Fabrication of high-precision cylindrical microlens arrays (CMLA) on single-crystal calcium fluoride (CaF2) substrates is critical for advanced lithographic illumination systems. However, the intrinsic soft-brittle characteristics and weak elastic anisotropy of CaF2 often induce brittle fracture and surface damage during ultra-precision machining, thereby limiting the achievable form accuracy and surface quality of microstructures. This study systematically investigates the coupling relationship between profile accuracy and surface integrity in the diamond machining of CaF2 CMLA, with the particular emphasis on the effects of material properties, auxiliary cutting strategies, tool geometric accuracy, and tool wear evolution. First, the fundamental mechanical properties of CaF2 are analyzed based on its elastic constants. The calculations show that CaF2 exhibits a Young's modulus of 110.04 GPa, a Poisson's ratio lower than 1/3, and a hardness of 5.71 GPa, confirming its typical soft-brittle nature. The elastic anisotropy factor ranges between 0 and 1, indicating relatively weak anisotropy compared with many crystalline optical materials. These characteristics imply that maintaining stable ductile-mode material removal during conventional cutting is challenging. To address these limitations, two auxiliary ultra-precision machining techniques, ultrasonic elliptical vibration cutting (EVC) and fly-cutting, are comparatively investigated. Experimental results show that EVC can locally improve surface finish by introducing intermittent cutting and reducing cutting forces. Nevertheless, the periodic reversal of tool motion in the elliptical trajectory inevitably generates scratches and defects at the groove bottom of the microlens structures. In contrast, fly-cutting consistently produces superior surface quality across the entire CMLA surface. This advantage is primarily attributed to the significantly reduced tool-workpiece contact time and the suppression of instantaneous cutting forces, which effectively mitigate brittle fracture during material removal. A systematic parametric investigation of the fly-cutting process is subsequently performed by varying spindle speed and feed rate. Within the investigated parameter range, the root-mean-square (RMS) profile error remained nearly constant at (208.2±5.76) nm, while the radius error is maintained at (0.23±0.04)%. The negligible influence of process parameters on form accuracy indicates that the dominant source of shape error originates from the direct replication of tool geometric inaccuracy onto the workpiece surface. This "direct replication" mechanism is further verified through post-processing correction of non-circular diamond tools. After tool contour calibration, the workpiece form error decreased dramatically from approximately 200 nm to an RMS value of 16.8 nm, with a peak-to-valley (PV) value of 72.4 nm, demonstrating that tool contour accuracy is the key factor governing microstructure formation accuracy. In addition, the evolution of surface roughness with cumulative cutting distance is investigated to evaluate the influence of tool wear on machining stability. The results reveal a distinct three-stage behavior: an initial stable stage with surface roughness below 2 nm for cutting distances shorter than 8 m, followed by a rapid degradation region between 12 and 18 m, and subsequently another relatively stable stage. This deterioration is mainly attributed to the progressive accumulation of localized tool wear. Post-machining characterization of V-shaped diamond tools reveals a highly non-uniform wear distribution, with severe wear concentrated at the tool tips, the primary cutting regions in multilayer cutting; while the central regions exhibit minimal wear. These results demonstrate that once the dominant geometric errors originating from tool fabrication are eliminated, tool wear, particularly in high-stress regions such as the tool tips, becomes the primary factor limiting process stability and surface quality. Consequently, ultra-precision machining of CaF2 CMLA exhibits a characteristic transition from a geometry-error-dominated regime to a wear-dominated regime. The findings confirm that the combination of form diamond tools and fly-cutting provides an effective and reliable approach for achieving high-precision, low-damage fabrication of CaF2 CMLA, offering valuable experimental guidance for the precision machining of CaF2 and other brittle optical crystals used in advanced optical systems.
Key words
calcium fluoride (CaF2) /
brittle optical materials /
cylindrical microlens array /
form tool cutting /
profile accuracy
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Funding
Key Project of the National Natural Science Foundation of China (U24B2061); Ten Key Technological Projects of Hunan Province, China in 2023 (2023GK1060)
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