08 August 2001
Sensor automates grinding of precision lenses
by Bob Felton
Faster, finer grinding could improve productivity and lower costs.
Faster, finer grinding could improve productivity and lower costs.
A new acoustic emission sensor will promote automation of the fine grinding that marks the last step in manufacturing the precision lenses used in military systems, medical equipment, and machine-vision systems, according to researchers at Lawrence Livermore Laboratory.
Presently, fine grinding requires direct human supervision. According to a report by the researchers who developed the sensor, "The relatively fast speed of the grinding tool must be slowed down as it approaches the lens for fine grinding. If the grinding tool strikes the optic at too high a speed, the optic's surface will be damaged. The speeds and motions of the grinding operation are computer controlled, but directing the fine-grinding tool toward the optical surface—a process known as in-feeding—requires human intervention for as long as several minutes. An operator gingerly feeding the fine-grinding tool toward the lens may not be sure if and when contact is actually made."
Further, because cooling fluids circulate constantly between the workpiece and the grinder, reliable measurements are inhibited.
The new sensor detects the separation between the grinding tool and the workpiece, allowing the grinder to approach the workpiece without fear of striking it.
In-feed rates, according to researchers, have increased by a factor of 10. The sensor is inside the spindle that supports the optic and is coupled to the optic by a spring contact probe that presses against the backside of the optic.
Because "the acoustic-emission levels generated in fine grinding prior to contact are at least 10 times less than background levels produced by typical grinding machines," according to the developers, "the entire assembly is acoustically isolated from the spindle and the machine." The sensor detects signals created by the movement of the grinding tool, the rotating glass optic, and the swirling coolant. "The turbulence created in the coolant generates signals that are strongly dependent on the gap between the moving tool and the workpiece," according to the inventors.
At present, many manufacturers of optical components omit the fine grinding step because it is labor intensive. The final polishing operation takes correspondingly longer, however. Because engineers can now automate optics manufacturing through the in-feeding stage, however, the cost to produce some optical components should drop.
Though designed specifically for the wet-grinding environment associated with fine optics, the developers said the sensor might also have application in the semiconductor industry, "which requires high surface quality and little or no subsurface damage in silicon wafers." They speculated, further, that the sensor "could greatly improve the economics of grinding wafers, enabling grinding to complement or even replace industry-standard chemomechanical finishing methods."
Additionally, "during a grinding operation, the acoustic emission signals may also be put to a completely different use: to determine when a grinding tool is worn and requires reconditioning to avoid producing lower-quality parts. This use of the acoustic emission instrumentation could significantly reduce the number of inspections needed to ensure proper tooling performance."
The sensor was developed jointly by researchers from Lawrence Livermore National Laboratory and OptiPro Systems, Inc. in Ontario, N.Y. OptiPro has begun integrating the sensor into a commercial line of lens-grinding equipment. Additional information is available at www.llnl.gov/str/Piscotty.html. IT
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