes

 Measured by the proportion of output that can be produced within design 

specifications

 What proportion of output will be 

expected to meet the specs?

 What factors contribute to variability?

 Component variability study ­ relative 

contribution of different sources of variation 

Upper specification

Lower

specification

Nominal value

Process Capability

Process distribution

Process is not capable

Upper specification

Lower

specification

Nominal value

Process distribution

Process Capability

Lower specification

Upper specification

Nominal value

Six sigma Four sigma Two sigma

Effects of Reducing  Variability on Process Capability

Process Capability

(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.

Cp = UTL - LTL

6 UTL - 3

Cpl = - LTL

3

Cpu =