Dynamic Performance Validation And Optimization For The Ultra-precision Flycutting Equipment

The ultra-precision diamond flycutting technology is the most effective method to finish large diameter KDP crystals.A fine optical surface with nanometer roughness can be directly produced in one processing.However,due to the characteristice of the copying process,waviness errors are introduced into the finished surfaces by vibrations and motion errors,which would deteriorate the surface quality of the elements.There are strict surface precision demands in full spatial frequency band for the KDP crystals applied as the frequency conversion elements of the high-energy laser system.Among them,the RSM value in the PSD1 band is required to be less than 5nm,which is close to the accuracy limit of our existing ultra-precision flycutting equipments and technologies.In order to further improve the machining accuracy,machining efficiency and yield,it is urgent to evaluate the stability and upgrade the existing equipments,especially to optimize the dynamic performance of the machine tool,so as to suppress the waviness error.In this paper,the dynamic performance of the spindle system and the whole machine tool of the flycutting machine tool was studied,and the mechanism of its influence on the surface waviness was explored.In the study of the dynamic performance of the aerostatic spindle system,the fluid-solid interaction simulation was performed to obtain the deformation of the spindle under the machining process,an accurate kinetic model of the spindle system was established based on the uneven air pressure distribution as well.In a series of dynamic analysis,it is found that the original spindle system is sensitive to machining in the sixth mode with the natural frequency of 550Hz.By optimizing the structure of the spindle system,the sensitive frequency becomes 850Hz in the tenth order,and the amplitude of vibration on the tool tip reduces greatly from 22nm to 7.7nm.Furthermore,an accurate dynamic model of the whole machine tool was established by the virtual material method,the modal analysis of the whole machine was conducted subsequently.The results indicate that insufficient bonding stiffness leads to severe modal vibrations,and the fifth mode of 269Hz is the most sensitive mode for processing.In order to verify the accuracy of simulations and explore the influence mechanism of dynamic performance on the waviness errors,a series of flycutting experiments were carried out and the surface topography features were extracted.The flycutting experiments with different spindle systems show that the modal vibration of the spindle system is the main cause of mid-spatial frequency waviness,which can be effectively suppressed by optimizing the spindle structure.The flycutting experiments with different bonding stiffness show that the insufficient bonding stiffness is the main reason to introduce the waviness of about 50mm,and such error can be eliminated by increasing the bonding stiffness appropriately.Finally,on the basis of the virtual material model,the modal optimization of the whole machine tool was carried out,and the optimal of the practical joint was proposed as increasing the preload torque of the beam-column bolts from 30N·m to 50N·m.