Tool-path Generation Of Direct NC Machining For Complex STL Surface Model

In the recent years, as the rapid development of the aerospace, automotive,consumerised products and mould industry, the complex surfaceed compoments have been more commomly used. Triagular mesh models can be used to present the shapes of the complex parts and have the advantages of fast generation, flexible and adaptable represention ability. At present, triangular mesh models have been widely used in rapid prototyping and additive manufacturing process, quality inspection and remanufacturing engineering and other related fields. Direct NC (numerical control)machining of the triangular mesh model is an important method for rapid manufacturing, which can avoid the problems like the complex manual operations and uncertain surface quality caused by the surface reconstruction process. However, since a triangular mesh model is composed of mesh facet information only, there are several problems in NC code generation, machining precision and machining quality. It is necessary to carry out an entensive study in direct NC tool-path generation from the triangular mesh model. In this thesis, the author focuses on the study of the tool-path generation directly from a triangular mesh model. On the basis of triangular mesh machining experiment and defects analysis for CL (cutter location) path, a new tool-path generation method was proposed for direct machining of the triangular mesh models. The method includes the vertex offset algorithm for mesh model, the tessellation error compensation algorithm for cutter location point generation, and the bi-directional spline interpolation algorithm for tool-path generation. With the implemention of these algorithms, a NC programming system was developed to realise the direct mesh machining. With the system developed, the tool-path for the complex STL (stereo lithography) surface model can be generated for the direct mesh machining. The main contents of this study include the following aspects:(1) The direct NC machining methods for the complex STL surface model were thorough investigated, the current research status at home and abroad of this field were also studied. Based on the analysis of the tool-path generation methods directly from the triangular mesh model, the key technical problems existed in this area were finalised, and the strategy and study plan of this thesis were determined.(2) Based on the structure description of STL model file, the influence of mesh quality to the tool-path generation was analyzed. To solve the mesh quality problem, a topologic mesh reconstruction method was proposed to provide an effective model data for the tool-path generation. On the basis of CL path generation methods, this thesis performed the actual machining with two types of mesh models: one was a normal mesh model, and the other was a subdivided mesh model. With the surface analysis of the machined models, the problems of overcut-slot defects, transit-pit defects and facet defects on the machined surface were found. Thus, the current mesh machining through the CL path generation existed the problem of machining quality,which needs to be solved.(3) This thesis studied the vertex offset method for mesh model machining. The correction algorithm of boundary vertex vector based on boundary vertex adjacent facets completion and the error compensation algorithm for vertex offset based on adjacent facets weighted error were proposed in the study. With the improvement of the triangular mesh model offsetting, this study focused on the overcut problem caused by the calculation of the deviation of boundary vertex vector. Through the completion operation of the boundary vertex adjacent facets, the boundary vertex vector was corrected, and the actual machining problems on the boundary area can be eliminated. Based on the actual offset distance of the offset model, the overcut problem caused by the insufficient offset distance was analyzed. Considering the weight of the adjacent facets error, the actual offset error of offset model was compensated.(4) The method of cutter location point generation on the offset model was explored,the tessellation error compensation algorithm based on approximate contour curve intersection was proposed, and the accuracy problem caused by tessellation error can therefore be avoided. On the basis of tool-path mode selection and scallop height calculation, the drive plane intersection method for cutter location point generation was determined. The position relation of drive planes and facets was investigated, and the calculation method of intersection points was provided. The accuracy of intersection points caused by tessellation error was analyzed, and the surface defects on the workpiece surface are intensively reasearched. Through the tessellation error compensation, the accuracy of cutter location points can be improved.(5) The algorithm of tool-path generation and tool-path interpolation were studied.The bi-directional spline interpolation algorithm for tool -path generation was proposed to generate the smooth tool-path. On the parallel direction to the drive plane,the tool-path can be generated and smoothed through the B-spline interpolation method. On the perpendicular direction to the drive plane, the connection problem of tool-path can be solved by the bi-directional spline interpolation. Based on the geometric relationship of the tool step and interpolation error, the approximation method of spline curve was analyzed. Through the linear approximation of equal parameters of spline curve, the interpolation points can be obtained.(6) Based on the study of the tool-path generation methods introduced above, the NC programming system was developed for directly machining triangular mesh model.The new tool-path generation method for triangular mesh model proposed in this thesis was implemented. Through the machining experiment and result analysis of several instance models, the effectiveness of the tool-path generation method proposed was verified, which provided an effective approach for the direct machining of complex mesh model.Finally, the conclusions of this thesis were summarized, and future research directions were presented in the field of direct mesh machining.