One of the common faults in turning is vibration. When the lathe vibrates, the normal cutting process of the process system is disturbed and destroyed, which not only seriously worsens the machining surface quality, but also shortens the service life of the machine tool and cutting tool. Therefore, it is necessary to take some measures to reduce or eliminate the vibration produced by the machine tool.
Today, I will mainly introduce the causes and elimination measures of low-frequency vibration caused by the deformation of workpiece system and tool rest system in the machining process.
01 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 self-excited vibration that does not change with the turning speed.
The main characteristics of low-frequency vibration are:
- low vibration frequency (50 ~ 300Hz), low noise during vibration
- The traces left on the cutting surface of the workpiece are deep and wide
- The vibration is violent, which often makes machine tool parts (such as tailstock, tool rest, etc.) loose and breaks the cemented carbide blade.
02 Causes Of Low Frequency Vibration
During the low-frequency vibration in turning, the workpiece system and the tool rest system are usually vibrating (but in most cases, the vibration of the workpiece system is large and plays a leading role). They sometimes separate from each other and sometimes approach each other, producing forces and reactions of equal magnitude and opposite directions. In the process of vibration, when the workpiece and the tool move apart, the cutting force F is separated 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 approaching cutting force F is negative.
During turning:
- friction between chips and the rake face of the tool
- The hardening degree of metal encountered by the tool when cutting in and exiting the workpiece is different
- The actual geometric angle of the tool changes periodically during the vibration process
- During vibration, the relative motion path of the tool to the workpiece is ellipse, which causes the periodic change of the cutting section
- The marks left by the vibration of the workpiece during the previous rotation cause periodic changes in the cutting section. These five conditions can cause periodic changes in cutting force and make f phase distance >f approach. In this way, in each vibration cycle, the positive work of 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) obtains energy supplement and generates self-excited vibration.
03 Elimination Measures Of Low Frequency Vibration
(1) In low-frequency vibration, the vibration is mainly caused by the change of cutting force caused by the vibration in the Y direction, which makes the f phase distance >f approach and produces vibration.
The following 5 measures are mainly taken.
① Tool principal deflection angle（ μ The larger the R angle), the smaller the FY force, and the less likely it is to produce vibration. Therefore, properly increase the main deflection angle of the tool to eliminate or reduce the vibration.
② Properly increasing the rake angle of the tool can reduce FY force and reduce vibration.
③ If the back angle of the tool is too large or the blade is too sharp, the tool is easy to bite into the workpiece, which is easy to produce vibration. When the tool is properly passivated, the rear cutter face can prevent the tool from “biting” into the workpiece, which can reduce or eliminate the vibration.
④ When turning, the position of the tool tip is too low (lower than the center of the workpiece) or when boring on the lathe, the position of the tool tip is too high, which will reduce the actual rake angle of the tool tip and increase the rear angle, which is easy to produce vibration.
⑤ If the tool rest system has negative stiffness, it is easy to “bite” the workpiece and produce vibration. Therefore, the vibration caused by the negative stiffness of the tool rest system on turning should be avoided as far as possible.
(2) When wide and thin chips are produced in the turning process, the vibration in Y direction causes 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 cross-section and cutting force. Therefore, vibration is easy to occur in this case. For example, when turning with a longitudinal cutter, the greater the cutting depth, 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 produce vibration. Therefore, when selecting the turning speed, we 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 reduce the cutting force of the trolley at the same time, appropriately increasing the feed rate and reducing the cutting depth also help to suppress vibration.
(3) Insufficient rigidity of workpiece system and tool rest system is the main reason for low-frequency vibration. The following measures can be taken to eliminate or reduce vibration:
① When clamping the workpiece with three 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 intermittent cutting or uneven cutting due to workpiece inclination.
② When machining thin and long workpieces that are easy to deform, bend and vibrate, the elastic center and auxiliary support are used, and coolant is added to cool the workpieces to reduce the thermal expansion deformation of the workpieces.
③ When clamping the workpiece, do not extend the workpiece too long. For the workpiece with insufficient rigidity, the reasonable center frame, tool heel rest and center and other auxiliary supports are used to increase the rigidity of the workpiece.
④ When using the top, the top and the taper hole of the top should match well to avoid bending the workpiece due to too large jacking force or swinging the workpiece due to too small jacking force, and pay attention to that the suspension of the tailstock sleeve should not be too long.
⑤ The bearing clearance of the machine tool spindle directly affects the rotation accuracy and stiffness of the spindle. If the clearance is too large and the stiffness is insufficient due to bearing wear during use, the bearing clearance should be adjusted and the preload should be applied to increase the stiffness of the workpiece system and eliminate vibration.
⑥ Regularly check the contact between the middle carriage and the big carriage, and the dovetail guide rail between the small knife rest and the middle carriage, and adjust the inclined inlay to maintain an appropriate gap to avoid crawling when the knife rest moves, causing the vibration of the knife rest system.
⑦ Each time the square tool holder is rotated to make the tool turn to the required position, the square tool holder should be pressed and fixed to prevent the square tool holder from loosening, reducing the stiffness of the tool holder system and causing vibration.