Concentricity is a kind of geometric position tolerance item that is commonly seen in the detection of mechanical products. It means that the relevant elements of the part (shaft and shaft, hole and hole, shaft and hole) are required to be coaxial, that is, to control the actual axis and the reference axis. The degree of deviation. For that regular shaft type parts, V-type brackets, steel balls and lever gauges, or special deflection gauges, etc., can be used to detect concentricity; for borehole parts, cores are generally used. Axis plus lever dial indicator or use a roundness meter to detect concentricity.
However, for some large parts (such as machine tool spindle, etc.) or irregular shaft parts and irregular parts of the box bore, using conventional methods to measure the coaxiality is very difficult to achieve or very troublesome. Therefore, using a coordinate measuring machine to measure concentricity is a good choice. Compared with the special coaxiality measuring instrument, the most important feature of the CMM measurement coaxiality is that there is no need to rotate the workpiece, no special mandrel or special bracket, no mechanical alignment, just use the probe probe to sample the workpiece, ie Can quickly output measurement results. However, when the coaxiality is measured by the CMM, measurements may sometimes occur due to differences in the understanding of the reference axis, different methods of measuring the axis of the measured element, or different methods of evaluating the coaxiality, or the influence of the CMM sampling point error. The result is a phenomenon with large errors and poor repeatability, that is, the measurement results cannot truly reflect the true coaxiality errors of the parts.
According to the definition of concentricity, when measuring the concentricity with a coordinate measuring machine, its measurement error can be examined from three aspects: 1. Acquisition and establishment of the reference axis; 2. Acquisition and establishment of the axis of the measured element; 3. Evaluation of the positional relationship between the reference axis and the axis of the measured element. There are mainly the following methods to reduce the error of measuring the coaxiality of a coordinate measuring machine:
1. Increase the distance between 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 inspected, the resulting error may be negligible.
2. Establish a common axis as a reference axis
When the reference cylinder and the measured cylinder are short and relatively far apart, a method of establishing a common axis as a reference axis may be adopted. That is, the middle section is measured on the reference cylinder and the measured cylinder, and the middle section is used as the 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. Approximate degree of straightness to replace concentricity
In the case of a short section of the workpiece under test, the linearity can be changed to replace the contingency alternative. Because the inclination of the shaft has little influence on the assembly of the workpiece in this case, and the shaft offset has a great influence on the assembly of the workpiece, the measurement axis offset actually measures the straightness of the connection of the shaft. The realization method is: measuring n cross-section circles on two small cylinders respectively, and then selecting the centers of these n circles to fit a three-dimensional straight line, and the straightness of the straight line can be used as a substitute for the concentricity. The shorter the working cross-section, the better the method is, and it is very convenient to make a comprehensive gauge to verify the accuracy of the method.
However, for some large parts (such as machine tool spindle, etc.) or irregular shaft parts and irregular parts of the box bore, using conventional methods to measure the coaxiality is very difficult to achieve or very troublesome. Therefore, using a coordinate measuring machine to measure concentricity is a good choice. Compared with the special coaxiality measuring instrument, the most important feature of the CMM measurement coaxiality is that there is no need to rotate the workpiece, no special mandrel or special bracket, no mechanical alignment, just use the probe probe to sample the workpiece, ie Can quickly output measurement results. However, when the coaxiality is measured by the CMM, measurements may sometimes occur due to differences in the understanding of the reference axis, different methods of measuring the axis of the measured element, or different methods of evaluating the coaxiality, or the influence of the CMM sampling point error. The result is a phenomenon with large errors and poor repeatability, that is, the measurement results cannot truly reflect the true coaxiality errors of the parts.
According to the definition of concentricity, when measuring the concentricity with a coordinate measuring machine, its measurement error can be examined from three aspects: 1. Acquisition and establishment of the reference axis; 2. Acquisition and establishment of the axis of the measured element; 3. Evaluation of the positional relationship between the reference axis and the axis of the measured element. There are mainly the following methods to reduce the error of measuring the coaxiality of a coordinate measuring machine:
1. Increase the distance between 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 inspected, the resulting error may be negligible.
2. Establish a common axis as a reference axis
When the reference cylinder and the measured cylinder are short and relatively far apart, a method of establishing a common axis as a reference axis may be adopted. That is, the middle section is measured on the reference cylinder and the measured cylinder, and the middle section is used as the 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. Approximate degree of straightness to replace concentricity
In the case of a short section of the workpiece under test, the linearity can be changed to replace the contingency alternative. Because the inclination of the shaft has little influence on the assembly of the workpiece in this case, and the shaft offset has a great influence on the assembly of the workpiece, the measurement axis offset actually measures the straightness of the connection of the shaft. The realization method is: measuring n cross-section circles on two small cylinders respectively, and then selecting the centers of these n circles to fit a three-dimensional straight line, and the straightness of the straight line can be used as a substitute for the concentricity. The shorter the working cross-section, the better the method is, and it is very convenient to make a comprehensive gauge to verify the accuracy of the method.
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