Into the Depths of Space
On an ULTRASONIC 200 from DMG MORI, Coherent produces one and a half meter hexagonal mirror segments for telescopes in terrestrial observatories.
Founded in 1971 and with a key location in Richmond, California, Coherent Aerospace & Defense specializes in the precision machining of optical elements and assemblies for aerospace and astronomical research. The company has 28,000 employees worldwide. Coherent has been relying on the fast high-precision ULTRASONIC technology from DMG MORI for glass machining, and with the acquisition of an ULTRASONIC 200, the economical production of mirror segments for giant telescopes is now also possible.
Precision machining for giant telescopes
New, larger optical telescopes with diameters of up to 30 meters are helping researchers look even deeper into space in order to answer fundamental questions in astronomy, astrophysics and cosmology. The challenge in building these giant telescopes is the machining of the hexagonal mirror segments from which the entire mirror is assembled. These massive light gathering mirrors are comprised of hundreds of segmented mirrors. “The initial shape of these segments is a round mirror with a diameter of one and a half meters and a thickness of just 45 mm,” explains Matthew White, Senior Manager of Manufacturing Engineering. “We shape this into a hexagonal form, known as hexing. Assembly functions, such as sensor pockets, are also incorporated.”
Up to ten times faster production thanks to ULTRASONIC technology
Conventional machining of such glass materials has been carried out for many years on machining centers with diamond tools, albeit at a relatively slow speed. “With the large mirror segments and the enormous quantities for us, it would take many years to equip an observatory this way,” calculates Mr. White. The ULTRASONIC technology from DMG MORI was seen as having the potential to drastically reduce the machining time. In Ultrasonic machining, the diamond tool oscillates along its axis at a high frequency and low amplitude – in addition to the tool's own high-speed rotation. “This enables us to remove material six to ten times faster. At the same time, the tool life increases significantly,” says White. The process has long proven its worth in the semiconductor industry, for example. Coherent has now transferred it to the machining of even larger glass components.
XXL machining on the ULTRASONIC 200
DMG MORI has the ULTRASONIC 200 in its range for the ultrasonic machining of large components. The machine is based on the DMU 200 gantry and offers travels of 4,000 x 2,000 x 1,200 mm. The stable foundation of the gantry construction is an optimal basis for the high-precision processing of the mirror segments. Coherent poured a new concrete slab for this purpose, which was additionally reinforced. “This ensures mechanical stability,” explains White. “We have also improved the temperature control in the hall in order to achieve constant ambient conditions.”
Reliable clamping and machining
The biggest challenge, however, lies in the enormous diameter and the low thickness. “In a normal clamping process, the component could warp,” says Mr. White. The component could jump out of the fixture after being released, which would jeopardize the dimensional accuracy. Due to the asymmetrical and aspherical shape, a simple vacuum chuck is also not a suitable solution, so the engineers at Coherent developed a proprietary tooling approach that uses a combination of support and clamping systems. White is very satisfied with the result: “This fixes the mirror segment in all degrees of freedom without causing unwanted bending.” He added that he is impressed by the machining results of the reliable process: “The ULTRASONIC technology significantly reduces the prevailing process forces thanks to the constant interruption of contact between the tool and workpiece. On the one hand, this ensures perfect surfaces and cutting edges because there is virtually no material chipping. On the other hand, it also maximally accommodates the complex clamping concept.”
Partnership-based cooperation with DMG MORI at all levels
The reasons for investing in the large ULTRASONIC model from DMG MORI were, on the one hand, the good experience with the innovative machining technology. On the other hand, there was already a good partnership with the machine tool manufacturer: “The response times in service are always good and the individual configuration options of the machines make selection easier,” says Matthew White, assessing the cooperation. He would like to continue this for future investments. “The next step will be to expand our ULTRASONIC capacities with an ULTRASONIC 80 eVo linear.”
Coherent Corp. - Global Headquarters
375 Saxonburg Boulevard, Saxonburg, PA, 16056
United States
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