High-frequency ultrasonic vibration-assisted sculpturing with a smoothed tool path for optical 3D micro-structured surfaces with sharp edges

The fabrication of complex 3D micro-structured surfaces on difficult-to-machine materials such as steel and tungsten carbide is the premise for mass production of glass components with functional surfaces by the glass moulding process. Ultrasonic vibration-assisted cutting (UVAC) is an effective method to machine 3D micro-structured surfaces on these difficult-to-machine materials. However, it is challenging and difficult for UVAC to machine microlens arrays with sharp edges, at which sudden acceleration of the z-axis slide is required to make the cutting tool follow the tool path. This could induce tool vibration and cause relative tool-work displacement, resulting in poor machining accuracy and surface finish of the machined 3D microstructure. In this paper, high-frequency ultrasonic vibration-assisted sculpturing (HFUVAS) with a smoothed tool trajectory is used to machine microlens array on steel mould for fine machining accuracy and surface quality. A comparison is made between the cutting performance of the circular microlens arrays machined by diamond turning and diamond sculpturing. The results found that relative tool-work displacement arising at sharp edge in diamond turning could be eliminated in HFUVAS with a smoothed tool path, demonstrating the effectiveness of a smoothed tool path in reducing relative tool-workpiece displacement in the fabrication of 3D micro-structured surfaces. Furthermore, a quadrilateral microlens array and hexagonal microlens array are also fabricated in order to extend the applications of HFUVAS, with a form error within 1 μm and surface roughness with an arithmetical mean height S_a of below 10 nm achieved.