Mechanism Research And Optimization Of Hard Milling Process For Hardened Steel Die

Manufacture of hardened steel molds of automobile panel dies is an important part in automobile manufacturing. However, high hardness, small machining allowance caused the phenomenon of “cutter sliding” in the finish machining of hardened steel molds, which often appears between the cutters and the workpieces. The accuracy of final dimensional and the surface quality are difficult to guarantee, which increase the time of the manual grinding. In addition,when using the cutters of cemented carbide to machine the high hardness in the process of finishing, the cutting efficiency is low and the cycle time of overall manufacturing is long. Therefore, improving the phenomenon of "cutter sliding" and carrying out the research on the applications of new cutter materials in finish machining of hardened steel molds are significant to improve the machining quality and efficiency of automobile panel dies, especially the level of automobile manufacturing industry.There is a blunt round of cutting edge in the manufacturing process of a cutter, which generates the minimum cutting thickness. When the actual cutting thickness, which is caused by the imprecise machining allowance, is less than the minimum cutting thickness, it generates the abnormal cutting between the cutters and workpieces and what’s worse, “cutter sliding” appears.Considering the effect of blunt radius of cutting edge in the process of finish machining about hardened steel molds, on the basis of analysis about the variations of cutting thickness and the surface topography of workpieces in the milling process, through dividing cutting-ploughing-sliding region of the surface topography, a method of determining the minimum thickness in the milling process was proposed and its feasibility was verified by experiments and simulations of machining. Basing on it, the minimum cutting thickness of different hardness hardened steel, such as Cr12 Mo V, was determined.Considering the cutting parameters, the curvature of workpieces, the deformations of cutters and so on, a predicted hardened steel die of limiting cutting depth, which was used in the process of finish machining of curved surface, was established by theoretical derivation. It was verified by the experiments of milling. The result provided a minimum finishing allowance of hardened steel molds in case of the phenomenon of “cutter sliding”.The machine ability in milling of hardened steel molds with PCBN cutters and cemented carbide cutters were compared by the experiments. Failure mechanism in intermittent machining, which generated in the process of using PCBN cutters to machine hardened steel ones, was studied.During the process of intermittent machining, it generated breakage of cutters, which can be divided into low speed damage and high speed one. The critical milling condition of avoiding low speed damage was given on the basis of disrepair mechanism.Through the observation of the fracture morphology of fatigue failure cutters in a cutting experiment, it revealed the fatigue failure mechanism in the process of intermittent machining under the effect of thermal-mechanical coupling and the preventive measures of fatigue damage were given.The shortcomings of using traditional ball end cutters to machine the small curved surface were analyzed. On this basis, types of double arcⅠ, double arcⅡ,sinⅠand sinⅡof PCBN cutting tools were developed for the finish machining of hardened steel molds, which were used for the small curved surface. The applicable conditions of new PCBN tools were provided.By taking hardened steel Cr12 Mo V, which is the common material of automobile panel dies as the object in this research, the phenomenon of “cutter sliding” in the finish machining of automobile panel dies and the mechanism of intermittent machining for hardened steel were studied by using the theoretical analysis, numerical simulations and machining experiments. The results can provide theoretical basis and technological instructions for improving finishing quality and efficiency of hardened steels molds of automobile panel dies as well as the high quality.