Autonomous calibration method of the zero-difference without using a standard gauge for a straightness-measuring machine

An autonomous method to calibrate the zero-difference for the three-point method of measuring surface straightness is proposed. A simple cone-disc, which can be made by a CNC turning machine, is used for the calibration. The calibration method is comprised of two steps. In the first step, the cone-disc rotates around and parallel to three displacement sensors built-in to a holder. In the second step, the geometrical parameters between the sensors and the cone-disc are simultaneously acquired during each predetermined movement of the sensors, and the zero-differences are computed by the proposed algorithm within a few revolutions.The conceptual design and the algorithm to realize the proposed method are explained with theoretical analyses and numerical simulations. The results of this study are as follows:(1)The repeatable accuracy of the zero-difference determination using the proposed method is at the same level of quality as that of the reverse method using the three-point method.(2)When the zero-difference is calibrated using a cone-disc gauge of 200 mm radius and 10 μm peak height, which can be manufactured by a CNC turning machine, the shape error of the object's surface (which is analyzed for its systematic error component and its repeatability error component) becomes less than 0.5 and 2 μm at measured lengths of 100 and 200 mm, respectively.