Principle and problems of measuring 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 in the calibration of the measuring needle, because the measuring ball of the measuring needle has its own size, the different positions of the measuring parts can be in contact with the different positions of the measuring parts of the 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 ball itself. Especially in the calibration of different length and position of the measuring needle, the size of the ball error is very important to the measurement results. 1. Principle of measuring needle calibration In the process of measurement and sampling, when the measuring needle touches the surface of the measured part, the measuring head system will display the signal as the target signal of measurement, and then inform the computer to collect data to obtain the coordinate value of the measuring point.

In addition, since the CMM is detected by the head measurement system, the resulting point coordinate value is the coordinate value of the center of the measurement ball. In order to obtain the actual size of the workpiece under test, the dynamic diameter value of the measuring ball needs to be increased or subtracted. After entering the head calibration procedure, measure the standard ball. The measurement method is the same as that of the ball in the geometric element measurement program. When the number of points meets the requirements, the measuring program will automatically calculate and process the measuring points, and automatically return the processed head calibration results to the head calibration dialog box interface. When the back button is pressed, the calibrated head measurements will be used as corrections for subsequent application data processing. The head measurement data after the reference includes the head measurement radius and sphericity error, as well as the coordinate value of the center of the ball with respect to the zero center.

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

(1) Preparation before measurement: First of all, according to the measurement range of the workpiece measurement direction and position, first determine the required measurement needle combination, including the diameter of the measurement needle, the number, the direction and whether to install the long rod, in the measurement process as far as possible not to replace the measurement needle.

(2) Detection and calibration: In the process of detection needle calibration, the selected detection needle should be contacted and measured at the large axial diameter of the standard ball to improve the accuracy of detection needle calibration. After calibration, pay attention to the diameter of the needle and the shape errors in the calibration process. If there is a big change, you need to find out why. More than 6 times of calibration are required to observe the duplicate data of the calibration results and analyze the standard deviation according to statistical principles.

(3) Repeated calibration: For the position calibration of a single measuring needle, it is necessary to observe the diameter and sphericity errors of the measuring needle. The diameter of the CMM measuring needle should be similar to the usual calibration, with good repeatability and small sphericity error. When calibrating the position of multiple measuring heads, in addition to observing the above results, the standard ball should be measured using the calibrated measuring needle at each position to observe the change in the spherical center coordinate value, which should be close to the indicating error or detection error. If the change range is (1 to 3) μm between the two, even if normal. Otherwise, it is necessary to re-check whether the installation of the probe, measuring base, connecting rod and standard ball is stable and reliable.

(4) Replacement of the measuring needle: Because the length of the measuring needle is an important parameter for the automatic calibration of the measuring head, if the automatic replacement of the calibration error, it will lead to the abnormal collision of the measuring needle, light will damage the measuring needle, heavy will lead to the measuring head (sensor) damage. During the process of changing the measuring needle, the coordinate system of the measuring frame can be reconstructed if the measuring chamber deviates from the original position. If the measuring head is too heavy and out of balance, try adding weight blocks in the opposite direction of the measuring head.

(5) Standard ball diameter: The standard ball has excellent circular error, 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 measurement needle calibration principle, it can be seen that the theoretical diameter value of the standard ball will directly affect the ball error of the measurement needle calibration. The software must obtain the measurement "equivalent diameter" of the CMM and automatically compensate the radius of the measuring ball.