| High-precision ceramic ball bearings are the key basic components of high-end machine tools,high-speed trains,wind turbines and other major equipment.Because they need to work under high temperature,high pressure,high speed,heavy load and corrosive conditions,any defects on the surface of ceramic balls may directly affect its performance.In order to adapt to the harsh working conditions,the machined ceramic balls are required to have nanometer surface roughness and submicron spherical errors,and there are no micro-cracks,scratches,changes in microstructure and residual stresses on the machined surface.At present,the related processing and equipment technology in China has not completely solved the problem of low polishing efficiency and poor surface integrity of ceramic balls,so the manufacturing cost of high-precision ceramic balls has been high.Based on the analysis and comparison of high-precision ball polishing method and magnetorheological finishing(MRF)technology at home and abroad,a new method for high-precision ceramic balls with global enveloped MRF of high-efficiency and low surface damage is proposed.The method realizes a flexible MRF sphere mode with controllable polishing force by constructing a three-dimensional global enveloped MRF pad,which improves the processing efficiency and surface integrity of the polishing process.This research illustrates the dynamic microstructural change law of polishing pad in the process of polishing,and explores the influence law of polishing force threshold,abrasive motion track,and cutting depth on polishing quality.The mechanism of variation of geometrical precision and removal of microstructure of ceramic sphere were also studied.The surface state control theory and MRF model of ceramic balls based on flexible controllable machining process were established.Thus,the high-efficiency and low surface damage polishing method of ceramic balls plus the related theories were ultimately achieved and proposed.The main tasks and results are summarized as follows:1)The mechanism of plastic domain removal of MRF was studied in this project.The mechanical behavior and brittle plastic transition characteristics of Si3N4 ceramic materials were studied by nano-scratch test and vickers indentation test.The critical load as well as depth of plastication of silicon itride ceramic spheres were obtained by nanoscratch experiments,which provided theoretical guidance for the selection of process parameters for high-efficiency and low surface damage polished ceramic balls.The experimental results show that the silicon nitride ceramic material will undergo three stages of elastic deformation,elasticoplastic deformation,and brittle fracture under the action of abrasive grain rubbing.In addition,when the load reaches about 9.86N or the abrasive cutting depth reaches about 8.3um,the microcrack initiation and propagation of the brittle removal mode would occur under the experimental conditions in this paper.2)A new MRF method of high-precision ceramic balls with high-efficiency and low-damage is proposed in this research.The method realizes the controllable and selective force of the MRF pad and the ceramic spherical surface contact process through a global magnetic uniformity design of the global enveloped surface and the new structure of the polishing plate,the driving method,and the dynamic microstructure control of the polishing pad.It could form a three-dimensional magnetorheological polishing pad,which can coat,clamp and transfer pressure flexibly,as a result rapidly reduce the surface roughness?spherical error,and surface damage of the sphere,and thus effectively improve the polishing efficiency and surface integrity.3)Based on the geometrical kinematics and dynamics analysis of the ceramic ball workpiece,the effect and relationship of each motion parameter of the sphere are obtained.Furthermore,the mechanical system analysis software of ADAMS is used to simulate the ball forming process.The high-speed camera is employed to monitor the movement track of the marking point during the processing of the work piece.The spherical motion model is established for the verification of motion parameters,such as the revolution angular velocity,rotation angular velocity and rotation angle of the ceramic ball,by both of the theory and experiment.The uniformity evaluation method of the polishing trajectory was then proposed.Finally,in order to optimize the uniformity of the grinding trajectory,the orthogonal experimental methods are used to find the best experimental solution.4)This research establishes the mathematical model of the polishing force in the global enveloped MRF method and the material removal model of the high-efficiency and low surface damage of silicon nitride ceramic balls by testing and analyzing the polishing force.Studies of the dynamic microstructure,magnetorheological effect pressure and hydrodynamic pressure of the MRF pad,and analyzing the maximum depth of penetration of the workpiece surface and the critical depth of the plastic/brittle transition were performed.5)The influences of machining gap,iron powder concentration and polishing plate rotation speed on material removal rate were systematically analyzed.The result shows that with the increase of machining gap,the material removal rate gradually decreases.When the machining clearance was 0.4mm,the material removal rate in the diameter direction of ceramic ball was 8.1 ?m/h.When the machining clearance was 1.2mm,the material removal rate was 0.6 ?m/h.Also,the material removal rate increases with the increasing iron powder concentration,then becomes steady.When the iron concentration increases to 20 vol.%,the influence become minimum with the concentration increasing or even decrease.The increase of the polishing plate speed also increases the material removal rate.But under the experimental conditions,when the speed is increased to 40 r/min,the material removal rate tends to be stable as about 3.8 microns per hour due to the decrease in the polishing force.6)The ceramic balls,which diameter is 9.525 mm,are used and polished by using the self-designed MRF experimental device in this research.Through a systematic analysis of major processing parameters,the scope of technological parameter is thus confirmed by single element experiments.Approaches including consideration of the reciprocal action on all factors and means of orthogonal experiments are used to optimize polishing parameters.This research selects the surface roughness,sphericity,surface appearance,and surface damage as the key indexes to evaluate the polishing effect.The silicon nitride ceramic balls were polished for 2.5 h with the self-designed magnetorheological polishing experimental device.The surface roughness Ra decreased from about 63 nm to 4.35 nm,and the sphericity could be down from 0.18?m to 0.11 ?m,the amount of mean diameter change by batch processing is 0.11?m,which reached the level of G5,the national standard for ceramic ball bearings. |
PDF Full Text Request