Vibration during turning interferes with and destroys the normal cutting process of the process system, which not only seriously deteriorates the surface quality, shortens the service life of machine tools and tools, but also produces harsh noise, which affects the physical and mental health of operators. Usually, in order to reduce vibration, the cutting amount has to be reduced, thus reducing the production efficiency. Therefore, it is very necessary to analyze the causes of vibration in turning and take corresponding measures to eliminate or reduce vibration. 1 Main types and phenomena of vibration in turning. After eliminating the vibration of the rotating parts and transmission system of the machine tool, the main type of turning vibration is self-excited vibration which does not change with the speed. It is mainly the vibration frequency caused by the deformation of workpiece system and tool rest system during machining.

Low frequency vibration and turning tool with frequency close to the natural frequency of workpiece.

2.

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3.

Vibration produced in metal cutting is a very harmful phenomenon. If vibration occurs during machining, there will be relative displacement between the tool and the workpiece, which will cause vibration marks on the machined surface and seriously affect the surface quality and performance of the parts. The process system will continue to bear the dynamic alternating load, and the tool is easy to wear (or even break), and the connection characteristics of the machine tool are destroyed, and even the cutting can not be continued in serious cases; The noise generated by vibration will also endanger the health of the operator. In order to reduce vibration, it is sometimes necessary to reduce the cutting amount, which reduces the production efficiency of machine tool processing. Therefore, it is necessary to analyze the causes of vibration in metal cutting and master the methods to control vibration.

1 causes of vibration phenomenon

Take turning as an example. When the vibration isolation foundation is installed in the lathe, the transmission system is free from defects and there is no impact during cutting, the main types of turning vibration are self-excited vibration (that is, chatter phenomenon) which does not change with the rotating speed, and the main reasons are low-frequency vibration (whose frequency is close to the natural frequency of the workpiece) caused by the deformation of the workpiece and the tool rest system during machining and high-frequency vibration (whose frequency is close to the natural frequency of the turning tool) caused by the deformation of the turning tool. This vibration often loosens the tailstock and tool rest of the machine tool, breaks the cemented carbide blade and leaves tiny marks on the cutting surface of the workpiece. The low-frequency vibration in turning is usually that both the workpiece and the tool rest are vibrating (the workpiece vibrates greatly), and they are sometimes far away (vibrating out) and sometimes close (vibrating in), resulting in equal and opposite forces and reaction forces (namely cutting force Fy and elastic restoring force F). The vibration motion of the tool rest is produced under the action of cutting force Fy. For vibration system, Fy is an external force. In the process of vibration, when the workpiece and the tool rest vibrate, the cutting force F vibrates in the same direction as the displacement of the workpiece, which makes positive work for the vibration system, and the vibration system absorbs a part of energy W from the cutting process and vibrates and stores it in the vibration system. The vibration motion of the tool holder is generated under the action of elastic restoring force F. When the tool holder vibrates, the vibration direction of F is opposite to the displacement direction of the workpiece, and the vibration system does work on the cutting process, that is, the vibration system consumes energy W to vibrate. Due to the periodic change of cutting force, w vibration >: w vibration in or f vibration out > f vibration in, so that the workpiece or tool can obtain energy to supplement and produce low-frequency self-excited vibration. At this time, in the relationship diagram between force and displacement, the vibration-out process curve is at the upper part of the vibration-in process curve (as shown in figure 1).

Figure 1

The reason of high-frequency vibration is that the friction between the tool flank and the chip in a certain speed range makes the cutting force Fy decrease with the increase of cutting speed V, that is, it has a downward characteristic, which leads to F vibration >: F vibration, so the machining system has self-excited vibration.

Two methods to control or reduce vibration

Self-excited vibration is related to the cutting process itself and the structural performance of the process system. Therefore, the fundamental way to control self-excited vibration is to reduce or eliminate the exciting force. The control method has the following aspects:

2. 1 Reasonable selection of cutting parameters

6? When the rotating speed of 1 is V=20~60m/min, natural vibration is easy to occur, and when it is higher or lower than this range, the vibration will be weakened. Therefore, low-speed cutting should be adopted for precision machining and high-speed cutting should be adopted for ordinary machining.

