High-Speed Milling Stability And Machining Accuracy

High-speed milling (HSM), which is one of the advanced manufacturing technologies, has been become the important part of modern manufacture technology in 21 century and used in many different fields, e.g. aeronautics and astronautics, automobile, die and mould, energy, and rail transit. While the material removal rate (MRR), reflecting the machining efficiency, is often limited by the occurrence of an instability phenomenon called chatter in HSM processes. In order to increase the chatter-free MRR and surface accuracy, the numerical analysis and experimental researches are carried out through underlying and systematacial investigation of the occurring sources of the different types of vibrations, influencing factors, and machining accuracy in HSM processes.An integrated four degree-of-freedom (DOF) dynamic model for HSM processes with solid carbide endmill is developed, including feed per tooth, tool eccentricity, regenerative effect, static deflection of tool-workpiece, and plouging effect. Dividing the cutting process into three cutting processes, the influence of the helix angle on dynamic cutting zone is considered. Unstable flip islands arise in the stability lobe diagram due to the helical flutes of the tool. The analysis of the directional cutting force factor gives a clear explanation for the existence of the unstable islands. The periodic delay differential equation with variable parameters for thin-walled workpiece milling process is proposed. Considering the feed motion and tool vibration, a 2-DOF dynamic model of regenerative chatter with state dependent time delay is developed in HSM processes. Based on the Frechet derivative theory, the linearization of periodic state-dependent delay different equation is investigated. For a system with practical milling paramters, it is shown that the incorporation of the state-dependent delay into the model does not essentially affect the linear stability properties of the system.Cutting force coefficients and tool or workpiece modal parameters are found by using experiments with variable feed rates and experimental modal analysis (EMA), respectively. The vibration frequencies and bifurcations during milling processes are described. The influences of cutting parameters on stability limits are investigated in detail. The three dimensional stability limit map for different spindle speeds, different axial depths of cut, and different radial depths of cut are obtained. Two types of instability are predicted corresponding to quasiperiodic (Hopf) and periodic (flip) chatter. The stability lobes of 1 -DOF, 2-DOF, and 4-DOF milling processes are shown. It is shown that the stability behaviour depends strongly on the flexibility of system. In this dissertation, a better delay approximation has been used in the modeling of variable spindle speed (VSS) milling processes, and the benefits of VSS milling operations are discussed by comparing the stability charts of VSS milling operations with those obtained for constant spindle speed milling operations. The three dimensional stability limit map for different frequencies and amplitudes of the cosine speed variation at the speed 12000rpm is shown. From the graph, it is possible to select variable parameters that are adequate for the cutting conditions required.The influences of different tool structural parametes on stability limits are investigated respectively. The tool frequency response function (FRF), which is required as input to existing stability lobe calculations, is determined analytically using receptance coupling substructure analysis and Euler-Bernoulli beams with step changes in cross section. The development of three-dimensional stability limit map is described, that combine the traditional dependence of allowable depth of cut on spindle speed with inherent dependence on tool overhang length, due to the corresponding changes of dynamics with overhang in the system. The stability of nonuniform pitch milling tools is predicted. Nonuniform pitch milling tool is a powerful technique to reduce the vibration level in milling when the system is unstable, while the influence of nonuniform pitch milling tools only plays an important part in period doubling lobes. Using FEM and EMA, the FRF of thin-walled workpiece depending on tool positions is obtained, and the three dimensional stability limit map of thin-walled workpiece milling process are obtained. An integrated theory of structural parameters design of solid Carbide endmill for HSM is established, which combines the milling uniformity with milling stability.Stability citeria for HSM processes is derived by the characteristic multipliers of the transition matrix which obtained by the Floquet theory, and a method for identifying stability limit lobes is also introduced. A novel method which may simply obtain stability limits lobes of milling processes is presented. The method is based on two theories: the first is that the stability boundary has a typical 'lobed' structure, with maxima stability located at spindle speeds corresponding to the integer fractions of the eigen-frequencies of the most flexible modes of the machine-tool-workpiece system; the second is theory of semi-bandwidth for resonant region. An updated method for dynamic optimation is proposed to determine the cutting parameters and structural parameters for increasing the chtter-free MRR and suface finish through considering the self-excited vibration and forced vibration in HSM processes. The objective function of the method is chatter-free MRR, constrains are chatter stability and surface finish, and optimizing variable are cutting parameters and structural parameters. The proposed method can be used in machining processes with lower surface finish, e.g. rough machining, semifinishing. Directional cutting force factors and stability limit charts are conducted for the down- and up-milling cases of various immersion rates. An in-depth investigation of the optimal stable immersion rates for down-milling in the vicinity of where the average cutting force changes sign is presented. Stability charts and performance contour in the parametric space for HSM processes are obtained. A new stable region-optimal stable region is definited in the tranditional conditional stable region, which is divided into three parts: unconditional stable region, optimal stable region, and new conditional stable region. The optimal cutting parameters are obtained, which are suitable for precision finishing or ultraprecision machining operations.Stable cutting conditions have been selected and the influence of forced vibrations on part geometric accuracy (Dynamic surface location error-DSLE) is investigated. The underlying theory, based on the situation of forced vibrations, is outlined. The DSLE is predicted by using the harmonic balance analyses during stable milling operations. Comparisons between the influences of system parameters (spindle speed, radial depth of cut, tooth number, helix angle, and tool overhang length) on the DSLE are included. Under the cases of stable and noresonant milling, stable and resonant milling, and unstable milling, cutting distortion, and time-dependent distortion are investigated by HSM experiments with thin-walled workpiece. Influences of vibration on machining distortion are explained in HSM monolithic component with uniform and nonuniform pitch milling tools.