Optimal process planning for multi-axis machining of complex parts

Abstract

Multi-axis machining has been widely used in the manufacturing industry and the multi-axis milling process is one typical machining technique owing to its flexibility and accuracy, which could be classified as point milling and flank milling. The process planning of multi-axis milling on complex parts is of great significance to manufacturing efficiency and accuracy. Nevertheless, the complexities of parts such as the non-height field, deep cavity, non-zero genus features, and weak structures of workpieces limit the effectiveness of conventional point or flank milling methods. In this thesis, aiming at these issues, we propose three process planning methods for multi-axis machining of complex parts. First, we present a novel volumetric field slicing method for rough and finish machining on a part with complex features such as deep cavities, non-zero genus as well as weak structures. For the roughing operation of point milling, rather than simply taking Z-level parallel planes as intermediate machining layers, we propose to use curved machining layers, which will eliminate the severe staircase effect on the in-process workpiece as suffered by the Z-level method, and hence significantly stabilize the cutting force and reduce the susceptibility to dynamic problems e.g., chattering. To facilitate the determination of the desired curved machining layers, an elaborate algorithm is presented to construct a geodesic distance field embedded in the volume-to-remove whose iso-surfaces are then naturally used as the machining layers, which are assumed to be smooth and have a uniform thickness. In terms of finishing, aiming at improving the stiffness of the in-process workpiece, and also facilitated by the prescribed geodesic distance field, we employed a new machining strategy of alternating between the roughing and finishing processes. Finally, collision-free tool paths are devised to machine the generated machining layers. Next, we present a quasi-developable approximation-based multi-pass flank milling method to improve the efficiency and accuracy on a relatively large complex freeform surface. We approximate the surface mesh with a series of quasi-developable strips on which efficient multi-pass flank-milling tool paths are generated. The proposed methodology enjoys generality – it can handle surface mesh with singular points and concave regions, compared with the conventional flank milling of a single parametric ruled strip whose application is limited when dealing with doubly curved surfaces. In specific, we calculate the normal curvature of each tangent direction on each vertex and select the flattest direction to build a curvature-induced vector field whose local variation assists the partition of the input mesh using n-cuts. We then cover each segmented patch with a set of quad strips whose ruling length is ECL, through an elaborate manipulation of scalar and vector fields and their iso-lines. These quad strips are then further optimized to increase their developability considering the constraints such as interference-free and semi-positive, forming the final cutter contact rulings of the flank milling tool path, follows by connecting the paths on each strip and path to output the G-codes. Finally, we present a collision-conscious multi-pass flank milling method to balance the continuity of the tool path and the feasibility of the tool orientation generated on an arbitrary complex freeform part, even with non-height field or non-genus structures. This method can automatically generate collision-free multi-pass flank milling tool paths and resolves the greedy-lock issue caused by the post-check of collision and the oversimplified binary collision condition. The key to the methodology is a stripification process on the offset surface of the part which considers the global collision information upfront when the tool path is computed, rather than avoiding collision by amending an already generated tool path. On the offset surface, each tangent direction reflects the possible tool axis, and the degree of collision is measured by a real number. On each vertex, we select an optimal tangent direction that has both a small collision degree and variation with adjacent directions, forming a smooth collision-conscious vector field. Again, along with singular points, the local variation of the field assists in partitioning the offset mesh by n-cuts. Each patch is then parameterized by the segmented vector field, whose iso-curves guide the potential postures of cutter location rulings. The rulings are then optimized sequentially to form a set of quad strips to serve as cutting passes of the final flank milling tool path, tending to the constraints of fairness, feasibility, and collision-free.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Kai TANG (Supervisor)