Views: 0 Author: Site Editor Publish Time: 2022-03-04 Origin: Site
Vibration in metal cutting is a very harmful phenomenon. If vibration occurs during machining, relative displacement will occur between the tool and the work piece, which will cause vibration marks on the machined surface and seriously affect the surface quality and performance of parts. The process system will continue to bear the effect of dynamic alternating load, the tool is easy to wear (or even edge breakage), the machine tool connection characteristics are damaged, and even the cutting process cannot continue in serious cases; Noise from vibration can also harm the health of the operator. In order to reduce vibration, sometimes the cutting amount has to be reduced, so that the production efficiency of machine tool processing is reduced. Therefore, it is necessary to analyze the reason of vibration in metal cutting and grasp the way to control vibration.
Take turning as an example. The main type of turning vibration is self-excited vibration that does not change with the change of turning speed (i.e., flutter phenomenon) under the condition that the isolation foundation is added when the lathe is installed, the transmission system has no defect and there is no impact in the cutting process. The main reason is the low frequency vibration (its frequency is close to the natural frequency of the work piece) and the high frequency vibration (its frequency is close to the natural frequency of the tool) caused by the deformation of the work piece and tool rest system in the process of machining. This kind of vibration often loosens the tail stock and tool rest of machine tools and breaks the carbide blades, leaving fine traces on the cutting surface of the work piece. The low-frequency vibration in turning is usually the work piece and the tool rest are vibrating (the work piece vibration is larger). Sometimes they are separated from each other (vibration), and sometimes they approach (vibration), resulting in equal and opposite acting force and reaction force.
The self-excited vibration is related to the cutting process itself and the structural performance of the process system. So the basic way to control self-excited vibration is to reduce or eliminate the excitation force. The control method has the following aspects:
1） Reasonable choice of cutting dosage
When the speed V=20~60m/min, it is easy to produce natural vibration. When the speed V is higher than or lower than this range, the vibration is weakened. Therefore, low speed cutting should be used in precision machining, and high speed cutting should be used in general machining. As the feed f increases, the natural vibration intensity decreases (as shown in FIG. 2A). FIG. 2B shows the relationship between cutting width Blim and feed f, indicating that feed F should be increased as much as possible under permissible conditions. When turning, the relation between cutting depth AP and cutting width B is b= AP/Sinf (f main deflection Angle), that is, the larger AP is, the larger cutting width B is, the more vibration tends to occur.
2） Reasonable selection of tool geometric parameters
The main influencing parameters are main declination Angle F and forward Angle G. As shown in FIG. 3, the amplitude is minimum when F =90°, and the cutting force is minimum in the Y direction and maximum in the X direction. Because the stiffness of the general process system is much better in the X direction than the Y direction, it is not easy to start vibration. As can be seen from FIG. 4, at the same cutting speed V, the cutting force and amplitude decrease with the increase of forward Angle G. Therefore, double front Angle vibration damper (see Figure 5) is usually used to reduce the cutting force, which can achieve good vibration reduction effect.
3） Reasonably improve the system stiffness
Turning slender shaft (L/D & GT; 12), the work piece rigidity is poor, easy to bend deformation vibration. At this time, the elastic center and auxiliary support (center frame or following tool rest) should be used to improve the anti-vibration performance of the work piece, while cooling liquid cooling to reduce the thermal expansion deformation of the work piece; When hole machining is carried out with slender tool rod, the middle guide support should be used to improve the anti-vibration performance of the tool. While increasing the stiffness of the process system, the weight of the component should be reduced as much as possible, and "maximum stiffness from the lightest mass" should be regarded as an important principle in structural design.
Reduces tool overhang length. In general, the length of the tool should not exceed two times of the height of the tool bar.
Uses a vibration damper and a vibration damper tool. If an elastic cutter bar with high tangential stiffness is used (see FIG. 7), it is not easy to produce high-frequency bending vibration of the cutter bar. When the tool is vibrating at high speed, it is advisable to increase the speed and cutting speed to improve the cutting temperature and eliminate the frictional decline of the tool surface and the natural vibration caused by it, but the cutting speed should not be higher than 1.33m/s(80m/min). For the machine tool spindle system, the bearing clearance should be appropriately reduced, and the rolling bearing should be appropriately prestressed to increase the contact stiffness and improve the anti-vibration performance of the machine tool.
Reasonably arranges the relative position of the cutter and the workpiece. The position of the tool installed on the lathe has a great influence on improving the stability of turning process and avoiding self-excited vibration. The results show that the stability is the worst when the lathe tool is installed on the horizontal plane, and the stability is the best when the lathe tool is installed on the azimuth of A =60°.
The vibration phenomenon can be reduced obviously and the surface quality of work piece and the technological capability of machine tool can be greatly improved by adopting the corresponding measures to reduce vibration in the process of turning. However, it is still necessary to study the vibration mechanism, find the cause and eliminate the vibration completely.