A SPINDLELESS INSTRUMENT FOR THE ROUNDNESS MEASUREMENT OF PRECISION SPHERES

This paper presents an automated instrument for high-precision roundne ss measurement of spheres. Instead of a rotating spindle, the surface of the sphere itself is used as a geometrical reference. The measureme nt method is based on the well-established three-point method. In our design, two points are realized as mechanical contacting points and on e as a capacitive gauge that directly faces the surface of the sphere. By this simple design-without any moving parts involved in the probin g process-we measured the roundness of 1-inch steel spheres with nanom eter resolution. After a short introduction into the basic mathematics of the three-point method, the design of the instrument is presented, including the automation of the measuring process and the means used to minimize environmental effects in an industrial environment. Result s are presented that prove the outstanding repeatability and accuracy of the instrument. In a comparison with conventional methods, the adva ntages and limitation of the method are discussed. (C) 1998 Elsevier S cience Inc.