PRODUCT DESIGN

Materials Selection

As stated earlier, the selection of a material for a machine part or structural member is one of the most important decisions the designer is called on to make. Up to this point in this chapter we have discussed many important material physical properties, various characteristics of typical engineering materials, and various material production

processes. The actual selection of a material for a particular design application can be an easy one, say, based on previous applications (1020 steel is always a good candidate because of its many positive attributes), or the selection process can be as involved and daunting as any design problem with the evaluation of the many material physical, economical, and processing parameters. There are systematic and optimizing approaches to material selection. Here, for illustration, we will only look at how to approach some material properties. One basic technique is to list all the important material properties associated with the design, e.g., strength, stiffness, and cost. This can be prioritized by using a weighting measure depending on what properties are more important than others. Next, for each property, list all available materials and rank them in order beginning with the best material; e.g., for strength, high-strength steel such as 4340 steel should be near the top of the list. For completeness of available materials, this might require a large source of material data. Once the lists are formed, select a manageable amount of materials from the top of each list. From each reduced list select the materials that are contained within every list for further review. The materials in the reduced lists can be graded within the list and then weighted according

to the importance of each property.

M. F. Ashby has developed a powerful systematic method using materials selection charts.16 This method has also been implemented in a software package called CES Edupack.17 The charts display data of various properties for the families and classes of materials listed in Table 2–4. For example, considering material stiffness properties, a simple bar chart plotting Young’s modulus E on the y axis is shown in Fig. 2–15. Each vertical line represents the range of values of E for a particular material. Only some of the materials are labeled. Now, more material information can be displayed if the x axis represents another material property, say density.

Mechanical Engineering

McGraw−Hill Primis

ISBN: 0−390−76487−6

Text:

Shigley’s Mechanical Engineering Design,

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The six phases of the generic development process

The generic product development process consists of six phases, as illustrated in

Exhibit 2-2. The process begins with a planning phase, which is the link to advanced research and technology development activities. The output of the planning phase is the project's mission statement, which is the input required to begin the concept development phase and which serves as a guide to the development team. The conclusion of the product development process is the product launch, at which time the product becomes available for purchase in the marketplace. One way to think about the development process is as the initial creation of a wide set of alternative product concepts and then the subsequent narrowing of alternatives and increasing specification of the product until the product can be reliably and repeatably produced by the production system. Note that most of the phases of development are defined in terms of the state of the product, although the production process and marketing plans, among other tangible outputs, are also evolving as development progresses.

Another way to think about the development process is as an information-processing system. The process begins with inputs such as the corporate objectives and the capabilities of available technologies, product platforms, and production systems. Various activities process the development information, formulating specifications, concepts, and design details. The process concludes when all the information required to support production and sales has been created and communicated.

A third way to think about the development process is as a risk management system. In the early phases of product development, various risks are identified and prioritized. As the process progresses, risks are reduced as the key uncertainties are eliminated and the functions of the product are validated. When the process is completed, the team should have substantial confidence that the product will work correctly and be well received by the market.

Exhibit 2-2 also identifies the key activities and responsibilities of the different functions of the organization during each development phase. Because of their continuous involvement in the process, we choose to articulate the roles of marketing, design, and manufacturing. Representatives from other functions, such as research, finance, field service, and sales, also play key roles at particular points in the process.

The six phases of the generic development process are:

o. Planning: The planning activity is often referred to as "phase zero" since it precedes the project approval and launch of the actual product development process. This phase begins with corporate strategy and includes assessment of technology developments and market objectives. The output of the planning phase is the project mission statement, which specifies the target market for the product, business goals, key assumptions, and constraints. Chapter 3, Product Planning, presents a discussion of this planning process.

1. Concept development:

In the concept development phase, the needs of the target market are identified, alternative product concepts are generated and evaluated, and one or more concepts are selected for further development and testing. A concept is a description of the form, function, and features of a product and is usually accompanied by a set of specifications, an analysis of competitive products, and an economic justification of the project. This book presents several detailed methods for the concept development phase (Chapters 4-8). We expand this phase into each of its constitutive activities in the next section.

