The correct method for measuring coaxiality error by coordinate measuring machine



Coaxiality is a common shape and position tolerance item in mechanical product testing. It indicates that the relevant elements of the part (axis and shaft, hole and hole, and shaft and hole) require coaxiality, that is, control the actual axis and the reference axis. The degree of deviation. For the regular shaft parts, V-type brackets, steel ball plus lever dial gauges or deflection instruments and other special inspection tools and combination aids can be used to detect the coaxiality; for the box hole parts, the core can be generally used. The shaft is added to the lever dial gauge or the roundness meter is used to detect the coaxiality.
    However, for some large parts (such as machine tool spindles) or irregular shaft parts and irregular inner holes of the box parts, it is difficult or very troublesome to measure the coaxiality by the conventional method. Therefore, using a coordinate measuring machine to measure concentricity is a good choice. Compared with the special coaxial measuring instrument, the biggest feature of the CMM measuring coaxiality is that there is no need to rotate the workpiece, no special mandrel or special bracket is needed, no mechanical alignment is required, and only the probe probe is used to sample the workpiece, that is, The measurement results can be quickly output. However, when measuring coaxiality with CMM, measurement may occur due to differences in the understanding of the reference axis, or different methods of measuring the axis of the measured element, or different methods of coaxiality evaluation, and the influence of CMM mining point error. The result is a large error and a poor repeatability, that is, the measurement result cannot truly reflect the true coaxiality error of the part.
    According to the definition of coaxiality, when measuring the coaxiality with a coordinate measuring machine, the measurement error can be investigated from three aspects: 1. acquisition and establishment of the reference axis; 2. acquisition and establishment of the axis of the measured element; Evaluation of the positional relationship between the reference axis and the axis of the measured element. The methods for reducing the coaxiality error of the coordinate measuring machine mainly include the following methods:
    1. Increase the distance between the reference sections
    When measuring the reference element, if the distance between the first section and the second section is increased, the proportion of error interference will be reduced accordingly. Therefore, the distance between the two sections should be as large as possible when measuring. If the reference is long enough and the reference section is adjacent to the section being examined, the resulting error can be negligible.
    2. Establish a common axis as the reference axis
    When the reference cylinder and the measured cylinder are shorter and are farther apart, a method of establishing a common axis as a reference axis may be adopted, that is, a middle section is measured on the reference cylinder and the measured cylinder, and a joint of the middle section is taken as a common axis, and then Calculate the concentricity of the reference cylinder and the measured cylinder relative to the common axis (reference axis), and take the maximum value as the coaxiality error of the workpiece. The evaluation results of this method compare the actual use of crystals.
    3. Replace the coaxiality with a straightness approximation
    In the case where the cross section of the workpiece to be tested is short, the straightness method of changing the straightness may be used instead of the coaxiality. Because the inclination of the shaft affects the assembly of the workpiece in this case, and the axial offset has a great influence on the assembly of the workpiece, the measurement of the axial offset is actually the straightness of the measurement of the axial connection. The method is implemented as follows: measuring n cross-sectional circles on two small cylinders respectively, and then selecting a center of the n circles to fit a three-dimensional straight line, and the straightness of the straight line can approximately replace the coaxiality. The shorter the working section, the better the method is, and it is convenient to make a comprehensive gauge to verify the accuracy of the method.
    The above is the relevant knowledge about the correct method of measuring the coaxiality error of the coordinate measuring machine for SI Rui measurement, and hope to help everyone. As one of the subsidiaries of Hexagon, the world's largest measurement technology group, Sirui relies on the global resource advantages and top-notch technology advantages. Sirui products have the highest cost performance, allowing customers to experience the quality of imported products, domestic products. s price.