IN the cost-conscious world of manufacturing, any new process that saves time and money is likely to be very popular. This is especially true for precision optical components, which would be more widely used if they cost less to produce. Livermore's award-winning OptiPro-AED (acoustic emissions detector) should help make the grinding process a reality.
The vast majority of optics produced in this country--for disposable cameras, binoculars, and gunsights--are relatively inexpensive to manufacture. But the highest quality systems--found in military night vision systems, medical endoscopes, and machine vision systems for aligning parts, for example--require higher quality lenses, which need additional grinding and polishing. Fine grinding, typically the last step before polishing, is now a relatively labor-intensive task that drives up the cost of precision optics.
Fine grinding is a sensitive operation. 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. Fluid to cool the grinding operation flows at several liters per minute, creating a torrent of fluid between the rotating tool and workpiece that makes visual proximity measurements impossible. Efficient and safe computer control of the process has not been possible in such an environment.

A Gentle Touch Does It
Livermore engineers Mark Piscotty, John S. Taylor, and Kenneth Blaedel (Figure 1, above), working with Mike Bechtold of OptiPro Systems Inc. of Ontario, New York, have developed a device known as the OptiPro-AED Proximity Sensor, which automatically detects the separation between the grinding tool and the lens (Figure 2, below) using an acoustic-emission detector (AED). With this device, the in-feeding operation may join the ranks of fully computer-controlled processes used to manufacture optical components. The grinding tool can automatically and efficiently approach the lens without risk of crashing into the fragile lens surface and with minimal loss of production time.
Funded under a cooperative research pact and by Lawrence Livermore's Small Business Program, this advance grew out of a project between the Laboratory, the Center for Optics Manufacturing, and the American Precision Optics Manufacturers Association, of which OptiPro Systems is a member.
Notes Piscotty, "The acoustic-emission levels generated in fine grinding prior to contact are at least ten times less than background levels produced by typical grinding machines. This device must therefore be extremely sensitive to be able to sense these smaller signals in a very noisy environment."
The OptiPro-AED is a piezoelectric acoustic-emission sensor mounted in a probe assembly that is inserted in the spindle of the optic being ground. The sensor is coupled to the optic through a lightly spring-loaded contact probe, which gently touches the backside of the optic. The entire assembly is acoustically isolated from the spindle and the machine. This unique design enables the small acoustic-emission signals to be detected in the presence of swirling coolant and background machine noise.
The sensor incorporates commercially available acoustic-emission equipment to detect signals generated by the relative motions of the rotating grinding tool, rotating glass optic, and coolant fluid. The turbulence created in the coolant generates signals that are strongly dependent on the gap between the moving tool and the workpiece.
OptiPro Systems Inc. has already begun integrating the OptiPro-AED into a commercial line of lens-grinding equipment. Recent tests show that the OptiPro-AED in-feeding process is more than ten times faster than traditional manual methods.
Some manufacturers of optical components have omitted the fine-grinding step because it is so labor intensive. But without fine grinding, the final polishing operation takes longer. The OptiPro-AED is expected to entice optics manufacturers to incorporate fine grinding into their production lines, improving productivity and lowering the cost of precision optics.






There's No Comparison
Other commercially available sensors, which detect when a grinding tool touches the piece being manufactured, are appropriate for use during coarse metal machining and grinding. If only a contact sensor is used during fine grinding of optical components, the workpiece and possibly the grinding tool can be damaged upon contact.
The OptiPro-AED measures the proximity of the tool to the workpiece, making it the only sensor useful for fine grinding of optical components. The proximity measurements tell the computer controls when to decelerate the tool prior to contact with the optic so that no damage occurs. This sensor is also the only one designed to work specifically under the optics industry's wet grinding conditions.

Expanding Use
The OptiPro-AED may also have applications in the semiconductor industry, which requires high surface quality and little or no subsurface damage in silicon wafers. The OptiPro-AED could greatly improve the economics of grinding wafers, enabling grinding to complement or even replace industry-standard chemomechanical finishing methods.
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 (dressing) 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.
If the precision optics industry adopts the OptiPro-AED, several spinoffs will ensue. The cost to produce some optical components already being manufactured should drop. More importantly, fully automating the fine-grinding process will improve the economics of producing precision optics in large volumes. If precision optics are less expensive, they will have much wider use than they do today--in defense, medical, and manufacturing applications as well as in a broad array of consumer goods.
--Katie Walter

Key Words: acoustic-emission detector, grinding, optics manufacturing, OptiPro-AED, proximity sensor, precision engineering.

For further information contact Mark Piscotty (925) 422-2052 (markp@llnl.gov), John S. Taylor (925) 423-8227 (jstaylor@llnl.gov), or Kenneth Blaedel (925) 422-0190 (blaedel@llnl.gov).


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