Research On Feedrate Optimization For Complex Surface Milling Based On Predictive Modelling Of Cutting Forces

With the rapid development of national economy and manufacturing industry, complex surface parts, due to its unique geometric shape and operational performance, are widely used in the key equipments of the industries such as shipping, die and mold, aerospace. The blade of controllable pitch propeller(CPP) is a typical part with complex surface, and its shape accuracy and machining performance directly affect the propulsion efficiency and service life of the rotary propeller in the dynamic positioning system. Under the ocean environment of service, the time-varying loads and environment mediums could lead to the failure occurrence of stress corrosion and fracture on the blade surface, which raises a strict requirement for surface integrity of the blade. High-performance machining technology is an important way to achieve the high precision, high quality, high efficient machining of the complex surface, and plays a vital role on understanding the formation discipline of the machining surface integrity and ensuring the service performance of the blade.Ni-Al bronze material is widely used to manufacture the propeller blade due to the characteristics of high strength, wear resistance and excellent resistance to stress corrosion, and the corresponding process variables in cutting process, especially finish-machining stage, have a direct effect on the surface quality of the component. However, related research regarding its predictive model has not yet been fully conducted. Additionally, there is no an efficient optimization method formed about blade machining, and the selection of the cutting parameters is mainly empirical for the operators. As a result, the machining accuracy and efficiency cannot be guaranteed, also less attention could be paid to the surface integrity. Therefore, based on the requirement of surface integrity on the machinability, this paper investigated the deformation characteristics of Ni-Al bronze material in cutting process, established predictive models of machining variables, and carried out the research on the feed-rate optimization method, in order to acquire the formation discipline of surface integrity and achieve the high quality, high efficient machining of the blade.In order to establish the thermo-mechanical constitutive relation of the Ni-Al bronze material under high strain-rate condition, a new methodology is proposed to accurately identify the constitutive parameters of Johnson-Cook model in the cutting process, combining the SHPB(Split Hopkinson Pressure Bar) tests, predictive model of cutting force and orthogonal cutting experiment. Firstly, quasi-static and SHPB tests were carried out to obtain the true stress-strain curves at various temperatures and strain rates. Then an objective function of the predictive and experimental flow stresses was put forward, which set the identified parameters of SHPB tests as the initial value, and utilized the PSO(particle swarm optimization) algorithm to identify the constitutive parameters inversely in machining. Finally, these identified parameters were verified to be feasible by comparing the values of cutting forces calculated from the predictive model and FEM simulation.A new analytical methodology of chatter model is proposed for the chatter vibration using a predictive model of cutting force in cutting processes, which solves the problem that the conventional chatter models cannot capture the thermo-mechanical properties of realistic cutting processes intuitively and the involved cutting force coefficients identified through experimental methods less accurately. This method regards the dynamic cutting process as the quasi-static one at each particular time with the input data of the material properties, tool geometry and cutting conditions, which the dynamic cutting forces can be determined with the equivalent cutting parameters and then the expression of dynamic cutting force coefficients is derived theoretically. In addition, dynamic model of the machine tool system for chatter vibration in cutting is formulated as a time delay-differential equation. The stability analysis of the proposed model is investigated and the stability lobe diagrams(SLD) of orthogonal cutting and milling are obtained further by the time domain semi-discrete and full-discrete methods respectively. Finally, comparisons among the proposed model, the semi-analytical model available in the literatures and experimental results are provided, which thus validates the effectiveness of the proposed analytical model.Cutting forces in milling process are generally calculated by the mechanistic or numerical methods which are considered time-consuming and need numerous works about identification for various cutting conditions and workpiece-tool pair. Therefore, an analytical method for the cutting forces model in ball end milling process is put forward based on a predictive machining theory, which regards the workpiece material properties, tool geometry, cutting conditions and types of milling as the input data and takes into account the effect of the edge radius, varying sliding friction coefficient and cutter runout on the cutting forces. The shear flow stress is estimated by introducing a modified Johnson-Cook constitutive law which considers the phenomenon of the work hardening, temperature softening and material size-effect. Each cutting edge of the cutter is discretized into a series of infinitesimal elements along the cutter axis and the cutting action of each element is equivalent to the oblique cutting process. The differential cutting force components applied on each element are predicted analytically using this predictive oblique cutting model. The entire cutting forces are obtained by the superposition of the infinitesimal cutting forces. Finally, the comparisons among the data available in the literatures and experimental results about plane milling and variable-depth complex surface milling to verify the effectiveness of the proposed model.By investigating the influence of feed-rate casting on the machining surface integrity(surface 3D topography, residual stress, micro-hardness), a feed-rate optimization method is proposed for milling process based on the control of constant cutting force. In this method, the objective function about the absolute value of the difference between the reference milling force and the maximum one calculated in each tool position is put forward, residual stress and process stability are regarded as the two main constraints. Then the method adopts Newton-Raphson iteration algorithm to alter the feed-rate in the CAM tool path file, thus ensures the constant cutting forces and enhance steadiness of machining process. The experiment is carried out about model propeller, the results show that the proposed optimization method ensures the machining time of the blade reduced by more than 20%, the surface residual stresses and micro-hardness machining accuracy to be increased, which has a clear effect on increasing the machining efficiency and improving the machining quality of the blade surface. This thus validates the effectiveness of the proposed optimization method.The proposed analytical models of cutting process variables and feedrate optimization method in the paper are universal, and can be extended to other materials and complex surface. The adopted theory can provide analytical method to the regulation of surface integrity, which could improve the surface quality and ensure the machining efficiency.