Research of Key Technology on Precision Grinding of Deep Conformal Sapphire Dome

Author:Wang Jin Hu

Supervisor:zhao qing liang


Degree Year:2018





The conformal optical domes is one of the key technologies for development of conformal optics,which can significantly improve the aerodynamic performance of high speed aircrafts and promote the environmental adaptability of optical systems when compared with spherical optical domes.The conformal optical dome technology involves material science,optics,mechanics,manufacturing technology,coating process and so on,and represents the advanced technology level of the country.Particularly,manufacturing technique is a key link that affect the equipment costs and cycles of conformal optical dome which combines the material science,mechanics and other subjects in itself.At present,the requirements for material strength,structure length ratio,and optical transmission of conformal dome are getting higher and higher with the development of national defense science and technology.However,the high hard and brittleness,large aperture,high steepness and thin-wall structure of conformal dome have brought great challenges to traditional precision and ultra-precision machining technology.In western developed countries,a series of processing technology and special equipment have been researched and exploited for conformal dome with different materials and structure,such as deterministic micro-grinding,belt grinding,sub-aperture polishing and so on,while domestic research on conformal dome is still concentrated in low hardness material such as Mg F2.Breakthrough in high efficiency and high precision machining technology of ALON,spinel,sapphire and other advanced infrared material for conformal optical dome is of great strategic significance to improve the technological level of conformal optical manufacturing and the preparation of high performance weapon in the future.In this study,the applicable grinding methods were discussed according to the geometric structure and material properties of deep sapphire conformal dome.Then grinding and scratching experiments were conducted to study the material removal characteristics of sapphire.Finally,precision grinding of deep sapphire conformal dome was finished based on in-situ dressing of hemispherical diamond wheel.The main contents of this paper include the following aspects:The applicability and limitations of vertical-spindle grinding,inclined-spindle grinding and normal grinding were discussed in terms of the structure characteristics of conformal dome’s internal and external surface.Then the grinding system composed of inclined-spindle and hemispherical diamond wheel was selected after planning the grinding path.After that,the wheel dimension and the relationship between grinding wheel’s active profile and workpiece’s profile were calculated and analyzed.Lastly,the influence of grinding wheel’s properties on the grinding surface quality of sapphire and the wear rate of grinding wheel was studied through plane grinding experimental research,then the selection principles for deep sapphire conformal dome precision grinding parameters were determined.The ground surface quality anisotropy was evaluated by macro and micro characterization along the C-and M-planes of sapphire.In addition,the generation mechanism of surface quality anisotropy of ground sapphire was uncovered through the scratch experiment along different crystal orientation and the indentation contact deformation theory.Besides,the characteristics and mechanism of the evolution from ductile flowing to brittle fracture in sapphire was studied,including the topographical change of chips and the initiation and extension of cracks.The in-situ non-contact test of the profile error was achieved for hemispherical grinding wheel.By the comparison to both the test result from contact profilometer and the methods of precutting workpiece,the accuracy of the test result was proved.Based on the dressing principles of roller wheel,the dressing error of hemispherical grinding wheel was modeled and analyzed,also the key factors and control standards influencing the grinding wheel’s dressing accuracy were determined.At last,the in-situ dressing procedures for the resin-bonded hemispherical grinding wheel were established and the precision dressing of a Φ50mm hemispherical wheel was achieved with a profile error less than 5 μm.Based on the grinding system composed of inclined-spindle and hemispherical diamond wheel,the grinding procedures were established for deep sapphire conformal dome after optimizing the proper matching of the grinding spindle speed and workpiece spindle speed.Then the deep sapphire conformal dome was ground precisely,with the form error achieving less than 3 μm on the top Φ50mm area of dome’s internal and external surfaces and surface roughness ranging from Ra 200 nm to 500 nm influenced by the material anisotropy.Besides,the wear distribution on hemispherical grinding wheel during precision grinding of conformal dome was predicted theoretically,which agreed well with the practical measurement results.The reason for wheel wear included bond breakage,grain micro fracture and grain erosion wear.