|Title:||Octree reinforced thin shell objects rapid prototyping|
Hong Kong Polytechnic University -- Dissertations
|Pages:||xi, 92 leaves : ill. ; 30 cm. + computer disk|
|Abstract:||Rapid prototyping has been widely accepted as an essential element in speeding up industrial practice in recent years. Rapid prototyping can be considered as a three dimensional printing process to produce prototype layer by layer. It converts the three-dimensional (3D) geometric data in computer aided design (CAD) model to a realistic prototype. The process is rapid because a prototype can be built in hours instead of days or weeks when compares with traditional methods. The comparison includes the tooling and part programming time in the prototype development cycle. The main advantage of rapid prototyping is to improve the 'time to market' and the design quality of a product. The information flow of rapid prototyping depends on the conversion of 3D CAD model to Stereolithography (STL) format. Then the contours of cross-section profiles are generated from STL data. The prototype is produced from the contour information. It is found that the tracing of the cross-sectional area is the most time consuming step of the rapid prototyping process. The process can be speeded up if the material volume to be traced can be reduced. There are two possible ways to reduce the material volume: 1. Pre-process the three dimensional CAD model before converting to STL format. 2. Post-process the solid cross-sections prior to produce the prototype. As the interfaces from cross-section slicing to prototype tracing are not disclosed by the rapid prototyping machine manufacturers, it is more convenient to reduce the material volume by pre-processing the CAD model. The material volume reduction can be achieved by extracting some empty volumes in the original solid. The empty volumes can be generated by negative offset so that the hollow solid has the volume of the original solid minus the offset solid. However, the generated hollow solid is subjected to the problem of lack of interior support. In this dissertation, the sub-boundary octree approach to produce a thin-shell-model using rapid prototypers model is investigated. A sub-boundary octree means an octree that is wholly in the interior of the original solid. The employment of solids of reduced volume with reinforced interior structuring is considered. The operation to difference the evaluated octree from the solid is studied. This is done by unioning the octree solid with the hollow solid from negative offset. The algorithms of sub-boundary octree skeleton extraction and edge merging are developed to create the octree skeleton with minimum number of Boolean operations. An efficient data structure is derived to represent the sub-boundary octree and octree skeleton. The system is implemented within Win'95 environment by utilizing Visual C++, AutoLISP and AutoCAD. Various thin shell models are produced in rapid prototypers to verify the concepts developed.|
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