2. System-level design:

The system-level design phase includes the definition of the product architecture and the decomposition of the product into subsystems and components. The final assembly scheme for the production system is usually defined during this phase as well. The output of this phase usually includes a geometric layout of the product, a functional specification of each of the product's subsystems, and a preliminary process flow diagram for the final assembly process. Chapter 9, Product Architecture, discusses

some of the important activities of system-level design.

3. Detail design:

The detail design phase includes the complete specification of the geometry, materials, and tolerances of all of the unique parts in the product and the identification of all of the standard parts to be purchased from suppliers. A process plan is established and tooling is designed for each part to be fabricated within the production system. The output of this phase is the control documentation for the product-the drawings

or computer files describing the geometry of each part and its production tooling,

the specifications of the purchased parts, and the process plans for the fabrication and assembly

of the product. Two critical issues addressed in the detail design phase are production

cost and robust performance. These issues are discussed respectively in Chapter 11,

Design for Manufacturing, and Chapter 13, Robust Design.

4. Testing and refinement:

The testing and refinement phase involves the construction

and evaluation of multiple preproduction versions of the product. Early

(alpha) prototypes are usually built with production-intent parts-parts with the same

geometry and material properties as intended for the production version of the product

but not necessarily fabricated with the actual processes to be used in production.

Alpha prototypes are tested to determine whether the product will work as designed

and whether the product satisfies the key customer needs. Later (beta) prototypes are

usually built with parts supplied by the intended production processes but may not be

assembled using the intended final assembly process. Beta prototypes are extensively

evaluated internally and are also typically tested by customers in their own use environment.

The goal for the beta prototypes is usually to answer questions about performance

and reliability in order to identify necessary engineering changes for the final

product. Chapter 12, Prototyping, presents a thorough discussion of the nature and use

of prototypes.

5. Production ramp-up: In the production ramp-up phase, the product is made using

the intended production system. The purpose of the ramp-up is to train the work force

and to work out any remaining problems in the production processes. Products produced

during production ramp-up are sometimes supplied to preferred customers and are carefully

evaluated to identify any remaining flaws. The transition from production ramp-up

to ongoing production is usually gradual. At some point in this transition, the product is

launched and becomes available for widespread distribution.

References and Bibliography

Many current resources are available on the Internet via

www.ulrich-eppinger.net

Stage-gate product development processes have been dominant in manufacturing firms

for the past 30 years. Cooper describes the modem stage-gate process and many of its

enabling practices.

Cooper, Robert G., Winning at New Products: Accelerating the Process from Idea to

Launch, third edition, Perseus Books, Cambridge, MA, 2001.

Composite Materials

Composite materials are formed from two or more dissimilar materials, each of which contributes to the final properties. Unlike metallic alloys, the materials in a composite remain distinct from each other at the macroscopic level. Most engineering composites consist of two materials: a reinforcement called a filler and a matrix. The filler provides stiffness and strength; the matrix holds the material together and serves to transfer load among the discontinuous reinforcements. The

most common reinforcements, illustrated in Fig. 2–14, are continuous fibers, either straight or woven, short chopped fibers, and particulates. The most common matrices are various plastic resins although other materials including metals are used.

Metals and other traditional engineering materials are uniform, or isotropic, in nature. This means that material properties, such as strength, stiffness, and thermal conductivity, are independent of both position within the material and the choice of coordinate system. The discontinuous nature of composite reinforcements, though, means that material properties can vary with both position and direction. For example, an epoxy resin reinforced with continuous graphite fibers will have very high strength and

stiffness in the direction of the fibers, but very low properties normal or transverse to the fibers. For this reason, structures of composite materials are normally constructed of multiple plies (laminates) where each ply is oriented to achieve optimal structural stiffness and strength performance.

High strength-to-weight ratios, up to 5 times greater than those of high-strength steels, can be achieved. High stiffness-to-weight ratios can also be obtained, as much as 8 times greater than those of structural metals. For this reason, composite materials are becoming very popular in automotive, aircraft, and spacecraft applications where weight is a premium.

