In order to realize thread milling, the machine tool must have the function of three-axis linkage. The helical interpolation is a function of the CNC machine tool. The machine tool controls the tool to realize the helical trajectory. The helical interpolation is formed by the circular interpolation of the plane and the linear motion perpendicular to the plane.
For example: the spiral trajectory from point A to point B (see Figure 1) is formed by the linkage of X-Y plane circular interpolation motion and Z-axis linear linear motion. For most CNC systems, this function can be realized through the following two different commands. G02: instantaneous needle circular interpolation command; G03: counterclockwise circular interpolation command.
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The thread milling motion (see Figure 2) is shown to be formed by the tool's own rotation and the helical interpolation motion of the machine tool. In the interpolation process of the grid circle, the required thread is processed by using the geometric form of the tool and combining the movement of the cutter along the Z axis to move the grid pitch.
Thread milling can use the following three plunge methods: arc plunge method, radial plunge method and tangential plunge method.
1. Arc cut method
Using this method, the tool cuts in and cuts smoothly, leaving no cutting marks and no vibration, even when processing hard materials. The programming of this method is more complicated than the radial cutting method, and it is recommended to use this method when machining precision threads.
1-2: Fast positioning; 2-3: The tool cuts in tangentially along the circular arc feed, and at the same time interpolates the feed along the Z axis; 3-4: 360° full circle thread interpolation movement, axially moving a lead ;4-5: The tool cuts in tangentially along the circular arc feed, and at the same time performs interpolation movement along the Z axis; 5-6: Rapidly retracts.
2. Radial cut method
Using this method is the easiest, but sometimes the following two situations occur: 1) There will be small vertical marks left at the entry and exit points, but it will not affect the quality of the thread. 2) When machining very hard materials, when the cutting is close to the full profile, due to the increase in the contact area between the tool and the workpiece, tool vibration may occur. In order to avoid vibration when cutting close to the full profile, the feed rate should be reduced to 1/3 of the helical interpolation feed.
1-2: Quick positioning; 2-3: Perform helical interpolation motion in a 360° full circle, and move one lead in the axial direction; 3-4: Return radially.
3. Tangential cut method
This method is very simple and has the advantages of the circular plunge method, but it is only suitable for milling of external threads.
1-2: fast positioning; 2-3: 360° full circle thread interpolation movement, axial movement of one lead; 3-4: fast retraction.
