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

 

 

 

 

 

 

1. The main characteristics of low frequency vibration

 

 

   After eliminating the vibration of the machine tool's rotary components and transmission system, the main type of turning vibration is self-excited vibration that does not change with the turning speed. Introduce the causes of low-frequency vibration caused by the deformation of the workpiece system and the deformation of the tool holder system and the elimination measures during the machining process.

The main characteristics of low frequency vibration are:

①The vibration frequency is low (50～300Hz), and the noise emitted during vibration is low;

②The marks left on the cutting surface of the workpiece are deep and wide;

③ Vibration is relatively severe, which often loosens machine tool components (such as tailstock, tool rest, etc.) and fractures the carbide blades.

 

 

 

 

 

 

2. Causes of low frequency vibration

 

 

   When the low frequency vibration in turning occurs, usually the workpiece system and the tool holder system are vibrating (but in most cases, the vibration of the workpiece system is larger and plays a dominant role), and they are sometimes separated and sometimes approached, resulting in equal directions Opposite force and reaction force. In the vibration process, when the workpiece moves away from the tool, the cutting force F separates in the same direction as the workpiece displacement, and the work done is a positive value. When the workpiece approaches the tool, the work done by the cutting force F approaching is negative.

During turning:

①The friction between the chips and the rake face of the tool;

②The degree of metal hardening encountered by the tool when cutting into and out of the workpiece is different;

③The actual geometric angle of the tool changes periodically during vibration;

④ During vibration, the relative motion path of the tool to the workpiece is elliptical, which causes periodic changes in the cutting section;

⑤ The mark left by the vibration of the workpiece in the previous revolution caused the periodic change of the cutting section.

These five conditions can cause periodic changes in the cutting force, and make the F phase away> F approach. 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 it does to the workpiece (or tool), so that the workpiece (or tool) obtains energy supplementation. Self-excited vibration.

 

 

 

 

 

 

 

3. Elimination measures of low frequency vibration

 

01

In the low frequency vibration, it is mainly due to the change of the cutting force caused by the vibration in the Y direction that the F phase is more than F approaching and the vibration is generated.

Mainly take the following 4 measures.

① The larger the entering angle (μr angle) of the tool, the smaller the Fy force, and the less likely it is to vibrate. Therefore, appropriately increase the tool entering angle to eliminate or reduce vibration.

②Appropriately increasing the rake angle of the tool can reduce the Fy force, thereby reducing vibration.

③The tool clearance angle is too large or the blade is too sharp, the tool is easy to bite into the workpiece, and it is easy to produce vibration. When the tool is properly passivated, its flank can prevent the tool from "gnawing into" the workpiece, which can reduce or eliminate vibration.

④The tool tip position is too low during turning (below the center of the workpiece) or too high when boring on the lathe, which will reduce the actual rake angle of the tool tip and increase the back angle, which is prone to vibration.

⑤ If the tool holder system has negative stiffness, it is easy to "gnaw into" the workpiece and cause vibration. Therefore, as far as possible to avoid the vibration of the negative stiffness of the tool carrier system on turning.

 

02

When a wide and thin chip is produced in the turning process, the vibration in the Y direction will cause the cutting force to change. When the cutting section is wide and thin, the vibration in the Y direction will cause a drastic change in the cutting cross-sectional area and cutting force. Therefore, in this case, vibration is very likely to occur. For example: in longitudinal turning, the greater the depth of cut, the greater the feed rate, and the smaller the entering angle, the wider and thinner the cutting section will be, and the easier it is to produce vibration. Therefore, when selecting the turning speed, avoid the medium-speed area where the cutting force decreases with the speed (when cutting carbon steel, the speed range is 30-50m/min), while reducing the turning back force and appropriately increasing the feed rate and Reducing the depth of cut also helps to suppress vibration.

 

03

The lack of rigidity of the workpiece system and the tool holder system is the main cause of low-frequency vibration. The following measures can be taken to eliminate or reduce the vibration:

①When clamping the workpiece with three-jaws or four-jaws, minimize the coaxiality error between the rotation center of the workpiece and the rotation center of the spindle as much as possible to avoid the vibration caused by the periodic change of the cutting force caused by the inclination of the workpiece and the intermittent cutting or uneven cutting .

②When processing thin and long workpieces that are easy to deform and produce vibrations, use elastic center and auxiliary support while adding coolant to cool the workpiece to reduce thermal expansion and deformation of the workpiece.

③When clamping the workpiece, do not extend the workpiece too long. For workpieces with insufficient rigidity, use reasonable auxiliary supports such as center frame, follower and top to increase the rigidity of the workpiece.

④When using the top, the top and the taper hole of the top should match well to avoid the bending of the workpiece due to too much top force or the too small top force to make the workpiece swing, and pay attention to the overhang of the tailstock sleeve.

⑤The bearing clearance of the machine tool spindle directly affects the rotation accuracy and rigidity of the spindle. If it is found that the clearance is too large due to bearing wear and the rigidity is insufficient, the bearing clearance should be adjusted and preloaded to increase the rigidity of the workpiece system to eliminate vibration.

⑥Regularly check the contact of the dovetail guide rail between the middle carriage and the large carriage, and between the small tool carriage and the middle carriage, and adjust the diagonal strips to maintain a proper gap to avoid crawling when the carriage is moving and cause vibration of the carriage system.

⑦Every time the square tool holder is rotated to make the tool turn to the desired position, the square tool holder should be pressed and fixed to avoid the looseness of the square tool holder and reduce the rigidity of the tool holder system to cause vibration.