| Hemispherical resonator gyro(abbreviated to HRG)is a new type of gyro without moving parts developed in 1960 s,which has the advantages of impact resistant,small size,light weight,resistance to radiation,high reliability and long life,and is widely used in inertial navigation system of aerospace,warships,and armament.Hemispherical resonator is the core component of HRG,and its machining accuracy and surface quality has directly influence on the performance of the gyro.At present,the machining accuracy of precision ground hemispherical resonator cannot meet performance requirements of the gyroscope,which has become a bottleneck technical problem that restricts the development of our hemispherical resonant gyroscope to higher precision and longer working life.This paper takes the key issues in the polishing process of hemispherical resonator as research object,a polishing method based on the principle of magnetorheological finishing(MRF)using a small polishing head is proposed,a MRF machine tool is developed,the MRF process using a small ball end permanent magnet polishing head is studied,the polishing trajectory error and material removal characteristics are analyzed,high accuracy and surface quality part is achieved.The polishing surface of hemispherical resonator includes internal and external center bar and spherical surface,which has a complex polishing trajectory.The conventional MRF machine cannot be used in finishing of small radius curvature surface parts by restrictions on the size of polishing wheel.A special structure form of polishing machine tool should be designed.The polishing method using a small polishing head based on MRF is proposed,the polishing machine tool is designed and developed,finite element model of the machine tool is built and static simulation is carried,static displacement of polishing tool and workpiece spindle under different position is analyzed,structure size of main bearing parts is optimized,polishing on fused quartz parts is carried out and fine surface roughness are obtained.The polishing head is 4mm in diameter made of rare earth permanent magnetic material.The MR fluid forms a solid-like flexible polishing film adhered to the surface of polishing head to achieve material removal.The polishing removal characteristics vary on the polishing path so that the polishing parameters need to be controlled to achieve high precision machining.A MRF process using a small ball end permanent magnet polishing head is proposed,the polishing head generates rotary symmetrical magnetic field which make MR fluid form a flexible polishing mold on the surface of the polishing head to removal material.MRF process using small ball end permanent magnet polishing head is studied,the influence of structure parameters on the magnetic field distribution is obtained based on magnetic field simulation,the structure size of the polishing head is determined,MR fishing fluid dynamics is analyzed,the relation of apparent viscosity and velocity with dynamic pressure is gained,and shear stress is adopted to establish the material removal function for the proposed MRF process,which provides the theoretical basis for the MRF forming of spherical surface of hemispherical resonator,experimental study on the influence of polishing parameters on material removal rate and surface roughness are carried out.The MRF of hemispherical resonator is based on the relatively movemonet of computer controlled polishing head and workpiece,using flexible MRF polishing mold in contact with the workpiece surface and residing at various points for a processing dwell time to achieve accurate material removal on workpiece surface,so that it improves surface quality while ensure the surface accuracy.Deviation of polishing path changes material removal characteristics on workpiece spherical surface during polishing process,which affects the shape accuracy.This paper designs the polishing trajectory of hemispherical resonator,analyzes the tool setting requirement and presents the methods of presetting polishing tool based on image processing,analyzes the influence factors of polishing path deviation,such as manufacturing and assembling deviation of polishing head,workpiece clamping deviation,image contour detection precision,machine tool movement accuracy and guide rail straightness,gains the permission deviation of the main factors influencing trajectory error.In order to ensure the polishing accuracy,material removal characteristics on polishing trajectory need to be realized.Based on analysis of the variation of magnetic field,the shear stress and velocity distribution on workpiece surface with different location are solved using the established model of material removal function.Polishing dwell time on machining trajectory are researched and planed.The polishing dwell time and feed speed on polishing trajectory are given.Orthogonal experiment are taken to investigate the combined influence of processing parameters on material removal rate and surface quality,and polishing process parameters are optimized.Optimization parameters of material removal rate are used for quickly removing the surface defects generated during grinding operations,while optimization parameters of surface roughness are taken to improve the surface quality.Nano scale surface roughness and sub-micron accuracy polishing surface of hemispherical resonator are achieved. |
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