Apparatus for measuring the roundness of a surface of an object

Abstract

The apparatus comprises a bifurcated head rotatably mounted upon a spring loaded arm. The head has two edge plates which can be brought into tangential contact with the periphery of a rotating object. When the head is brought into contact with objects of different diameters, the points of contact with the edge plates progress along location locus lines which, when produced, intersect an an apex. A displacement transducer mounted upon the head has a spring loaded plunger with a probe which is contacted to and detects inaccuracies upon the periphery of the object. In order to accommodate test objects of different diameters, the displacement transducer is bodily movable along an axis of adjustment relative to the head. The point of contact between the probe and test object will, between objects of different diameter, progress along a straight measurement locus line parallel to or coincident with the axis of adjustment. This measurement locus line passes through the apex but does not bisect the included angle between the location locus lines. This arrangement is mechanically simple yet gives unequal angular spacings between the points of contact which do not depend on the diameter of the test object. This facilitates the application of accurate harmonic (Fourier) reconstruction of the roundness profile of the test object. No precision turntable or spindle is required and the method is therefore suitable for in situ measurement. The diameter of the test object need not be known.