The directionality of properties of composite materials increases the complexity of structural analyses. Isotropic materials are fully defined by two engineering constants:

Young’s modulus E and Poisson’s ratio ν. A single ply of a composite material, however, requires four constants, defined with respect to the ply coordinate system. The constants are two Young’s moduli (the longitudinal modulus in the direction of the fibers, E1, and the transverse modulus normal to the fibers, E2), one Poisson’s ratio (ν12, called the major Poisson’s ratio), and one shear modulus (G12). A fifth constant,

the minor Poisson’s ratio, ν21, is determined through the reciprocity relation, ν21/E2 = ν12/E1. Combining this with multiple plies oriented at different angles makes structural analysis of complex structures unapproachable by manual techniques. For this reason, computer software is available to calculate the properties of a laminated composite construction.

Mechanical Engineering

McGraw−Hill Primis

ISBN: 0−390−76487−6

Text:

Shigley’s Mechanical Engineering Design,

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Budynas−Nisbett

A Generic Development Process

A process is a sequence of steps that transforms a set of inputs into a set of outputs. Most people are familiar with the idea of physical processes, such as those used to bake a cake or to assemble an automobile. A product development process is the sequence of steps or activities which an enterprise employs to conceive, design, and commercialize a product. Many of these steps and activities are intellectual and organizational rather than physical.

Some organizations define and follow a precise and detailed development process, while others may not even be able to describe their processes. Furthermore, every organization employs a process at least slightly different from that of every other organization. In fact, the same enterprise may follow different processes for each of several different types of development projects.

A well-defined development process is useful for the following reasons:

• Quality assurance: A development process specifies the phases a development project will pass through and the checkpoints along the way. When these phases and checkpoints are chosen wisely, following the development process is one way of assuring the quality of the resulting product.

• Coordination: A clearly articulated development process acts as a master plan which defines the roles of each of the players on the development team. This plan informs the members of the team when their contributions will be needed and with whom they will need to exchange information and materials.

• Planning: A development process contains natural milestones corresponding to the completion of each phase. The timing of these milestones anchors the schedule of the overall development project.

• Management: A development process is a benchmark for assessing the performance of an ongoing development effort. By comparing the actual events to the established process, a manager can identifY possible problem areas.

• Improvement: The careful documentation of an organization's development process often helps to identifY opportunities for improvement.

References and Bibliography

Many current resources are available on the Internet via

www.ulrich-eppinger.net

Stage-gate product development processes have been dominant in manufacturing firms

for the past 30 years. Cooper describes the modem stage-gate process and many of its

enabling practices.

Cooper, Robert G., Winning at New Products: Accelerating the Process from Idea to

Launch, third edition, Perseus Books, Cambridge, MA, 2001.

Plastics

The term thermoplastics is used to mean any plastic that flows or is moldable when heat is applied to it; the term is sometimes applied to plastics moldable under pressure. Such plastics can be remolded when heated.

A thermoset is a plastic for which the polymerization process is finished in a hot molding press where the plastic is liquefied under pressure. Thermoset plastics cannot be remolded.

Table 2–2 lists some of the most widely used thermoplastics, together with some of their characteristics and the range of their properties. Table 2–3, listing some of the thermosets, is similar. These tables are presented for information only and should not be used to make a final design decision. The range of properties and characteristics that can be obtained with plastics is very great. The influence of many factors, such as cost, moldability, coefficient of friction, weathering, impact strength, and the effect of fillers and reinforcements, must be considered. Manufacturers’ catalogs will be found quite helpful in making possible selections.

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The Challenges of Product Development

Developing great products is hard. Few companies are highly successful more than half the time. These odds present a significant challenge for a product development team. Some of the characteristics that make product development challenging are:

• Trade-offs: An airplane can be made lighter, but this action will probably increase

manufacturing cost. One of the most difficult aspects of product development is recognizing, understanding, and managing such trade-offs in a way that maximizes the success of the product.

• Dynamics: Technologies improve, customer preferences evolve, competitors introduce new products, and the macroeconomic environment shifts. Decision making in an environment of constant change is a formidable task.

• Details: The choice between using screws or snap-fits on the enclosure of a computer can have economic implications of millions of dollars. Developing a product of even modest complexity may require thousands of such decisions.

• Time pressure: Anyone of these difficulties would be easily manageable by itself given plenty of time, but product development decisions must usually be made quickly and without complete information.

