تقسیم چند ضلعی برای برنامه ریزی عملیات ماشینکاری جیبی

This paper presents a new approach to improve tool selection for arbitrary shaped pockets based on an approximate polygon subdivision technique. The pocket is subdivided into smaller sub-polygons and tools are selected separately for each sub-polygon. A set of tools for the entire pocket is obtained based on both machining time and the number of tools used. In addition, the sub-polygons are sequenced to eliminate the requirement of multiple plunging operations. In process planning for pocket machining, selection of tool sizes and minimizing the number of plunging operations can be very important factors. The approach presented in this paper is an improvement over previous work in its use of a polygon subdivision strategy to improve the machining time as well as reducing the number of plunges. The implementation of this technique suggests that using a subdivision approach can reduce machining time when compared to solving for the entire polygonal region.

Pocket machining strategies for complex pockets have primarily been focused on using a single tool approach; the main concern being manual tool change time. The advent of rapid tool changers has reduced this tool change time to a minimum, offering new approaches to process planning. There has been significant research on pocket milling, too broad of a topic to discuss sufficiently without a review paper (Chuang and Lin, 1997, Hansen and Arbab, 1992, Held, 1991, Held et al., 1994, Jeong and Kim, 1999a, Jeong and Kim, 1999b and Persson, 1978). Various techniques have been developed, ranging from simple polygon offset generation to more sophisticated approaches using Voronoi diagrams. However, most researchers have focused on design strategies using a single tool. In the case of simple pockets, a single tool approach can be efficient but it is less effective as the complexity of the pocket increases. Once again, advances in CNC machining have slowly lead to more advanced designs in products, for both function and aesthetics. However, more complex pocket designs have lead to the need for more complex process planning strategies. In many of these cases, a multiple tool approach can overcome the tool change time by machining larger areas using large tools and smaller tools only as needed. While designing a multiple tool strategy, it is desirable to select an optimal set of tools in order to reduce the initial plunge cutting of these tools. Plunging typically requires significantly slower feed rates, and equally important, requires center-cutting tools. This paper focuses on presenting a multiple tool strategy for the pocket machining of freeform pockets that may contain islands. It also presents a tool selection and sequencing strategy to minimize machining time, and to reduce or eliminate plunge cutting.

This paper presented a method for polygon subdivision, tool selection and tool sequencing. In polygon subdivision, the pocket is analyzed in order to determine necks, which can have adverse effects on tool selection. The tool selection procedure determines all possible subsets of tools and selects the best based on general machining performance. The polygon subdivision approach is shown to be a suitable method for improved process planning and the current implementation shows that the use of multiple tools can significantly reduce the machining time. Of course, machinists are well aware of this, which is why they instinctively use large tools first to remove much of the stock material and then use small tools as needed. This paper is not intended to argue that using multiple tools is better; however, the subdivision method is shown to be an enabling method to determine how to select multiple tools. In other words, by breaking a complex pocket into several sub-polygons, one effectively opens up the design space to allow for more tool choices. In addition, the polygon sequence that is generated as an output from the method could eliminate the need for excessive plunge cutting operations. If used effectively, the sequence could yield the shortest path through each neck, essentially passageways, leading into the next sub-polygon. As such, a small tool could machine the pathway through each neck, allowing the roughing tool to begin machining the next region without plunge cutting first. This would be extremely beneficial when the roughing tools used are not center-cutting tools, as in the case of through-spindle coolant tools. Although the current approach performed well for the sample pocket geometries tested, there are opportunities for improvement. One limitation is that the islands were assumed to be convex in the current version of this method and hence were ignored when calculating the smallest tool diameter. This would be a relatively straightforward improvement, but it was deemed to add little technical merit to the current work and was ignored. With respect to process planning, the present approach calculates machining time based on area alone, and uses an approximate approach to tool paths. Although this was sufficient to show relative improvement across sample tests, the results may not be suitable for selecting actual process plans in a Computer Aided Process Planning System. A better approximation for machining parameters and machining time estimates would be needed in a commercial application. Computation time within the algorithm for computing the subdivision and tool selection was on the order of 60 s per model. Although not a focus of the current research, a practical implementation may require reduced computation time so that the results would be readily available during tool selection inside a CAD/CAM system. As a more elaborate extension to this paper, one could investigate a subdivision method for more complex geometry beyond the 2-1/2D pockets as in this paper. This would consider the more complex problem of 3D pocket geometry, such as complex tooling molds.