DESIGN FOR MANUFACTURING:DESIGN AND DESIGN ALTERNATIVES

1. DESIGN AND DESIGN ALTERNATIVES

The essence of design is that it is a plan to achieve a purpose or to satisfy a need. In mechanical design, the plan is a representation, such as a set of drawings defining the configuration (geometry and material) of physical elements.

The immediate purpose of a specific set of physical elements or a specific design is the functional requirement. The design process, at this level, is to start with the known functional requirement to plan or search for the design configurations.

The design solution is almost always not unique. Conceptually, the design process can be con- sidered a mapping process between the ‘‘purpose space’’ and the ‘‘functional space’’ and between the ‘‘functional space’’ and ‘‘configuration space.’’ The ability to develop alternative physical designs is of fundamental importance to design for manufacturability.

Alternative physical designs may be developed by knowing the functional requirement. A design, in general, can be decomposed into subfunctional requirements for each of its subsystems. Each subfunctional requirement, again, can be used to characterize and develop the design alternatives of each subsystem. By repeating this process, a functional design hierarchy can be developed with the possible design alternatives at various levels of functional requirements, for the product (the assem- bly), the subassembly, and parts. This design hierarchy is shown in Figure 4 (Liu and Trappey 1989).

The properties of the design hierarchy are as follows:

1. All the subfunctions are required to form a design, and only one among all the physical design alternatives is needed to satisfy a specific functional requirement.

2. Each and every possible physical design, for a system or a subsystem, has place in the hier- archy. Therefore, the hierarchy serves as a guide for design knowledge acquisition, as a struc- ture for design knowledge storage, and as an indexing system for design knowledge retrieval. This has important application in serving as a software tool for concurrent engineering.

3. Upstream reasoning from the physical design can be conducted by answering the question ‘‘What is the design for?’’ Then the higher level functional requirement may be reached.

4. Downstream reasoning from functional requirement can be done by answering the question ‘‘How can the functional requirement be satisfied?’’ Then the physical design alternatives may be generated.

5. Upstream–downstream reasoning forces the designer to analyze the functional requirements and higher purposes. Thus, it can be used for managing the design process and yet, in the meantime, allow for the individual designer’s creativity (see Figure 2).

6. The hierarchical system can serve as structured blackboard for design communication, con- sultation, and retrieval.

The application of the design hierarchy by one of the authors (Liu) has led to very significant product innovation. When the same method was applied by the students in his classes, general improvement in design creativity was observed. However, the results varied tremendously among the individuals.

More discussions and elaboration of the proposed functional–physical design hierarchy were done in Liu and Trappey (1989) and Trappey and Liu (1990).