| Nowdays parts with complex surface and difficult-to-machine materials are widely used in many fields such as die industry, aviation, and shipbuilding, automobile manufacturing industry, medical devices, and so on. Electrochemical machining (ECM) is an anodic dissolution process to machine workpiece in flowing electrolyte where the shape of cathode (as tool) is copied onto the workpiece (as anode). It has many advantages over traditional machining such as its application regardless of material hardness and strength, no mechanical cutting force, no tool wear, and good surface quality, so ECM is a widely-employed unconventional machining method in manufacturing industry. Numerical control electrochemical machining (NC-ECM) is one of the hot research areas in nontraditional machining, which uses a universal tool-electrode with a simple shape to move along the designed path to obtain the required shapes, and combines the technical characteristics of CNC machining and electrochemical machining. This process not only has the advantage of eliminating the expensive design and manufacture cost for electrodes with complicated shapes, but also has the advantage of increasing machining accuracy and surface quality. NC-ECM is an effective method for machining parts with difficult-to-cut materials and complex geometry, for example, turbine blades, engine casting, dies and molds. The interiorly research focuses on roughing and finishing surface with the linear edge cathode, and the technology using spherical cathode machining surface in actual production haven’t been seen in foreign research. Taking into account the spherical processing parts of spherical cathode, the cathode trajectory relative to the workpiece can be controlled by CNC system to machine any surface, so the key technology research of NC-ECM in machining complex surface with spherical cathode is carried out. This technology has important application value on machining complex surface of parts with difficult-to-cut materials, the main contents as follows:1. The overall design proposal of NC-ECM machine tool is carried out.An ordinary milling machine is transformed with three parallel movement axis and one revolving axis and one oscillating axis, and the calculations are finished in order to design the key structure of machine tool. The inner spraying rotating spherical cathode and electrolyte system are developed and their performance is stabile and reliable to meet the experiment requirements of NC-ECM with spherical cathode. The control system of NC-ECM machine tool based on PMAC (Programmable Multi-Axis Controller) is developed by use of Visual C++6.0software. The software with good man-machine interface has functions such as initialization, parameter settings, manual control, automatic processing, status display, analog quantity collecting and so on. These functions meet the requirements of control system and the foundation is laid for the smooth process of experiments.2. Based on Computational Fluid Dynamics (CFD) method, the flow field of inner-spraying spherical cathode is used as research object and the research on numerical simulation of flow field is carried out. The numerical calculation model of flow field in the machining gap is established. Two-dimension (2-D) field and three-dimension (3-D) field of flow field model are simulated respectively. The2-D flow field simulation of initial option is carried out. According to the simulation result, there are some defects in the flow field, such as, the lower electrolyte velocity in the inter-electrode gap, the larger range of the velocity at the contraction of flow diameter and the outlet of spherical surface.The2-D flow model are established at the different initial conditions and the simulation results are achieved and analyzed, which show that there are the same defects in the flow field. In order to solve the field defects, the optimal designs of cathode are carried out and the2-D flow field of optimal options is simulated, conclusions as fellows:1) the defects that the larger range of the velocity at the outlet of spherical surface and the lower electrolyte velocity in the inter-electrode gap are solved by means of adding a hole to the center of the spherical surface;2) the defect that the larger range of the velocity at the contraction of flow diameter is solve by means of replace the ladder-shaped cavity with the cross-section of the cone-shaped cavity of the cathode;3) the optimum design is obtained. The3D flow field simulation of initial option is carried out based on the2D flow field simulation. According to the simulation result, there are some defects in the flow field, such as, the lower electrolyte velocity at center of the machining area, the larger range of the velocity at the diameter contraction of the3D model profile and the outlet of spherical surface. The3-D flow model are established at the different initial conditions and the simulation results are achieved and analyzed, which show that there are the same defects in the flow field. In order to solve the field defects, the3-D flow field simulation of optimal options is carried out and the results show that the conclusions obtained separately from2-D and3-D numerical simulation make no difference. It is indicated that both2-D and3-D numerical simulation can used to optimize cathode designs and analyze flow field characteristics, but the3-D numerical simulation results are more accurate.3. Based on the simulation results of flow field and the optimal design of cathode, four kinds of cathode are used respectively to carry the orthogonal test out. The working voltage, the electrolyte pressure, the initial inter-electrode gap, the cathode rotate-speed, the cathode feed rate are chosen as the technological parameters. The surface roughness and the cutting depth are chosen as the technological indicators. According to the comparisons of experiment results, the machining effects of option three are the best relatively and the numerical simulations are consistent with the experiment results. It is indicated that the computational fluid dynamics (CFD) method can be applied to simulate the flow field, and the optimization design of the cathode can be guided according to the results of simulation. According to the orthogonal test, the experiments are carried out and the curves that the single technological parameter influences on surface roughness and cutting depth are obtained. The technological laws of NC-ECM with spherical cathode are summarized.4. Based on the basic theory of electrochemical machining, the research on shaping law of NC-ECM with spherical cathode is carried out. The mathematical model of the ideal machining process is established and solved by MATLAB, and the relation curves between different technological parameters and the variation of inter-electrode gap are obtained. In order to solve the difficulty in analyzing the shaping law of NC-ECM with spherical cathode, the method and the thinking of processing simulation are presented based on the finite element method (FEM). The two-dimensional analysis model of the electric field with spherical cathode built in ANSYS software is solved. The current density distribution and surface shape in different time on the anode are obtained. The experiments based on the simulation parameters are carried out, and the dimensions of the machined surface are measured, which are compared with the theoretical values. It is indicated that the simulation method meets the accuracy of the engineering calculations. The method and theory for further in-depth research on the process simulation in NC-ECM with spherical cathode are provided. Based on the above analysis, this method is used to solve the current density distribution in the case of cathode under motion state. The variation curve of workpiece contour is achieved. This method is of great significance for the prediction of workpiece surface under certain technological conditions in NC-ECM. The FEM model is verified by comparisons between experiment results and calculation results. The foundation is laid for the practical application of this technology.5. The spherical surface, the circular-arc surface and the blade surface are machined and measured respectively. It is showed that these surfaces basically meet the design requirements and the process of electrochemical machining surface with multi-axis linkage can be carried out on this machine tool. The repeated adjustment and improvement are carried out on machine tool, control system and other aspects in the experiment. The problems that impede the experiment will be solved continually in the further research. |
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