Two-point diameter calibration of a sphere using a micro-coordinate measuring machine at NMIJ

Abstract The National Metrology Institute of Japan (NMIJ) has developed a highly accurate two-point diameter measurement method of a sphere using a micro-coordinate measuring machine (µ-CMM) with a low-force two-dimensional touch probe. The measured sphere is used as a reference to calibrate the probe of other µ-CMM in industry. Our strategy for measuring sphere’s diameter is that the probe radius of NMIJ’s µ-CMM is calibrated using a reference gauge block, and the NMIJ’s µ-CMM using the calibrated probe measures the mean two-point diameter of a sphere. To achieve the highly accurate calibration of the reference gauge block, we have adopted a double-sided interferometer (DSI) and an atomic force microscopy (AFM). The DSI measures the length of gauge block without wringing. The phase correction value of the gauge block is evaluated by measuring the surface roughness using the AFM. Using the DSI and AFM, the mechanical length of reference gauge block was obtained with the expanded uncertainty of 13 nm (k = 2). Then, the probe radius of NMIJ’s µ-CMM was also calibrated using the reference gauge block calibrated by the DSI and the AFM. The µ-CMM’s probe radius was obtained with the expanded uncertainty of 10 nm (k = 2). Finally, the µ-CMM measured the mean two-point diameter of a sphere with the expanded uncertainty 21 nm (k = 2) was demonstrated.