The Study Of Uniform Polishing Method Of Abrasive Flow Machinning With Minimal Differential Pressure

The advantages of abrasive flow machining (AFM) such as the flexible profiling, high efficiency, easy to be operated, it has unique technological advantages in complex surface polishing, which can be widely applied in polishing of complex hardware components surface, inner cavity surface, the mould cavity, hydraulic valve parts and so on. But some defects including "over-polishing","off-polishing" or "round off" may occur in polishing process, which restricts the application in high polishing of performance and precision machining parts such as precision bearing raceway, impeller blade etc., How to reduce the above defects in AFM process is one of the technical problems to be urgently solved.In this paper, the precision polishing of complex inner cavity surfaces was taken as the research object. Based on the digestion and absorption of advanced research results at home and abroad, the flow characteristics of AFM process and its material removing mechanism was first be theoretically analized. And then, a new kind of Uniform Polishing Method of AFM with Minimal Differential Pressure was invented, in which the pressure defference at the inlet and outlet of the abrasive flow in AFM process is controled by means of adding to the back pressure and uniform abrasive flow pressure is adjusted by means of ratio control strategy between the hydraulic cylinder, grinding cylinder and abrasive flow medium. Furthermore, the rotating-abrasive flow machining (R-AFM) equipment was developed, which was used for the polishing experiments of large size rolling bearing ring raceway and large size of the hollow roller rolling bearing. This research on abrasive flow polishing methods provides a new way of precisely, efficiently polishing high performance parts with complex surfaces. The main work and achievements are as follows:In the view and experience of the "over-polishing", "off-polishing" and "round off" problems in the traditional abrasive flow polishing process, the paper presents a polishing method with minimal pressure differential and uniform flow field. The paper analyzes the changes of flow characteristics after increasing the back pressure and implementing rotation in the flow field and outlines pressure and velocity laws followed. Based on Navier-Stokes equations, and considering of viscosity and elasticity of abrasive flow, the flow characteristics of abrasive flow was analyzed. It also deduces the mathematical models of pressure, velocity and flow rate. Together with the adhesive wear model of Holm and Burwell and the sliding wear model of Hailing, the material removal model was derived under the condition of back pressure and rotation using infinitesimal analysis method and it was proved its conformity with the Preston equation. Using Fluent software, the simulation reveals the effects of back pressure increase and rotation on the uniformity of the flow field.For the hydraulic extrusion transmission system of traditional abrasive flow, this paper presents a hydraulic proportional control strategy with closed-loop feedback function where transmission parameters can be precisely controlled by sensors of displacement and pressure. Control models of hydraulic and abrasive systems are established in the paper and they clearly reveal the transmission of abrasive flow from the hydraulic system to the abrasive system. Matlab simulation shows that the stability of polishing speed can be improved by the hydraulic cylinder-abrasive cylinder hydraulic proportional control strategy and can be controlled by PID. The effects of cylinder cross-sectional area and material cylinder piston stroke were analyzed on the overall equipment size and the power of the hydraulic system. Cylinder volume is chosen as the main parameter. In addition, the effects of the four-pillar structure to the vibrations, shocks, static loads and dynamic loads through simulation were investigated. Based on the theories above, an abrasive flow machining equipment of minimal pressure differential and rotation is developed. In order to verify the stability of the developed equipment, experiments under conditions of back pressure control and rotation motion are done respectively and simulation results are further verified.Combined with the theoretical analysis and equipment development, to test the effects on the problems like "over-polishing", "off-polishing" and "round off" of increasing back pressure, we polish the inner hole of the hollow roller of a large-size rolling bearing. Comparison between the situations of the import and export of the roller of whether increasing the back pressure or not verifies the improvement. Through experiments, it provides the strategy of matching section scale with pressure when a cavity is formed by the fixture and the equipment of abrasive flow. It also figures out the size range of the clearance during the processes of polishing the complex surface or inner face of a deep hole using abrasive flow.A reverse design method of the fixture of complex surface parts is proposed at the same time. To begin with, the 3D data of the complex surface of the processed part was got through 3D scanning. Then the 3D model from the data was built with the help of related software. Finally, a cavity of the fixture which is consistent with the complex surface of the processed part using 3D printing technology was made. And using a fixture which is consistent with the processed work-piece surface will provide us a geometric uniform flow field in the abrasive flow extrusion polishing process.In view of the ring raceway surface polishing of the large-size rolling bearing, experiments of positive- and reverse- designed fixtures are separately carried on. Comparative analysis of the characters of the polished surfaces is carried out, including surface features like microstructure and roughness and machining characteristic of the bearing ring raceway. The results prove the feasibility of polishing the bearing ring raceway using abrasive flow. The study shows abrasive flow is used to polish the global inner face of the raceway of the large-size bearing and the roughness is decreased from 0.4μm to less than 0.2μm. New grains are formed and they are favorable for the formation of oil film, which improves the abrasion process.