Views: 2 Author: Site Editor Publish Time: 2022-04-08 Origin: Site
Vibration is one of the common faults in turning. When the lathe produces vibration, the normal cutting process of the process system is disturbed and destroyed, which not only seriously deteriorates the machining surface quality, but also shortens the service life of the machine and the tool. Therefore, it is necessary to take some measures to reduce or eliminate the vibration of machine tools.
After eliminating the vibration of rotary assembly and transmission system, the main type of turning vibration is self-excited vibration which does not change with the turning speed. Today we mainly introduce the low frequency vibration caused by workpiece system deformation and tool rest system deformation in the process of machining and its elimination measures.
The main characteristics of low-frequency vibration are as follows:
(1) the vibration frequency is low (50 ~ 300Hz), the noise emitted 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 make the machine tool parts (such as tailstock, tool rest, etc.) loose and make the carbide blade broken.
When the low frequency vibration in turning, usually the workpiece system and tool rest system are vibrating (but in most cases, the vibration of the workpiece system is larger and plays a leading role), sometimes they are separated from each other, sometimes they approach, and produce equal and opposite force and reaction force. In the process of vibration, when the workpiece moves in phase separation with the tool, the phase separation of the cutting force F is in the same direction as the workpiece displacement, and the work done is positive. As the workpiece approaches the tool, the work done by the cutting force F approaches is negative.
In the turning process:
(1) The chip and tool rake face friction cutting tool in cutting and exit artifacts at different levels of metal hardening,
(2) In the process of vibration cutting tool geometry angles cyclical change,
(3) Vibration, the workpiece relative motion trajectory is an ellipse tool, thus cause cyclical change,
(4) The workpiece cutting section in the previous turn traces left by the vibration, the cutting section changes periodically.
These four conditions can cause periodic changes in cutting force and cause F phase separation. F approach. In this way, in each vibration cycle, the positive work done by cutting force on the workpiece (or tool) is always greater than the negative work done by it on the workpiece (or tool), so that the workpiece (or tool) obtained energy supplement to produce self-excited vibration.
(1) In the low-frequency vibration, mainly due to the Y direction of the vibration caused by the cutting force changes, so that F phase separation > F approaches and vibrates.
l The following four measures are mainly adopted.
The larger the main deflection Angle (μ R Angle), the smaller the Fy force, the less likely to produce vibration. Therefore, the vibration can be eliminated or reduced by properly increasing the main deflection Angle of the tool.
Appropriately increasing the rake Angle can reduce the Fy force and weaken the vibration.
The tool Angle is too large or the blade is too sharp, the tool is easy to bite into the workpiece, easy to produce vibration. When the tool is properly passivated, the tool surface can prevent the tool from "gnawing" into the workpiece, which can reduce or eliminate the vibration.
When turning tip position is too low (lower than the workpiece center) or when boring on the lathe tip position is too high, will make the actual front Angle of the tip decreases and the back Angle increases, easy to produce vibration.
If the tool rest system has negative stiffness, it is easy to "chew" into the workpiece to produce vibration. Therefore, the vibration caused by the negative stiffness of tool rest system on turning can be avoided as much as possible.
(2) When wide and thin chips are produced in the turning process, the vibration in the Y direction causes the change of the cutting force. When the cutting section is wide and thin, the vibration in the Y direction will cause the drastic change of the cutting cross-sectional area and the cutting force. Therefore, it is very easy to produce vibration in this situation. For example: in longitudinal knife turning, the greater the cutting depth, the larger the feed, the smaller the main Angle, the wider the cutting section, the thinner, the more likely to produce vibration. Therefore, the selection of turning speed should avoid the middle speed zone 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, appropriately increasing the feed and reducing the cutting depth can also help to suppress vibration.
(3) Insufficient stiffness of workpiece system and tool rest system is the main cause of low-frequency vibration. The following measures can be taken to eliminate or reduce vibration:
With three or four claw clamping workpiece as far as possible to minimize the coaxiality error of workpiece rotation center and spindle rotation center, avoid workpiece tilt and intermittent cutting or uneven cutting caused by periodic changes in the vibration of cutting force.
Thin and long and easy to deformation bending vibration of the workpiece, the use of elastic center and auxiliary support and cooling liquid cooling can reduce the thermal expansion deformation of the workpiece.
When clamping the workpiece, do not extend the workpiece too long. For the workpiece with insufficient stiffness, reasonable center frame, heel rest and top auxiliary support are used to increase the stiffness of the workpiece.
When using the top, the top and the top cone hole should be well matched, to avoid the workpiece bending force is too large or the top force is too small to support the workpiece swing, and pay attention to the tail seat sleeve suspension can not be too long.
Machine tool spindle bearing clearance directly affects the spindle rotation accuracy and stiffness, such as found in the use of bearing wear clearance is too large and insufficient stiffness, should adjust the bearing clearance and apply preload, to increase the stiffness of the workpiece system to eliminate vibration.
Regularly check the dovetail rail contact between the middle pallet and the big pallet, the small tool rest and the middle pallet, adjust the inclined insert to keep the appropriate gap to avoid the creeping of the tool rest when moving, causing the vibration of the tool rest system.
Every time the rotation of the tool rest to the required position, should be pressed and fixed, to avoid the loosening of the tool rest to reduce the stiffness of the tool rest system vibration.