module 5 lecture 2 final

7/30/2019 Module 5 Lecture 2 Final

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Module

5

Design for Reliability andQuality


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Lecture

2Design for Quality


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Instructional Objectives

By the end of this lecture, the students are expected to learn how to define quality, the

importance ofdesign for quality, and various methods that are followed to achieve the same.

Defining Quality

According to Joseph Juran, the term quality of a part(or product or component)should refer to

the product features that meet customers needs and satisfaction, and to avoidance from

deficiencies that would minimize the chance of failure of the part. David Garvin in 1987 also

defined quality in eight basic dimensions for a manufactured part which is outlined in Table

5.2.1 [2].

Table 5.2.1 Severity and corresponding ranks of failures

Dimensions Description

PerformanceDoes the product perform to its standards? Does the product provide the

intended service?

What additional benefits may be added to the product? Will there be any

tangible or non-tangible benefit?Features

ReliabilityIs the product consistent? Will it perform well over its lifetime and perform

consistently?

How durable is the product? Will it last with daily use?Durability

Conformance Does your product meet with any agreed internal and national specifications?

Is the product easy to service?Serviceability

Aesthetics Is the product appealing to the eye?

What sort of quality perception does the marketing team want to convey in

the marketing message? Will price charged reflect the quality of the product?

Perceived

Quality


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Importance ofDesign for Quality

Design is more responsible for the quality than anything else. The designers determine the

number of component in a specific part/product, decide which are to be procured externally,

design the rest of the components and specify indirectly how they can be manufactured,

determine how the parts must be assembled, and specify the overall function of the components

in the final assembled part. In other words, the designers largely influence the entire

procurement, manufacturing and assembly cycles of any small part or large component.

Although manufacturing processes are often linked to the final quality of a part, both design and

manufacturing are responsible for the final quality inherited by a part or component. If the

quality is envisaged appropriately in the design procedure, the quality in manufacturing can also

be ensured at lesser expenses and the cost of inspection reduces significantly. This leads to the

concept ofDesign for Quality.

Benefits of Design for Quality (DFQ)

(1) The DFQ process allows the engineer to identify, plan for and manage factors that impactthe robustness and reliability of the products in the design process.

(2) DFQ reduces or eliminates the cost of quality that can be envisaged as the cost incurred inthe inspection and rework, in the procurement of replacement materials. Appropriate DFQ

procedure can also avoid defects and errors, scrap, degradation of factory/machine

capacity, re-qualifications/re-certifications expenses, and overhead demands

(3) Improved and consistent quality of parts provide better appeal to the customers thatobviously lead to greater stability of the manufacturing shops and can create greater

amount of opportunities.

Figure 5.2.1 schematically outlines the Demings Chain Reaction depicting salient features of

design for quality. In particular, the various factors that affect and in turn, get influenced by

design for quality are clearly indicated in Figure 5.2.1.


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Strategy to implementDesign for Quality

Following are a few frequently used techniques that are used to ensure design for quality.

Understand past quality issues

The root causes of any past quality issue should be realized and resolved thoroughly with the

help of a multi-functional team having representatives from all the departments. Such a team

would brainstorm solutions not only to resolve the previous quality issues but would also come

up with new possible design ideas that might improve the quality further.

Designing the product rightly in the first time

It is always suggested to take utmost care so as to design the product right from the first time.

Further, standard manufacturing techniques must be followed so that the quality in

manufacturing can also be obtained from the first time itself. If quality is not assured by the

initial design, then expensive changes would be required in later stages of the product cycle

wasting valuable engineering resources and time.

Figure 5.2.1 Schematic outline of Demings chain reaction with respect to design for quality


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Simplify the Design

Simplify the design such that the product can be built from the smallest number of parts.

Minimize Cumulative effect of Part Quality and Quantity

The quality of a product or component can be approximated by the average quality level of theparts following the expression as:

1nap )Q(Q

+

= (1)

where Qp refers to the quality level of the product, Qa is the average quality level of parts andn

is the number of parts in the product. Equation (1) states that the quality of the product ( the first-

pass accept rate) will be equal to the quality level of the parts to the exponent of the number of

parts assuming perfect manufacturing processing. Therefore, high-quality parts and simplified

design which give fewer parts would help to attain higher quality product. This is also known asminimizing the exponential cumulative effect of part quality and quantity. For example, a

product consisting of 17 parts with an average quality level (Qa) of 98% would lead to a product

quality level (Qp 70.0)98.0(18) of . In other words, only 70% of the products will be good with

an average quality level of the parts as 98%. This assumes perfect factory quality. Other

unforeseen factory quality problems will lower the level of product quality even further.

Select Parts for Quality

Too often parts are selected for functionality and cost. However to ensure quality by design,parts must also be selected for quality.

Optimize processing

Be sure that the manufacturing process selected is robust enough and can produce high quality

products in production quantities. Also automating the process can be a good option. Automated

production lines often help to produce better and more consistent quality parts / products than

manual production.

Reuse Proven Design and

Use proven standard parts and design features that have been used successfully before and would

be most likely to minimize risk and assure quality. A key goal for the designer should be to use

proven design, parts and modules.

