Analysis and performance of a parallel axis flatness measuring instrument

This article describes the design, analysis, and performance of a flatness inspection instrument to measure workpieces with up to 1mm departure from flatness. The instrument uses two air bearing spindles arranged with parallel axes to simultaneously rotate a workpiece and slowly pass a capacitance probe over the spinning surface. Capacitance probes offer user-selectable sensitivity to provide multiple combinations of measurement range and resolution. In tests with a high sensitivity probe, the instrument demonstrated measurement repeatability of 25nm on a ∅75mm workpiece. This article presents a complete homogeneous transformation matrix analysis of the propagation of errors into the measurement as well as sample measurements on diamond turned workpieces.