Correct method of measuring coaxiality error with CMM

Coaxiality is a common form and position tolerance item in mechanical product testing. It indicates that the relevant elements of parts (shaft to shaft, hole to hole, shaft to hole) require coaxiality, that is, to control the deviation degree between the actual axis and the reference axis. For regular shaft parts, special inspection tools and combined auxiliary tools such as V-shaped support, steel ball and lever dial indicator or deflection instrument can be used to detect coaxiality; For box hole parts, mandrel plus lever dial indicator or roundness instrument can be used to detect coaxiality.

Correct method of measuring coaxiality error with CMM

However, for some large parts (such as machine tool spindle) or irregular shaft parts and irregular inner holes of box parts, it is difficult or troublesome to measure coaxiality by conventional methods. Therefore, using CMM to measure coaxiality is a good choice. Compared with the special coaxiality measuring instrument, CMM is characterized by no need to rotate the workpiece, no need for special mandrel or special support, and no need for mechanical alignment. It can quickly output the measurement results only by sampling the workpiece with a probe. However, when using CMM to measure coaxiality, due to the differences in the understanding of the reference axis, or the different measurement methods for the axis of the measured elements, or the different evaluation methods for coaxiality, as well as the influence of CMM sampling point error, sometimes there are large errors and poor repeatability of the measurement results, that is, the measurement results can not truly reflect the real coaxiality error of the parts.

According to the definition of coaxiality, when measuring coaxiality with CMM, the measurement error can be investigated from three aspects: 1. Acquisition and establishment of reference axis; 2. Acquisition and establishment of measured element axis; 3. Evaluation of the positional relationship between the datum axis and the axis of the measured element. The main methods to reduce the coaxiality error measured by CMM are as follows:

1. Increase the distance between datum sections
When measuring the reference element, if the distance between the * section and the second section is increased, the proportion of error interference will be reduced accordingly. Therefore, the distance between two sections shall be increased as much as possible during measurement. If the datum is long enough and the datum section is adjacent to the tested section, the error can be ignored.

2. Establish a common axis as the datum axis
When the reference cylinder and the measured cylinder are short and far from each other, the method of establishing a common axis as the reference axis can be adopted, that is, measure the middle section on the reference cylinder and the measured cylinder, take the connection of the middle section as the common axis, and then calculate the coaxiality of the reference cylinder and the measured cylinder relative to the common axis (reference axis) respectively, and take its * value as the coaxiality error of the workpiece. The evaluation results of this method are compared with the actual application of crystallization.

3. Replace coaxiality with straightness
When the section of the measured workpiece is short, the alternative method of measuring straightness instead of coaxiality can be used. In this case, the inclination of the shaft has little impact on the workpiece assembly, while the axis offset has a great impact on the workpiece assembly. Measuring the axis offset is actually measuring the straightness of the axis line. The realization method is: measure n cross-section circles on two small cylinders respectively, and then select the 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 is, the better the effect of this method is, and a comprehensive gauge can be easily made to verify the accuracy of this method.