Abstract: With stylus type measuring instruments for the measurement of surface roughness or form an improved method of setting up employs a reference body having a cylindrical or part-cylindrical surface. By probing the curved reference surface a datum point is established. The reference body is then moved by a predetermined amount with respect to this datum and acts as a secondary reference standard.

Abstract: A roundness measuring instrument of the type having a workpiece table and a pick-up carried on a pick-up mount which is radially adjustable to move the pick-up radially with respect to a fixed axis about which the table is rotatable, in which there is provided a displacement transducer sensitive to the movement of the pick-up mount in the radial direction. A device for summing the signals from the displacement transducer and from the pick-up is used to provide a wide range signal usable in metrological determinations on workpieces, such as determinations of radius and separation of two plane faces. A computer for calculating the value of a predetermined algorithm may also be provided to obviate the necessity for centering either a reference body or the workpiece with respect to the axis of rotation of the table.

Abstract: A method of determining the coordinates of the center of curvature of the curved surface of a workpiece on a surface measurement instrument of the type having a workpiece table rotatably mounted on a main spindle, and a sensor for contacting the surface of a workpiece mounted on the table. The method comprises the steps of traversing the sensor over the workpiece surface and deriving three signals representing the value of the radial distance from the spindle to the workpiece surface at three points spaced along the traverse. From these values, and a knowledge of the angular separation of the three points an approximation for the coordinates of the center of the curved surface of the workpiece with respect to the spindle can be derived using the algorithm ##EQU1## WHERE X AND Y RESPECTIVELY REPRESENT THE COORDINATES OF THE CENTER OF THE WORKPIECE, R.sub.1, R.sub.2, R.sub.3 REPRESENT THE RADIAL DISTANCE FROM THE SPINDLE AXIS TO THE WORKPIECE SURFACE AT THE THREE POINTS ALONG THE TRAVERSE, AND .phi.