Abstract
This paper describes a high-precision coordinate measurement machine, which we call "A-Ruler." It will be used to measure the shape of freely curved surfaces with an accuracy of 5nmRMS at a measurement volume of 350×80×50mm^3. Studies on margin of error indicate that the measurement of a spherical surface is influenced by almost all types of errors. This indicates taht the performance of A-Ruler can be assessed by measuring a spherical surface. The influences of scanning velocity and the sphericity of the tip-sphere are also investigated using a concave sphere workpiece made of Zerodur^<TM>. Its curvature radius is 104.508mm. The repeatability is 3.8nmRMS perφ120mm area. The accuracy is assessed by comparison with an interferometer. The difference between A-Ruler and the interferometer is 4.5nmRMS perφ120mm area.
