Computer-controlled polishing (CCP) with computer numeric controlled (CNC) machines offers optics manufacturers a volume fabrication technology. Working the edges of nonround components using CCP techniques presents a significant challenge, however. Often, the tool removal profile does not reach all the way to the edge, leaving a raised region. Push the tool close enough to the edge to catch the raised region and you run the risk of a rolled edge in the clear aperture.
By raster scanning a magnetorheological finishing (MRF) system, engineers at QED Technologies (Rochester, NY) have developed a high-convergence tool that can address this challenge. The machine uses a magnetorheological fluid to automatically polish rectangular mirrors, windows, and prisms to λ/25. Unlike the company's earlier versions, which use a spiral tool path to polish round parts, the raster version is symmetric with rectangular parts. "The main distinction for this approach versus the spinning approach is the aperture," says QED president Don Golini. "If the part is round, you can spin it. The raster can do rectangular or cylindrical parts."
In MRF, the work piece sits at a fixed distance from a rotating spherical wheel, so that the optical surface and the wheel surface form a converging gap. An electromagnet placed below the moving wheel generates a nonuniform magnetic field gradient normal to the wheel in the vicinity of the gap. Magnetorheological polishing fluid that becomes highly viscous in the presence of a magnetic field is delivered to the wheel just above the pole of the electromagnet. The fluid acquires the wheel velocity and essentially becomes a subaperture polishing tool.
"The big difference is that in this process, the slurry doesn't seem to change its removal characteristics over fairly long periods of time," says Peter Mumola, vice president of optics for Zygo Corp. (Middlefield, CT). "In slurry CCP, as pH changes, the slurry loads up. That doesn't seem to happen with MRF."
Zygo Corp. engineers have used the raster-scanned machine to finish rectangular windows of Zerodur, a low-expansion glass ceramic from Schott Glass (Duryea, PA). "Behind the face we were polishing was a lot of [structuring for] light weighting," says Mumola. "When we tried to polish it conventionally we ended up with print through."
To develop the raster scan machine, QED built an MRF machine around a 5-axis CNC platform, replaced the software engine, and revamped the algorithms to make the device operate in a raster scan rather than the spiral scan. The lines of the raster scan are much more closely spaced that the tool footprint, to ensure even coverage. Since July 2000, Mumola and his colleagues have used the equipment on a multi-shift basis to produce plano reference mirrors for high-accuracy stage positioning systems. "The machine has done an excellent job in producing parts with a high flatness requirement," he says. "We're getting better than λ/20 p-v, and it does it routinely and efficiently."
Zygo has used the technology on materials as diverse as sapphire, silicon carbide, and calcium fluoride (CaF2). In the case of CaF2, lens blanks of the material are cross sections of a crystal and exhibit spatially varying degrees of mechanical hardness, so fabrication using conventional polishing techniques can be very difficult. "MRF is an enabling technology for volume production of calcium fluoride optics at 193- and 157-nm wavelengths," says Golini, citing manufacturers such as the Silicon Valley Group (San Jose, CA), Carl Zeiss (Oberkochen, Germany), and Tropel Corp. (Fairport, NY) as customers.
Mumola, for one, is bullish on the technology. "We had two CNC machines that were configured for CCP, and we basically had QED turn them into MRF machines," he says with a laugh.