Mechanisms Of The Tiny-grinding Wheel Finishing Based On The Electro-magneto-rheological (EMR) Synergistic Effect

With the development of the micro electro mechanical systems (MEMS) technology, the demand for micro apparatuses made of hard brittle materials, which have miniature size, complex shape and high surface quality, is increasing. Micromachining is the key supporting technology that has to be developed to meet the increasing accuracy requirements of product miniaturization and industrial realization of nanotechnology. As a new type of smart materials, the electro-magneto-rheological (EMR) fluid combines the characteristics of both the electrorheological (ER) fluid and the magnetorheological (MR) fluid, and exhibits not only the ER effect but also the MR effect under the applied fields. In particular, the EMR fluid can produce a synergistic effect under an electro-magnetically coupled field, which exhibits that the shearing stress of the EMR fluid can be increased more than the combination of the shearing stresses under the electric or magnetic field.Based on the machining principles of ER finishing and MR finishing, an EMR synergistic effect-based tiny-grinding wheel finishing (EMR finishing) method is presented. In the EMR finishing, the EMR fluid dispersed with micron-sized finishing abrasives is used as a polishing fluid to form a flexible EMR tiny-grinding wheel at the end of the cone-shaped tool when an electro-magnetically coupled field is applied, and the machining performance of the EMR tiny-grinding wheel can be controlled effectively by using the EMR synergistic effect. The EMR finishing can employed to polish the surface or machine the microstructure of brittle materials with a high efficiency.To solve the problem that the water-based ER fluid with poor insulating property cannot form a stable ER effect, the mechanism of ER effect is investigated thoroughly, and a noval ER polishing method, where the electrode and the ER fluid is separated by insulating materials, is presented. Using the impoved ER polishing device, the water-based ER fluid can form a stable ER effect and a good machining effect can be obtained. Based on the principle of the EMR finishing, the experimental equipment of the EMR finishing is developed to machine the workpiece under the paralled or cross combination of the electric and magnetic fields.Experiments of the EMR finishing are conducted to study the influences of some parameters on the polishing effect of the glass workpiece at a fixed-point machining mode. With the increase of the applied field intensity, the extension of the machining time and the decrease of the machining gap, the removal ability of the EMR tiny-grinding wheel will enhance. Influence of the rotational speed of the tool on material removal depends on the combined action of the cutting speed and the centrifugal force of the EMR tiny-grinding wheel. The presence of the abrasives can enhance the efficiency of material removal remarkably, however, too much diamond abrasives have little effect on the increase of the polishing performance of the EMR tiny-grinding wheel. The hardness of the abrasives determines the material removal ability of the EMR tiny-grinding wheel. The greater the abrasive hardness, the higher the material removal efficiency. The grain size and density of the abrasives directly influence the structure of the EMR particle chains and then change the efficiency of material removal. The optimum size and density of the abrasives should be chosen according to the field strength, the centrifugal force of the abrasives and the grain size of the dispersed particles.The distribution of the electric and magnetic field is simulated using Ansys software. According to the dipole model of particle energy interaction, the formulae of the field attraction forces between the EMR particles (including the dispersed particles and the abrasives) are derived, and the mechanical model of the EMR particle in the EMR tiny-grinding wheel is put forward. Furthermore, the distribution model of the polishing pressure and the material removal model are established. The real profiles of the machined surface at a fixed-point machining mode confirm the material removal model of the EMR finishing.The improved mechanical model of the EMR particle in the rotational EMR tiny-grinding wheel is established on the basis of the experimental results under the different electro-magnetically coupled fields and the calculating results of mutual particle interactions. This model clarifies the synergistic effect mechanism of electro-magnetically coupled field and the formation mechanism of the EMR tiny-grinding wheel. During the EMR finishing, the combined effect of the electric force, the magnetic force and the spin couple generated by the Lorentz force determines the stability of the EMR particle chains, the electric force and the magnetic force sustain the stability of the EMR particle chains, whereas in situ vibration of the EMR particles generated by the spin couple will affect the stability of the chains. In situ vibration of the EMR particles has two effects on the material removal performance of the EMR tiny-grinding wheel. On the one hand, in situ vibration of the EMR particles will impact regularly on the workpiece surface and promote material removal. On the other hand, in situ vibration will break the particle chains and reduce material removal when the interaction force of the particle chains is weak or the spin couple is large. When the electric force and the magnetic force are close in the value and match the spin couple, a good synergistic effect of the EMR finishing can be achieved. Results of orthogonal tests of two rotational modes further confirm the synergistic effect mechanism of electro-magnetically coupled field. At the rotational tool mode, the rotational motion of the EMR polarized particles in the magnetic field will generate the Lorentz force and the spin couple, and the EMR particles will vibrate in situ due to the effect of the spin couple, then material removal will increase. The couple mode of electric and magnetic field has a significant influence on the machining efficiency of the EMR finishing, and there is a good synergistic effect of the EMR finishing in the condition of high voltage of electric field and low excitation voltage of magnetic field.