"4+2"axis ultra-precision diamond shaping for partially overlapping Fresnel lenses based on an ultra-precision machine tool and two additional rotational axes

Optical surfaces and microstructures with high form accuracy and excellent surface quality are widely utilized nowadays and the diamond machining technology is still preferable for the fabrication of these workpieces. However, when machining surfaces featuring a high aspect ratio or intricate microstructures like monolithic microstructural optics, the machining efficiency is quite low and the cutting tool interference is inevitable for the existing diamond turning techniques. Hence, a novel method is proposed to machine such microstructures by introducing two additional rotational motions into the traditional ultra-precision machine tool. In this way, the arbitrary adjustment of the tool posture may be achieved in the machining process and intricate microstructures like monolithic microstructural optics can also be machined by the side cutting edge with high efficiency. The whole mechanical system and machining principle are presented firstly. Next, the tool path planning and the compensation for the tool tip offset are introduced. Then, the partially overlapping Fresnel lens array consisting of two segmented Fresnel lenses are fabricated and inspected. Finally, the measurement data are analyzed and the proposed method is validated to be able to fabricate such complex microstructures with high profile accuracy and surface quality successfully.