Abstract
Tool edge wear has a significant effect on machining accuracy. Therefore, to obtain high machining accuracy, the tool edge geometry must be measured with high accuracy. On-machine measurement of tool edge geometry is desirable to reduce re-installation errors and improve throughput. Although conventional optical measurement methods can generally achieve high measurement accuracy in a clean measurement environment, on-machine measurement is difficult because the cutting fluid adhering to the tool interferes with the measurement. We have proposed a tool edge profile measurement method based on confocal detection of fluorescence generated by laser excitation of cutting fluid, and have demonstrated its validity. In this study, we focused on the phenomenon of saturated fluorescence to further improve the measurement accuracy. The intensity of fluorescence from the cutting fluid depends on the intensity of the excitation laser. When the intensity of the excitation laser is relatively low, the fluorescence intensity is proportional to the intensity of the excitation laser. At higher excitation laser intensities, however, the fluorescence intensity saturates due to the limited number of fluorescent molecules that can be excited. In other words, when a focused laser beam is used to excite the cutting fluid, the fluorescence intensity saturates only in a localized region in the vicinity of the focal spot. Therefore, detecting only the saturated fluorescence improves the spatial resolution of tool edge geometry measurement. In this study, the excitation laser was time-modulated to detect the saturated fluorescence. When the excitation laser is modulated at frequency f, fluorescence is also emitted at frequency f when the fluorescence is not saturated. On the other hand, when the fluorescence saturates, it is known that a frequency component twice the modulation frequency of the excitation laser appears in the fluorescence intensity. In other words, by modulating the excitation laser at frequency f and extracting only the 2f frequency component of the emitted fluorescence intensity using a lock-in amplifier, only the saturated fluorescence can be detected. In this study, the geometry of a worn throw-away tip was measured to validate the proposed method. Compared to the conventional method that detects non-saturated fluorescence, the proposed method has a steeper rise in fluorescence intensity when the focal spot is scanned from the air to the cutting fluid and a steeper fall in fluorescence intensity when the focal spot is scanned from the cutting fluid to the tool. This experimentally confirmed that the measurement resolution of the tool surface was improved. The tool edge geometry was measured by scanning the focal spot at many points on the tool. As a result, wear of a few micrometers to several tens of micrometers could be measured with high accuracy. The proposed method is expected to be implemented on machine tools.