6? The feed f of 1 increases, and the natural vibration intensity decreases (as shown in Figure 2a). The curve of the relationship between the cutting width blim and the feed f is shown in Figure 2b, which shows that the feed f should be increased as much as possible under the permitted conditions. The relationship between cutting depth ap and cutting width b during turning is b=ap/sin? 8? 9(? 8? 9 is the main deflection angle), that is, the larger the ap is, the larger the cutting width b is, and the more inclined it is to vibrate.

A b

Figure 2

A b

Fig. 3 Influence of main deflection angle on vibration

Fig. 4 Influence of rake angle

Fig. 5 Double front angle shock absorber.

Fig. 6 Chamfer damping turning tool Fig. 7 Elastic arbor turning tool

2.2 Reasonable selection of tool geometric parameters

6? 1 What is the main influence parameter? 8? 9 and the front corner? 8? 0。 As shown in Figure 3, when? 8? When 9 = 90, the amplitude is the smallest, the cutting force in Y direction is the smallest, and the cutting force in X direction is the largest. Because the stiffness of general process system in X direction is much better than that in Y direction, it is not easy to start vibration. As can be seen from figure 4, at the same cutting speed v, with the rake angle? 8? With the increase of 0, the cutting force decreases and the amplitude decreases. Therefore, the double rake angle damper (see Figure 5) is usually used to reduce the cutting force, which can achieve good vibration reduction effect.

6? 1 reducing the back angle is beneficial to vibration reduction. Generally, the clearance angle is 2 ~ 3. If necessary, grind the damping edge of the clearance surface to form a chamfered damping turning tool (as shown in Figure 6). Its characteristics are that the tip of the knife is not easy to cut into metal, the back angle is small, it has the function of vibration reduction and good stability when cutting.

2.3 Reasonable improvement of system stiffness

6? 1 turning slender shaft (aspect ratio >; 12), the workpiece has poor rigidity and is prone to bending deformation and vibration. At this time, elastic center and auxiliary support (center frame or heel support) should be used to improve the anti-vibration performance of the workpiece, and cooling liquid should be used to reduce the thermal expansion deformation of the workpiece. When machining holes with slender tool holders, intermediate guiding support should be adopted to improve the anti-vibration performance of the tool.

6? 1 It is necessary to reduce the weight of components as much as possible while increasing the stiffness of the process system, and take "obtaining the maximum stiffness with the lightest mass" as an important principle of structural design.

6? 1 Reduce the protruding length of the tool. In general, the extension length of the cutter should not exceed twice the height of the cutter bar.

6? 1 Use shock absorber and shock absorber tool. If an elastic arbor with high tangential stiffness is used (see Figure 7), it is not easy to generate bending high-frequency vibration of the arbor.

6? 1 When the tool vibrates at high speed, it is advisable to increase the rotational speed and cutting speed to increase the cutting temperature and eliminate the reduction of friction on the tool flank and the resulting natural vibration, but the cutting speed should not be higher than 1.33m/s(80m/min).

6? 1 For the spindle system of the machine tool, the bearing clearance should be appropriately reduced, and the rolling bearing should be properly prestressed to increase the contact stiffness and improve the vibration resistance of the machine tool.

6? 1 Reasonably arrange the relative position of the tool and the workpiece. The installation orientation of turning tool on lathe has great influence on improving the stability of turning process and avoiding self-excited vibration. The test shows that the stability of ordinary turning tool installed on a horizontal plane is the worst, while it is installed on a horizontal plane. 8? In the direction of 4 = 60, the stability of turning process is the best.

3 Conclusion

By taking corresponding vibration reduction measures to the causes and basic laws of vibration in turning, the vibration phenomenon in cutting process can be obviously reduced, and the surface quality of the workpiece and the technological ability of the machine tool can be greatly improved. However, in order to completely eliminate the vibration phenomenon, it is necessary to further study the vibration mechanism, find out the reasons and take measures to eliminate it.