In their quest for the twin grails of high production volume and extreme precision in the manufacture of both spherical and aspherical optical surfaces, manufacturers have been stymied by the difficulty of translating measurement results obtained from metrology tools to adjustments of grinding and polishing processes. In the past, this process involved manual analysis by highly skilled technical personnel — in other words, error-prone humans.
Advanced CNC fabrication and metrology equipment utilize existing shop data networks to translate measurement results into toolpath adjustments that not only speed corrections to surfaces in production, but also feed that information back to control process drift as tools age and wear. The result is quality components made more quickly and reproducibly.
This technology makes it possible to create an optical fabrication shop in which nearly all of the manufacturing processes are done on CNC equipment that utilizes feedback from metrological equipment via the shop’s network. The entire production, from optical blank to certified product, can be intelligently informed by PC-based metrology. Direct communication between machines allows for more accurate process corrections. The final result is a more predictable process for the manufacture of both spherical and aspheric optics. A state-of-the-art optics shop includes CNC platforms for grinding, polishing, and final polishing. Metrology equipment includes an interferometer and, in shops producing aspheres, a profilometer. All of these pieces of equipment can be networked so that they have access to a shared storage location or can communicate directly with one another. In this way, advanced CNC platforms can translate part errors detected by metrology into correction factors for producing subsequent parts. The process can be repeated as additional corrections may be required to keep pace with tool wear.
The technology starts with state-of-the-art optical fabrication equipment, such as Schneider Optics’ SCGA121 shown in Figure 1. The system is a seven-axis CNC platform for grinding optical spheres
and aspheres. It uses automatic part-loading and tool-changing to generate spherical surfaces in series. Standard quality tests for work piece parameters, such as radius of curvature and center thickness, are built into the machine. An operator-manipulated spherometer feeds directly into the machine’s control system, which adjusts the toolpath to achieve the desired radius of curvature. Internal process controls automatically test each part for center thickness by a mechanical probe. As the grinding tools wear and center thickness drifts, the system automatically adjusts itself to compensate.
Having a machine with internal process controls is helpful, but in the integrated- shop environment, how well that machine plays with others is more important. Specifically, devices made by different manufacturers must work together. This is especially true for the manufacture of aspheres.
While the Schneider system has a native probe to measure the generated asphere, more accurate measurements can be made using more sophisticated standalone profilometers, such as the PG I1240 from Taylor Hobson shown in Figure 2. This system uses a 2-μm diamond-tipped stylus to trace the profile of an asphere with a vertical displacement resolution of 0.8-nm. Once this tool has made a trace on an asphere that has been through the pre-grind and finegrind processes, it sends the figure error measurement over the shop network to the SCGA121, which then uses this data to compute a correction to the toolpath. The SCGA121 uses the same control process during the finish grinding step.
An asphere that has gone through the three-step grinding process on the SCGA121 will have a fine ground surface and a figure error in the 1-2 μm range. To produce a specular surface and improve this figure, the asphere goes through a polishing step on the SCGA121 using a subaperture urethane pad tool. The subaperture tool reduces the smoothing of the aspheric shape that occurs when full-aperture polishing tools are used. However, since the polishing pad cannot perfectly match the asphere’s ever-changing local radius of curvature, the polishing process tends to induce another figure error, which typically appears at the center or edges of the part.
Close coordination of the generating and metrology equipment makes it possible to avoid or reduce the need for additional grinding to correct this polishing error by feeding the error measured in an initial part to the generator, which then adjusts the grind on the subsequent parts. The new grinding toolpath creates an inverted figure error in the next part during the finish grinding step, compensating for the error that will be induced by polishing.
While process-control feedback can help stabilize the production process and reduce the amount of grinding and polishing needed to make corrections, tool wear and toolpath errors that cannot be predicted introduce a random component into the process. Correcting errors unique to a single part calls for a specialized process. A leading technology for making unique surface corrections is magnetorheological finishing (MRF). The QED Q22-Y MRF machine, shown in Figure 3, polishes the part with a subaperture tool composed of magnetically sensitive polishing compound flowing over a spinning wheel in the presence of a strong magnetic field. The resulting removal function does not change over time, resulting in predictable polishing.
In the case of spherical polishing, a laser-based interferometer, such as the Zygo VeriFire, can characterize the Q22-Y’s removal function and the surface figure error of an individual part. That surface topology data again goes from the metrology tool to the MRF machine via the shop network. The MRF machine’s control software includes algorithms to calculate the dwell time required to correct the figure error.
A similar process is used for the final finishing of aspheric optics, although the measurement process is complicated by the aspheric form. Since contact profilometers work equally well on ground and polished surfaces, the PG I1240 may be used to inform the MRF process. Since the PG I1240 only produces two-dimensional data, however, it cannot help correct asymmetrical errors. Nonetheless, the equipment described here may be used to produce aspheres with figure errors below 1⁄2-wave peak-to-valley.
When this level of precision is not enough, computer-generated holograms may be used in conjunction with the VeriFire to produce 3D measurements to feed to the MRF. The greater expense of this measurement process makes it suitable only when the less expensive profilometer technique cannot produce the optical quality needed.
This article was written by Matthew Tardiff, Optical Manufacturing Engineer at Edmund Optics. For more information, call 1-800-363- 1992, or visit http://info.ims.ca/5293-231.