| Energy has been the material basis for human survival and economic development,but the problem of energy shortages has occurred in most countries and even globally.Inertial Confinement Fusion(ICF)is one of the most effective ways to solve the problem of energy deficiency because of its clean and economical characteristics.Potassium dihydrogen phosphate(KH2PO4 or KDP),an optical crystal,has been widely used as the frequency multiplication elements in ICF projects due to its good quality of growth and large electro-optic coefficient.However,micro-defect spots would appear on the surface of crystal easily after continuous irradiation of high energy laser beams and ultra-precision machining.These micro-defects will keep growing and lead to the failure of optical components if any effective measure has not been taken.As is well known,KDP crystal is one of the difficult-to-machine optical materials due to the unique properties(extremely soft,fragile,deliquescent and easy to craze),and the defects repairing is a meaningful way to prolong the life of optical components and improve its optical performance.Micro-milling has been widely recognized as a very effective micro-defect repairing method.In this paper,influence of geometric parameters of micro ball end mill and cutting parameters on the machined surface quality was studied in-depth,and the optimal geometric parameters of micro ball end mill and cutting parameter were also obtained by combining the methods of finite element simulation,theoretical analysis and experiments.The existing problems in the design of micro end mill were summarized,and two micro ball end mills with relatively small and large negative rake angle were designed by fully considering the soft and brittle properties of KDP crystal.Through the deeply study of material parameters setting,rule of separation between the chip and workpiece,selection of steps,setting of contact conditions and so on in finite element simulation,the three-dimensional(3D)finite element models were established to simulate the micro ball end-milling of KDP crystal within the ductile region.The correctness of the simulation model was verified by comparing the cutting force in the finite element simulations and cutting experments.In order to investigate the influence of geometric parameters of micro ball end mill on milling process,the cutting force and distribution of stress and strain on the KDPcrystal and micro mills were selected as optimization objectives.3D finite element models with different rake angles,relief angles and cutting edge radii to simulate the micro ball end-milling of KDP crystal were established,and influence of these three factors on milling process was systematically analyzed.The simulation results showed that the cutting performance changed from good to bad with the increase of rake angle,and the optimal rake angle was-45°.With the increase of relief angle,the cutting performance became better,and the influence of relief angle on cutting performance became weak when relief angle is greater than 9°.Taking into account the stiffness of the tool,the optimal relief angle was 9°.The cutting edge radius which is between 1.5 ?m and 2 ?m was regard as the most reasonable value by comprehensive analysis of cutting force,tensile stress on cutting area and sharpening technology.The PCD micro ball end mills were fabricated by wire electrical discharge machining and diamond grinding.The cutting edge radius could reach 1.66 mm.Through the cutting experiments among three self-fabricated PCD micro ball end mill and commercial CBN micro ball end mill,the simulation results was further verified,and the cutting performance of selffabricated PCD micro ball end mill was also verified.To study the minimum undeformed chip thickness and size effect in micro endmilling of KDP crystal,the undeformed chip thickness model was established,and specific cutting force was also calculated.By combining the periodicities of undeformed chip thickness,minimum undeformed chip thickness and cutting force together,the chip formation model was established,and the ratio of minimum undeformed chip thickness to cutting edge radius was predicted between 0.43 and 0.48.The minimum undeformed chip thickness increased with the augment of cutting edge radius,and the minimum undeformed chip thickness for KDP crystal was larger than metel materials because of the soft-brittle properity of KDP crystal.The cutting specific energy,morphologies and surface roughness Ra value of micro grooves were studied carefully,and the results showed that severe size effect appeared when the ratio of feed per tooth to cutting edge radius is less than 0.5.Only slight size effect appeared when this ratio is between 0.5 and 0.7.With this ratio continues to increase(0.7~1),the size effect almost disappear,but micro cracks and bumps began to form.The brittle cutting emerged when this ratio is greater than 1.In order to study the influencing factors of the machined surface quality in micro ball end-milling of KDP crystal,the undeformed chip thickness model in micro ballend-milling of KDP crystal was established,and the mapping relationship between undeformed chip thickness and cutting parameters was obtained.Based on the size effect study in Section 4,the size effect and brittle-ductile transition under different feed rates and depth of cut were studied by comprehensively analyzing cutting force,specific cutting force,machined surface quality and chips in micro ball end-milling of KDP crystal.The size effect,ductile cutting and brittle cutting zones divided by the size effect and brittle cutting boundaries were traced on a graph drawn between the ratio of feed per tooth to cutting edge radius and depth of cut.In this paper,the cutting parameters which lets the ratio of the maximum undeformed chip thickness to cutting edge radius around 0.2 were the relatively optimal cutting parameters.Among these cutting parameters,depth of cut of 2 mm,the ratio of feed per tooth to cutting edge radius of 1 and spindle speed of 60,000 r/min were the optimal cutting parameters.The Gaussian Surfaces with contour diameters of 600 and 1000 ?m were machined with the optimized cutting parameters.The Gaussian surfaces had hardly any micro cracks,which indicated that the cutting was within the ductile domain.The surface roughness Ra values were less than 32 nm,proving the rationality of the optimized cutting parameters. |
PDF Full Text Request