برنامه ریزی عملیات عملیات برای ماشینکاری کف 2½D پاکت بر اساس یک الگوی مسیر ابزار مارپیچ تبدیل شده

This work proposes a process planning for machining of a Floor which is the most prominent elemental machining feature in a 2½D pocket. Traditionally, the process planning of 2½D pocket machining is posed as stand-alone problem involving either tool selection, tool path generation or machining parameter selection, resulting in sub-optimal plans. For this reason, the tool path generation and feed selection is proposed to be integrated with an objective of minimizing machining time under realistic cutting force constraints for given pocket geometry and cutting tool. A morphed spiral tool path consisting of G1 continuous biarc and arc spline is proposed as a possible tool path generation strategy with the capability of handling islands in pocket geometry. Proposed tool path enables a constant feed rate and consistent cutting force during machining in typical commercial CNC machine tool. The constant feed selection is based on the tool path and cutting tool geometries as well as dynamic characteristics of mechanical structure of the machine tool to ensure optimal machining performance. The proposed tool path strategy is compared with those generated by commercial CAM software. The calculated tool path length and measured dry machining time show considerable advantage of the proposed tool path. For optimal machining parameter selection, the feed per tooth is iteratively optimized with a pre-calibrated cutting force model, under a cutting force constraint to avoid tool rupture. The optimization result shows around 32% and 40% potential improvement in productivity with one and two feed rate strategies respectively.

Significant use of 2½D pocket machining in aerospace and automotive industries has motivated engineers and researchers to search persistently for new methods that increase process productivity and reliability. Process planning is a critical component in process optimization and improvement and has wide flexibility in choosing the tool path geometry as well as applicable machining parameters. The large number of studies dedicated to the process planning issues for 2½D pocket machining (Banerjee, Feng, & Bordatchev, 2007) only reinstates its enormous research popularity as well as the associated complexities. A typical pocket geometry with length (l), width (w), and height (h) can be represented in terms of elemental machining surface: Floor, Corner and Wall, as shown in Fig. 1a with their connectivity shown with the adjacency graph in Fig. 1b. Floor machining operation involves the maximum material removal and machining time compared to the other EMSs. The primary planning requirement is that the tool completely sweeps the entire Floor surface area in minimum possible time without over-limit tool wear and rupture. In order to perform Floor machining, the following parameters have to be simultaneously considered – feed rate (V), step over (s), tool diameter (d) and the material (dm) to be left from the 2½D pocket boundary as shown in Fig. 1c. The major planning tasks involved are tool selection ( D’Souza et al., 2001 and Lee and Chang, 1995), tool path generation ( Bieterman and Sandstrom, 2003, Bruckner, 1982, Choi and Kim, 1997, Gupta et al., 2001, Stori and Wright, 2000 and You et al., 2001) and machining parameter selection ( Bae et al., 2003, Kloypayan and Lee, 2002, Lee and Cho, 2003, Smith et al., 1991 and Weck et al., 1994). The tool selection involves determining the tool diameter, tool path geometry generation deals with cut pattern, path interval and axial cuts and machining parameters to be selected are radial and axial depth and feed rate. Full-size image (35 K) Fig. 1. Representation of 2½D pocket in terms of: (a) elemental machining surfaces; (b) adjacency graph; and (c) Floor. Figure options Recently, high speed milling has become popular to minimize machining time and maintain high material removal rates. However fluctuations in the feed rate vector leads to significant jerks in the machine tool. It would seem that high speed machining for generating Floor is suitable, if a tool path can be selected such that feed rate fluctuation along it is minimized. For this purpose, a spiral tool path is being considered in this work due to its capability of achieving smoother and consistent machining. The feed rate along such a tool path can be smoothly adjusted (Bieterman & Sandstrom, 2003) owing to its consistent cutter engagement (Stori & Wright, 2000) and lower curvature with G1 continuity. The spiral cut pattern had been generated by creating the offsets of the pocket boundary ( You et al., 2001) with a consistent engagement angle and minimal curvature ( Stori & Wright, 2000). A partial differential equation based morphed spiral cut pattern ( Bieterman & Sandstrom, 2003) has been proposed which can overcome the short comings of previous methods, such as complicated self-intersection calculations and limited pocket geometries. However, the morphed spiral tool path is represented as a spline interpolation which in spite of tremendous research and development effort is still not widely available in common commercial Computer Numerical Control (CNC) machine tools. This limits the applicability of such spline-based tool path trajectory to only a limited number of machine tools. To maximize the practical applications of tool path composed of higher order splines, the tool path needs to be converted into a combination of the piecewise continuous circular and linear segments which can readily be implemented by the common CNC Linear interpolation functions. This can be achieved with the help of biarc (Bolton, 1975 and Su and Liu, 1989) and arc spline (Meek & Walton, 1992) concepts, which are G1 continuous composite curves using two arcs and an arc and a line, respectively. The G1 continuous tool path enables a constant feed rate without slowing down or stopping the tool. According to a maximum permissible limit on the feed rate. This feed rate limit can be calculated based on the geometry of the tool path and the dynamic properties of the machine tool servo system ( Pateloup, Duc, & Ray, 2004). In this work, the process planning of Floor machining is to be presented. The planning task of tool path generation addressing the geometric and the machining issues is presented in Section 2. The machining parameter selection for the minimization of machining time under a cutting force constraint is discussed in Section 3. In Section 4, the calculated tool path length and measured dry machining time obtained by the proposed tool path is compared with other commercially available strategies. The simulation of instantaneous cutting forces targeting optimal feed per tooth selection with single and double feed strategies is also provided. In the last section, contributions of the present work and future recommendations are presented.

In this paper, a new process planning for Floor machining is introduced with two major planning tasks: tool path generation and optimal feed selection for a given cutting tool. In tool path generation, a morphed spiral cut pattern is realized by a proposed composite biarc and arc spline tool path trajectory without the need of spline interpolations. The G1 continuity of the proposed tool path trajectory allows selection of a constant feed rate over the entire path. As an extension to existing method ( Bieterman & Sandstrom, 2003) the generation of morphed spiral tool path was also extended to pockets with islands. The overall tool path length and machining time of the proposed tool path was compared with other tool paths generated by commercially available CAM packages using identical geometric and machining parameters. The results show a much shorter tool path length of the proposed tool path trajectory due to more consistent distance between neighboring tool path trajectories. The shorter path length and a capability of constant feed rate being maintained over the entire G1 continuous tool path without acceleration/deceleration ramps lead to potential significant machining time savings. Using instantaneous cutting forces calculated by an existing mechanistic, a single feed strategy was implemented to optimize the feed per tooth taking into account the rupture strength limit of the cutting tool. The simulation results clearly indicate the more consistent machining that can be achieved with the proposed tool path with a constant single feed over the entire tool path compared to that generated by Mastercam™. Further, a double feed strategy was also developed and implemented to provide even higher productivity. The process planning approach can be extended to pockets with islands using morphed spiral tool path strategy introduced in this work. With more advanced cutting force models for sculptured surfaces, optimal machining plans for pockets with 3D Floor surfaces can be automatically generated.