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The design of a mechanical product usually takes place primarily in a top-down fashion, where the designer first generates a rough, overall sketch of the product and its main components. Later, the designer refines the sketch to a detailed level while taking into account the relevant requirements posed by strength, cost, manufacturability, serviceability, and other similar considerations. Current computer-aided design (CAD) systems provide only limited support for this kind of work. For instance, they cannot deal with geometric or other information at varying levels of detail, nor do they capture explicitly geometric relationships among components intended to be joined together in an assembly. This paper describes early results of ongoing research on supporting top-down design of mechanical products and discusses the major requirements for CAD systems used for top-down design. A prototype design system is described that provides the following characteristics not usually found in ordinary CAD systems: structuring of product information in several layers, according to the stage of the design process; representation of geometric information about components at several levels of detail; and representation and maintenance of geometric relationships of components by means of a constraint-satisfaction mechanism.
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