1 Performance and characteristics of mechanical processing vibration
Vibration is divided into two types: forced vibration and self-excited vibration. The specific performance and characteristics are as follows.
1.1 Forced vibration Forced vibration is the vibration produced by an object subjected to a periodic changing external force. For example, during the grinding process, due to the imbalance of the motor, high-speed rotating grinding wheel and pulley, the thickness or length of the V-belt is inconsistent, the oil pump is not working smoothly, etc., it will cause forced vibration of the machine tool, which will stimulate the interaction between various parts of the machine tool. The relative vibration amplitude affects the accuracy of machine tool processing of workpieces, such as roughness and roundness. For cutting tools or machine tools that perform rotary motion, vibration will also affect the rotation accuracy.
The characteristics of forced vibration are: ① The forced vibration itself cannot change the interference force, and the interference force generally has nothing to do with the cutting process (except for the forced vibration caused by the cutting process itself). The interfering force is eliminated and the vibration stops. For example, if the interference force generated by external vibration sources is eliminated, the interference force causing vibration will naturally disappear. ②The frequency of forced vibration is the same as the frequency of external periodic interference force, or an integral multiple of it. ③When the ratio of the frequency of the interference force to the natural frequency of the system is or is close to 1, extreme vibration occurs and the amplitude reaches the maximum value. At this time, it has the greatest impact on the machine tool processing process. ④The amplitude of forced vibration is related to the interference force, the stiffness and damping of the system. The greater the interference force, the smaller the stiffness and damping, the greater the amplitude and the greater the impact on the machining process of the machine tool.
1.2 Self-excited vibration (chatter) The non-attenuating vibration caused by the alternating force generated by the vibration system itself during the vibration process is self-excited vibration. Vibrations will occur even without any external periodic interference forces. For example, the friction caused by the grinding wheel against the workpiece during the grinding process will cause self-excited vibration. Poor rigidity of the workpiece and machine tool system, or improper selection of grinding wheel characteristics will increase friction and intensify self-excited vibration. Or vibration caused by poor tool rigidity or incorrect tool geometry angles are all self-excited vibrations.
The characteristics of self-excited vibration are: ①The frequency of self-excited vibration is or is close to the natural frequency of the system. According to the frequency, it can be divided into high-frequency chatter (generally frequency is 500-5000Hz) and low-frequency chatter (generally frequency is 50-500Hz). ② Whether self-excited vibration can be generated and its amplitude depends on the comparison between the energy obtained by the system and the energy consumed by damping in each vibration. ③Since the interference force of continuous self-excited vibration is excited by the vibration process itself, when the vibration stops, the interference force and energy replenishment process disappear immediately.
2 Analysis of the causes of vibration
The causes of vibration are complex and changeable. Based on the vibration phenomena in the machining industry and the manifestations of two different types of vibrations, the reasons are analyzed and roughly As follows:
2.1 Causes of forced vibration: ① Periodically changing centrifugal force caused by unbalanced rotating parts on machine tools. For example, it is caused by unbalanced circulation of the motor, chuck, or belt. ② Periodically changing transmission force caused by defects in machine tool transmission parts. Such as periodic vibration caused by loose machine tool components such as tool holders, spindle bearings, and carriage plugs, or manufacturing errors in transmission parts such as gears and bearings. ③ Periodically changing cutting forces caused by the unevenness of the cutting process itself. Such as turning polygonal or uneven surface workpieces and processing irregular-shaped blank workpieces on lathes. ④Inertial impact caused when the direction of movement of the reciprocating parts changes. For example, the direction of the surface grinding process changes or the rotation direction of the machine tool changes instantly. ⑤Interference force from other external vibration sources. Around the casting workshop, the vibration of the air hammer causes forced vibration and even direct vibration of other machine tools.
2.2 Causes of self-excited vibration: ① During the cutting process, the change in friction between the chip and the tool, and the tool and the workpiece. ②The hardness inside the cutting layer metal is uneven. The uneven hardness phenomenon that occurs when turning the outer circle or end face after repair welding often causes tool collapse and self-vibration of the lathe. ③The installation rigidity of the tool is poor. If the tool bar is too small or extends too long, it will cause the tool bar to tremble. ④The workpiece has poor rigidity. If the workpiece with poor rigidity such as a slender shaft is processed, corrugation or taper will appear on the surface of the workpiece.
