Brief introduction of the hottest machining vibrat

Introduction to machining vibration

I. The Influence of vibration on the machining process. In the machining process, the process system often vibrates, that is, between the workpiece and the cutting edge of the tool, in addition to the nominal cutting motion, there will also be a periodic relative motion. When vibration occurs, the normal cutting process of the process system will be disturbed and destroyed, resulting in vibration marks on the machined surface of the parts, which reduces the machining accuracy and surface quality of the parts. Waviness will occur when the frequency is low, and micro unevenness will occur when the frequency is high. Strong vibration will make the cutting process impossible, and even cause the tool to "collapse". For this reason, the cutting parameters are often forced to be reduced, resulting in the insufficient performance of machine tools and cutting tools, which limits the improvement of productivity. Vibration also affects the durability of cutting tools and the service life of machine tools, makes noise, worsens the working environment, and affects the health of workers

there are three kinds of vibration according to their causes: free vibration, forced vibration and self-excited vibration. According to statistics, forced vibration accounts for about 30%, self-excited vibration accounts for about 65%, and free vibration accounts for a small proportion. Free vibration is often caused by the sudden change of cutting force or the impact of other external forces. This kind of vibration can generally decay rapidly, so it has little impact on the machining process. Forced vibration and self-excited vibration are not natural attenuation and harmful vibration. The following is a simple analysis of these two forms of vibration

II. Forced vibration in machining

(I) causes of forced vibration

forced vibration work in machining is a vibration caused by periodic alternating exciting forces (i.e. vibration sources) inside or outside the process system. The technical personnel of our company will introduce to you the purchase and measurement precautions of pressure testing machine. The exciting force of forced vibration of the system mainly comes from the following aspects

1. The high-speed rotating parts on the machine tool are unbalanced. There are many high-speed rotating parts on the machine tool, such as motor rotor, belt wheel, spindle, chuck and workpiece, grinding wheel of the grinder, etc, Excitation force F (i.e. centrifugal inertia force) is generated due to imbalance

2. errors in the transmission system of machine tools the gears in the transmission system of machine tools produce periodic exciting forces due to manufacturing and assembly errors. In addition, the forced vibration of the process system may be caused by various factors, such as belt joints, poor size of bearing rolling elements, and oil pulsation in hydraulic transmission

3. The non-uniformity of the cutting process itself. The intermittent characteristics of the cutting process, such as milling, broaching and turning intermittent surfaces with keyways, cause periodic changes in the cutting force due to intermittent cutting, thus arousing vibration

4. The external vibration source is transmitted from the strong vibration of adjacent equipment (such as stamping equipment, gantry planer, etc.) through the foundation, causing the process system to produce forced vibration of the same (or integral multiple) frequency

(II) ways to reduce forced vibration

forced vibration is caused by external periodic interference forces. Therefore, in order to eliminate forced vibration, we should first find out the vibration source, and then take appropriate measures to control it

1. reduce or eliminate the exciting force of the vibration source

the parts with a speed of more than 600r/min must be balanced, especially the parts with high-speed rotation, such as the grinding wheel, are easy to cause the vibration of the spindle due to its uneven distribution of sand particles and uneven surface wear during operation. Therefore, the new grinding wheel must be balanced twice before and after trimming

improve the manufacturing accuracy and assembly accuracy of gears, especially the working stability accuracy of gears, so as to reduce the vibration caused by periodic impact and reduce noise. These materials are used in the fields of railway and Aerospace; Improve the manufacturing and assembly accuracy of rolling bearings to reduce the vibration caused by the defects of rolling bearings; Choose the transmission belt with the same length and uniform thickness

2. Avoid the frequency of excitation force close to the natural frequency of the system to prevent resonance

if the speed of the motor is changed or the speed of the main shaft is changed to avoid the resonance zone; Improve the accuracy of the contact surface and reduce the roughness of the joint surface; Eliminate the gap and improve the contact stiffness to improve the stiffness and natural frequency of the system

3. Adopt vibration isolation measures

such as flexible connection between the motor of the machine tool and the bed to isolate the vibration of the motor itself; Separate the hydraulic part from the machine tool; Adopt hydraulic buffer device to reduce the impact of component reversing; Use thick rubber and wood to isolate the machine tool from the foundation, and use anti vibration ditch to separate the connection between the foundation of the equipment and the ground, so as to prevent the surrounding vibration source from being transmitted to the machine tool through the ground and foundation

III. self excited vibration in machining

when the system is triggered by some accidental instantaneous interference force from the outside or itself, the cutting force changes periodically for some reason in the vibration process itself, and the periodically changing dynamic force in turn strengthens and maintains the vibration, making the vibration system supplement the energy consumed by the damping effect. This type of vibration is called self-excited vibration. The self-excited vibration produced in the cutting process is a strong vibration with high frequency, which is usually called chatter

