Two-point diameter calibration of a sphere by a micro-coordinate measuring machine using a silicon gauge block as a reference standard

Abstract

The National Metrology Institute of Japan (NMIJ) developed an accurate two-point diameter measurement system for a sphere by using a micro-coordinate measuring machine (μ-CMM) with a gauge block for calibrating the radius of the μ-CMM probe. The surface roughness of end faces of the gauge block is a key uncertainty factor and, to reduce this in calibrating the μ-CMM probe, a newly fabricated silicon gauge block with a polished surface roughness of a few nanometers or less was used. To overcome poor repeatability caused by interaction forces acting on the probe, we developed a μ-CMM probe with T-shaped sharp styli and an 8-μm tip radius, which minimized the contact area between the stylus tip and gauge block. A calibrated μ-CMM probe (expanded uncertainty of 7.4 nm; k = 2) equipped with the newly developed T-shaped sharp styli was used to measure the mean two-point diameter of a sphere with an expanded uncertainty of 15 nm (k = 2).