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Online Interferometer Keeps Machining 'Stress-Free'

Photonics Spectra
May 1999
Daniel C. McCarthy, News Editor

Precision is essential to the diamond-turned infrared optics and aspheres machined by Pilkington Optronics. However, the company occasionally had found that imprecise mounting of optics on the diamond-turning machine resulted in inaccuracies in finished parts, a fact revealed only when the part was measured off-line. To avert costly remachining operations, the company mounted a Fisba µPhase microinterferometer directly onto its diamond-turning machines to perform online metrology of optics during manufacture.
The company also wanted to ensure that the fixture would hold the part free from distortion before it came off the machine. "It's not a question of whether the machine is good enough, it's a question of whether the [machined] part is good enough," said Gary Owen, manufacturing manager at Pilkington.
Pilkington's three-axis machines mount the spherical face of an optical element on a chuck and hold it in place by applying a vacuum. If the vacuum applies too much stress before the lens is machined, a well-turned surface will become a badly turned surface once the vacuum is released. Because Fisba's micro- interferometer is mounted directly on the toolpost of the machine, it monitors stress to the chuck, the machined substrate and the performance of the machine itself, including the condition of the diamond tool. This enables operators to detect and correct spindle, mounting or tool errors online.
"Previously, we relied on good practices to get a reasonably good mounting surface," Owen said. He added that standard interferometers can measure the spherical surface of a lens before it is mounted, but this measurement cannot account for the lens' reaction once it is mounted on the machine. Fisba's interferometer performs online to minimize the effects of unwanted stress before the surface is diamond-turned.
Owens said the interferometer has proved particularly useful for machining very large optics, where size renders more flexibility to the lens. "We're now able to make lenses up to 250 mm in diameter to an accuracy of 0.3 µm peak to valley, which is large for infrared lenses," he said.
Fisba Optik's Twyman-Green type of two-beam interferometer is slightly larger than a matchbox. Its integrated charge-coupled device camera digitizes interference images that can be computer-analyzed by the phase-shift method. Fisba also offers a number of interchangeable modular objectives that can measure planar as well as spherical surfaces.

The science of measurement, particularly of lengths and angles.
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