In an important step forward for the optics industry, University of Rochester engineers have solved a long-standing problem by developing the first formula to predict how well different glasses stand up to the grinding process, a key step in making high-quality optical components unblemished by cracks, waves and other defects. Engineers working with the University's Center for Optics Manufacturing (COM) presented the results at a meeting of the Optical Society of America in Portland, Ore., earlier this month.
"This is a significant breakthrough in our ability to predict and control what will happen on the factory floor," says Harvey Pollicove, director of COM. "In 50 years of research, no one has come close to accurately predicting surface finish -- until now."
Our lives depend on a series of lenses, prisms, and mirrors ground and polished to within a hair's breadth of perfection to transmit light from point to point. "Smart" bombs and airplane guidance systems, laser printers and fax machines, cameras and telescopes, the lasers that zip our voices and data across the globe or cut into our bodies -- good optics stand behind it all.
But creating high-quality optical surfaces on glass has always been more an art than a science, with the grinding process dependent on master craftsmen drawing on years of experience to size up the more than 250 types of glass they might work on, and working by trial and error to gauge new glass materials. The new work, performed by a team led by mechanical engineer John Lambropoulos, makes it possible for technicians with very little optics background to take almost any brittle glass and predict the outcome of the grinding process. Lambropoulos and his students found that just two properties, hardness and fracture toughness, determine how a type of glass holds up under deterministic microgrinding, a process developed at COM.
"Glass varies tremendously from type to type," says Lambropoulos, professor and chair of the Department of Mechanical Engineering. "It's like cooking: Even if you start out with the same materials, what you end up with depends on several factors, such as the mix of ingredients and the processing steps. We've brought science into the picture, so that technicians can predict the consequences of grinding by making two very simple measurements."
Lambropoulos and graduate students Tong Fang and Su Xu analyzed experimental data collected by engineers in COM's Manufacturing Sciences Group who subjected three dozen types of glass to precise grinding tests and then modeled the tests on a computer. They found that hardness and fracture toughness together account for variations in the grinding process. A glass's hardness is routinely provided by glass manufacturers, while its fracture toughness can be measured using very inexpensive equipment, says Lambropoulos.
"This explains why some glasses become very smooth with very little damage, while some become rough and damaged, even though you're using the same machine, the same operator, and the same settings," says Lambropoulos. To perform its tests the team, whose work is funded by the American Precision Optics Manufacturers Association and the U.S. Army, used a new grinding machine developed by COM, the Opticam. In a process known as deterministic microgrinding, the computer-controlled Opticam allows engineers to grind optical elements with a precision previously unknown in the industry. A dozen of the machines are now in use in companies such as Texas Instruments and Eastman Kodak, and two new models will be introduced later this year.
The new result is an important step in COM's mission to modernize and automate optics manufacturing. Pollicove says the next step is to develop a "manufacturing map" that lays out the properties and grinding temperament of each type of glass. Eventually, a computer operator will input the type of glass into the Opticam, which will use the information to decide on the speed and force of its equipment while grinding.
"This work will have an impact throughout the optics manufacturing world, especially on smaller businesses," says Pollicove. "Sometimes a shop will lose money grinding a new material that has unexpected qualities; some shops won't even bid on a material that they haven't had experience with. This work breaks down such barriers." tr Note to editors: Color slides are available.Deterministic microgrinding
To transform a rough piece of glass into a glistening, crystal-clear optical component, opticians grind and then polish the piece. Traditionally, opticians use small abrasive particles such as aluminum oxide mixed in a water-based slurry to gradually remove imperfections and shape a rough piece of glass into the optical shape that is needed to precisely direct light rays from a distant point. The grinding process leaves tiny cracks just below the surface as well as small crater-like imperfections that make the glass piece hazy and prone to breakage. To remove the damaged layer and produce a clean optic, the piece goes through extensive finishing or polishing.
"Polishing is the most time-consuming and expensive part of the optics process," says Lambropoulos.
The computer-controlled Opticam microgrinding system developed at COM produces remarkably cleaner pieces. An optical work piece is held firmly in place; the grinding tool is made of bronze or some other rigid material, with an abrasive material such as diamond flakes embedded in it. The flakes are as small as 1-2 microns wide, smaller than even the blood cells that squeeze through our smallest blood vessels.
"The Opticam's stability and precision lets you control the process in a way previously unheard of in the optics industry," says Pollicove.
In a work piece produced by the Opticam, only the top one or two microns (millionths of a meter) are damaged and need to be removed, compared to many times that amount with conventional grinding techniques. This reduces polishing time and expense. For many applications, such as night vision systems or equipment to monitor wind shear or smokestack emissions, microgrinding eliminates the finishing step altogether.
"Microgrinding gives you the quality of polishing for many applications, but it's as inexpensive as grinding," says Lambropoulos. tr