Research On Key Technology Of Ultra-Precision Grinding Of Monocrystalline Silicon Biconical Free-Form Mirrors

With the rapid development of the design concept of high-end optical systems,freeform optics with excellent reflection and imaging performance breaks the limitation of rotation and translation symmetry of the traditional optical surface.Free-form optics has been widely used in various advanced optical systems to effectively reduce the wavefront error,simplify the system structure,optimize the energy utilization rate,improve the fields of view and image quality,and realize the functions that are difficult to be achieved by traditional optics.However,the free-form optics made of hard and brittle materials have the extreme requirements of complex surface shapes,sub-micron profile accuracy,nano-surface finish,uniform and controllable of scallop-height,and plastic machined surface,which bring unprecedented great challenges to ultra-precision grinding.At present,the ultra-precision manufacturing of large-scale free-form optics made of hard and brittle materials is faced with many technical problems,such as immature grinding process,difficult to guarantee accuracy,no effective on-machine measurement technology,overly long processing time,and difficult to converge full-frequency error.This dissertation aims to break through the processing technical problems of micronlevel profile accuracy,nano-level surface roughness,controllable intermediate frequency scallop-height,and low-damage machining surface of the monocrystalline silicon biconical free-form mirror in ultra-precision grinding.The research on the key technology of ultra-precision grinding of monocrystalline silicon biconical free-form mirror was carried out.This thesis mainly includes the following aspects:(1)The damage mechanism during ultra-precision grinding of monocrystalline silicon was studied using the proposed AE signal processing method in this paper,and an on-machine AE monitoring and acquisition system for ultra-precision grinding of monocrystalline silicon was established.The effects of grinding depth and feed speed on the damage type of monocrystalline silicon and the energy spectrum characteristics of AE signal were analyzed through the ultra-precision grinding experiment monitored by AE.The material removal mode and the corresponding time-frequency characteristics,the material damage characteristics and the critical characteristics of AE signals during material removal mode transition in the process of ultra-precision grinding of monocrystalline silicon were studied.The subsurface damage depth was successfully predicted at different grinding depths and feeding speeds.(2)Based on the difficulty of truing the arc-shaped grinding wheel with coarse abrasive grain metal bond,a bi-directional feed compensation dressing strategy was proposed and verified.The on-machine truing system of arc-shaped grinding wheel and the non-contact on-machine measurement system for profile accuracy and geometric parameters were established.Based on the bidirectional feed compensation truing strategy and non-contact on-machine measurement method,the on-machine precision truing technology of the arc-shaped grinding wheel was studied.The wear mechanism and wear resistance of dressing rollers with different particle sizes were analyzed.(3)The grinding strategy and processing technology of ultra-precision raster grinding for off-axis biconical free-form mirror were studied.The experimental platform for ultra-precision raster grinding of biconical free-form mirror and the on-machine measurement system of profile errors were established.The profile error scanning strategy of biconical free-form surface was studied,and the profile error evaluation model of free-form surface was established and verified.The ultra-precision raster grinding experiments of off-axis biconical free-form surfece of monocrystal silicon were carried out to verify the correctness of the profile error measurement system and the profile error evaluation model.The profile error compensation machining technology of ultraprecision raster grinding of monocrystalline silicon biconical free-form surface based on on-machine measurement was developed,and the experiments of profile error compensation machining were successfully carried out.(4)The slow-tool servo grinding process of hard and brittle material free-form surface optics was proposed,and the grinding wheel trajectory generation strategy of slow-tool servo for grinding biconical free-form surface was studied in detail.The generation process of the surface topography of slow tool servo wheel grinding was modeled and analyzed.The slow-tool servo grinding wheel grinding experiments of nonoff-axis biconical free-form surface mirror were carried out,and the higher profile accuracy and nanoscale surface roughness were achieved.By establishing the scallopheight error model,putting forward the precision adjustment method of the center error and calculating the critical grinding parameters,the slow-tool servo grinding experiment of the off-axis biconical free-form mirror was carried out.Finally,the low,medium and high frequency error fast convergence and the plastic grinding of the monocrystalline silicon off-axis biconical free-form mirror were realized.