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Generating the medial surface for a general boundary representation model raises several difficulties. Problems might emerge from the complexity of the resulting equations, singularities caused by unforeseen relative boundary element positions and orientations, etc. The majority of the current algorithms are based on the topology of the boundary representation model and produce wireframes composed of straight lines regardless of the real medial surfaces. Many of the solids used in engineering can be represented by extrusions, delimited by a cross-section and an extrusion distance. This paper develops a fast and efficient method for creating the facetted approximations of the medial surfaces of extrusions generated by sweeping along the normal direction to the generating cross-section.

During the conceptual phase of product design, rapid generation of the rough assembly description and capture of the product layout should be supported. This work introduces a top-down approach for creating the product layout and facilitating subsequent detailed design. For this purpose design spaces are introduced. These design spaces represent the conceptual boundary of the components as well as references for defining relationships between the functional parts of the assembly. These functional units can be connected together by means of interface features and constraints. Once the design spaces and the relationships have been set up correctly the task of refining the components geometrically can be distributed and done in parallel. This concurrent work is controlled with access rights and well-defined responsibility areas. Since the layout and the component level are strictly delimited, kinematic analysis as well as layout modifications can be done at any stage of the product design without any explicit information about the geometry of the components. The presented top-down method, called 3D Layout Modeller (3DLM), facilitates capturing the human intention of the constructional design by providing tools and methods for doing this in a natural manner. This means that the design process starts with the overall definition of the functional behaviour of the product and the detailed description of the components remains for the design refinement stages (“minimum commitment modelling”). Since there must be well defined connections between the components, the project can be separated into different tasks. It is possible that a task can also be further divided, which indicates the hierarchical nature of the method.

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