Development of an intelligent grinding system for fabricating aspheric glass lenses

Abstract

Precision optical systems, such as lithographic projection lens or cameras for satellites, typically composed of glass or ceramics optical components. In the manufacturing processes of glass and ceramics components, grinding is a key technique for curvature generation (CG) of optical surface. However, dimension errors and sub-surface damage (SSD) of components occur during the CG process. High material removal rates during the CG leads to deeper SSD on optical surface. The SSD must be removed by subsequent lapping and polishing processes to ensure high optical surface quality. However, these processes are complex, and a precise process to remove SSD from the aspheric surfaces with tens or hundreds of microns of departure from best fit sphere (BFS) is required. Thus, an efficient precision lens fabrication method can reduce the depth of SSD and help to control the dimension error during the CG process. We developed a five-axis grinding system with intelligent machining techniques to improve the quality of CG process. The intelligent grinding system based on the Tongtai five-axis machine (GT-630) was combined with two measurement modules of the Renishaw measurement probe. A customized computer-aided manufacturing software was developed and applied to this grinding system. Spherical and aspherical components with a maximum diameter of 600 mm can be fabricated using the grinding system. To improve the grinding efficiency, an ultrasonic-assisted module was combined with the BBT 40 tool arbor. The frequency range, amplitude, and power of the ultrasonic device were 15–40 kHz, 0.2–2 μm, and 1000 W, respectively. The intelligent grinding system included several vibration, temperature, and acoustic emission sensors and a microphone to monitor the dynamic status of machine during the grinding process and help to quality control. The wear of the diamond grinding wheel could be predicted immediately by analyzing the collected data in this system. A five-axis machine was applied to an intelligent grinding system for large glass lenses. The fabrication time of 200 mm diameter aspheric lenses reduced about 3 weeks by using this intelligent grinding system to integrate aspherization processes into curvature generation to become an aspheric curvature generation step.