Evaluation of the Effective Parametric Errors in Coordinate Measuring Machines Using the Locus of Stylus on the Horizontal Plane

Abstract

A new method is proposed for the evaluation of effective parametric errors in three-axis coordinate measuring machines (CMMs), using the locus of stylus of the measuring probe on an ideal horizontal plane. The CMM base, which is mostly made of granite or steel, can be used as a good reference surface for obtaining the locus of stylus of the CMM probe, after the flatness calibration has been performed. Rectangular grid measurement with electronic level has been applied for the flatness measurement, then analysed either by the least squares technique or by the minimum separation technique. The CMM probe was moved and took the reading (vertical scale coordinates) on the corresponding grid points on the precalibrated base, giving the base probing data. The discrepancy data between the flatness and the base probing data become the locus of stylus, which is to be processed to give a mean plane plus the deviation data. The mean plane is called the locus plane which is the metrological reference plane for a specific machine, the deviation data (called the error of the locus of stylus) indicate a unique error feature of the specific machine in the vertical direction. This error of the locus of stylus can be decomposed into three effective parametric error components such as vertical straightness errors of the horizontal axes, and the roll error of a horizontal axis. As the parametric errors are evaluated in the effective sense, this technique can be applied to the modern CMMs which have a software compensation scheme, which is the advantage over the conventional parametric error method. Two cases of ‘on site’ calibrations have been shown in order to demonstate the efficiency and validity of the developed method. The computer aided feature is fully achieved, so that the data acquisition, analysis, and representation are performed on line with the aid of a microcomputer (IBM/PC/XT/AT compatible).