Development of a Novel Artifact as a Reference for Gear Pitch Measuring Instruments

Abstract

The pitch accuracy of a gear is graded on the order of 0.1 μm in ISO 1328-1; therefore, it is necessary for gear measuring instruments (GMIs) to be able to measure gears with the required high accuracy. GMIs are evaluated by measuring a calibrated gear or a gearlike artifact. It is, however, difficult to obtain a measurement uncertainty of less than 0.1 μm. The reason for this difficulty is that a gear artifact has a form error and surface roughness, and that the measurement position on the gear face differs slightly from the calibrated position. In view of this situation, we propose a novel multiball artifact (MBA), which is composed of equally spaced pitch balls, a centering ball, and a datum plane. The pitch balls are assumed to act as gear teeth by calibrating the angular pitch between the centers of each pitch ball. The centering ball and the datum plane are used to set a reference axis of the virtual gear. We manufactured an MBA with the pitch balls arranged on a curvic coupling. The angular pitch deviation between the centers of each pitch ball was calibrated using a coordinate measuring machine (CMM) and adopting the multiple-orientation technique. A master gear was also calibrated for comparison. The measurement uncertainty for the cumulative angular pitch deviation was 0.45 arc sec for the MBA and 1.58 arc sec for the master gear. The MBA could be calibrated with small uncertainty compared with the master gear. After the calibration, a virtual gear of the MBA was built using the calibration value. The virtual gear was measured using the gear-measuring software on the CMM. The measurement value was equal within the range of uncertainty of calibration value. It is verified that the superiority of the MBA to the gear artifact is due to the following reasons: (1) The balls can be manufactured with an accuracy of several tens of nanometers. (2) The calibrated result for the MBA is almost independent of a probe-positioning error because the centers of each pitch ball can be measured at multiple points. (3) In setting the reference axis, the gear artifact generally uses a datum cylinder, in contrast, the MBA uses more accurate ball.