One of the common failures in turning operations is vibration. When the lathe vibrates, the normal cutting process of the process system is disturbed and destroyed, which not only seriously deteriorates the quality of the machined 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 rotary components and transmission system of the machine tool, the main type of turning vibration is the self-excited vibration that does not change with the turning speed. Introduce the causes and elimination measures of the low-frequency vibration caused by the deformation of the workpiece system and the deformation of the tool holder system during the machining process.
The main characteristics of low frequency vibrations 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;
③The vibration is relatively severe, which often loosens the machine tool components (such as tailstock, tool holder, etc.) and breaks the cemented carbide blade.
Second, the cause of low frequency vibration
During low-frequency vibration 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). Opposite action and reaction forces. During the vibration process, when the workpiece and the tool move away from each other, the cutting force F is in the same direction as the displacement of the workpiece, and the work done is positive. When the workpiece approaches the tool, the work done by the cutting force F is negative.
During turning:
①Friction between chip and tool rake face;
② The degree of metal hardening encountered by the tool when cutting into and exiting the workpiece is different;
③ During the vibration process, the actual geometric angle of the tool changes periodically;
④ When vibrating, the relative motion trajectory of the tool to the workpiece is an ellipse, which causes periodic changes in the cutting section;
⑤The marks left by the vibration of the workpiece during the previous revolution caused periodic changes in the cutting section.
These five situations can cause periodic changes in the cutting force, and make the F phase move away from>F. In this way, in each vibration cycle, the positive work done by the cutting force on the workpiece (or tool) is always greater than the negative work it does on the workpiece (or tool), so that the workpiece (or tool) is supplemented by energy. self-excited vibration.
3. Measures to eliminate low frequency vibration
01
In the case of low-frequency vibration, it is mainly due to the change of cutting force caused by the vibration in the Y direction, which makes the F phase approach to > F and generate vibration.
Mainly take the following four measures.
①The larger the cutting edge angle (μr angle) is, the smaller the Fy force is, and the vibration is less likely to occur. Therefore, appropriately increase the cutting edge angle of the tool to eliminate or reduce vibration.
② Properly increasing the rake angle of the tool can reduce the Fy force, thereby reducing vibration.
③The back angle of the tool is too large or the blade is too sharp, the tool is easy to bite into the workpiece, and it is easy to generate 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 position of the tool tip is too low during turning (lower than the center of the workpiece) or the position of the tool tip is too high when boring on a lathe, which will reduce the actual rake angle of the tool tip and increase the rear angle, which is prone to vibration.
⑤If the tool holder system has negative stiffness, it is easy to "gnaw into" the workpiece and generate vibration. Therefore, the vibration caused by the negative stiffness of the tool holder system on turning should be avoided as much as possible.
02
When wide and thin chips are produced during turning, the vibration in the Y direction causes changes in the 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, it is extremely easy to generate vibration in this case. For example: in longitudinal tool turning, the greater the depth of cut, the greater the feed rate, and the smaller the main deflection angle, the wider and thinner the cutting section, and the easier it is to generate vibration. Therefore, when selecting the turning speed, you should avoid the medium-speed zone where the cutting force decreases with the speed (when cutting carbon steel, the speed range is 30-50m/min), and at the same time reduce the turning force, and appropriately increase the feed rate and Reducing the depth of cut also helps dampen vibration.
03
Insufficient rigidity 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 vibration:
① When clamping the workpiece with three claws or four claws, try to minimize the coaxiality error between the workpiece rotation center and the spindle rotation center, and avoid the vibration caused by the periodic change of cutting force caused by the workpiece tilt and intermittent cutting or uneven cutting .
② When processing thin and long workpieces that are easy to deform and bend to generate vibration, use elastic tops and auxiliary supports while cooling with coolant to reduce thermal expansion and deformation of the workpiece.
③When clamping the workpiece, do not make the workpiece protrude too long. For workpieces with insufficient rigidity, use reasonable auxiliary supports such as center frame, counter rest and top to increase the rigidity of the workpiece.
④ When using the tip, the tip and the tip taper hole should be well matched to avoid bending the workpiece due to too much jacking force or too little jacking force to support the workpiece, and pay attention to the overhang of the tailstock sleeve not being too long.
⑤ The 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 and the rigidity is insufficient due to bearing wear during use, the bearing clearance should be adjusted and the preload should be applied to increase the rigidity of the workpiece system to eliminate vibration.
⑥Regularly check the contact condition of the dovetail guide rail between the middle carriage and the large carriage, and between the small tool holder and the middle carriage, and adjust the inclined inserts to maintain an appropriate gap to avoid crawling when the tool holder moves, causing vibration of the tool holder system.
⑦ Every time the square tool holder is turned to turn the tool to the required position, the square tool holder should be pressed and fixed to avoid vibration caused by loosening of the square tool holder and reducing the rigidity of the tool holder system.
