Glass-removal coefficients emerge from the realm of black arts
Material removal from optical glass during grinding can be characterized by glass mechanical properties. Researchers from the University of Rochester Center for Optics Manufacturing (Rochester, NY) reported at the OSA Annual Meeting in Portland, OR, how they used a computer-controlled grinding machine to perform bound-abrasive-grinding experiments on a variety of optical glasses, including fused silica, crown, and flint glasses (paper #MBBB1). These experiments included bound-abrasive tools bearing 2-4-, 15-, and 60-µm grit sizes. By correlating the resultant surface finish with mechanical properties of the glass, they were able to relate ductility index (a quantity based on glass hardness and fracture toughness) to surface finish, predicting postgrind surfaces to a 95% accuracy. Says group leader John Lambropoulos, "We`ve brought science to optics manufacturing by showing the relationship between the quality of the surface after manufacturing and the fundamental mechanical properties of the glass."
To verify the theory, Lambropoulos used it to derive lapping-hardness data for loose abrasive grinding of many common optical glasses and found it agreed with values obtained empirically by optical-glass manufacturers. Accurate removal rates are critical to computer-controlled grinding operations, and the Preston classic material-removal equation is directly proportional to a coefficient that in the past needed to be determined experimentally. "Now," says Lambropoulos, "if you tell me the grit size and shape, the glass, the coolant, and the backing plate type for brittle glass removal, I can tell you the Preston coefficient."