Two Examples of Tool Influence and Compensation of CNC Turning Precision

Foreword CNC lathe processing parts are controlled according to programmed instructions to control the tool movement, and the preparation of machining program is generally based on the tool tip as a programming point, but the actual tool tip is round, which leads to the tool's walking trajectory In the event of a change, the effect on machining accuracy is even more pronounced, especially in control systems where there is no tip circular arc compensation. Therefore, this factor must be taken into account when programming. The following describes several workpieces when the shape changes, the radius of the cutting edge radius of the processing accuracy and take appropriate measures.

figure 1

figure 2

image 3

1 The workpiece machining surface transitions from the outer cylindrical surface to the conical surface. Figure 1 shows the positional relationship between the tool and the machined part. When the cutter finishes converting the cylinder to the cone, the programmed control tool should theoretically be at the position of O1, but due to the influence of the tool tip arc r, the point of the cutting cone is actually converted from the original E point to A. point. It can be seen that the cones processed according to this do not correspond to the cones of the design drawings and are slightly larger than the designed cone size, that is, errors occur in the axial and radial directions, that is, the coordinate difference between point A and point C. . From Figure 1 available: Axial error ∆X=r-rcosa Radial error ∆Z=r-rsina
Among them: r—the radius of the tool tip arc a—the angle between the machining cone busbar and the turning center of the component. To figure out the cone that corresponds to the design drawing, it can be seen from Fig. 1 that the tool must be at the O2 point. , which is equivalent to the tool tip along the Z-axis direction, moving Z∆ value forward, so from Figure 1 available: According to the geometric relationship: ∠BO2D=a ∠CO2D=a/2 So CD=rtana/2 Z∆=r-rtana /2 In programming, this error is often ignored. If this error is taken into account, the processing accuracy of the cone can be guaranteed. According to the above calculation, similarly, when the cylinder and the cone are connected in the form shown in Figure 2, the tool should move from the programming point O1 down to the O2 position, and the moving value x∆ is: =r-rtan(90-a) /2 In summary, these errors must be accounted for when the machining surface transitions from a cylinder to a cone or when the cone changes into a cone. Otherwise, due to the axial and radial errors, a small platform is generated at the joint when machining, and the machining accuracy of the parts is extremely poor. 2 The workpiece machining surface transitions from the outer cylinder to the spherical surface. Figure 3 shows the positional relationship between the tool and the machined part. When the tool starts to cut the ball, the tool is at point O1 and the cutting point will be point B. From the figure, there is a difference AB in the radius between the processed sphere and the design drawing sphere. As can be seen from Fig. 3, when the radius of the tool nose increases, the AB value increases; conversely, the AB value decreases. When the radius of the processing sphere increases, the AB value decreases. To eliminate this error in machining, it is necessary to change the cutting point of the tool, that is, move the position of O1 to the position of O2, that is, move the Z value to the left. According to the geometric relation: OE=(R+r)cosa Z∆=EH=OH-OE
=(R+r)-(R+r)cosa
=(R+r)(1-cosa)
Where: R is the radius of the machined part r is the radius of the tool nose arc a is the angle between the center of the machined part and the center of the tool nose and the coordinate of the Z axis When programming, the cylindrical surface of the machined cylinder is converted to the Z axis coordinate With the value of Z∆ as the starting point of the cutting sphere, the quality of the sphere can be guaranteed.

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