Discussion on the problem of CNC turning machining process (3)

4 Determination of the knife compensation value when “knife”

For thin-walled workpieces, especially thin-walled workpieces with difficult-to-cut materials, the phenomenon of “knife-making” is severe during cutting, which causes the size of the workpiece to be changed to change. Generally, the outer circle becomes larger and the inner hole becomes smaller. The “knives” are mainly caused by the elastic deformation of the workpiece during machining. The degree of “knife” is closely related to the amount of back-feeding during cutting. Use the "equal back knife depth method" and use the knife value to make a small range adjustment to reduce the impact of the "knife" on the machining accuracy. As shown in Fig. 3, the outer circle size to be processed is A, and the double-sided margin is 2t. In the test cutting, half of the t value is taken as the amount of backing knife during cutting, and the test cutting is performed on the whole length of the surface. After the test cutting, the program is arranged to stop, and whether the outer circle size is equal to A+t is measured. The error size adjusts the tool's tool compensation value, and then continues to run the program to complete the finishing pass. Since the finishing process and the test cutting process use the same back-feeding amount and the same cutting speed and feed speed, the cutting resistance is the same, and the corresponding elastic deformation of the workpiece is the same. The input tool compensation value can just offset the “knives” The resulting deformation ensures the dimensional accuracy of the turned workpiece.

5 chip breaking problem during turning

CNC turning is automated machining, and if the chip's chip breaking performance is too poor, it will seriously hinder the normal processing. In order to solve this problem, the chip breaking performance of the tool itself should be improved as much as possible. Secondly, the cutting amount of the tool should be reasonably selected to avoid the occurrence of strip-shaped chips that hinder the normal processing. In CNC turning, the most ideal chips are spiral-shaped chips with a length of 50-150mm and a small diameter, or pagoda-shaped chips, which can be regularly removed in a certain direction for easy collection and removal. If the chip breaking is not ideal, a pause can be arranged in the program if necessary to force the chip breaking: the chip breaking table can also be used to enhance the chip breaking effect. When using the upper press type of the indexable insert, the platen can be pressed together with the insert at the same time: when the inner hole is used, the cutting edge of the tool can be used to improve the chip removal.

6 Choice of indexable tool blade shape

Compared with the ordinary machine tool processing method, CNC machining puts higher requirements on the tool, which not only requires good rigidity and high precision, but also requires dimensional stability, high durability, good chip breaking and chip discharging performance: at the same time, it requires easy installation and adjustment. This meets the high efficiency requirements of CNC machine tools. The tools selected for CNC machine tools often use tool materials that are suitable for high-speed cutting (such as high-speed steel, ultra-fine-grained carbide) and use indexable inserts.

Machine clamp indexable tools are widely used in CNC turning, which is an important means to improve the productivity of CNC machining and ensure product quality. There are a wide variety of indexable turning inserts, the most widely used diamond-shaped inserts, followed by triangular inserts, round inserts and grooving inserts. Diamond-shaped inserts have three types of 80°, 55° and 35° according to their sharp angles.

The 80° diamond blade has a moderately pointed angle. The blade has good strength, heat dissipation and durability, and can be assembled into a tool with a main angle of slightly more than 90° for end faces, outer circles, inner holes and steps. machining. At the same time, the blade can be clamped well, and can be positioned on the bottom side of the blade and the adjacent two sides of the 80° blade angle on the non-cutting position. The positioning method is reliable, and the position accuracy of the blade is only the accuracy of the blade itself. Correlation, high indexing accuracy, suitable for CNC turning.

The 35° diamond-shaped insert is used for the complex profile of the workpiece or the trench for excavation because of its small tool nose angle and low interference.

7 grooving path

The deep groove type is usually processed by a grooving knife on a CNC lathe. If the width of the knife is equal to the groove width required for machining, the grooving knife cuts the cutting position once, and if the groove is processed by a narrow grooving knife, the groove type is processed. , it should be divided into multiple times. A reasonable cutting route is: cut the middle first, then cut left and right. Because the radius of the fillet on both sides of the blade is usually smaller than the radius of the fillet of the bottom and side walls of the workpiece, when the left and right cutters are cut, when the cutter approaches the bottom of the groove, each arc needs to be taken. If the middle knife does not cut in advance, it will not be able to create the necessary conditions for the two arcs. Even if the radius of the fillet on both sides of the blade is the same as the radius of the fillet on both sides of the groove of the workpiece, it is better to cut the knife first in the middle. Because the knife is cut, the load on both sides of the blade is equal. The next two knives, one knife is The load on the left side is heavy, the load on the right side is heavy, and the wear of the tool is even. Machine-clamping grooving knives should not be arranged with cross-cutting knives.

8 Summary

The NC machining program is the instructional file of the CNC machine tool. The CNC machine tool is controlled by the program instructions. The whole process of machining is automatically performed according to the program instructions. The CNC machining program not only includes the process of the parts, but also the cutting amount and the route. The tool size and the movement process of the machine tool, the CNC machining process is more detailed, and the quality of the process plan directly affects the efficiency of the machine tool and the quality of the part processing, and should be paid special attention in actual production.

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