Calibration principle and method of star measuring needle

When the CMM is used to detect a series of parameters such as the inner and outer diameters in the depths of parts, as well as the grooves and inner cavity lines in the inner cavity, it is necessary to detect the positions that cannot be detected by a single measuring needle through the star measuring needle, so as to achieve the detection effect. The star measuring needle consists of five vertical spherical measuring needles. In order to ensure the accuracy of the measurement process, it is necessary to systematically correct the measuring needle before each measurement begins.

1. Purpose of star needle measurement correction

(1) By correcting the measuring needle, it can help relevant personnel to clarify the diameter value and position parameters of the measuring needle, and ensure that the data in the contact can be correctly compensated accordingly. In addition, in the application process of CMM, the staff should fully consider the length and azimuthal value of the measuring needle, so that the relative position of the measuring needle center is accurate, to ensure the accuracy of the coordinate relationship.

(2) In the correction process, the staff should also pay attention to the establishment of the coordinate conversion relationship between the measuring needles of different angles, so as to effectively improve the accuracy of the measured value. During the operation of the measuring machine, the software catches the contacts, and the captured contacts are the coordinates of the center of the ruby ball. In the process of measurement, the staff summarized the coordinate value of the contact point between the ruby ball and the workpiece, and the distance between the two points represented the radius length of the measuring needle. In view of the above analysis, the personnel should compensate to some extent during the calibration of the measuring head.

2. Principle of star needle measurement

The star needle calibration mainly adopts the standard ball, the diameter of the standard ball is between 10mm and 50mm, and its diameter and shape errors are calibrated. During the calibration of measuring needles, the number of detection points for each measuring needle should be evenly distributed and the number should be about 5. At the same time, in order to supplement the bending amount of the measuring head in the correction process and fully reduce the adverse influence brought by the error, 2 points should be measured at the position of the measuring head axis and 4 points should be measured at the circumference direction perpendicular to the axis, a total of 6 points should be measured. Automatic mechanical equipment will automatically store the calibration results of the measuring needle and then calibrate other measuring needles.

Analysis of detection Angle range for Samsung needle calibration

Throughout the calibration process, the following will often occur: when the sub-measuring needles in one direction are calibrated in the field, the remaining sub-measuring needles will create some friction with the standard ball, which will then interfere with the calibration results. It has some negative effect on the calibration result of star measuring needle.

(1) Determine the Angle range of measurement needle correction. In the calibration process of measuring needle, the determining factor of the detection point is relatively single, only through the calibration of the measuring needle's axial direction, and has no direct relationship with the spatial direction of the calibration ball in the measuring machine. When the spatial direction of the calibration ball is corresponding, the measuring needle in a certain Angle area often cannot be calibrated. The specific calibration range is shown in Figure 1.

When the calibration needle axis is located in the plane of the calibration ball, the specific formula is as follows:

In the formula, D represents the diameter length of the calibration ball, D1 represents the diameter length of the mounting rod in the calibration ball, and d the diameter length of the measuring needle ball is the value of the mounting Angle in the calibration ball.

Through the analysis of the figure above, we can see that under this installation position, the range of measuring needle Angle is the shaded part in the picture. The specific formula is as follows:

(2) Installation conditions. If the Angle between the measuring needle, X, Y projection line and the mounting rod axis of the calibration ball is corrected on the plane, the mounting Angle in the calibration ball is greater than the Δα calculated above, then we can infer that the value of the calibrable measuring needle A Angle is about ±180° to meet the installation conditions.

In addition, in the actual calibration process, there will be certain deviations in the ball top detection and auxiliary point acquisition, needle ball diameter and head seat installation, so in the calibration process, the staff should pay attention to properly adjust the Angle range of the needle, to ensure that it is less than the standard calculated value, to ensure that the automatic calibration in a good running track.

4 Star needle measurement calibration method

During the calibration of star measuring needle, there are often some interference phenomena. In order to eliminate such adverse effects, during the correction process, the staff should correct a sub-measuring needle, adjust the supporting direction of the correction ball and the Angle direction of the measuring needle, make the position relationship between the two parallel, and then complete the automatic correction of five measuring points according to the system function.

As can be seen from Figure 2, the detection point is perpendicular to the top of the ball A and the calibration needle axis B, C, D, E on the calibration ball. There are corresponding auxiliary points in these detection points, which are A, B, C, D, E, respectively. The detection work should first start from the auxiliary point, and then return to the auxiliary point after the detection work, and then return to the vertex A, above the ball, and then start to move down a little bit. The whole movement path is tapered, which can prevent irregular collision in the detection process.