• Economics: Developing, producing, and marketing a new product requires a large investment. To earn a reasonable return on this investment, the resulting product must be both appealing to customers and relatively inexpensive to produce. For many people, product development is interesting precisely because it is challenging. For others, several intrinsic attributes also contribute to its appeal:

• Creation: The product development process begins with an idea and ends with the production of a physical artifact. When viewed both in its entirety and at the level of individual activities, the product development process is intensely creative.

• Satisfaction of societal and individual needs: All products are aimed at satisfying needs of some kind. Individuals interested in developing new products can almost always find institutional settings in which they can develop products satisfying what they consider to be important needs.

• Team diversity: Successful development requires many different skills and talents. As

a result, development teams involve people with a wide range of different training, experience,

perspectives, and personalities.

• Team spirit: Product development teams are often highly motivated, cooperative groups. The team members may be colocated so they can focus their collective energy on creating the product. This situation can result in lasting camaraderie among team members.

References and Bibliography

A wide variety of resources for this chapter and for the rest of the book are available on the Internet. These resources include data, templates, links to suppliers, and lists of publications. Current resources may be accessed via www.ulrich-eppinger.net Wheelwright and Clark devote much of their book to the very early stages of product development, which we cover in less detail. Wheelwright, Stephen c., and Kim B. Clark, Revolutionizing Product Development: Quantum Leaps in Speed, Efficiency, and Quality, The Free Press, New York, 1992. Katzenbach and Smith write about teams in general, but most of their insights apply to product development teams as well. Katzenbach, Jon R., and Douglas K. Smith, The Wisdom of Teams: Creating the High-Performance Organization, Harvard Business School Press, Boston, 1993. These three books provide rich narratives of development projects, including fascinating descriptions of the intertwined social and technical processes. Kidder, Tracy, The Soul of a New Machine, Avon Books, New York, 1981.

Sabbagh, Karl, Twenty-First-Century Jet: The Making and Marketing of the Boeing 777, Scribner, New York, 1996. Walton, Mary, Car: A Drama of the American Workplace, Norton, New York, 1997.

Duration and Cost of Product Development

Most people without experience in product development are astounded by how much time and money are required to develop a new product. The reality is that very few products can be developed in less than 1 year, many require 3 to 5 years, and some take as long as 10 years. Exhibit 1-1 shows five engineered, discrete products. Exhibit 1-3 is a table showing the approximate scale of the associated product development efforts along with some distinguishing characteristics of the products.

The cost of product development is roughly proportional to the number of people on the project team and to the duration of the project. In addition to expenses for development effort, a firm will almost always have to make some investment in the tooling and equipment required for production. This expense is often as large as the rest of the product development budget; however, it is sometimes useful to think of these expenditures as part of the fixed costs of production. For reference purposes, this production investment is listed in Exhibit 1-3 along with the development expenditures.

References and Bibliography

Sabbagh, Karl, Twenty-First-Century Jet: The Making and Marketing of the Boeing 777, Scribner, New York, 1996. Walton, Mary, Car: A Drama of the American Workplace, Norton, New York, 1997.

Copper-Base Alloys, Brass

When copper is alloyed with zinc, it is usually called brass. If it is alloyed with another

element, it is often called bronze. Sometimes the other element is specified too, as, for example, tin bronze or phosphor bronze. There are hundreds of variations in each category.

Brass with 5 to 15 Percent Zinc The low-zinc brasses are easy to cold work, especially those with the higher zinc content. They are ductile but often hard to machine. The corrosion resistance is good. Alloys included in this group are gilding brass (5 percent Zn), commercial bronze (10 percent Zn), and red brass (15 percent Zn). Gilding brass is used mostly for jewelry and articles to be gold-plated; it has the same ductility as copper but greater strength, accompanied by poor machining characteristics. Commercial bronze is used for jewelry and for forgings and stampings, because of its ductility. Its machining properties are poor, but it has excellent cold-working properties. Red brass has good corrosion resistance as well as high-temperature strength. Because of this it is used a great deal in the form of tubing or piping to carry hot water in such applications as radiators or condensers.

