Time delay compensation in high-speed diamond turning of freeform surface using independent fast tool servo with a long stroke

The demand for wearable device applications has continuously grown in recent years, especially with the significant rise of augmented and virtual reality technologies. Freeform optics plays a crucial role in these devices by enhancing optical performance, shortening the light path, and reducing the weight, all while allowing for smaller, lighter systems with higher efficiency. The independent fast tool servo (FTS)-based diamond-turning method stands out as a highly effective technique for fabricating freeform shapes with high accuracy and productivity. However, microsecond-order time delays occur within the system, significantly impacting form accuracy as machining speeds increase. This study explores the sources of form errors in freeform surface fabrication associated with the FTS diamond-turning process, with particular attention to the effects of clocking angle errors caused by the time delay. These errors were found to greatly affect form accuracy, especially at higher machining speeds. The FTS position data were analyzed, and time delays under various operational conditions due to servo control were confirmed. To precisely identify the extent of the time delay, a cylindrical surface was machined under high-speed conditions, and the clocking angle error was measured using a non-contact chromatic probe. Results showed that time delays originating from the machine platform had a significant effect on form accuracy. By accurately identifying and compensating for these time delays, the clocking angle error was eliminated. To validate the effectiveness of the time-delay compensation strategy, a cylindrical freeform surface was machined after the compensation, and the clocking angle error was minimized down to 0.00014° evaluated by on-machine measurement. The form accuracy of the freeform machining result after compensation was achieved at 0.85 μm PV. This study establishes a methodology for identifying and compensating for time delays in an independent FTS system, contributing to improved form accuracy in freeform optics fabrication. • In FTS diamond turning of freeform surfaces, clocking angle error is a crucial error factor. • Time delay of FTS is composed of three components where servo control delay is dominant. • The C-axis encoder signal processing time delay cannot be recognized with the FTS system and its calibration is essential. • After compensation, clocking angle error was reduced to 0.00014° and form error to 0.85 μm. • The proposed time delay identification methodology is applicable to various freeform shapes.