Proceedings of the
9th International Conference of Asian Society for Precision Engineering and Nanotechnology (ASPEN2022)
15 – 18 November 2022, Singapore

A Novel Calibration Method of Angular Misalignment in FTS-Based Diamond Turning

Shigeru Tanikawa1 and Jiwang Yan1,a

1Department of Mechanical Engineering, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, 223-8522, Japan


With technological progresses in optics fields, such as VR and 3D sensing technologies, the demand for optical lenses with complex shapes, like freeform surfaces, has significantly expanded in recent years. Ultra-precision diamond turning is one of the main manufacturing methods for freeform optics. There are two tool motion mechanisms in freeform diamond turning: slow tool servo (STS), which moves the entire tool rest, and fast tool servo (FTS), which only drives the diamond tool tip. Especially, in FTS-based diamond turning, high-frequency and long-stroke FTS units driven by a separate control system from the machine tool controller are recently developed as a new trend. However, this kind of independent FTS control system causes a time delay between the machine tool instruction and the FTS tool motion. Even a small time delay on the microsecond scale can cause a large angular misalignment due to the high spindle rotation speed during FTS machining. In this study, a novel method was proposed to measure the time delay from the machined surface with microsecond scale accuracy and to compensate for the misalignment based on the measured time delay. To measure the time delay, a spherical dimple in the workpiece center and eight spherical dimples at an angular space of 45° on the outer region of the workpiece were machined by using the STS, and another eight spherical dimples were machined by using the FTS. The angular misalignment and time delay values were successfully obtained by comparing the measured positions of the spheres machined by the FTS and the STS. Based on the measured time delay, detected C-axis tool positions were corrected to be at the position after the time delay by considering the cutting velocity during machining, and the angular misalignment was reduced from 1.149° to 3.47 × 10-4°. This proposed calibration method of angular misalignment for independent FTS unit is expected to significantly improve the accuracy of ultra-precision machining by FTS-based diamond turning.

Keywords: Diamond turning, Fast tool servo, Precise machining, Freeform surface, Ultra-precision cutting

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