Brass with 20 to 36 Percent Zinc

Included in the intermediate-zinc group are low brass (20 percent Zn), cartridge brass (30 percent Zn), and yellow brass (35 percent Zn). Since zinc is cheaper than copper, these alloys cost less than those with more copper and less zinc. They also have better machinability and slightly greater strength; this is offset, however, by poor corrosion resistance and the possibility of cracking at points of residual stresses. Low brass is very

similar to red brass and is used for articles requiring deep-drawing operations. Of the copper-zinc alloys, cartridge brass has the best combination of ductility and strength. Cartridge cases were originally manufactured entirely by cold working; the process consisted of a series of deep draws, each draw being followed by an anneal to place the material in condition for the next draw, hence the name cartridge brass. Although the hot-working ability of yellow brass is poor, it can be used in practically any other fabricating

process and is therefore employed in a large variety of products.

When small amounts of lead are added to the brasses, their machinability is greatly improved and there is some improvement in their abilities to be hot-worked. The addition of lead impairs both the cold-working and welding properties. In this group are low-leaded brass (32 1 /2 percent Zn, 1/ 2 percent Pb), high-leaded brass (34 percent Zn, 2 percent Pb), and free-cutting brass (35 1/2 percent Zn, 3 percent Pb). The low-leaded brass is not only easy to machine but has good cold-working properties. It is used for

various screw-machine parts. High-leaded brass, sometimes called engraver’s brass, is used for instrument, lock, and watch parts. Free-cutting brass is also used for screwmachine parts and has good corrosion resistance with excellent mechanical properties. Admiralty metal (28 percent Zn) contains 1 percent tin, which imparts excellent corrosion resistance, especially to saltwater. It has good strength and ductility but only

fair machining and working characteristics. Because of its corrosion resistance it is used in power-plant and chemical equipment. Aluminum brass (22 percent Zn) contains 2 percent aluminum and is used for the same purposes as admiralty metal, because it has nearly the same properties and characteristics. In the form of tubing or piping, it is favored over admiralty metal, because it has better resistance to erosion caused by highvelocity water.

Brass with 36 to 40 Percent Zinc

Brasses with more than 38 percent zinc are less ductile than cartridge brass and cannot be cold-worked as severely. They are frequently hot-worked and extruded. Muntz metal (40 percent Zn) is low in cost and mildly corrosion-resistant. Naval brass has the same composition as Muntz metal except for the addition of 0.75 percent tin, which contributes to the corrosion resistance.

Mechanical Engineering

McGraw−Hill Primis

ISBN: 0−390−76487−6

Text:

Shigley’s Mechanical Engineering Design,

Eighth Edition

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Who Designs and Develops Products?

Product development is an interdisciplinary activity requiring contributions from nearly all the functions of a firm; however, three functions are almost always central to a product development project:

• Marketing: The marketing function mediates the interactions between the firm and its customers. Marketing often facilitates the identification of product opportunities, the definition of market segments, and the identification of customer needs. Marketing also typically arranges for communication between the firm and its customers, sets target prices, and oversees the launch and promotion of the product.

• Design: The design function plays the lead role in defining the physical form of the product to best meet customer needs. In this context, the design function includes engineer; ing design (mechanical, electrical, software, etc.) and industrial design (aesthetics, ergonomics, user interfaces).

• Manufacturing: The manufacturing function is primarily responsible for designing, operating, and/or coordinating the production system in order to produce the product. Broadly defined, the manufacturing function also often includes purchasing, distribution, and installation. This collection of activities is sometimes called the supply chain.

Different individuals within these functions often have specific disciplinary training in areas such as market research, mechanical engineering, electrical engineering, materials science, or manufacturing operations. Several other functions, including finance and sales, are frequently involved on a part-time basis in the development of a new product.

Beyond these broad functional categories, the specific composition of a development team depends on the particular characteristics of the product. Few products are developed by a single individual. The collection of individuals developing a product forms the project team. This team usually has a single team leader, who could be drawn from any of the functions of the firm. The team can be thought of as consisting of a core team and an extended team. In order to work together effectively, the core team usually remains small enough to meet in a conference room, while the extended team may consist of dozens, hundreds, or even thousands of other members. (Even though the term team is inappropriate for a group of thousands, the word is often used in this context to emphasize that the group must work toward a common goal.) In most cases, a team within the firm will be supported by individuals or teams at partner companies, suppliers, and consulting firms . Sometimes, as is the case for the development of a new airplane, the number of external team members may be even greater than that of the team within the company whose name will appear on the final product. The composition of a team for the development of an electromechanical product of modest complexity is

shown in Exhibit 1-2.

