Tool Path Generation For Single Crystal Germanium Freeform Surface Turning Based On Ductile Region Cutting Mechanism

Optical freeform surfaces have a powerful ability to correct aberrations and optimize system structures,and are known as revolutionary components of modern optical systems.With the rapid development of ultra-precision machining and highprecision measuring technique,the manufacturing ability of optical freeform surface components has been improved,and their applications in optical systems are also increasing.Among them,infrared optical brittle materials have very important application value in key fields such as space exploration,aerospace,national defense and military industry due to their special waveband characteristics.However,due to the inherent low fracture toughness and high brittleness of infrared optical brittle materials,brittle fracture is extremely easy to occur during the process of turning freeform surface components,and surface quality and surface accuracy are difficult to control.In order to take into account both its high-precision machining requirements and the brittle nature of materials,it is necessary to systematically study the ductile domain cutting theory and machining methods of this type of freeform surface.At present,research on ultraprecision machining of freeform surfaces of infrared optical brittle materials is relatively limited.There is a lack of systematic research on the cutting mechanism,tool path generation,and basic technology of freeform surfaces of such materials,which limits the wide application of freeform surface components of brittle materials and the rapid development of infrared imaging.It is necessary to systematically study the ductile domain cutting theory and machining methods for such freeform surfaces.Focusing on the problems of difficult control of surface quality and surface accuracy,low machining efficiency,and weak machining ability of large diameter components in the ultra-precision turning process of infrared freeform surfaces,the research goal was to achieve submicron surface accuracy and nano surface roughness in infrared optical freeform surface turning of single crystal germanium.Combined with the advantages of long-stroke fast tool servo turning technology in infrared freeform surface machining,this thesis systematically studies the material ductile region cutting mechanism and the method of tool path generation under ductile cutting constraints during freeform surface turning of single crystal germanium.Specifically,the contents of this research are as follows:(1)The ultra-precision turning process of optical freeform surfaces was analyzed.The concepts of optimal comparison of rotational surfaces,non rotational surfaces,and degree of deviation were proposed,and the surface decomposition strategy for freeform surfaces was studied.The method of tool parameter selection for freeform surface machining was studied,a new method for analyzing the constraints of freeform surface shape on tool geometric parameters was proposed.Finally,an infrared freeform surface turning process represented by single crystal germanium material is proposed.(2)The ductile cutting mechanism of single crystal germanium infrared freeform surfaces based on long stroke fast tool servo turning were studied.A ductile region cutting model considering feed rate,cutting depth,tool radius,and radial slope of freeform surface was proposed to explore the mechanism of ductile removal and crack formation of single crystal germanium materials.The ductile to brittle transition(DBT)critical depth was determined using a taper cutting tests,and freeform surface turning experiments were conducted on single crystal germanium under different cutting parameters.The effectiveness of the proposed model was verified by measuring and analyzing the micromorphology,chip morphology,and surface roughness of the machined surface.(3)The research on tool path generation for optical freeform surface turning was carried out,and a tool path generation method based on surface analysis model and multiobjective optimization was proposed.The relation between tool path point sampling details and the resulting Interpolation error of freeform surfaces was analyzed and simulated.Then,the non-dominated sorting genetic algorithm(NSGA-II)was used to solve the path generation problem to obtain the pareto optimal solution of the tool path point sampling parameters for freeform surface turning.Next,based on the optimization results of freeform surface tool paths,the turning tool paths of freeform surface components were generated,and the tool paths generated by different sampling methods were evaluated,respectively.To demonstrate the effectiveness of the proposed method,turning tests of XY polynomial freeform surfaces were attempted on single-crystal germanium using the optimization method.(4)The technology of tool path generation for ductile cutting of large aperture infrared freeform surface components was studied.Taking the turning of a 160 mm diameter single crystal germanium infrared freeform surface component as the research object,a variable feed rate tool path generation method based on the maximum chip thickness constraint was proposed.The feed rate of the tool path was adjusted according to the maximum chip thickness constraint,ensuring that the entire turning process achieves material removal in a ductile cutting manner.Based on the proposed variable feed rate tool path generation method,the tool path for turning freeform surface components was generated and analyzed theoretically.Experiments on freeform surface turning of single crystal germanium were carried out to verify the effectiveness of the new method.