Understanding the friction behavior and surface characteristic in multiple ball-end milling passes of soft-brittle KH₂PO₄ optics

The utilization of a ball-end cutter to fabricate multi-layer microstructures is the preferred technique for eliminating surface defects in soft-brittle KH₂PO₄ (KDP) optics. However, the previous milling pass would inevitably affect the tool-workpiece friction behavior and surface characteristics in the next milling process, which poses a great challenge to obtaining a defect-free microstructure surface. In this study, the effect of the multiple ball-end milling (MBEM) passes on material removal mode is explored according to tensile stress, residual stress and cutting force, which is verified from the perspectives of surface characteristics and roughness. The spindle speed, feed rate and milling depth of each-pass machining process are 50,000 r/min, 3 mm/s and 2 μm. The evaluation models of tool-workpiece friction behavior during the MBEM passes are developed, and the influence of soft-brittle materials on tool wear is analyzed. The results show that with when the number of milling passes (i) rises, the normalized plowing friction coefficient, tool-chip contact length and area increase, indicating that the tool-workpiece friction becomes severe. Meanwhile, with the rise in the i, the stress concentration area and surface roughness increase, which is attributed to the intensified tool-workpiece friction leading to an increase in cutting forces, thereby elevating the tensile stress exerted on the brittle KDP materials. When the i reaches 4, the brittle-plastic transition behavior occurs and edge chippings appear on the down-cut side. Furthermore, after the MBEM passes (the length of the milling path reaches 330 mm), there is no crack at the grain boundary, single grain spalling or even tool fracture on the CBN tool surface. Nevertheless, the low hardness of the KDP materials (2.18 GPa), coupled with the thermal softening effect, results in many chips adhering to the tool surface. The chemical composition of the built-up edge (BUE) consists of K, P and O. In particular, the built-up edge is easy to be generated in the tip zone with lower cutting speed (39.0 m/min) for the ball end cutter. This work can provide theoretical support and parameter basis for formulating the recycling strategy of the large-aperture KDP optics in high-power laser devices.