Design Process

Before any discussion of CAD, it is necessary to understand the design process in general (Fig. 3). What are the series of events that lead to the beginning of a design project? How does the engineer go about the process of designing something? How does one arrive at the conclusion that the design has been completed? We address these questions by defining the process in terms of six distinct stages:

1. Customer input and perception of need

2. Problem definition

3. Synthesis

4. Analysis and optimization

5. Evaluation

6. Final design and specification

A need is usually perceived in one of two ways. Someone must recognize either a problem in an existing design or a customer-driven opportunity in the marketplace for a new product. In either case, a need exists which can be addressed by modifying an existing design or developing an entirely new design. Because the need for change may only be indicated by subtle circumstances—such as noise, marginal performance characteristics, or deviations from quality standards—the design engineer who identifies the need has taken a first step in correcting the problem. That step sets in motion processes that may allow others to see the

need more readily and possibly enroll them in the solution process.

Once the decision has been made to take corrective action to the need at hand, the problem must be defined as a particular problem to be solved such that all significant parameters in the problem are defined. These parameters often include cost limits, quality standards, size and weight characteristics, and functional characteristics. Often, specifications may be defined by the capabilities of the manufacturing process. Anything that will influence the engineer in choosing design features must be included in the definition of the problem.

Careful planning in this stage can lead to fewer iterations in subsequent stages of design. 

Once the problem has been fully defined in this way, the designer moves on to the synthesis stage, where knowledge and creativity can be applied to conceptualize an initial design. Teamwork can make the design more successful and effective at this stage. That design is then subjected to various forms of analysis, which may reveal specific problems in the initial design. The designer then takes the analytical results and applies them in an iteration of the synthesis stage. These iterations may continue through several cycles of

synthesis and analysis until the design is optimized. 

The design is then evaluated according to the parameters set forth in the problem definition. A scale prototype is often fabricated to perform further analysis and to assess operating performance, quality, reliability, and other criteria. If a design flaw is revealed during this stage, the design moves back to the synthesis / analysis stages for reoptimization, and the process moves in this circular manner until the design clears the evaluative stage and is ready for presentation.

Final design and specification represent the last stage of the design process. Communicating  the design to others in such a way that its manufacture and marketing are seen as vital to the organization is essential. When the design has been fully approved, detailed engineering drawings are produced, complete with specifications for components, subassemblies, and the tools and fixtures required to manufacture the product and the associated costs of production. These can then be transferred manually or digitally using the CAD data to

the various departments responsible for manufacture.

In every branch of engineering, prior to the implementation of CAD, design has traditionally been accomplished manually on the drawing board. The resulting drawing, complete with significant details, was then subjected to analysis using complex mathematical formulas and then sent back to the drawing board with suggestions for improving the design. The same iterative procedure was followed, and because of the manual nature of the drawing and the subsequent analysis, the whole procedure was time consuming and labor intensive. CAD has allowed the designer to bypass much of the manual drafting and analysis that was previously required, making the design process flow much more smoothly and efficiently. 

It is helpful to understand the general product development process as a stepwise process. However, in today’s engineering environment, the steps outlined above have become consolidated into a more streamlined approach called concurrent engineering. This approach enables teams to work concurrently by providing common ground for interrelated product development tasks. Product information can be easily communicated among all development processes: design, manufacturing, marketing, management, and supplier networks. Concurrent engineering recognizes that fewer iterations result in less time and money spent in  moving from concept to manufacture and from manufacturing to market. The related processes  of design for manufacturing (DFM) and design for assembly (DFA) have become integral parts of the concurrent engineering approach.

DFM and DFA methods use cross-disciplinary input from a variety of sources (e.g., design engineers, manufacturing engineers, suppliers, and shop-floor representatives) to facilitate the efficient design of a product that can be manufactured, assembled, and marketed in the shortest possible period of time. Often, products designed using DFM and DFA are simpler, cost less, and reach the marketplace in far less time than traditionally designed products. DFM focuses on determining what materials and manufacturing techniques will result in the most efficient use of available resources in order to integrate this information early in the design process. The DFA methodology strives to consolidate the number of parts, uses gravity-assisted assembly techniques, and calls for careful review and consensus approval of designs early in the process. By facilitating the free exchange of information, DFM and DFA methods allow engineering companies to avoid the costly rework often associated with repeated iterations of the design process.

In an attempt to define the stages or components of the design process, many definitions exist today which can vary from one to another. However, they all share a common thread that includes a needs statement by identifying the problem, a search for possible solutions, analysis and development of the solutions, testing, and finally usage of the final product. 

These kinds of descriptions of the design process are commonly called models of the design process. Figure 4 is an example of a model created by Pahl and Beitz in 1984 which was composed of four main phases:

• Clarification of Task. Involves collecting information about the design requirements and the constraints on the design and describing these in a specification.

• Conceptual Design. Involves establishment of the functions to be included in the design and identification and development of suitable solutions.

• Embodiment Design. The conceptual solution is developed in more detail, problems are resolved, and weak aspects are eliminated.

• Detail Design. The dimensions, tolerances, materials, and forms of individual components of the design are specified in detail for subsequent manufacture.

Figures 4 and 5 both depict two traditional methods of describing the design process, where there are sequential stages of design, with the manufacturing to follow. However, the current trend in manufacturing is to expedite the process by encouraging the design, development, analysis, and preparation of manufacturing information to be done simultaneously.

This kind of engineering has previously been used in companies that produced established products or are constantly producing new products. The terms concurrent engineering, and simultaneous engineering have been coined to describe this type of engineering. This will be addressed in more depth in Chapter 17. Here, we will keep the sequential models of the design process during our discussion of modeling and communication in design.

Emory W. Zimmers, Jr. and Technical Staff

Enterprise Systems Center

Lehigh University

Bethlehem, Pennsylvania

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz

Copyright  2006 by John Wiley & Sons, Inc.

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