Data-structure for the description and handling of engineering drawings

Abstract

The specifications of a data-structure are essentially linked to the representative model, to the functional links to be obtained amongst the various data and the action to be taken on such structure 1,2. An engineering drawing is the two-dimensional representation obtaibed according to precise accepted standards of three-dimensional objects. The data-structure, therefore, which represents an engineering drawing must contain all the connections among the two-dimensional graphic elements of which the drawing is composed. It should also contain a complete description of the three-dimensional object represented to allow action on the drawing at any level through its two-dimensional representation. On the other hand, an analysis of the various phases of the design process which lead to the production of final working drawings will show that the three-dimensional model of the mechanical part becomes increasingly detailed and complex, as are the drawings used to illustrate it. The quantity of drawings which accompany the various phases of the design process increases and the types of action to be taken on such drawings refer to an ever greater extent to details which do not substantially alter the piece from a functional point of view, but which complicate its shape to an appreciable degree. Furthermore, at such a level, the drawing contains a large amount of data of a technological and constructive nature which are also continually changed or amended. The use of computer-aided drafting systems is justified where large numbers of considerably complex drawings are involved. During the final stage of mechanical design where changes essentially concern details (from the point of view of the shape of the piece); it would be unreasonable at this level to use for the storage of the drawing, a data-structure which contains a complete and faithful representation of the illustrated mechanical part with its great complexity 3. It will therefore be sufficient for the data-structure for handling the drawings to contain only those functional links which are of interest at the detailed design stage.