In addition to the four geometric errors of positioning error, straightness error, angular motion error, and perpendicularity error, the CMM also has four geometric errors due to force deformation, thermal deformation, measurement system, probe system, control system, data recording and processing. Errors generated by the system:
1. Force deformation error
As a precision measuring instrument, the CMM should have high rigidity, so that the force deformation does not become an important factor affecting the measurement accuracy of the CMM. However, with the acceleration of the production rhythm in various industries, the requirements for the efficiency of three-coordinate measurement have also increased, including higher movement speed and detection speed, which requires its moving parts to be lighter in mass, but this is related to improving the stiffness of the components. contradiction.
2. Thermal deformation error
To understand the thermal deformation of the CMM, it is first necessary to measure the temperature of the instrument, the object to be measured and the environment. In order to realize thermal deformation error compensation, it is necessary to stick temperature measuring elements on the three rulers and the measured part, and also need to place temperature measuring elements in the air to check whether the ambient temperature and temperature gradient meet the requirements. Only when the ambient temperature meets the requirements, it is meaningful to test the performance of the measuring instrument, and the measurement accuracy error of the three-coordinate measuring instrument will be greatly reduced. If conditions permit, Sirui Measurement suggests that you can place the instrument in a constant temperature room and control the indoor temperature and humidity.
In order to achieve error compensation, in addition to measuring temperature, the equivalent linear expansion coefficient of the scale and the DUT must also be known. The so-called equivalent linear expansion coefficient refers to the fact that the actual machine is not only related to the material, but also related to the fixing method, so the equivalent linear expansion coefficient obtained through the actual measurement can not only better reflect the use state of the scale and the current status of thermal deformation errors, but also conform to the same Because the same temperature measuring element is used to determine the equivalent linear expansion coefficient and perform error compensation, the influence of the temperature calibration error of the temperature measuring element is eliminated.
3. Probe and accessories error
The displacement z of the CMM is the sum (measurement of outer dimension) or difference (measurement of inner dimension) of the measured dimension L and the equivalent diameter d of the measuring end, so the calibration of the equivalent diameter of the measuring end is of great significance. The method for calibrating the equivalent diameter of the measuring end is to use the probe to measure the calibrated standard ball or gauge block, and the difference between the movement of the instrument and the size of the standard ball or gauge block is the equivalent diameter of the measuring end. It should be pointed out that since the elastic deformation of the measuring rod has a great influence on the equivalent diameter of the measuring ball, after replacing the measuring rod, adding the extension rod, connecting the adapter or the rotation angle of the rotating body, the equivalent diameter of the measuring end must be changed. Recalibrate the diameter.
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