⑤Built-up edge appears and disappears from time to time, and the rake angle of the tool and the cross-sectional area of ??the cutting layer change from time to time during the cutting process. ⑥ Vibration caused by inappropriate cutting amount. Wide and thin cutting is prone to vibration.
3 Methods to prevent and eliminate vibration
3.1 Measures to reduce forced vibration: ① Balance (static balance and dynamic balance) parts that rotate at high speed (above 600r/min) or Set up automatic balancing device. Or use vibration reduction devices. ②Adjust the clearance between bearings and inserts to change the natural frequency of the system so that it deviates from the excitation frequency; adjust the motion parameters so that the vibration source frequency that may cause forced vibration is far away from the natural frequency of the weak mode of machine tool processing. ③ Improve the stability of the transmission device. For example, if a belt with few joints or no joints is used on a lathe or grinder, the transmission belt should be chosen in a consistent manner. Use helical gears instead of spur gears, install flywheels on the main shaft, etc. ④ Use vane pumps instead of gear pumps on precision grinders, and use buffer devices in the hydraulic system to eliminate motion impact. ⑤Separate the power source of high-precision machine tools and the machine tool body on two foundations to achieve vibration isolation. Commonly used vibration isolation materials and vibration isolators include rubber vibration isolators, foam rubber, wool adhesive, etc. ⑥ Properly select the hardness, grain size and structure of the grinding wheel, and properly rest the grinding wheel to reduce grinding wheel clogging and reduce fluctuations in grinding force. ⑦ Select the milling cutter diameter, number of teeth and helix angle appropriately according to uniform milling conditions; increase the number of milling cutter teeth; use down milling instead of up milling; use an equidistant tooth structure to destroy the periodicity of the interference force. ⑨ Scrape and grind the contact surface to improve contact stiffness; use tool rests, steady rests, etc. to enhance the rigidity of the process system. Choose a better grinding wheel frame guide rail form ⑨ Use basic parts with bonded structure and bed with thin-walled sand sealing structure to increase damping and improve vibration resistance. ⑩ To isolate the influence of external vibration, take vibration isolation measures, such as placing an elastic wood board or hard rubber between the grinding wheel motor base and the backing plate.
3.2 Measures to reduce self-excited vibration: ①Adjust the position of the small-stiffness spindle of the vibration system so that it is outside the angle range between the cutting force F and the normal direction of the machined surface, such as when boring. When flattening the boring bar and turning the outer circle, the turning tool should be installed backwards. ② Reduce the overlap coefficient by changing the cutting amount and tool geometry, such as using a right-angle offset tool to turn the outer circle. ③ Reduce the cutting speed and increase the feed, main deflection angle and rake angle; ④ Increase the cutting speed appropriately; improve the machinability of the material being processed. ⑤ Increase cutting damping; appropriately reduce the clearance angle of the tool; grind a vibration-absorbing edge on the flank surface; appropriately increase the chisel edge of the drill bit; appropriately make the tool tip higher than (the outer circle of the car) and lower than (the inner hole of the car). ) centerline of the workpiece to obtain a small working relief angle. In order to reduce the high-frequency vibration of the tool, the relief angle and rake angle of the tool should be increased. ⑥Adjust the cutting speed to avoid critical cutting speed. When cutting off, facing or using wide-blade tools, forming tools and threading tools, the cutting speed should be lower than the critical cutting speed. When longitudinal turning and cutting the end face of an annular workpiece, the cutting speed is greater than the critical cutting speed, etc. ⑦Improve the stiffness of the process system and improve the vibration resistance. When installing the turning tool, it should not be extended too long. The boring tool should be as short and thick as possible; the extension length of the tailstock sleeve should be shortened as much as possible; when machining a slender shaft, use a center frame or a follower tool holder, or use a main declination angle. Very large slender shaft turning tool to eliminate vibration. ⑧ Try not to use a cutting speed that easily produces built-up edge. ⑨ Use appropriate cutting amount. The cutting width can be reduced while the cutting thickness can be increased.