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Tool Path Optimization Bolsters Precision Machining of Freeforms

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Researchers from Keio University developed a tool path generation method for driving an independently controlled fast tool servo (FTS) for freeform surface machining. Without necessity of trial and error, the method enabled rapid manufacturing of high-precision freeform optics for various products, ranging from AR/VR systems to components used in aerospace and biomedical engineering.

The method could help eliminate barriers to the use of independently controlled FTS for diamond turning, and lead to more ultraprecision machining technologies for freeform optics.

Due to their flexibility and compatibility with most commercial machine tools, independently controlled FTS units have the potential to contribute significantly to advanced freeform optics fabrication — though historically there has been no established tool path generation method for these FTS units.

The researchers generated the tool path program using the ring method and mesh method instead of the conventional spiral tool path. To achieve nanometer-level accuracy, they optimized the tool paths generated by the ring and the mesh methods by predicting the machined surface profile through simulation and minimizing the deviation between the predicted and the designed surfaces.

Researcher Yusuke Sato said, “This study aims at proposing novel methods to generating and optimizing the tool path for the independent FTS control system to reduce form error of a machined surface caused by two-dimensional interpolation.”

First, according to Sato, the researchers pre-generated control point clouds in two different methods — the ring method and mesh method. To optimize the layout of the control points, the researchers determined the number of control points and the optimal aspect ratio of the layout parameters.


Layouts of control point clouds generated by (a) the ring method and (b) the mesh method. Courtesy of the International Journal of Extreme Manufacturing (2022). DOI: 10.1088/2631-7990/ac5f12.
Based on the distribution of the control points, the final machined surface profile is predicted and interpolated by simulation, Sato said. Once this step is complete, according to Sato, comparing the simulated surface with the designed surface, the form error is obtained.


“By repetitively adjusting the parameters of the control points, the form error was minimized to the desired tolerance,” Sato said.

To demonstrate the tool path generation method, the researchers conducted cutting tests of a 2D sine wave and a microlens array and compared the results. After tool path optimization, the peak-to-valley form error of the machined surface was reduced from 429 to 56 nm for the 2D sine wave by using the ring method, and from 191 to 103 nm for the microlens array by using the mesh method. The tool path generation method reduced the shape error from submicron to 10-nm level for the 2D sine wave.

According to professor Jiwang Yan, FTS-based diamond turning is an excellent candidate method to fabricate freeform surfaces with high efficiency. However, conventional FTS units driven by piezoelectric actuators have very small strokes in micrometer scale. This limits their applications, Yan said.

“In recent years, long-stroke FTS units, equipped with voice coil-driven air bearings, have been developed, which enable millimeter-level working strokes, and in turn, greatly expand the applications of FTS diamond turning,” Yan said. These voice coil-based FTS units are independently driven by separate control systems. This improves the system compatibility and stability, according to Yan.

The tool path generation method introduced by the Keio team could provide a foundation for advancing ultraprecision machining technologies for freeform optics through diamond turning, by using an FTS unit with a separate controller to achieve high accuracy without the need for trial and error.

Beyond AR/VR systems and certain components, the method supports the machining of freeform optics for use in cameras, scanners, and head-mounted displays.

The research was published in the International Journal of Extreme Manufacturing (www.doi.org/10.1088/2631-7990/ac5f12).

Photonics Spectra
Dec 2022
GLOSSARY
optical
Pertaining to optics and the phenomena of light.
opticsopticaloptical machiningmachiningoptics fabricationfreeform opticsservofast toolfast tool servoDisplaysAR/VRResearch & TechnologyeducationAsia PacificKeio Universitysurface inspectionmanufacturingpoint cloudsdiamond turningnanoTechnology News

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