Why does the Lathe Produce Vibration? INTRODUCES SEVERAL ELIMINATION MEASURES!

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

1

Main characteristics of low frequency vibration

The main type of turning vibration is the self-excited vibration which does not change with the turning speed. Today we mainly introduce the causes of low-frequency vibration in machining process due to the deformation of workpiece system and tool holder system and its elimination measures.

The main characteristics of low-frequency vibration are: 1 the VIBRATION FREQUENCY IS LOW (50 ~ 300HZ) , the noise produced by vibration is low; 2 the marks left on the cutting surface of the workpiece are deep and wide; 3 the vibration is violent, often causes the machine tool component (like the tailstock, the knife rest and so on) to loosen and causes the hard alloy blade to fragment.

2

Cause of low frequency vibration

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 large and plays a dominant role) , creating equal and opposite forces and reaction. During vibration, when the workpiece moves away from the tool, the cutting force F moves away from the workpiece in the same direction, and the work done is positive, as shown in Fig. 1(a) . As the workpiece approaches the tool, the work done by the cutting force F approaches a negative value, as shown in Fig. 1(b) .

During the turning process: 1 the friction between the chip and the tool front face, 2 the different degree of metal hardening encountered by the tool when cutting into and withdrawing from the workpiece, 3 the actual geometric angle of the tool is changed periodically during the vibration process, 4 when the tool vibrates, the relative motion locus of the tool to the workpiece is ellipse, which causes the periodic change of the cutting section, and the trace left by the vibration of the 5 workpiece in the previous rotation, which causes the periodic change of the cutting section. All these five conditions can cause the periodic change of cutting force and make the phase of f closer to that of F. Thus, in each vibration cycle, the positive work done by the cutting force on the workpiece (or tool) is always greater than the negative work done by the cutting force on the workpiece (or tool) , so that the workpiece (or tool) gets energy supplement and produces self-excited vibration.

3

Measures to eliminate low frequency vibration

(1) in the low frequency vibration, it is mainly due to the variation of cutting force caused by the vibration in the y direction, which makes the F phase closer to f than F and produces vibration.

Take the following four main measures.

1 The larger the main Deflection Angle (r angle) of the tool, the smaller the FY force, and the less vibration is produced. Therefore, properly increasing the main deflection angle of the cutter can eliminate or reduce the vibration.

2 properly increasing the rake angle of the cutting tool can reduce the FY force and reduce the vibration.

3 The back angle of the cutting tool is too large or the cutting edge is too sharp, the cutting tool is easy to gnaw into the workpiece, easy to produce vibration. When the tool is properly passivated, the back surface of the tool can prevent the tool from "GNAWING" the workpiece, which can reduce or eliminate the vibration.

4 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, the actual rake angle of the tool tip will decrease and the back angle will increase, which is easy to produce vibration.

When the tool rest system has negative stiffness, it is easy to "GNAW" the workpiece to produce vibration. Therefore, as far as possible to avoid the negative stiffness of the tool rest system on the vibration of turning.

(2) when wide and thin chips are produced in the turning process, the vibration in y direction will cause the change of cutting force. When the cutting section is wide and thin, the vibration in y direction will cause the drastic change of cutting section area and cutting force. Therefore, it is very easy to produce vibration in this case. For example: In the longitudinal tool turning, the deeper the cutting, the greater the feed, the smaller the main deflection angle, the wider and thinner the cutting cross-section, the more prone to vibration. Therefore, when selecting the turning speed, we should avoid the middle speed zone where the cutting force decreases with the cutting speed (when cutting carbon steel, the speed range is 30 ~ 50m/min) , and reduce the turning back force, proper increase of feed rate and decrease of cutting depth are also helpful to restrain vibration.

(3) the stiffness deficiency of workpiece system and tool rest system is the main reason of low frequency vibration. The following measures can be taken to eliminate or reduce the vibration:

When the workpiece is clamped with three or four jaws, the coaxiality error between the rotating center of the workpiece and the rotating center of the spindle is minimized, avoid workpiece tilt and intermittent cutting or non-uniform cutting caused by periodic changes in the cutting force caused by vibration.

2 When machining thin and long workpieces, which are easy to be deformed and bend, the thermal expansion deformation of workpieces can be reduced by using elastic tip and auxiliary support and cooling liquid.

Do not extend the workpiece too long when clamping the workpiece. For the workpieces with insufficient rigidity, the reasonable auxiliary supports such as center frame, tool carrier and top point are adopted to increase the rigidity of the workpieces.

4 when using the top, the top and the top cone hole should cooperate well, avoid the top force is too big to cause the work piece to bend or the top force is too small to play the supporting role to make the work piece swing, and pay attention to the tailstock sleeve overhang can not be too long.

The bearing clearance of machine tool spindle directly affects the rotating accuracy and rigidity of spindle. If it is found that the clearance is too big and the rigidity is not enough because of bearing wear, the bearing clearance should be adjusted and the pre-tightening force should be applied, to increase the stiffness of the workpiece system to eliminate vibration.

6 check regularly the contact of dovetail guide between Middle Carriage and large carriage, between small carriage and middle carriage, and adjust the inclined inlay bar to keep proper clearance to avoid creeping when the carriage moves, causing vibration of the cutter carriage system.

The square tool rest should be pressed tightly and fixed each time the square tool rest is rotated to the desired position to avoid the square tool rest loosening and reduce the stiffness of the tool rest system to produce vibration.