Research on the CAM Technique for Sheet Metal Incremental Forming
|School||Huazhong University of Science and Technology|
|Course||Materials Processing Engineering|
|Keywords||Sheet metal incremental forming (SMIF) The robustness of CAM algorithm Boolean operation Invalid loops removal Interference-free cutter locationpoint Unit tool-path Software architecture|
Sheet metal incremental forming (SMIF) is an emerging and influential flexible metal forming process with the computerized numerical controlled (CNC) technology, in which complex shapes can be manufactured directly from computer-aided design (CAD) data without dedicated dies. Compared to the traditional stamping process, the SMIF process can achieve higher forming limits and provide lower forming costs. It is well suitable for small batch, more varieties production and rapid prototyping (RP) in many industrial areas, such as automobile, aerospace, and medical applications. The tool-path for SMIF has a direct effect on the dimensional accuracy, surface finish, thickness variation and processing time. Thus, the tool-path generation is a key issue in SMIF. The main task of the CAM technique for SMIF is to generate tool-path for SMIF. However, any related report has not been seen for researching the specific CAM software for SMIF at present. In order to promote the SMIF technology and the corresponding specific CAM software to various areas, this paper studies two core aspects in CAM technique for SMIF in-depth and systematically on the basis of the early achievements. The two core aspects are the robustness of the basic geometric algorithms for CAM and the tool-path generation for SMIF. Then, the main research contents and results are as follows:The 2D polygons intersection algorithm is a key step for implementing 2D polygons Boolean operation and invalid 2D loops removal algorithm. This paper presents an optimal algorithm for 2D polygons intersection based on the concept of hot point. The hot point rounding technique with finite precision computation is an exact geometric computation method, so that the robustness of the proposed algorithm is achieved. By using the advantages of computation based on finite precision, all intersections in 2D polygons are caculated by a sweep-line algorithm for strictly monotone chains, and the intersection between an edge and a hot point region is judged by the sign of vector cross product, then the efficiency of the proposed algorithm is achieved. The test of the performance shows that the proposed algorithm is highly efficient and strongly robust.In the process of calculating cutter location contours, the 2D polygons Boolean operation should be called frequently. The G-H Boolean algorithm is recognized as the most efficient algorithm, which is based on the concepts of entry point and exit point. However, the G-H algorithm exists a serious robustness problem, since it does not provide an effective solution for degenerate cases. This paper proposes a new method for 2D polygons Boolean operation. According to the location of edges at corresponding intersection point, all degenerate cases can be divided into tangential case, overlapping edge case and overlapping intersections case. By proposing the concepts of virtual interseciont point and virtual polygon, all degenerate cases are generalized. Then, the criterion of tracing-direction-changing at intersection is proposed and the Boolean algorithm based on the proposed method is implemented. Evaluation results show that the robustness problem in the G-H algorithm is effectively solved by the proposed method, and the efficiency is steadily raised 6-8 times by comparison with the KBool algorithm. Furthermore, besides self-intersecting polygons, this method can be applicable to handle any other types of polygons. Therefore, the proposed method is high-performance.In tool-path generation, the cutter contact contours and the cutter location contours are the basic elements. Invalid loops in these contours should be removed before planning tool-path. According to the characteristic of intersection, all invalid loops can be divided into general case and degenerate case. However, a systematical solution for these invalid loops has not seen at present. This paper presents a unified invlid 2D loops removal approach for all general and degenerate cases based on the thought of decomposing complex polygons into simple polygons. All degenerate cases can be integrated with the general case by proposing the concept of virtual self-intersecting polygon and the criterion of priority ecit-point direction determination. In the mean time, this unified method can be applicable to both cutter contact contours and cutter location contours by adoting the concept of valid orientation. The corresponding invalid loops removal algorithms for a compex polygon and polygons are implemented respectively. Finally, the performance tests show that this unified method is also efficient and robust.The SMIF process includes positive forming and negative forming, and it introduces the principle of Layered Manufacturing in Rapid Prototyping technology. But existing approaches do not consider these unique characteristics. This paper presents an effective tool-path generation method by adopting the thought of generating cutter location data directly from corresponding cutter contact data. By analyzing the interference characteristics between fillet-end tool and model surface and considering tangential case and intersection case comprehensively, a discrete computational model is proposed to calculate sinlge-layer interference-free cutter location contour. Then, various types of tool-paths can be planned based on these calculated cutter location contours to meet different SMIF process requirements. This method solves the tool-path generation problem for both positive and negative forming, and it is applicable to various tool shapes and suitable for STL model with defects. Implementation tests prove that the proposed method is effective, and the tool-path achieved is highly precise.As the SMIF process would require a long time process, thus this paper presents a unit tool-path generation method. First, the applicable unit tool-path planning principle for SMIF is analyzed. Second, the STL model is sliced and a series of Z-constant cutter location contours are calculated, and then the topological relation between the contours in every cutter location contour (C-Net data structure) and between all cutter location contours (Z-C-Net data structure) are built. Finally, conbined with the unit tool-path planning principle, the total length of the tool-paths is minimized by extracting the unit sequence from the Z-C-Net data structure, performing every unit with suitable sub-path planning style, and the valid linking paths among the sub-paths are obtained. The test results show that the forming time can be significantly reduced on the basis of ensuring the forming quality for multi-contour feature model.According to the degree of module related to CAM knowledge, a flexible and hierachical software architecture is proposed, and a specific CAM software called AFS2 is developed for the SMIF process. The research contents and results mentioned above have been implemented and integrated into AFS2. This software has been applied in the automobile industry at present.