Basics of Highest Accuracy Roundness Measurement – R. Thalman

Abstract:

A primary roundness measurement is independent of any standard due to the application of error
separation techniques, and it is based on a complete characterization of all relevant influence quantities. The
described roundness tester at METAS is based on a commercial instrument, but has undergone several modifications
in order to achieve highest roundness measurement accuracy. It is equipped with a rotating spindle
with an incremental encoder and an oil-hydrostatic bearing, driven by a vibration free DC motor with variable
rotation speed. It further exhibits low noise amplifier electronics with selectable gain for the LVDT probe, new
software for data acquisition and evaluation, and a heat protection shield. The software offers access to all
relevant signals and allows for data manipulation such as averaging, noise analysis, drift monitoring, error
separation with multi-step procedure, Gaussian filtering in the Fourier space, spectral analysis or mapping of
the pre-determined spindle error. Experimental results on a standard sphere and on a multi-wavelength standard
are presented and the uncertainty budget is discussed.