Factors Affecting the Accuracy of CMMs

In addition to the measurement accuracy error caused by the failure of the CMM itself, there are many factors that may affect the accuracy of the CMM, resulting in measurement errors. Operators should understand these factors to avoid affecting the accuracy of part measurements.

1. Temperature stability of the machine room

As mentioned earlier, temperature is a major factor affecting the accuracy of the measuring machine. An annual accuracy correction does not guarantee that the measuring machine can measure accurately in the event of temperature changes. Especially when the season changes, the temperature of the computer room has been different from the calibration time. It is necessary to use a temperature correction factor to correct errors caused by differences in grating temperature and gauge block temperature when the temperature in the computer room is stable. This correction can be made based on season or ambient temperature conditions.

2. Correct compensation file

The compensation file is a compensation file generated by using a dual frequency laser interferometer to detect the positional, straightness, and angular errors of the measuring machine. Measurement software compensates for errors based on it. Often, some operators forget to install compensation files or lose backup files when installing the measuring machine software, resulting in poor accuracy of the measuring machine and recalibration of the machine accuracy. Therefore, the operator should keep the backup file and know how to install the compensation file.

3. Correct and accurate probe calibration The purpose of probe calibration is to correct the diameter of the ruby ball of the measuring rod (probe tip), correct the probe at the measuring point, and obtain the positional relationship between different probe positions. All errors generated during probe calibration will be added to the measurement. Therefore, it is necessary to ensure correctness and accuracy in this link.

The following points should be noted:

Keep the standard ball and measuring rod clean.

Ensure that the measuring base, measuring head, measuring rod, and standard ball are firmly fixed.

Enter the correct rod length and standard ball diameter.

Judge the accuracy of the correction based on the shape error and the corrected diameter and repeatability of the gemstone ball (the diameter of the gemstone ball after correction will vary depending on the length of the extension rod).

When using different probe positions, after correcting all probe positions, check the calibration accuracy by measuring the coordinates of the spherical center of the standard sphere. The probe file saved after the probe calibration can be retrieved and used without moving the probe and the rod. However, if the measurement accuracy is relatively high, it is recommended to calibrate the probe again.

4. Measuring changes in mechanical parts

The working state of the measuring machine After a long period of use, especially in situations where the environment is relatively poor and temperature fluctuations are relatively large, there may be changes in the mechanical part, such as poor perpendicularity, and poor measurement accuracy of the leg end attached to the measuring machine with a long beam length. At this point, it is necessary to carry out overhaul and accuracy verification of the measuring machine.

5. The shape error of the measured part and the correctness of the reference

Because the principle of CMM measurement is to collect points first, and then the software calculates the error by fitting the collected points. Therefore, when measuring with a measuring machine, there are certain requirements for the shape error of the parts. When the measured part has obvious burrs or trachomas, the repeatability of the measurement becomes significantly worse, so that the operator cannot give accurate measurement results. In this case, on the one hand, it is required to control the shape error of the measured part, and on the other hand, it is also possible to appropriately increase the diameter of the measuring rod gemstone ball, but the measurement error is obviously larger. One of the important steps in measurement is to adopt the correct measurement method and select the correct measurement reference during the three-dimensional coordinate measurement. For example, when measuring circles and lines, it is necessary to select a projection plane or work plane, correctly select a primary and secondary datum when establishing a part coordinate system, and how to select a reference axis when calculating coaxiality. This requires mastering the measuring principle of the measuring machine and taking the correct methods and steps according to the actual situation.