| Summary: | 博士 === 國立中正大學 === 機械系 === 92 === This thesis describes the development of a CAM system for the high speed NC machining of triangulated sculptured surfaces in STL (stereo lithography) format using a generalized cutter. This is important because the use of STL format for representing a CAD model has been widely accepted in industry for quite some time. It is not just because of application such as RP which specifically requires the use of it, but also due to the fact that complex STL models can now be directly created by the digitization and reverse engineering process. Although many CAD/CAM software systems support the translator of STL files, but only a few papers have addressed the issue of NC machining directly from STL file. Therefore, the objective of this study is to develop a general tool path generation system of STL models for 3-axis NC machining. Firstly, the computing algorithm of interference-free Z-projection cutter location (CL) by using a generalized APT (Automatically Programmed Tools) cutter is developed. It is general in the sense that it can be applied to various cutters including ball, flat and fillet end-mills. In addition to the Z-projection method, for the general requirements of accuracy, efficiency and robustness for a triangulated sculptured surfaces machining system, a Z-level contour tool path is generated for contour-machining. The procedure contains four steps: horizontal slicing; offset vectors intersection detection; computation of CL point; and loops intersection removal. This approach is proved to be very successful by using the existing cutting-simulation software. When the Z-level method is used for rough machining or pocket machining, it is important to remove material efficiently without uncut. A contour-parallel tool path generation method is developed to create non-intersection offset paths. The offset method is implemented in a linear time.
Four themes are introduced here. For the first one, the tool path generation algorithm is developed for the z-map machining of STL model. Based on the topology of APT cutters and triangular surfaces, the computation of CL point is divided to nine different types. Some key parameters are used to decide the exact computing process. An efficient method for the region query of a tessellated mesh is also presented to find the overlap triangles under cutting area during the computation. The computation procedure presented here can be use for the z-map tool path generation for 3-axis machining with different tool path topology.
For the second theme, the z-constant contour tool path generation of STL model is developed for high speed machining (HSM) by using an APT defined cutter. The original contour lines are generated by horizontal slicing and the tool paths are generated by offsetting from the slicing lines. When it is machined with a generalized cutter, the radius of cutter profile is not the same at every cutter contact point, hence the conventional iso-distance offsetting method is not suited here. Therefore, a new tool path generation approach is developed based on the use of the generalized cutter. Firstly, a non-intersection offset direction at local range is found for CL point computation. Here an offset-vector intersection detection algorithm is developed for completing the work. And then procedures are executed to compute CL points and link the tool paths.
The third is the contour-parallel tool path generation algorithm. It consists of two main methods: the offset vector intersection detection algorithm and the positive offset turn algorithm. The offset vector intersection algorithm is used to generate an offset path without local-intersection. And the positive offset turn algorithm is used to remove the global intersections existing in the offset paths.
The last is discussion about tool path error. A general tool path error model is created. The tool path error model can support for tool path interference detection and also can be used for raw tool path planning.
Simulations as well as real machining examples are given to illustrate the effectiveness of the proposed methods. The research scheme of efficiently hybrid tool path generation strategy and pocketing tool path optimization are also discussed in the last section.
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