Freeform surfaces manufacturing of optical glass by ultrasonic vibration-assisted slow tool servo turning

The optical glass is a typical difficult-to-cut material for single-point diamond turning because of its high hardness and low fracture toughness. The application of high-frequency vibration-assisted technology can make it a machining method for processing optical glass materials. However, due to the lack of understanding of intermittent cutting characteristics, it is still a challenge to achieve a high-precision and crack-free freeform surface on hard-brittle glasses. Therefore, the freeform surfaces of optical glasses are processed by ultrasonic vibration-assisted slow tool servo turning (UVSTS). The crack evaluating model of UVSTS turning is established based on the theory of specific cutting energy to predict fracture formation at all cutting position points of the freeform surface. Furthermore, the mechanisms of fracture initiation are analyzed and discussed deeply according to different factors. Finally, the bidirectional sinusoidal surface as a representative freeform surface is successfully fabricated on N-BK7 glass materials. The surface roughness can reach 1 nm Sa only by UVSTS turning without requiring any subsequent processing (including grinding or polishing). The research findings show the UVSTS turning technology can be used as a new machining method with high precision and high efficiency to replace traditional grinding and polishing technology for processing the optical elements of the freeform surface of glass materials.