Part


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Document thoroughly and completely

In the rush to develop products, many designers fail to document every aspect of the design

thoroughly. Drawings, manufacturing instructions, and bills-of-material sent to the

manufacturing or vendors need to convey the design unambiguously for manufacture,

tooling, and

Implement incentives that reward quality

inspection. Imprecise drawings invite misunderstandings and interpretation, which

add cost, waste time, and may compromise quality. Centralize the most current data with good

product data management.

Many a times there are a lot of incentives for achieving the production deadline. Similar

incentives should be defined to meet quality standards as well.

Utilize Quality Function Deployment

Quality function deployment (QFD) can be used define products to capture thevoice of the

customer the first time without the cost and risk of changing the design. QFD is a tool for

systematically translating the requirements of the customer into product design specifications

and resource prioritization. Its strength is to translate the objective and the subjective wants of

the customer into objective specifications that engineers can use to design products. The basic

structure is a table with "Whats" as the labels on the left and "Hows" across the top. The roof of

the table is a diagonal matrix of "Hows vs. Hows" and the body of the table is a matrix of "Whats

vs. Hows". Both of these matrices are filled with indicators of whether the interaction of the

specific item is a strong positive, a strong negative, or somewhere in between. Additional

annexes on the right side and bottom hold the "Whys" (market research, etc.) and the "How

Much". The rankings based on the Whys and the correlations are used to calculate priorities for

theHows. Figure 5.2.2 depicts a typical structure of a QFD table.


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Figure 5.2.2 Schematic presentation ofquality function deployment(QFD) house / table

Figure 5.2.3 provides a physical insight how the QFD table / house can be prepared for a new

part. Figure 5.2.4 depicts a complete QFD table / house for a new part.

Hows vs. Hows

Hows

Whats vs. Hows

WhyWhats

How much


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Figure 5.2.3 Basis of the development ofquality function deployment(QFD) house / table


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Figure 5.2.4 Typical quality function deployment(QFD) house / table for a part


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Utilize

Poka-yoke is a Japanese term that means mistake-proofing. The

Poka-Yoke

Poka-Yoke principles to product

design are meant to prevent mistakes by design

Understand the design

in addition to the traditional manufacturing

techniques or to prevent incorrect assembly or fabrication. The Poka-yoke principles ensure that

proper conditions exist before actually executing a process step, preventing defects from

occurring in the first place. It refers to techniques that can identify and keep away defects out of

products and processes and, substantially improve quality and reliability. It can be thought of as

an extension of FMEA. The step-by-step process in applyingpoka-yoke can be envisaged as

Analyze and understand the ways a product can fail. Decide the right poka-yoke approach, such as using a

o shut out type (preventing an error being made by modifying the design), oro an attention type (highlighting that an error has been made by adding more

features to the design)

Do appropriate modifications in the design to incorporate the above approach Trial the method and see if it works Finalize the design and proceed ahead

Figure 5.2.5 depicts a typical example of applyingpoka-yoke principle.

Figure 5.2.5 Application ofPoka-Yokeprinciples


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Proactively minimizing all types of risk

Proactively minimize all types of risk, not just functionality. Use Failure Modes Effects Analysis

(FMEA), which is one of the techniques used to identify and analyze failure and actions that

need to be taken to reduce their occurrence.

Optimize tolerances

Optimize tolerances

for a robust design using Taguchi Methods to ensure the high quality by

design. This is a systematic way to optimize tolerances to achieve high qualityat low cost, which

is often achieved by using the principles ofDesign of Experiments to analyze the effect of all

tolerances on functionality, quality, and manufacturability. The procedure can identify critical

dimensions that need tight tolerances and precision parts, which can then be taken care of

appropriately. The unique strength of this approach is that it can minimize cost while assuring

high quality by identifying low demand dimensions that can have looser tolerances and cheaper

parts. Such a design would be consideredrobustso that it could be manufactured predictably

with consistently high quality and perform adequately in all anticipated usage environments.

Without a methodical way to determine tolerances, the alternatives would be either to make all

tolerances tight which is expensive or inadvertently (or deliberately) make tolerances too loose,

leading to manufacturability and quality problems.


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Exercise

Develop a QFD table for a pen.

Reference

[1] David M. Anderson and David M. Anderson, Design for Manufacturability and ConcurrentEngineering, CIM Press, 2004.

[2] G Dieter, Engineering Design - A Materials and Processing Approach, McGraw Hill, NY,2000.

[3] http://upload.wikimedia.org/wikipedia/en/3/3e/A1_House_of_Quality.png[4] http://www.mistakeproofing.com/example4.html[5]

http://www.impacture.com/qfdwhatis.htm

[6] http://thequalityportal.com/pokayoke.htm
http://upload.wikimedia.org/wikipedia/en/3/3e/A1_House_of_Quality.pnghttp://upload.wikimedia.org/wikipedia/en/3/3e/A1_House_of_Quality.pnghttp://www.mistakeproofing.com/example4.htmlhttp://www.mistakeproofing.com/example4.htmlhttp://www.impacture.com/qfdwhatis.htmhttp://www.impacture.com/qfdwhatis.htmhttp://thequalityportal.com/pokayoke.htmhttp://thequalityportal.com/pokayoke.htmhttp://thequalityportal.com/pokayoke.htmhttp://www.impacture.com/qfdwhatis.htmhttp://www.mistakeproofing.com/example4.htmlhttp://upload.wikimedia.org/wikipedia/en/3/3e/A1_House_of_Quality.png

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