Throughout this book we assume that the team is situated within a firm. In fact, a for-profit manufacturing company is the most common institutional setting for product development, but other settings are possible. Product development teams sometimes work within consulting firms, universities, government agencies, and nonprofit organizations.

References and Bibliography

Exercises

Wheelwright and Clark devote much of their book to the very early stages of product development, which we cover in less detail.

Wheelwright, Stephen c., and Kim B. Clark, Revolutionizing Product Development: Quantum Leaps in Speed, Efficiency, and Quality, The Free Press, New York, 1992.

Katzenbach and Smith write about teams in general, but most of their insights apply to product development teams as well.

Katzenbach, Jon R., and Douglas K. Smith, The Wisdom of Teams: Creating the High-Performance Organization, Harvard Business School Press, Boston, 1993.

These three books provide rich narratives of development projects, including fascinating descriptions of the intertwined social and technical processes.

Kidder, Tracy, The Soul of a New Machine, Avon Books, New York, 1981.

Sabbagh, Karl, Twenty-First-Century Jet: The Making and Marketing of the Boeing 777, Scribner, New York, 1996.

Walton, Mary, Car: A Drama of the American Workplace, Norton, New York, 1997.

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Titanium

Titanium and its alloys are similar in strength to moderate-strength steel but weigh half

as much as steel. The material exhibits very good resistence to corrosion, has low thermal conductivity, is nonmagnetic, and has high-temperature strength. Its modulus of elasticity is between those of steel and aluminum at 16.5 Mpsi (114 GPa). Because of its many advantages over steel and aluminum, applications include: aerospace and military aircraft structures and components, marine hardware, chemical tanks and processing equipment, fluid handling systems, and human internal replacement devices. The disadvantages of titanium are its high cost compared to steel and aluminum and the difficulty of machining it.

Mechanical Engineering

McGraw−Hill Primis

ISBN: 0−390−76487−6

Text:

Shigley’s Mechanical Engineering Design,

Eighth Edition

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Characteristics of Successful Product Development

From the perspective of the investors in a for-profit enterprise, successful product development results in products that can be produced and sold profitably, yet profitability is often difficult to assess quickly and directly. Five more specific dimensions, all of which ultimately relate to profit, are commonly used to assess the performance of a product development effort:

• Product quality: How good is the product resulting from the development effort? Does it satisfy customer needs? Is it robust and reliable? Product quality is ultimately reflected in market share and the price that customers are willing to pay.

• Product cost: What is the manufacturing cost of the product? This cost includes spending on capital equipment and tooling as well as the incremental cost of producing each unit of the product. Product cost determines how much profit accrues to the firm for a particular sales volume and a particular sales price.

• Development time: How quickly did the team complete the product development effort?

Development time determines how responsive the firm can be to competitive forces and to technological developments, as well as how quickly the firm receives the economic returns from the team's efforts.

• Development cost: How much did the firm have to spend to develop the product? Development

cost is usually a significant fraction of the investment required to achieve the profits.

• Development capability: Are the team and the firm better able to develop future products as a result of their experience with a product development project? Development capability is an asset the firm can use to develop products more effectively and economically in the future.

High performance along these five dimensions should ultimately lead to economic success; however, other performance criteria are also important. These criteria arise from interests of other stakeholders in the enterprise, including the members of the development team, other employees, and the community in which the product is manufactured.

Members of the development team may be interested in creating an inherently exciting product. Members of the community in which the product is manufactured may be concerned about the degree to which the product creates jobs. Both production workers and users of the product hold the development team accountable to high safety standards, whether or not these standards can be justified on the strict basis of profitability. Other

individuals, who may have no direct connection to the firm or the product, may demand that the product make ecologically sound use of resources and create minimal dangerous waste products.