Research On Modal Parameters And Its Influence On Cutting Performance Of High Speed Face-Milling Cutter

High-speed face-milling cutter is mainly used for machining flat surface, and widely used for rough and finish machining of cast iron, aluminum. For its high cutting speed, face-milling cutting process has been widely used in automotive, aerospace, mold and other manufacturing fields. With high speed CNC machining centers and other high-performance machine tools upgraded, more and more technical requirements has been purposed for improve the dynamic performance of face-milling cutter.High speed face-milling cutter has the features of large diameter, more teeth, and its cutting process is intermittent. When cutting teeth periodically cut in and off of the workpiece, there exits phenomenon of the vibration and impact. The chip thickness of each cutting teeth changes with rotation angle, hence cutting force is cyclical and time varying. The forced and self-vibration generated by centrifugal force and dynamic cutting force during cutting process, has significant effect on milling stability and workpiece surface quality.By applying modal analysis on face-milling cutter, the modal parameters are extracted in this paper. By analyzing the effect of modal parameters on milling stability and workpiece surface roughness, the relationship between the dynamic characteristic and cutting performance are established, which is the main research content of improve cutting stability and workpiece surface quality.By analyzing the nature of face-milling vibration system and damping characteristic, the multi-freedom structural proportion damping vibration system differential equation is built. By solving of eigenvalue, the analytical expression of FRF is got, which can supply theory basis for FEM modal analysis and experimental modal analysis for face-milling cutter.Owing to the assembly of structural features of indexable face-milling cutter, ABAQUS software is selected in this paper for its contact modeling function to improve the accuracy of the modal analysis results.Because cutting tool body is the largest part of the face-milling cutter, in this paper, the effects on modal parameters by the materials’properties and the density of cutting teeth are investigated. There are five different cutting tool body materials and three different densities of cutting teeth for different face-milling cutters. The first six natural frequencies and modal of these face-milling cutters are analyzed in this paper. Results show that, for the five different materials, the natural frequencies from high to low are: carbon fiber reinforced plastics,40CrMo,45#steel, AlCuMg2and Ti6A14V.Density of cutting teeth are important factors of cutter structure, the lower of the density of the teeth, the higher of the natural frequency and the larger spindle speed range for safe and stable cutting within the recommended teeth range.According to structural characteristics of the face-milling cutter, the principle and method of experimental modal analysis are determined. Modal experiments adopt single input and output method of frequency domain identification. Exciter adopts electronic impact hammer to induce repeatability and adjustability impact force. Supporting methods adopt two ways including free modal and work modal. For the FRF of the tool tip, component analysis is adopted to identify the parameters. The modal parameters get from experimental modal analysis are compared with FEM analysis results to validation the FEM model.According to composite structure system design method and two-component response coupling substructure analysis, based on the FEM modal analysis and experimental modal analysis, the joint surface parameters between spindle/shank and face milling cutter are get (linear stiffness Kx, linear damping Gx, rotation stiffness Kg and rotation damping Ge). The contact surface parameters are then valid by experiments. When using work modal to get the FRF of tool tip, by using the contact surface parameters, the modal parameters of face-milling cutter can be get indirectly. The joint surface parameters and the equivalent FEM model can be used to analyze the dynamic characteristic of the face milling cutter fixed on the specific machine tools.The stability of the face-milling process is important to estimate and evaluate the cutting performance of the face-milling cutter. By analyzing the effect of face-milling cutting angle on milling process, based on mechanism of renewable flutter, the model of radial instantaneous chip thickness and dynamic face-milling cutting force are established. The matrix of dynamic milling force coefficient reflects the features of milling force:intermittent, time-varying and periodicity.According to the FRF negative real part of face-milling system, the critical axial depth-of-cut is calculated. The diagrams which reflect the relationship between axial depth-of-cut and spindle speed have also been drawn in this paper. The stability limit lobes of face-milling process can forecast and control the phenomenon of chatter.In order to verify the validity and reliability of the stability limit lobes, face-milling experimental system and parameters are designed. Cutting force in three directions are tested, work hardening capacity are also measured and calculated, the two and three dimensional picture reflecting workpiece surface integrity are get. By comparing different stability limit lobes, it is found that with the decrease of natural frequency and modal damping, the stable cutting region decreases; with the increase of density of cutter teeth, the stability of the face-milling process decreases within the recommended teeth range.The workpiece surface quality can reflect the cutting performance of face-milling cutter and be evaluated by the the surface roughness of workpiece. By studying the influencing factors’effect on surface roughness during face-milling process, including radial and axial runout of the teeth in the form of flat blade, feed per tooth, cutting edge length, nose radius, cutting lead angle, axial depth-of-cut, axial displacement between cutting tooth and workpiece caused by forced vibration, the surface roughness prediction models are established in the direction of the maximum value of surface roughness.According to the measured radial and axial runout of the teeth in the form of flat blade, the trajectory of cutting teeth is calculated based on the static surface roughness model. By combining experimental and modal analysis results, the vibration differential equations are solved and the axial relative displacement between cutting teeth and workpiece are get. By integrating the prediction results of static and dynamic models, the surface roughness and contours are predicted. The predicted results can be used for estimation and evaluation of surface quality. The prediction surface roughness model is verified by experiments. At the same time, it is found that the surface roughness increases and the workpiece surface quality deteriorates with the increase of density of cutter teeth under the same cutting conditions within the recommended teeth range.The project is supported by Major Science and Technology Program of High-end CNC Machine Tools and Basic Manufacturing Equipment (2011ZX04016-031).