Theoretical And Experimental Research On Hydrodynamic Suspension Ultra-smooth Machining For Hard-brittle Materials

With the development of science and technology, hard-brittle materials are widely used in comprehensive fields such as optical instrument, electronics, aeronautics and astronautics, instruments and civil industries. And the requirement of surface quality is higher and higher. So, The technology of ultra-precision machining for hard-brittle materials has received much attention. However, hard-brittle materials have an outstanding specialty of high brittleness and low fracture toughness. And their elasticity limitation and intensity are very close. Fracture failing occurs when load exceeds the elasticity limitation. Then the quality and performance of machined surface are affected. That is why the ultra-precision machining for hard-brittle materials is still difficult nowadays.Lots of scientists in different countries are dedicating their lives to the research on grasping super-smooth surface and propose many new processes which mostly base on the improvement of traditional lapping or polishing with fixed or loose particles, such as chemical-mechanical polishing, elastic emission machining, floating polishing, plasmas-assisted chemical etching and so on. The core of these processes is lessening the removal rate for less surface deteriorating and roughness. To meet the requirements of machining precision and automation, a hydrodynamic suspension ultra-smooth machining technology based on CNC equipment is presented. In the ultra-smooth machining system, machining tool and workpiece are non-contact. And the clearance of them is controlled by CNC equipment and machining apparatus. In the machining process, the combined process of hydrodynamic pressure, impacting of nanoscale particles and chemical is the key to obtain nanoscale smooth workpiece surfaces.In this paper, theoretical and experimental researches were done on hydrodynamic suspension machining. It mainly includes the followings:Firstly, theory research on hydrodynamic suspension ultra-smooth machining was done. In the machining process, hydrodynamic pressure generated in the slight clearance played a great role. It can be concluded that the higher the pressure becomes, the higher the velocities of abrasive particles get, and the higher the dynamic energy of abrasive particles obtains. In addition, as the velocities of abrasive particles become higher, the collision number of abrasive particles per unit time becomes larger. Consequently, it possesses enough power to remove atoms of surface layers. The contact mathematical model of impacting nanoscale particles on the micro surface asperity was set up. Based on the theories of dynamic nanoindentation and particles impacting wear, the formulas of removal volume when single particle impacting surface asperity were obtained. Assumption that the distribution of the asperity heights is assumed to be Gaussian, the mathematical model of impacting process between nanoscale particles and workpiece surface is built by statistics method. With the course of machining, the height of workpiece surface asperity is declining and the volume of asperity is decreasing. And then the surface quality is increasing.Secondly, the hydrodynamic suspension ultra-smooth machining system was established, which is made up of CNC equipment and machining apparatus. The machining apparatus is mounted on the headstock of the NC machine tool. The machining wheel is mounted on the spindle of a high-speed motor. The base equipment is a three-axis CNC machine tool, which provides the automatic feed of machining. The machining wheel rotates with the high-speed motor, which is the main motion of machining. The vessel with machining slurry is placed on the X-Y worktable. In the machining process, the whole workpiece is all in the machining slurry, which is composed of nanometer particles, carried liquid and surfactant. And then many experiments for K9 glass were conducted. The correlation between the surface roughness of workpiece and the machining time, motor rotate speed, the density of abrasive particles is revealed. It is found that with the machining time increased, the surface roughness decreased. And the higher the rotational speed is, the lager the decreasing magnitude of surface roughness is. The surface roughness decreases as the density of abrasive particles come higher. From the surface morphology before and after machining, it can be seen that surface roughness decreased from about Ra 50nm to about Ra 10nm, and surfaces of high quality without crystallite dislocation and microcracks were achieved. Moreover, orthogonal experiments were done. The ultra-smooth surface under Ra 1nm was obtained. Four factors chosen in orthogonal experiments respectively are: medium, machining time, speed rotation of the electromotor and machining slurry concentration. After orthogonal experiment analyzing, the most excellent combination of process parameters is gained. It is shown that experimental results are consistent with the theoretical analyses. The hydrodynamic pressure and nanoscale abrasives play the most important role in hydrodynamic suspension machining.Then, many machining experiments for silicon wafers were done. The correlation between the surface roughness of workpiece and machining temperature, machining time, motor rotate speed, density of abrasive particles is revealed. Experimental results indicate that the machining effects are the best when the machining time 60 min and motor rotate speed 6000 r/min and density of abrasive particles 30 g/L were chosen. After machining the surface roughness can be Ra1.55 nm at the most.A compliant machining apparatus was developed to improve the original machining system. Force sensor and piezoelectricity piles were added. To realize compliant machining, the hydrodynamic pressure could be adjusted by the displacement servo of piezoelectricity piles. Many experiments were conducted to reveal the correlation between the hydrodynamic pressure in the machining zone and motor rotate speed, machining clearance. It is shown that with the increasing of motor rotate speed, the hydrodynamic increased gradually and then decrease relatively after it reached certain value. When the motor rotate speed is 6000 r/min and machining clearance is 10μm, the hydrodynamic pressure obtained max value. Meanwhile, machining experiments were conducted to find the influence of machining clearance on the hydrodynamic pressure. It is shown that in the condition of same motor rotate speed, the machining clearance has a great influence on the machining effects. When the machining clearance is between 5 and 40μm, the machining effect is distinct. And in the condition of maintaining clearance 10μm, the performance is optimal. It can be concluded that under the he condition of certain combination of machining parameters, the higher hydrodynamic pressure is, the better machining effects are.In addition, identification model of hydrodynamic pressure equipollent with the theoretical model of real process was set up. The identification results showed that the error between outputs of identification model and experimental value was very small. It is proved that the identification model of hydrodynamic pressure has a high accuracy. The force/position decomposition-isomorphic compliant control strategy of the hydrodynamic suspension machining system based on CNC equipment was presented. Through compliant machining apparatus system, work condition can be decomposed into orthogonal force control space and position control space. Furthermore, the orthogonal subspace can be converted to position servo space through the machining apparatus. Force/position interaction relationship was dealt with between environment space and motion servo space so that compliant control system is decoupled to multivariate control system in the position servo space. According to the normal force control error, whole cutter location data is modified and nanoscale displacement compensation is realized. Then, compliant control of free-form surfaces realized by transferring the numerical motion control instructions.Lastly, many experiments were conducted to reveal machining rules of hydrodynamic suspension ultra-smooth compliant machining. It is shown that when the motor rotation speed was chosen as 6000 r/min, the hydrodynamic pressure can be maintained to be stable through adjusting the machining clearance by the displacement servo of piezoelectricity. When the hydrodynamic pressure wan maintained to be 5N, the surface roughness of machined workpiece can be reduced to about Ra 5nm after machining for 60min, and then it keeps decreasing gradually and slowly until it gets stable. The consistent and uniform machined effects could be realized.