DESIGN FOR MANUFACTURING:DESIGN FOR ASSEMBLY

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DESIGN FOR ASSEMBLY

The goal of DFA is to ease the assembly of the product. Boothroyd et al. (1994) propose a method for DFA that involves two principal steps:

• Designing with as few parts as possible. This is accomplished by analyzing parts pairwise to determine whether the two parts can be created as a single piece rather than as an assembly.

• Estimating the costs of handling and assembling each part using the appropriate assembly process to generate costs figures to analyze the cost savings through DFA.

• In addition to the assembly cost reductions through DFA, there are reductions in part costs that are more significant. Other benefits of DFA include improved reliability and reduction in in- ventory and production control costs. Consequently, DFA should be applied regardless of the assembly cost and product volume.

10. COMPUTER SOFTWARE TOOLS: OBJECT-ORIENTED PROGRAMMING AND KNOWLEDGE-BASED SYSTEMS

Modern CAD / CAM systems and computer-aided processing planning systems for machining are well known and are very important for integrating design and manufacturing. However, more work is needed to develop them into tools for helping design for manufacturability. We need a software system that can be easily modulized, expanded, alternated in its structures and contents, and integrated partially or fully. The key technology is a recently developed style and structure of programming called object-oriented programming (OOP).

Object-oriented programming supports four unique object functions or properties:

1. Abstraction: Abstraction is done by the creation of a ‘‘class protocol description’’ that defines the properties of any object that is an instance of that class.

2. Encapsulation: An object encapsulates all the properties (data and messages) of the specific instance of the class.

3. Inheritance: Some classes are subordinate to others and are called subclasses. Subclasses are considered to be special cases of the class under which they are grouped in the hierarchy. The variables and methods defined in the higher-level classes will be automatically inherited by the lower-level classes.

4. Polymorphism: Allows us to send the same message to different objects in different levels of class hierarchy. Each object responds in a way that is inherited or redefined with respect to the object’s characteristics.

With these properties, integrated and expandable software for supporting designs, including design for manufacturability, can be developed. An example is shown in Trappey and Liu (1990), who developed a system shell for design using the object-oriented programming language, SMALLTALK- 80 (Goldberg 1984).

Another key software technology for design for manufacturability, such as automated rule check- ing, is knowledge-based systems, or expert systems. The general methodology for building these systems roughly consists of five steps: identification, conceptualization, formalization, implementa- tion, and testing (Hayes-Roth et al. 1983). An example of this approach for fixture design for ma- chining is shown in Ferreira et al. (1985).

SNAG-0013

Current CAD systems have been oriented to detail design, while the application of DFM guide- lines to early design stages yields the largest benefits. Research is needed to lay the foundation for the CAD system for conceptual design so that DFM and CAD can be integrated successfully. Mu- kherjee and Liu (1995) propose a promising method. In the proposed representation, called sketching abstraction, the discretionary geometry of the part having functional relevance is captured using functional features, while the nondiscretionary geometry is represented using a linkage mechanism. The functional features are related to the part function using data structures called function–form matrices. They attempt to bridge the gap between function and form representations and provide the designer with a tool that can be used for generating design alternatives. Figure 6 is an example of this representation, which facilitates manufacturability evaluation in conceptual design (Mukherjee and Liu 1997).

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