This study proposes a non-contact measurement method for evaluating a micro cutting tool diameter at a sub-micrometer precision in atmospheric environment. Practically, non-contact tool measurements are widely used in optical measurement methods. However, the generally used geometrical optical measurement methods do fundamentally not have sub-micrometer precision due to well-known light diffraction phenomenon, notably when measuring a micro tool. We have been proposing and developing a measurement method especially for the micro tool utilizing laser diffraction. In this study, we subtracted a transparent light component from a laser light distribution diffracted by the micro tool to be measured to enhance diffraction pattern characteristics. Consequently, micro rods having diameters of 15, 20, and 30 μm could be precisely measured. Furthermore, in order to verify the method validity, a two-helical-fluted micro tool (20 micrometer in diameter) was also measured while rotating (4 min^-1) with less than 400 nm difference compared to images from a scanning electron microscopy (SEM) image. Finally, a trial to measure a micro drill diameter was also carried out during high-speed tool rotation (136,800 min^-1) with our developed apparatus that is enough compact to be installed to machine tools, in order to perform the measurement in real rotation without stopping the tool rotation.

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