Principle and problems of needle calibration in CMM

The measuring accuracy and working efficiency of CMM are closely related to the calibration and selection of measuring needle. Before the measurement work, we must reasonably choose the measuring needle and do a good job of calibration of the measuring needle, because the measuring ball of the measuring needle has its own size, and different positions of the measuring parts may contact with different positions of the measuring ball. Therefore, the measurement data contains the value of the measurement ball itself, and the measurement needle calibration is the process of measuring the size of the measurement ball itself. Especially when calibrating the measuring needle of different length and position, the sphericity error of the measuring ball calibration result is very important to the measurement result.

1. Measuring needle calibration principle

During the measurement and sampling process, when the measuring needle touches the surface of the measured part, the measuring head system will display the signal as the target signal of the measurement, and then inform the computer to collect data to obtain the coordinate value of the measured point.

In addition, because the CMM is detected through the probe system, the obtained point coordinate value is the coordinate value of the center of the measuring ball. To obtain the actual size of the workpiece under test, the dynamic diameter value of the test ball needs to be added or subtracted. After entering the probe calibration procedure, the standard ball is measured in the same way as the ball in the geometric element measurement procedure. When the number of sampling points meets the requirements, the measuring program will automatically calculate the measuring points, and the processed probe calibration results will be automatically returned to the probe calibration dialog box interface. When the back button is pressed, the calibrated probe data will be used as a correction for subsequent application data processing. The datum data includes the probe radius and sphericity errors, as well as the coordinates between the center of the probe and the center of the zero probe.

2. When calibrating the measuring needle, attention should be paid to the following problems:

(1) Preparation before measurement: according to the measurement range of the workpiece and the direction and position to be measured, determine the required measurement needle combination, including the diameter, number, direction of the measurement needle and whether to install the long rod, try not to replace in the measurement process.

(2) Detection and calibration: When measuring needle calibration, the selected measuring needle should contact the maximum axial diameter of the standard ball respectively to improve the accuracy of measuring needle calibration. Note the diameter and shape errors of the measuring needle after calibration. If there is a big change, you need to find out why. More than 6 times of calibration should be performed to observe the repeated data of calibration results, and the standard deviation should be found for analysis according to the statistical principle.

(3) Repeated calibration: To calibrate the position of a single measuring needle, it is necessary to observe the diameter and sphericity error of the measuring needle; the diameter of the three-coordinate measuring needle should be similar to the normal calibration with good repeatability and small sphericity error; to calibrate the position of multiple measuring heads, besides observing the above results, the calibration needle should also be used to measure the standard ball and observe the change of the coordinate value of the center of the ball. The value should be similar to the indicating value error or detection error. The range is (1 to 3) μm, even if normal. Otherwise, re-check whether the installation of probe, measuring base, connecting rod and standard ball is stable and reliable.

(4) Measuring needle replacement: Because the length of the measuring needle is an important parameter for the automatic calibration of the measuring head, if the automatic calibration is wrong, it will lead to abnormal collision of the measuring needle, light damage to the measuring needle, heavy damage to the measuring head (sensor). If the measuring nacelle box deviates from its original position, the initial measuring frame coordinate system can be re-established. If the measuring head is too heavy and out of balance, try adding counterweight blocks in the opposite direction of the measuring head.

(5) Standard ball diameter: The roundness error of the standard ball is very good, and the diameter value is used as the standard in the calibration process. Be sure to enter the theoretical diameter of the standard ball correctly. According to the previous principle of measuring needle calibration, it can be seen that the theoretical diameter value of the standard ball will directly affect the sphericity error of measuring needle calibration, and the software must obtain the "equivalent diameter" of the measuring ball of the CMM to automatically compensate the radius of the measuring ball.