Three coordinate star needle calibration method

When the CMM is used to check the depth sounding parameters such as the inner diameter, outer diameter and inner cavity groove, a single measuring needle cannot detect the required detection position, so the star measuring needle is needed for detection. It usually consists of five vertical spherical measuring needles. When using the star measuring needle, in order to better ensure the measurement accuracy, improve the measurement accuracy, should be calibrated before measurement.

1: Star needle measurement correction Purpose:

(1) Obtain the exact diameter and position coordinates of the Zeiss measuring needle to ensure correct compensation for each contact data. When measuring the Zeiss CMM, it is necessary to select different length, diameter and orientation of the measuring needle to achieve the purpose of touch. It is also required to determine the relative position relation between the spherical center and the known measuring needle to ensure that the geometric elements measured by different measuring needles have the correct coordinate relation.

(2) Establish the coordinate conversion relationship between different Angle measurement needles to ensure the accuracy of measurement data when different Angle measurement needles are used in parts measurement. When CMM is measuring, the software can obtain the coordinates of the center point of the ruby ball when each contact is obtained. We can obtain the coordinates of the contact point between the ruby ball and the workpiece surface. The distance between the two points is the measurement radius value in the direction of the contact point, which needs to be compensated by the calibration of the measuring head.

2: Star needle calibration principle:

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. When the measuring needle is corrected, each measuring needle should detect more than 5 points uniformly on the standard ball. In order to correct the bending of the measuring head and reduce the influence of error, the measuring head axis direction 2 points, perpendicular to the circumference of the axis 4 points, a total of 6 points. The machine automatically completes the calibration of the currently selected measuring needle, automatically saves the calibration result, and calibrates other measuring needles in turn.

3. Analysis of detection Angle range of star needle calibration:

In the actual calibration process of the star measuring needle, other measuring needles may collide with the standard ball and interfere with it, so that the star measuring needle cannot be corrected. (1) : Determine the calibration Angle range of the measuring needle. When the measurement needle is calibrated, its detection point is only determined by the axial direction of the calibration needle, and has nothing to do with the spatial direction of the calibration ball in the CMM. Corresponding to each spatial direction of the calibration ball, there is always an angular area where the measuring needle cannot be calibrated. The Angle range of the measuring needle during calibration is shown in Figure 1.

The axis of the needle of the calibration ball is provided on the mounting plane of the calibration ball:

D Calibrate the ball diameter; D1 is the diameter of the mounting rod of the calibration ball; d the nominal diameter of the needle ball is; Alpha is the mounting Angle of the calibration ball. As can be seen from Figure 1, the range of calibrable needle A Angle at the installation position is the shaded part in the figure, i.e. -(α-△x)-180(α+△α)(2)* Good installation conditions. If the included Angle between the projection line of the corrected measuring needle axis on the XY plane and the projection line of the axis of the calibration ball mounting rod on the XY plane and the installation Angle of the calibration ball are greater than the above calculated △a, theoretically corresponding to the Angle of the calibrated measuring needle A ±180, it is said that such installation meets the best installation conditions. In actual calibration, due to the possible deviation of ball top detection and auxiliary point acquisition, head seat installation, calibration ball mounting plane and calibration needle Angle estimation, and needle diameter, the calibration needle Angle range should be smaller than the calculated value to ensure the safe operation of automatic calibration.

4: Star needle measurement correction method

In order to eliminate the interference during star-shaped needle correction and ensure the smooth correction, when correcting a sub-needle of star-shaped needle, the supporting direction of the correction ball must be parallel to the Angle direction of the needle, and then the 5 measuring points of the selected direction should be automatically corrected according to the system function. The detection points are the vertex A of the sphere and the four points B.C.D.E of the calibration sphere perpendicular to the circle of the large section of the calibration needle axis * (as shown in Figure 2). Each probe point is preceded by a secondary point, respectively a secondary point. A', B', C', D', E", each point is probed from the corresponding auxiliary point of that point. At the end of the probe, return to the auxiliary point, then return above the apex of the ball (point A), and then move down, i.e. along a tapered path, so that * avoid abnormal collisions.

After repeated correction practice of star measuring needle of CMM, the correction method of star measuring needle is summarized, and the correct and rapid correction of measuring point position on standard ball and Angle range is determined reasonably.