Research On High-Speed External Cylindrical Grinding Mechanism Of Difficult-to-Cut Materials And Its Surface Integrity

High-speed grinding is considered as the most effective way to significantly improve the processing quality based on the substantial increase in processing efficiency, also is the core technology to resolve the grinding bottleneck for different materials. In high-speed grinding, the numerous abrasives on the workpiece and wheel are again and again high-speed impact with each other, the transient effect of the shear force is limited to a micro-region, and the energy dissipation in the micro-local temperature of the material may reach the melting or near the molten state. The effect of positive feedback between workpiece and abrasive makes the local adiabatic shear even more enhanced. The higher the grinding speed, the stronger the role of adiabatic shear, close to the speed of sound to the extreme conditions of high speed grinding, resulting in number of new mechanism and the mechanism of surface integrity are to be studied and practice.Mechanism of high speed grinding is important and difficult area of research on grinding. With respect to the surface grinding has been extensively studied, the two-way rotation of the cylindrical grinding wheel spindle and the workpiece axis presents greater challenge to its research. In the extreme operating conditions of high-speed cylindrical grinding, the sliding-plowing-cutting theory in the traditional surface grinding process has been difficult to apply in high-speed external cylindrical grinding. And in recent years with the rapid development of the aerospace, aviation, automotive, military, instrumentation, communications engineering, electronic industry and machinery manufacturing industries, new materials used become more and more, and these materials are high mechanical properties, chemical stability, resistance to high temperature and high pressure, can be adapted to a variety of special working environment, they become an important factor to promote technology development in these areas. However, many of these new materials are difficult using conventional machining methods for processing, which restrict the application of these materials, improve the product processing costs, and usually such materials are called as difficult-to-cut materials.Therefore, focusing on the typical difficult-to-cut materials for high-speed cylindrical grinding, in order to explore the chip formation mechanism, thermal conduction mechanism, surface integrity characteristics, the subject aims to be through the theoretical modeling, the combination of simulation and experiment of high-speed cylindrical grinding, to conduct the in-depth and systematic research on the theory of material removal in high-speed cylindrical grinding of difficult-to-cut materials. Based on the available conventional grdinding mechanism of materials, it aims to be ultimate realization of high quality, high efficiency, low consumption, and green grinding. The main contributions of this dissertation are as follows:(1) It studied and revealed the dynamic behavior and high-speed characteristics of high-speed cylindrical grinding of difficult-to-cut materials. By the simulation study and engineering experiments on single grain grinding of TC4 titanium alloy, it has been found the chip formation phenomenon of single of the five stages of the three forms in single grain grinding, revealed the effect of wheel speed on the mechanisms of single grain cutting and its high-speed characteristics, constructed the calculating models of arc length with different forms, and introduced the wheel speed in the material removal rate model which has important impact on the high-speed grinding efficiency, and provides a theoretical basis for the interpretation and quantitative study of the efficiency characteristics of high-speed grinding.(2) Through the research and experiments on the high-speed cylindrical grinding force of difficult-to-cut materials, this study explored the impact mechanisms of the two important process parameters, including the grinding wheel speed, workpiece speed in grinding process, on the grinding force, analyzed the variation and characteristics of the grinding force, grinding force ratio, and specific grinding energy with the specific removal rate. Experimental results showed that grinding TC4 titanium alloy, the normal force makes the workpiece "softening" difficult to be removed, its main mode of material removal is completed by the tangential force; in the case of certain wheel speed, it found the increase in grinding force is less than the increase in the specific removal rate, so in a reasonable range of grinding force, increasing the workpiece feed rate can enhance the specific removal rate; under the conditions ensuring the constant specific removal rate, workpiece speed can be properly controlled to reach stable grinding force; in addition, it also can be integrated to consider raising the wheel speed and workpiece feed rate to increase grinding efficiency and surface quality for achieving the perfect combination.(3) Based on cylindrical grinding of undeformed chip thickness, it proposed the assumption that the grinding arc area source model is for the quadratic curve, thereby constructing the calculating model of workpiece surface and sub-surface temperature to study the grinding thermal mechanism under different conditions. It developed the temperature test sensor in cylindrical grinding arc that suitable for different-to-cut materials, and solved the problem that the high-speed cylindrical grinding surface temperature is difficult to be effectively measured. Grinding temperature test results showed that the calculating results by the quadratic curve heat flux distribution model optimized with Rw model were consistent with the experimental results. In high speed grinding conditions, the energy partition entering the workpiece was approaxiately between 30%-60%. Compared with the conventional grinding, it reduced 24%-54%(compared with the average value 84%). Here, the workpiece has the largest influence on the temperature, the whlle speed makes the temperature rise slowy, and the influence by the depth of cut locates between the othere two factors. Thus the influence of the above three factors on the temperature is in order:workpiece speed, depth of cut, and wheel speed. The results showed that the wheel speed is not the main considerable factor in grinding difficult-to-cut materials, while should take into account the combination of grinding parameters to achieve the grinding quality and efficiency. In addition, it was also found that along the depth direction of workpiece, the grinding temperature also gradually moves to the cut-in side of the grinding arc, grinding heat is mainly concentrated at the workpiece surface and sub-surface near the cut-in side, the above findings provide scientific basis for cooling effect, optimization design and installation in high-speed grinding.(4) It discussed the influence of grinding process parameters on the surface roughness and residual stress. The experimental results showed that the grinding speed ratio q has greater impact on the surface roughness, there is a critical speed ratio that can achieve the combination of grinding quality and efficiency, at the same time, the increase in wheel speed will make the surface residual stress transit to tensile stress, thus accelerating the wear of ceramic materials, affecting its life. Therefore, the blind pursuit of wheel speed can not achieve the desired processing quality requirements, but should be taken into account the physical and chemical properties of these materials. In addition, it also found there exists a reasonable wheel speed range can achieve the effective control of grinding thermal damage, thus it should comprehensively take the grinding power and energy partition into account.Through the above study, the author hopes this research can provide the scientific basis for the design and manufacturing of high-speed grinding equipment, prepare the theoretical paths for processing various difficult-to-cut materials in high speed grinding, and further hopes to guide a clear direction for the follow-up researchers in the field of high-speed processing.