Research On Sphere-shaping Mechanism And Precision Balls Processing With Variable-radius Groove

Precision balls are the key components in the high-level fundamental machine parts such as high performance bearing,high precision ball screw and high precision linear guide.Its accuracy and quality will directly affect the performance and service life of mechanical fundamental parts,thereby affecting the high-end equipment.At present,aiming at the technical problems in batch processing with probability sphere-shaping,a new batch processing method for precision balls with variable-radius groove(VRG)is proposed.In this method the movement posture of the ball is changed through the change of the groove curvature to realize the high efficiency and high consistency processing of precision balls.Aiming at the problem of the processing trajectory can be distributed over the entire spherical surface after a single processing cycle.The kinematic model of a ball in VRG method was established based on pure rolling motion hypothesis and geometric kinematics theory.Coordinate matrix of trajectory points at each time can be calculated by coordinate transformation.Simulation results indicate that the trajectory can be distributed over the entire spherical surface after a processing cycle with VRG,which was proved by an experimental observation,so then the kinematic model is valid.Aiming at the problem of processing trajectory evaluation algorithm is complex,and the evaluation accuracy is not high.A quantitative evaluation method of the trajectory distribution uniformity was presented for the VRG method.The standard deviation of the trajectory point density(SD)is used as the evaluation index.The smaller the SD value is the better distribution the trajectory has.The influence of the combination of the ratio of lapping plate rotating speed and eccentricity on the movement posture of a ball and trajectory uniformity were analyzed.The effects of geometric parameters of variable-radius groove on trajectory uniformity were also investigated.The results indicate that the trajectory uniformity is better when eccentricity is two times of the ball diameter and the groove half angle is 45°.The trajectory distributes better with the increase of the polar radius of groove and the spacing factor of groove.The results of orthogonal test show that eccentricity is the significant influence factor.Aiming at the problem of the dynamic forming process of spherical surface is difficult to quantitatively describe.Based on Hertz contact and Preston’s formula,the computational model of sphericity,which combining processing trajectory and ball surface material removal,was established to reveal the practical sphere-shaping of workpiece in VRG method.The better the spherical surface can be formed with the smaller sphericity.According to the analysis of simulation results,the trend of sphericity with the processing time gradually convergence in VRG method.The sphericity can be improved quickly under the higher the loading pressure,but the final stable value is higher.The sphericity with the plate rotation speed decreased first and then increased.Based on the above research,the experimental platform for VRG method was set up.Bearing steel ball GCr15 and silicon nitride ceramic ball as processing object,the processing result with eccentric variable-radius v-groove is better than with variable-radius v-groove.Under the optimized processing conditions,after fine lapping and polishing stage the average value and the deviation of the processing precision of the bearing ball and the ceramic ball are obviously improved by eccentric variable-radius v-groove.The sphericity average value of0.122μm,the surface roughness Ra average value of 11 nm and the variation of ball lot diameter of 0.25μm can be obtained for the bearing steel ball.The sphericity average value of0.115μm,the surface roughness Ra average value of 12 nm and the variation of ball lot diameter of 0.19μm can be obtained for the silicon nitride ball.The experimental results are better than accuracy requirements of G5 grade balls.Research results confirm that the high efficiency and high consistency processing for precision balls can be realized by the change of the groove curvature in variable-radius groove processing method.