Ultrasonic Vibration Micro Milling System Establishment And Research Of Milling Force And Residual Stress

With the development of science and technology, the micro mechanics system has been extensively applied in high technology fields such as aerospace, minisatellite, and modern medicine. The performance and the service life of the micro mechanics parts are affected greatly by the manufacturing process. In micro mechanics machining field, the traditional manufacturing approach of micro mechanical parts is to use large precision machine tool to mill. But the cost and the mechanical creep are difficult to control, and the wasted resources and energy are too large. With the emergence and continuous improvement of small machine tools, the micro mechanical parts are machined mainly by small machine tools. Compared with traditional manufacturing approach, there exist the problems that the surface roughness in the milling process is big, there are flanging and micro burrs on the surface, and there also exists surface residual tensile stress. These factors are unfavorable to improve the precision of the product and their service life, so it is necessary to improve the machining process. In this respect, some scholars have proposed that the combination of ultrasonic vibration and the micro-cutting can effectively improve the milling quality of workpiece. This method is mainly applied in the ultrasonic vibration micro drilling, ultrasonic vibration micro planing and the ultrasonic vibration micro turning. But there are few reports on ultrasonic vibration micro milling by now. The ultrasonic vibration can reduce the value of surface roughness and surface micro flanging and burr appeared in micro milling process. At the same time, it can improve the state of the surface residual stress, and improve the accuracy and surface quality, corrosion resistant properties and the service life of the workpiece.The purpose of this paper is to solve problems appeared in the micromilling process of the metal and brittle materials to improve the machining accuracy and surface quality, and put forward the method of the combination of ultrasonic vibration and micro milling in this paper. This paper is mainly developed from the following aspects.Firstly, the ultrasonic vibrator is designed based on the ultrasonic vibration theory and the ultrasonic amplitude transformer theory. And then the torsional vibration table is designed. Because the ultrasonic vibration micro milling system in this project is used in milling hard alloy, the milling force and ultrasonic vibration amplitude has to be big. The reverse spring ensures that the vibration frequency of the torsional work table is as same as the ultrasonic vibrator. The ultrasonic amplitude transformer designed based on the ultrasonic wave theory can not meet the experiment need, so the amplitude of vibration has to be further amplified.So the end of the vibrator should be added the second level amplification transformer. The step-style ultrasonic amplitude transformer is adopted. The vibrator designed through this method can satisfy the requirement of the experiment in theory. But in practice, because the vibrator load changes in a certain range, the load changes makes directly the natural frequency of the vibrator offset, and the incentive frequency of the vibrator remains unchanged, so in some cases the vibrator is difficult to work properly. Since the vibrator frequency bandwidth depends directly on the size of the vibrator, in order to increase the bandwidth and ensure the normal work of the vibrator, the finite element software ANSYS is adopted in this paper to optimize the vibrator size and its natural frequency, and then a proper size of the ultrasonic vibrator is finally got.The research is based on cutting force mathematical theory of the micro milling and ultrasonic vibration micro milling carbide steel. Simplifing the cutting process properly, the plural coordinates of the milling cutter contact point is got. The cutting point coordinate of the later period minus that of the fronter is the cutting thickness in micro milling process. Multiplying the cutting thickness and the cutting coefficient measured in the experiment is the milling force. Similarly the ultrasonic vibration micro milling force equation is obtained. The dynamic fracture mechanics theory is applicated in explaining the crack extension of workpiece materials how to influent the cutting force, then the cause of ultrasonic vibration micro milling force decrease by the crack propagation mechanism is explained in this paper.After constructing the theory model of the ultrasonic vibration micro milling, the finite element model of the ultrasonic vibration micro milling force is carried on in this paper. It is concluded that ultrasonic vibration micro milling force reduces in finite element simulation results. It is shown that the results of the simulation analysis and the theoretical analysis are in good agreement.The simulation model of the surface residual stress in ultrasonic vibration micro milling is created. And the milling cutter and the workpiece are simplified properly. The Johnson-Cook constitutive equation is choosed to describe the workpiece material performance. The Johnson-Cook damage law is adopted as a chips-forming rule. The distribution nephogram of the residual stress after removing the workpiece constraint is obtained by means of explicit calculation. The residual stress of the same reference point in the direction of the S22 and S11 is extracted respectively. Then the regularity that the surface residual stress changes with the micromilling cutter geometrical parameters, the milling parameters and the vibration parameters in the ultrasonic vibration micro milling is gained by simulation.The ultrasonic vibration micro milling experiment system is established by assembling the ultrasonic vibrator, the ultrasonic vibration work table, the ultrasonic generator, the dynamometer, and a CCD camera in the 5-axis small machine tool. The slotting experiment on 4340 steel and glass is performed in this experiment system. The slotting experiment verifies the conclusion that micro milling force reduces after adding ultrasonic vibration. The regularity that the milling surface roughness, the milling cutter deformation, and the corrosion resistance are improved after adding ultrasonic vibration is obtained by observing the surface topography of the workpiece under the laser confocal microscopy. The conclusion that the surface residual stress reduces after adding ultrasonic vibration is obtained after measuring the surface residual stress by using the X-ray diffraction equipment. The glass slotting experiment verifies the conclusion that the micro milling force of the brittle material reduces significantly, the edge breakage of the brittle material reduces and the milling process of glass is improved after adding ultrasonic vibration.A basis for manufacturing micro mechanical parts in future is provided in this paper to improve the quality of the workpiece machined by adding ultrasonic vibration.