Study On Force And Surface Topography Modeling And Process Optimization Of Meso-Scale End-Milling

The miniaturization has become a trend in design and development of products in these years. The component dimensions of miniature products range from 0.1mm to 10mm, and the dimensions of geometrical features range from 0.01mm to 1mm. This range belongs to meso-scale. At present, the main technologies for meso-scale manufacturing are micro-electro-mechanical systems (MEMS) and ultra-precision manufacturing technologies. However, MEMS technology has some obstacles, such as fewer categories of feasible machining materials, relatively poor precision, unable to make 3D features, low relative precision, expensive machine tools, etc, and conventional precision mechanical machining technology suffers from poor workpiece-to-machine volume ratio, consequently poor overall efficiency and high cost, so these two technologies can not satisfy the need of modern manufacturing system. Micro/Meso Mechanical Manufacturing technology, which combines the advantages of conventional mechanical machining technology and miniaturization technology, uses miniaturized machine tool system, so that it can obtain much less thermal distortion error, quick responding speed and high relative precision; Also, it can fabricate complex 3D components with any material, and has the advantages of low cost machine tools, high workpiece-to-machine volume ratio, low energy consumption rate and low machining cost, so it is a good choice for meso-scale manufacturing.Based on the comparison and evaluation of micro manufacturing research in the world, a micro three-axis CNC milling machine system was developed. Using the machine as a test platform, meso-scale milling mechanics research is performed to develop a three-dimensional force model of meso-scale end-milling considering size effects, propose a surface topography model and a surface roughness prediction model, and optimize end-milling process using genetic algorithm. To fulfill the above research objectives, the following four aspects of efforts are performed:(1) Development of micro three-axis CNC milling machine systemThe developed micro three-axis CNC milling machine system consists of five subsystems, namely stages, motion controller, spindle, milling cutters and dynamic force measurement. The structure and specification of five subsystems will be introduced, and then the capabilities of the machine tool and precision control will be analyzed and evaluated. This micro milling machine is the test platform of this research.(2) Modeling and simulation of three-dimensional cutting forces for meso-scale end-milling operationThere exist special process phenomena and mechanical characteristics in meso scale end-milling due to significant size effects. Considering the combination of exact trochoidal trajectory of tool tips and tool run-out, a single-edge-cutting phenomenon is indicated as a prevalent one in meso-scale end milling operations. A criterion of single-edge-cutting phenomenon, a model of nominal instantaneous uncut chip thickness and its corresponding algorithm are proposed. Then accumulative uncut chip thickness during actual milling process can be obtained by integrating the intermittence of chip formation, which is unique in meso scale milling process and induced by minimum chip thickness phenomenon, into nominal uncut chip thickness model. The cutting process is divided into elastic-plastic deformation phase and chip formation phase dominated by ploughing forces and shearing forces, respectively, and three-dimensional cutting forces are respectively modeled according to the corresponding phases. Based on the modeling and simulation technologies introduced, a simulation system for predicting three-dimensional cutting forces in meso scale end-milling process is developed. Cutting experiments are performed on the self-developed micro three-axis CNC milling machine, and the simulation results show a very good agreement with those data from milling experiments. The proposed force model overcomes the poor simulation precision using conventional milling force model, and can be utilized to guide the design and optimization of cutting conditions in meso scale machining.(3) Modeling and simulation of surface topography of meso-scale end-millingIn order to predict milled surface topography and machining precision under given process conditions, the micro cutter is simplified as a two-stage cantilever beam, and the principle of virtual work is used to model the flexible deflection of end-mill. Integrating the flexible deflection into trajectory equations of the cutting edges, a milled surface generation model for meso-scale cylindrical helical end-milling operations is developed. Then an algorithm for the simulation of three-dimensional end-milled surface topography is presented, and simulation examples are also given. Taking milling method, feedrate, spindle speed and radial depth of cut as four factors, the orthogonal experiments are designed to validate the proposed model. Also, the effects of machining parameters on milled surface finish are analyzed. The surface prediction model contributes to the theoretical and practical understanding of meso-scale cutting mechanism, ensuring machining quality and the optimization of cutting conditions.(4) Machining precision and efficiency oriented process optimization of meso-scale end-millingFor meso-scale end-milling operation, high surface finish should be ensured, and on the other hand, the machining efficiency should also be improved in practical milling processes. On the basis of previous research achievements, the surface finish and material remove rate are regarded as optimization objective. Milling method, feedrate, spindle speed, radial depth of cut are chosen as optimization variables, and an optimization procedure of end-milling process using genetic algorithm on Matlab platform is proposed. Through the comparison with the experimental results, high machining precision can not only ensure, but also improve machining efficiency and reduce production cost. The optimization procedure contributes to the application of meso-scale end-milling operations in practical manufacturing.