Spherical surface machining usually requires high profile accuracy and surface roughness. It is difficult to obtain a spherical surface with high profile accuracy and high surface quality by ordinary machining methods. The conventional machining methods of spherical surface mainly include turning and milling. The milling efficiency is relatively low, and the machining accuracy and surface roughness are not high. Therefore, the parts with high accuracy requirements usually do not use this machining method. In contrast, turning efficiency is much higher, and the machining accuracy and surface roughness are also very good. It is an ideal machining method without special equipment. However, the turning accuracy is affected by many factors such as machine tool accuracy, numerical control system, tool wear and so on. It is difficult to quickly process a sphere with high contour accuracy and uniform surface quality. Therefore, in order to obtain a sphere with high contour accuracy, some special equipment and some special processing methods are usually used. This paper mainly introduces a method of using Siemens five axis turning and milling machine tool to process the outer spherical surface. This method is similar to the turning and milling compound processing. The surface quality of the machined spherical surface is uniform and the spherical contour precision is high.
2 Principle Analysis
According to the geometric principle, when a sphere and a plane intersect, it will form a section, and its section intersection line is a circle (see Figure 1). The closer the plane is to the center of the ball, the greater the radius of the circle. When the center of the ball is on the plane, the radius of the circle is the largest (equal to the radius of the ball). Conversely, any circle can be passed by countless balls with radius no less than its radius. According to this principle, we use the circle formed by the rotation of the milling cutter to cut the rotating workpiece. In this way, a sphere will be machined. The processing method is shown in Figure 2.
Figure 1 cross section line
Figure 2 processing method
3 data calculation
Theoretically speaking, a circle can be passed by countless balls with different radii. Therefore, when we process a ball, we should first determine the size of the ball, and then determine the tool diameter and tool rotation angle according to the range of the spherical surface. The calculation of spherical machining dimension is shown in Figure 3. O is the center of the ball, R is the radius of the ball, a is the vertex of the ball, B is the termination position of the ball, D is the diameter of the section circle at the termination, L1 is the rotation axis of the ball, β is the angle between OB and L1, then the straight line AB is the rotation diameter of the milling cutter, L2 is the rotation axis of the milling cutter and passes through the center of the ball, α is the rotation angle of the cutter, and H is the final cutting position of the cutter. It can be concluded that the tool rotation angle is
Then the radius of milling cutter r = rsin α and the position of lower cutter H = RCOs α.
Fig. 3 Calculation of spherical machining dimension
4 processing method
(1) Processing equipment in this paper, the horizontal turn milling compound machining center is used, and the structure of the machine tool is shown in Figure 4. The machining center is equipped with Siemens 840D CNC system, with RTCP five axis linkage function and dual channel function.
Figure 4 machine tool structure
(2) The processing tool is the sleeve milling tool as shown in Figure 5.
Figure 5 milling tool
(3) Processing method Firstly, the turning tool is used to turn out the ball shape with allowance; then the tool diameter is calculated according to the ball size, and the tool diameter is adjusted to the corresponding size; the zero point of the workpiece coordinate system is set at the ball center, and the tool axis is rotated to the machining angle position (α); the workpiece coordinate system is rotated to make the Z axis parallel to the tool axis, and the positioning tool axis coincides with the rotated Z axis Start the five axis function to make the tool in the linkage state, rotate the tool and make the workpiece rotate slowly, control the tool to feed along the z-axis direction after rotation at the cutting speed until it reaches the expected processing position (H), and make the tool return along the feed axis after processing.
(4) Processing procedure
The processing procedure is as follows.
N11 ROT Y225
5 key points
1) In order to ensure the spherical integrity of the machining part, the tool tip rotation path must pass through the spherical vertex, and the tool rotation center must pass through the spherical center.
2) Starting five axis function in the process of machining can accurately control the tool tip position and facilitate programming.
3) After reaching the machining depth, the workpiece should be rotated at least 360 ° to ensure the integrity of spherical machining.
It is not only easy to obtain better surface roughness and shape accuracy by milling method, but also the special structure of turn milling machine and the powerful function of Siemens 840D make the programming and processing more simple, no special fixture is needed, and the processing technology is simplified.