Application of Tool Compensation in CNC Lathe Machining Process (2)

May 25, 2022

The control point of the lathe is the center of the tool holder, so tool position compensation is always required. The tool position compensation is used to realize the conversion between the center point of the tool nose arc and the reference point of the tool holder, corresponding to the conversion between A and S in Figure 3, but in fact we cannot directly measure between the two center points. The distance vector, but only the imaginary tool tip! The distance from the tool holder reference point $.

For the sake of simplicity, it is assumed that the cutter radius r=0, and the tool length measuring device can be used to measure the coordinate sum of the imaginary tool nose point P relative to the tool holder reference point and stored in the tool parameter table.

In the formula: ——— imaginary tool tip P point coordinates;

(X, Z) ——— The coordinates of the tool holder reference point A.

At this point, it is easy to write out the formula for calculating the tool position compensation.

The coordinates of the imaginary tool tip P in the formula are actually the coordinates of the machining part trajectory point, which can be obtained from the numerical control machining program. At this time, after the part contour trajectory is compensated by the formula (2), it can be realized by controlling the tool holder reference point A.

For the case of r≠0 in Fig. 3, when performing tool position compensation, it is necessary to consider not only the compensation of the radius of the cutter head but also the installation method of the tool (see 2.2).

2 tool radius compensation

When programming a machining program, the tool tip is generally regarded as a point. However, in fact, the tool tip has a circular arc. When cutting the inner hole, the outer circle and the end surface, the tool edge arc does not affect the machining size and shape, but When cutting the taper surface and the arc, the tool's travel trajectory does not match the programmed trajectory, and there is a difference. Figure 4 shows the trajectory of a circular tool nose with radius compensation and no radius compensation. It can be seen from the figure that when programming with the imaginary tool tip P, the center track of the tool arc is as shown by the double-dotted line in Fig. 4, and there is an error, the error size and the radius of the arc of the actual machining path of the tool and the contour shape required by the workpiece. r related. If the tool arc center is programmed and the radius compensation function is used, the path of the center of the tool arc is the thin solid line in Figure 4, and the machining path and the required contour of the workpiece are equal.

Fig. 4 The trajectory of the arc tool tip with radius compensation and no radius compensation

Because the installation and geometry of the turning tool is more complicated, the following is further elaborated in several aspects.

2.1 Determination of the orientation of the imaginary tool tip P

The orientation of the imaginary turning tool tip P relative to the center of the arc is related to the moving direction of the tool, which directly affects the calculation result of the circular turning tool compensation. Figure 5 is the imaginary tool nose orientation and code of the circular turning tool. As can be seen from the figure, there are eight kinds of orientations of the tool tip P, which are represented by 1~8 eight digit codes respectively. At the same time, when the tool tip takes the center position of the arc, the code is 0 or 9, which can be understood as no circle. Arc compensation.

Figure 5 Circular tool imaginary tool tip orientation and code

2.2 Relationship between arc radius compensation and position compensation

If the tool point A is used as the programming starting point and the arc radius compensation is not considered, the X-axis and Z-axis compensation values of the turning tool are determined according to the method shown in Figure 1(b). It is necessary to consider the position compensation of the turning tool and the arc radius compensation. At this time, the position compensation value of the turning tool in the X axis and the Z axis can be determined according to the method shown in Fig. 6, and the arc radius r value of the tool is placed. In the corresponding storage unit, the numerical control device automatically performs arc radius compensation during machining. In the memory unit corresponding to the compensation number in the tool code T, a set of data is stored: the length compensation value of the X-axis Z-axis, the arc radius compensation value, and the imaginary tool nose orientation (0~9). During operation, the four data of each tool can be input into the storage unit corresponding to the tool compensation number to realize automatic compensation (Table 1).