One of the common faults in turning machining is vibration. When the lathe generates vibration, the normal cutting process of the process system is disturbed and damaged, which not only seriously deteriorates the surface quality of the machining, but also shortens the service life of the machine tool and cutting tools. Therefore, it is necessary to take some measures to reduce or eliminate the vibration generated by the machine tool.

Today, we will mainly introduce the causes and elimination measures of low-frequency vibration caused by deformation of the workpiece system and tool holder system during the machining process.

After eliminating the vibration of the rotating components and transmission system of the machine tool, the main type of turning vibration is self-excited vibration that does not change with the turning speed.

The main characteristics of low-frequency vibration are: ① low vibration frequency (50-300Hz), and low noise emitted during vibration; ② The marks left on the cutting surface of the workpiece are deep and wide; ③ The vibration is quite intense, often causing machine tool components (such as tailstock, tool holder, etc.) to loosen and causing hard alloy blades to break.

When low-frequency vibration occurs in turning, both the workpiece system and the tool holder system are usually vibrating (but in most cases, the vibration of the workpiece system is relatively large and plays a dominant role). They sometimes move apart and sometimes approach, generating equal and opposite forces and reactions. During the vibration process, when the workpiece and tool move apart, the cutting force F moves in the same direction as the workpiece displacement, and the work done is positive. When the workpiece approaches the tool, the work done by the cutting force F approaches is negative.

During the turning process: ① the friction force between the chip and the front face of the tool; ② The degree of metal hardening encountered by the cutting tool when entering and exiting the workpiece is different; ③ During the vibration process, the actual geometric angle of the tool changes periodically; ④ During vibration, the relative motion trajectory of the tool towards the workpiece is elliptical, causing periodic changes in the cutting section; ⑤ The marks left by the vibration of the workpiece during the previous rotation cause periodic changes in the cutting cross-section. These 5 situations can cause periodic changes in cutting force and cause F to separate> F approaches. In this way, in each vibration cycle, the positive work done by the cutting force to the workpiece (or tool) is always greater than the negative work done by it to the workpiece (or tool), thus allowing the workpiece (or tool) to obtain energy supplement and generate self-excited vibration.

（1）During low-frequency vibration, it is mainly due to the variation of cutting force caused by the vibration in the Y direction, which causes F to separate from each other; F approaches and generates vibration.

The following four measures are mainly taken.

①Tool main deviation angle（ μ The larger the r angle, the smaller the Fy force, and the less likely it is to vibrate. Therefore, increase the main deviation angle of the tool appropriately to eliminate or reduce vibration.

②By appropriately increasing the tool rake angle, Fy force can be reduced, thereby reducing vibration.

③If the back angle of the tool is too large or the blade is too sharp, the tool can easily bite into the workpiece and cause vibration. When the tool is properly passivated, the back face of the tool has the effect of preventing the tool from "biting" into the workpiece, which can reduce or eliminate vibration.

④When turning, if the position of the tool tip is too low (lower than the center of the workpiece) or too high when boring on the lathe, it will cause the actual front angle of the tool tip to decrease and then increase, which is prone to vibration.

⑤If the tool holder system has negative stiffness, it is easy to "bite" the workpiece and generate vibration. Therefore, try to avoid the negative stiffness of the tool holder system from causing vibration during turning as much as possible.

（2）When wide and thin chips are generated during the turning process, the vibration in the Y direction causes changes in cutting force. When the cutting section is wide and thin, the vibration in the Y direction will cause drastic changes in the cutting cross-sectional area and cutting force. Therefore, vibration is highly likely to occur in this situation. For example, when turning with a longitudinal cutting tool, the deeper the cutting depth, the greater the feed rate, and the smaller the main deviation angle, the wider and thinner the cutting section, and the easier it is to generate vibration. Therefore, when selecting the turning speed, it is necessary to avoid the medium speed zone where the cutting force decreases with the speed (when cutting carbon steel, the speed range is 30-50m/min). At the same time, reducing the back cutting force of the car, appropriately increasing the feed rate, and reducing the cutting depth can also help suppress vibration.

（3）The insufficient stiffness of the workpiece system and tool holder system is the main cause of low-frequency vibration, and the following measures can be taken to eliminate or reduce the vibration:

①When clamping the workpiece with three or four claws, the coaxiality error between the workpiece rotation center and the spindle rotation center should be minimized as much as possible, to avoid vibration caused by periodic changes in cutting force caused by intermittent or uneven cutting due to workpiece tilting.

②When processing thin and long workpieces that are prone to deformation, bending, and vibration, elastic centers and auxiliary supports are used while cooling with coolant to reduce the thermal expansion deformation of the workpiece.

③When clamping the workpiece, do not extend the workpiece too long. For workpieces with insufficient stiffness, reasonable auxiliary supports such as center frames, tool holders, and tips are used to increase the stiffness of the workpiece.

④When using a tip, the tip should be well matched with the top cone hole to avoid bending the workpiece due to too much jacking force or swinging the workpiece due to too little jacking force. It is also important to note that the tail seat sleeve overhang should not be too long.

⑤The bearing clearance of the machine tool spindle directly affects the rotational accuracy and stiffness of the spindle. If it is found that the clearance is too large and the stiffness is insufficient due to bearing wear during use, the bearing clearance should be adjusted and pre tightening force should be applied to increase the stiffness of the workpiece system and eliminate vibration.

⑥Regularly check the contact between the middle carriage and the large carriage, as well as the dovetail guide rail between the small knife holder and the middle carriage. Adjust the diagonal insert to maintain appropriate clearance to avoid crawling during the movement of the knife holder, which may cause vibration in the knife holder system.

⑦Every time the square tool holder is rotated to rotate the tool to the desired position, it should be compressed and fixed to avoid loosening the square tool holder and reducing the stiffness of the tool holder system, resulting in vibration.