Study On The Fan-Shaped Distribution Of Surface Roughness In Turning Single Crystal Germanium

Single crystal germanium belongs to brittle optical crystal material,which has the characteristics of high infrared refractive index and low dispersion index.It is widely used in aerospace,infrared optics and other fields,and is often used to manufacture optical lenses and windows.However,in single point diamond turning,the machined surface roughness often presents a phenomenon of alternating distribution of light and dark sectors,which not only affects the transmission of infrared light,but also prolongs the processing time and increases the production cost.Therefore,in view of this phenomenon,this paper calculates the critical undeformed chip thickness and undeformed chip thickness of dynamic brittle plastic transition of single crystal germanium as the theoretical judgment of this phenomenon,which is verified by single point diamond turning test.First of all,through the experimental analysis and SEM observation,it is found that the interphase distribution of light and dark sectors is a manifestation of alternating plastic removal and brittle failure on the machined surface of single crystal germanium,which is caused by the relationship between the critical undeformed chip thickness of brittle plastic transition and the undeformed chip thickness.The results show that the critical undeformed chip thickness of dynamic brittle-plastic transition of single crystal germanium varies periodically.When the undeformed chip thickness is between the peak and valley of the critical undeformed chip thickness of brittle-plastic transition,plastic removal and brittle failure will occur alternately on the machined surface of single crystal germanium,resulting in the interphase distribution of light and dark sectors,This phenomenon can be avoided when the undeformed chip thickness is not greater than the valley of critical undeformed chip thickness of brittle plastic transition.Therefore,according to the relationship between undeformed chip thickness and feed speed,cutting depth,tool tip radius and rotation speed,the mathematical calculation method of undeformed chip thickness is obtained.Secondly,because the cutting force can reflect the quality of the machined surface to a certain extent,the SPH Simulation method is used to simulate the plastic cutting process of single crystal germanium-25°and the tool back angle is 10°.When the cutting force is small and the fluctuation is small,the chip is easy to flow out,so it is easier to realize the plastic removal of the machined surface of single crystal germanium,and avoid the phenomenon of alternating distribution of light and dark fan-shaped regions in the machined surface roughness.Finally,the relationship between the critical undeformed chip thickness and the undeformed chip thickness is verified by a single point diamond turning test with some tool parameters obtained from the simulation.The relationship between the critical undeformed chip thickness and the undeformed chip thickness can be used as the theoretical judgment of this phenomenon,It provides a theoretical basis for the plastic removal of the machined surface of single crystal germanium and the avoidance of the interphase distribution of light and dark sectors.