- Detail

Analysis and prevention of grinding wheel vibration grinding with grinding wheel is a common method for manufacturing mechanical parts, and it is also a complex process of force and vibration. Vibration is an inevitable and harmful phenomenon in the grinding process. It can reduce the accuracy and surface quality of the workpiece, and in serious cases can lead to the fracture of the grinding wheel and the damage of the processing system. Therefore, reducing and preventing vibration is an important measure to improve the grinding quality. The main reasons for vibration are:

1) when grinding parts, each abrasive grain on the working surface of the grinding wheel is equivalent to a micro blade with a negative rake angle, However, the irregular shape of each abrasive grain leads to the change of grinding force

2) the eccentric, unbalanced, high-speed rotation of the grinding wheel and the elastic deformation of the system cause the vibration of the grinding wheel and the processing system

3) internal vibration of the grinding system (such as the vibration of the power part, the vibration of the transmission part and the vibration of the support part) and external vibration (the vibration caused by the external vibration source transmitted to the grinding system)

4) vibration of the ground part

1 vibration analysis

the innovative workshop of the U.S. Department of energy of the grinding wheel: the critical materials Institute recently invented a new chemical process. Imbalance is the main cause of forced vibration and is relatively easy to detect. Therefore, the following mainly analyzes the forced vibration caused by the imbalance of the grinding wheel. As shown in Figure 1, the rigidity of the machined part (part) is very large. Obviously, the vibration of the grinding wheel in the normal direction (x-axis) of the ground surface has the greatest impact on the accuracy and surface quality of the workpiece. Therefore, the vibration in the x-axis is mainly studied

Figure 1 grinding vibration model

Figure 2 unbalance of grinding wheel

as shown in Figure 2, the grinding wheel is unbalanced due to asymmetry. If the unbalanced part is concentrated at one point, its mass is m, and the rotation radius of M is r, the centrifugal force generated is

f0=mr ω 2=mr (2 π n/60) 2 (1)

where: ω Is the angular velocity of the grinding wheel (rad/s); N is the rotational speed of the grinding wheel (r/min)

the projection of cardiac force F on the x-axis is fx=sin（ ω t+ β)，β Is the initial phase angle. Let the static stiffness of the system be K, and the motion equation of the vibration system be (2)

the equation is solved to obtain

where: wn is the natural frequency of the system, wn= (k/m), a=c/(2m), and C is the damping coefficient; wd=(w2-a2) ； Damping ratio z=a/wn; Frequency ratio l=w/wn; q=arctan 2xl/(1-l2) ； V0 is the initial velocity of particle m; X0 is the projection of the initial position of particle m on the x-axis

static deformation d= F0 = F0 K Mw2 (4) steady state vibration response h= d [(1-l2) 2+ (2XL) 2] (5)

steady state vibration response h shows the inherent vibration characteristics of the grinding system. Reducing the steady state vibration response is a very effective measure to reduce vibration

2 vibration prevention measures

1) improve the dynamic characteristics of the grinding system

a. improve the rigidity of the grinding system

it can be seen from equations (4) and (5) that increasing the rigidity K of the grinding system can significantly reduce the steady-state vibration response H. Due to the limitation of spatial position and geometric dimension, the extension rod of internal grinder is generally thin and long (as shown in Figure 3), which reduces the stiffness of the extension rod and is the main cause of vibration. Grind the inner hole of a part on the M2110A internal grinder. The part is quenched steel, and the hole diameter is 50.4mm. The ceramic bonded parallel grinding wheel with internal and external diameters of 6mm and 30mm respectively is selected. The grinding wheel speed is 14400r/min, and the workpiece speed is 20or/min. Figure 4 shows the relationship between the extension length of the connecting rod l of the internal grinder and the steady-state vibration response h of the grinding wheel. Obviously, the larger L, the smaller the stiffness K of the grinding system, and the larger the steady-state vibration response h of the grinding wheel

Fig. 3 Diagram of internal grinding connecting rod

Fig. 4 Relationship between the extension length L of the connecting rod and the steady-state vibration response h of the grinding wheel

b. increasing the damping of the grinding system and avoiding the resonance zone

it can be seen from equation (5) that increasing the damping of the grinding system can significantly reduce the steady-state vibration response H. Most of the damping on the grinder is generated on the guide rail surface and the connecting surface. The value should not be greater than 0.5mm on the guide rail surface to establish an oil film. The main shaft is supported by sliding bearings, which can significantly improve the damping of the grinding system

when the vibration frequency ω Close to the natural frequency of the system ω 0, λ ≈ 1, the system resonates and the amplitude of the grinding wheel increases sharply, which should be avoided as far as possible

2) when cylindrical roller parts with a diameter of 20mm are ground on m1040 centerless grinder with a vibration damping device

the surface produces prismatic ripples. It is found that when the vibration frequency is 160 ~ 18ohz, the amplitude of the grinding head is the largest and resonance occurs. During the excitation test on the grinding head, it is found that the natural frequency of the system is 17lhz. In order to reduce the natural frequency of the system, a damping damper (as shown in Figure 5) is installed at the outer end of the grinding wheel spindle of centerless grinder. The shell of the damper is rigidly connected with the grinding wheel spindle, the additional mass is sliding on the shaft of the damper, and the hydraulic oil with certain damping is filled between the shell and the additional mass. After installing the shock absorber, it is found that the natural frequency of the system is reduced to 105Hz, and the stability of the system is significantly improved, The quality of parts is also high (2) the installation foundation is not firm or the foundation screws are loose after 10 years of research and development; in order to improve (as shown in Figure 6) 。

Fig. 5 vibration damper is installed on the grinding wheel

Fig. 6 amplitude frequency characteristic curve before and after the vibration damper is installed

3) timely clean the grinding surface of the grinding wheel

when grinding a step long axis, the grinding effect is good at the beginning, and strong vibration occurs after a period of time. After careful inspection, it is found that the surface of the grinding wheel is blocked, and the vibration is significantly reduced after finishing with a diamond pen (as shown in Fig. 7)

Figure 7 amplitude of grinding wheel before and after dressing

in a word, vibration is a common phenomenon in grinding, and it is also a complex physical process, which seriously affects the quality of parts. Especially with the development of technology, grinding is developing towards high-speed and high-quality, and anti vibration is more important in grinding. (end)

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