SURFACE MICROROUGHNESS OF OPTICAL-GLASSES UNDER DETERMINISTIC MICROGRINDING

Deterministic microgrinding of precision optical components with rigid , computer-controlled machining centers and high-speed tool spindles i s now possible on a commercial scale. Platforms such as the Opticam sy stems at the Center for Optics Manufacturing produce convex and concav e spherical surfaces with radii from 5 mm to infinity, i.e., planar, a nd work diameters from 10 to 150 mm. Aspherical surfaces are also bein g manufactured The resulting specular surfaces have a typical rms micr oroughness of 20 nm, 1 mu m of subsurface damage, and a figure error o f less than 1 wave peak to valley. Surface roughness under determinist ic microgrinding conditions (fixed infeed rate) with bound abrasive di amond ring tools with various degrees of bond hardness is correlated t o a material length scale, identified as a ductility index, involving the hardness and fracture toughness of glasses. This result is in cont rast to loose abrasive grinding (fixed nominal pressure), in which sur face microroughness is determined by the elastic stiffness and the har dness of the glass. We summarize measurements of fracture toughness an d microhardness by microindentation for crown and flint optical glasse s, and fused silica. The microindentation fracture toughness in nonden sifying optical glasses is in good agreement with bulk fracture toughn ess measurement methods. (C) 1996 Optical Society of America