Research On High Speed Milling Of Thin-wall Graphite Workpiece

High speed milling (HSM) technologies have many advantages comparing with the conventional milling in the field of graphite electrode manufacturing because better surface quality of workpiece can be obtained by HSM. HSM is favorable to machine thin-walled workpiece due to lower cutting force. However, the deflection and fracture of thin-wall workpiece are the problems in high speed milling of graphite. At present, the research on the thin-walled graphite workpiece processing technology with high speed milling is not enough. The programming strategies and cutting parameters are chosen based on the engineer's programming experience in the workshop, which lead to lower machine accuracy and the fracture of workpiece. Therefore, the approach to solving the problems of the deflection and fracture of thin-wall workpiece is the key to perform the high speed milling of thin-wall graphite workpiece.In this thesis, based on previous studies, systematic researches on the deflection and fracture in high speed milling of thin-wall graphite workpiece were conducted, using the combination of theoretical, experimental and finite element analysis.To study the effects of cutting force on the deflection and fracture of thin-wall graphite workpiece, the effects of cutting parameters on cutting force have been analyzed by orthogonal experiment. The cutting force experience formulas were established by the means of regression analysis. The results show that, the cutting force increase with increasing of cutting foce, feed per tooth, axial depth and radial depth of cut, but the effect of cutting speed on cutting force is not obvious.On the deflection of workpiece in high speed milling thin-wall graphite parts, the effects of the deflection of workpiece on the machine qualities were discussed. Then the effects of cutting parameters on the deflection of workpiece were studied by using of orthogonal array and single factors experiments, and the notable factors were found and analyzed. The major results show that, in high speed milling of thin-wall graphite workpiece, poor quality occur on the top of workpiece; the effects of all cutting parameters on the deflection of thin-wall part are obvious but cutting speed; the deflection goes up with increase of feed per tooth and axial depth of cut in down milling, and the trend of increased of deflection is obvious using small; on the other hand, the deflection decrease with increase of radial depth of cut. Therefore, in order to reduce the the deflection of workpiece, higher allowance should be leaved after rough machining, so that larger radial depth of cut can be taken to finish machining.To analyze dynamic deflection characteristic of cutting points in high speed milling of thin-wall parts, finite element model of the deflection of thin-wall graphite workpiece have been built. The results of simulation show that, the deflection increased gradually to the top along the height direction of workpiece; along the length direction, the the deflection decreases from maximum on the boundary to minimum on the middle of thin-walled parts, and then increases to maximum again. The effects of cutting parameters and machining strategies on the machine errors were obvious, especially using small radial depth of cut. In addition, the deflection of workpiece using machine strategy of cutting from both sides were smaller than that of cutting from one side.The fracture process of graphite workpiece in high speed milling was studied by the online photography method. The results show that, one of the reasons of fracture of graphite workpiece is the large the deflection and the serious rebound of workpiece; the workpiece might break from the base due to stress concentration in the instant of cutting in by the tool; the fracture occurs if the feed is too high on the turn-off while using the strategy of cutting from both sides. Besides, the fractography was observed by SEM, the results show that the reason of the fracture on the corner of workpiece is tensile stress.Based on previous results in the thesis, process plans of the technology of machining thin-wall graphite workpiece to reducing the deflection and fracture of workpiece have been proposed. The plans have been proved through the practical examples of machining, and a thin-wall graphite workpiece with thickness of 0.1mm and thicknees/height ratio of 1: 250 was achieved by high speed milling technologies.