Views: 0 Author: Site Editor Publish Time: 2022-08-03 Origin: Site
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 efficiency requirements for 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 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 object, 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 realize error compensation, in addition to measuring temperature, it is also necessary to know the equivalent linear expansion coefficient of the scale and the measured object. 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 better reflect the use state of the ruler and the status of thermal deformation error, and it is consistent with the 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. Error of probe and accessories
The displacement z of the three-coordinate measuring instrument is the sum of the measured dimension L and the equivalent diameter d of the measuring end (measuring the outer dimension) or difference (measuring the inner dimension), 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 measure the calibrated standard ball or gauge block with a probe, 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.
It is one of the important aspects of the measurement to take the correct measurement method and correctly select the measurement benchmark.
1. Select the projection plane or the work plane when measuring circles and lines, correctly select a datum and a second datum when establishing the part coordinate system, how to choose the datum axis when calculating the coaxiality, and so on. This requires mastering the measurement principle of the CMM and taking correct methods and steps according to the actual situation.
2. Use the shortest possible stylus of the CMM. According to the user's measurement experience, the shorter the stylus, the more accurate the measurement result. However, because the use of a long stylus can make the measurement more convenient, many operators will ignore this problem, but for more precise measurements, a shorter stylus must be used for measurement.
3. Minimize the number of joints and long rods, and the measurement accuracy will also be improved. This is also the experience accumulated by operators who have operated the CMM for a long time.
4. When measuring with a three-coordinate measuring instrument, try to make the diameter range of the side head as large as possible. Because the three-coordinate measuring instrument is used for measurement, the probe is an important measurement accessory, which will directly affect the measurement results.
5. To choose the correct measurement method, different measurement methods are used for different measurement objects, and whether the measurement method is correct or not will directly affect the measurement accuracy of the CMM. Therefore, when using the CMM for operation, it is necessary to use the appropriate measurement method, and also to do the measurement according to the correct operation procedure to avoid the loss caused by the error.