(I) characteristics of self-excited vibration

1. Self excited vibration is a kind of vibration that does not decay. The vibration process itself can cause a periodically changing force, which can periodically obtain energy supplement from non alternating energy to maintain this vibration

2. The self-excited vibration frequency is equal to or close to the natural frequency of the system, that is, it is determined by the parameters of the system itself

3. The amplitude of self-excited vibration depends on the ratio of energy obtained by the system to energy consumed in each vibration cycle. When the energy obtained is greater than the energy consumed, the amplitude will continue to increase until the two energies are equal. Conversely, the amplitude will continue to decrease. When the energy obtained is less than the energy consumed, the self-excited vibration also disappears

so far, there is no fully mature theory to explain the causes of self-excited vibration under various conditions. At present, the way to overcome and eliminate the self-excited vibration in machining is still through various experiments, in terms of equipment, tools and practical operation

(II) ways to control self-excited vibration

1. Reasonable selection of cutting parameters

figure 5-5 shows the relationship curve between cutting speed and amplitude A during turning. In the range of 20 ~ 60m/min, a increases rapidly, and when it is higher or lower than this range, the vibration gradually weakens

figure 5-6 shows the relationship curve between feed rate f and amplitude A. when f is smaller, a is larger, and a decreases with the increase of F

figure 5-7 shows the relationship curve between the back cutting amount and the amplitude A, the larger the A is, the larger the A is

bearing clearance, apply a certain preload to the rolling bearing, and improve the grinding quality of the top hole, etc. When machining slender shafts, good vibration reduction effects can be achieved by using center frame or tool heel, shortening the overhang of boring bar and tool as far as possible, replacing live center with dead center, and using elastic tool bar

4. use vibration damping device. When the above measures still fail to achieve the purpose of vibration elimination, the use of vibration damping device can be considered. Vibration damping devices are usually attached to the process system to absorb or consume the energy during vibration, so as to achieve the purpose of vibration damping. It is also effective in suppressing forced vibration and flutter. It is an important way to improve the vibration resistance of the process system, but it can not improve the stiffness of the process system. There are mainly two types of shock absorber and damper

(1) principle and application of damper damper is to use solid or liquid damping to consume vibration energy and realize vibration reduction. Figure 5-9 shows a dry friction damper that uses multi-layer spring plates to rub against each other to eliminate vibration energy. The damping effect of the damper is related to its speed and stroke. The faster the motion is and the longer the stroke is, the better the damping effect covers all links from concept stage, design, simulation, processing to component testing. Therefore, the damper should be installed at the place where the relative motion of the vibrating body is the largest

(2) the principle and application of vibration absorbers. Vibration absorbers are divided into dynamic vibration absorbers and impact vibration absorbers

① dynamic vibration absorber uses elastic elements to connect an additional mass block to the system, and uses the dynamic action of the additional mass to counteract the force added by the elastic elements to the system and the exciting force of the system, so as to weaken the vibration

figure 5-10 shows the dynamic vibration absorber for boring bar. This kind of vibration absorber uses microporous rubber liner as elastic element and has additional damping effect, so it can get better vibration elimination effect

② impact vibration absorber is composed of a shell rigidly connected to the vibration system and a mass that can impact freely in the shell. When the system vibrates, the shell is impacted by the reciprocating motion of the free mass, which consumes the vibration energy, so the vibration can be reduced

figure 5-11 shows the bolt shock absorber. When the tool vibrates, the free mass 1 also vibrates, but because the free mass is elastically connected with the tool, the vibration phase difference is 180. When the tool flexes downward, the free mass moves upward against the force of spring 2. At this time, a gap is formed between the free mass and the cutter bar. When the tool moves upward, the free mass moves downward at a certain speed, causing impact and consuming energy

5. Reasonably adjust the stiffness ratio of vibration mode. According to the vibration mode coupling principle, the vibration of the process system is also affected by the stiffness ratio of each vibration mode and its combination. By reasonably adjusting the relationship between them, the vibration resistance of the system can be effectively improved and the self-excited vibration can be suppressed

figure 5-12a shows the flat boring bar, and the cutter head 2 is fixed on any angle position of the boring bar with screws 3. The thickness of the flattened part of boring bar 1 a= (0.6 ~ 0.8) d, where D is the diameter of the boring bar. After the boring bar is flattened, the two mutually perpendicular main mode shapes have different stiffness K1 and K2, and then the azimuth angle with high stability can be found through the transposition adjustment of the cutter head 2 on the boring bar α （ α Is the included angle between the normal direction of the machining surface and the vertical line of the boring bar)

take boring bar a =0.8d, and the overhang length of boring bar is 550mm. It can be seen from figure 5-12b that there was no self-excited vibration at that time. The "8" shaped area shown in figure 5-12c shows that the most suitable azimuth angle is between

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