Tech Briefs

Optical component production becomes more stable and predictable when metrology results inform the manufacturing process.

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

Figure 1. The SCGA121 is a Seven-Axis CNC Platform for grinding and polishing optical surfaces.

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.

Figure 2. The PG I1240 Profilometer uses a diamond-tipped stylusto trace the profile of an asphere.

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.

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