Reliable measurements of the diametrical dimension over balls

摘要

Due to very high accuracy requirements placed on gears, traceable measurements of the manufactured parts are indispensable during production processes. Often tactile measuring instruments are used, since they provide highest accuracy and universal properties. For the diametrical dimension over balls, the measurement points are determined in two opposite tooth spaces each in double-flank contact. In the following this process is referred to as gap measurement. In order to compensate systematic errors from the measuring process a new substitution method is presented. The method offers a practical application for reducing the measurement uncertainty by comparison measurements based on a calibrated measurement standard which is similar to the workpiece in form and dimension. In general, gap measurements are performed in the self-centering probing mode. In order to keep the measurement uncertainty as small as possible, the exact knowledge of the probe condition and the surveillance of the sensitive probing process in the gap are necessary. In addition to the quality of the gear, also stability and characterization of the probing system are crucial points. In coordinate metrology common characterization procedures of the probing systems use single-point contacts to describe the probe behavior. In opposite, the presented substitution method characterizes the probing system considering the realistic double-flank contact. The occurring forces result in elastic deformations and significant length measuring deviations. Wear, cracks or adhesions of dirt are often detected only insufficiently and thus not taken into account in the measuring process. In order to compensate any length measuring deviations resulting from effective probing behavior considering the actual probe configuration, a novel measurement standard, the so-called gap measurement standard, has been developed at PTB. The design of the gap measurement standard comprises at least two (or four) calibrated precision balls which are mounted on a thermally invariant base plate. Each pair of balls forms a gap similar to the gear geometry expected at the contact points. The diameter and the distances of the balls can be calibrated with small measurement uncertainties by means of a laser-interferometric measuring system. Using this measurement standard, the important parameters of the probe characteristic for gap measurements are detected and the influences from the self-centering probing process are compensated. In the following, the most important probe influences by measuring the diametrical dimension over balls are discussed, and solutions for a reduction of the uncertainty components are presented.