Vibration control of ultra-precision machine tools Harbin Institute of Technology Wang Jiachun Dong Shen Li Dan axis carried out a more detailed theoretical analysis, and through experiments, the vibration of HI-type ultra-precision lathe is very small.
As we all know, in the machining of machine tools, the vibration is extremely harmful. Especially for ultra-precision machine tools, when using diamond tools for ultra-precision cutting, the machine tool is required to work extremely smoothly and the vibration is extremely small. Otherwise, it is difficult to ensure high machining accuracy and ultra-smooth Surface Quality. Therefore, the prevention and control of vibration has become a key technology to ensure the processing quality in ultra-precision machining.
Taking the H I type ultra-precision lathe developed by Harbin Institute of Technology as an example, the main part is briefly discussed.
The structure of the 1 H I ultra-precision lathe H 1 ultra-precision lathe structure is shown in Figure 1.
1. Absorbing material 2. Earth 3. Concrete foundation 4. Air spring 5. Machine bed 6. Air-floating X-direction slide 7. Tool holder 8. Aerostatic bearing spindle 9. Air-floating Z-direction slide whole machine The air spring 4 is placed on the concrete foundation 3, and the concrete foundation is placed in the vibration isolation groove through the layer vibration absorbing material 1. X, the slides 6, 9 are suspended above the air float rail, the aerostatic bearing spindle is placed on the slide 9 and the knife holder 7 is placed on the X slide.
2 H I type ultra-precision lathe anti-vibration 2. 1 separate foundation, vibration isolation groove In order to make the ground vibration not directly transmitted to the machine tool, the machine uses a separate concrete foundation, through the vibration isolation groove and the vibration isolation groove The material isolates the ground vibrations. The vibration isolation groove has a length of 3.5, a width of 3.5, and a depth of 2. 5. Adding a vibration absorbing material such as pebbles and sand to the groove. Table 1 shows the vibration comparison between the earth and the concrete foundation.
2. 2 Use of granite Since the granite has good vibration damping capacity and a small coefficient of thermal expansion compared to steel, the main components of the machine are made of granite.
Ground concrete foundation attenuation amplitude () X to Y to Z direction such as bed, housing, X-direction slide and Z-direction slide. The main shaft is made of dense jade. Table 2 shows the performance comparison of several machine tool structural materials.
Ceramic cast iron steel indium steel granite artificial granite Young's elastic modulus ratio stiffness vibration logarithmic decay rate thermal expansion coefficient thermal conductivity 2.3 air spring due to metal spring, rubber vibration isolation pad for low frequency vibration isolation ability, and the ground vibration Containing a fairly low frequency component, the machine uses an air spring to isolate the vibrations from the foundation. Figure 2 shows the air spring vibration isolation system. The air spring can use the mechanical model shown in Figure 3. In the figure: F is the spring load V is the air spring body volume V is the additional air chamber volume d is the air density R is the flow damping coefficient is the damper damping coefficient k is the spring stiffness N is the multi-deformation P is the air spring The pressure P is the atmospheric pressure A, and the effective area a of the air spring is the volume ratio.
1. Height control valve 2. Machine tool 3. Air spring 4. Damping hole 5. Auxiliary chamber When the vibration displacement is small, the influence of k is small, so only the vertical ground direction vibration is considered. A damped air spring vibration isolation system is simplified as shown in Figure 4.
Assuming that the mass of the whole machine is evenly distributed to 4 air springs, then = /4. The free vibration equation is the characteristic equation. The natural frequency f of the system is the vibrational signal x is transmitted to the ground. x sin (wt), then the relative transfer rate T of the mass is the curve in the equation. 5 Hz. The natural frequency of the vertical ground direction is 4. 1 Hz.
2. 4 Aerostatic bearing The spindle type ultra-precision lathe uses an aerostatic bearing spindle (Fig. 6). The main shaft of the aerostatic bearing, because the rigidity of the main shaft is much larger than the stiffness of the air film, the main shaft can be regarded as a rigid body. And because the bearings are rigidly mounted, it can be assumed that only finite stiffness and damping are present in the bearings.
1. Spindle 2. Static pressure gas bearing 3. Air intake hole 4. Air outlet According to the static pressure gas lubrication theory, it is assumed that the gas flows without inertia and isothermal layer, and the gas has no flow in the circumferential direction, and the viscosity coefficient is assumed to be a constant value. Then the equation of motion is the mass continuous equation for the state equation P / d = P to derive bearing bearing capacity W =). Where L is the bearing width D is the bearing diameter is the dimensionless bearing capacity coefficient is the inlet pressure P is the atmospheric pressure.
Let the bearing clearance be h and the eccentricity be e, then the eccentricity X = e / h. Also know that when X ≤ 0.5, XW approximates the linear relationship [2], so the bearing stiffness can be approximated as constant: K = W / (Xh).
The damping coefficient is given as the dimensionless damping force when the spindle speed W is X=0. Then the constant vibration grinding of the vibration camshaft of the main shaft is based on the same theory of Yanshan University.
In the camshaft profiling process, if the camshaft rotates at a constant speed, the grinding speed and acceleration of the cutting point change very much, and the maximum grinding speed differs from the minimum grinding speed by 16 times [1]. The change of the grinding speed is The main cause of cam profile error. The position of the maximum cam profile error, near the maximum grinding speed. The segmented shifting method is currently used to reduce the maximum grinding speed, thereby reducing the maximum error of the cam profile. In order to improve the machining accuracy of the cam, this paper gives the calculation method of constant speed grinding during camshaft profiling. A key technical theory is provided for the design of constant speed grinding of profiled camshaft grinding machines and retrofitting existing machine tools.
1 Basic principle of camshaft profiling In the cam profiling process, if the manufacturing error of the cam template is not considered, the motion of the cam template has no effect on the grinding speed of the cam to be machined. Under this premise, this paper performs motion analysis on constant speed grinding of camshaft profiling.
It is known from the literature [2] that the contouring of the camshaft is shown in Fig. 1. The camshaft 2 and cam template (omitted in Figure 1) are rigidly coupled to the motor shaft. They rotate around the pendulum 1 with a relative length of point O. The pendulum 1 swings around the O point relative to the frame 5. A grinding wheel of radius R that rotates around point O relative to frame 5. The distance from point O to point o is a. 3. When the camshaft rotates relative to the pendulum 1 at angular velocity k, the interaction between the cam template and the roller it contacts (omitted in Figure 1) causes the pendulum 1 to wrap around O. Point swing. At this time, the cam 2 is in contact with the grinding wheel 3 at the point K.
The equation is where the rotor mass is the damping constant of a rotor. k is the static stiffness of a bearing.
If the natural frequency is k, if the external disturbance power is p=Psinkt, the rotor vibration amplitude is smooth due to the low friction characteristics of the air bearing. If the bearing parameters are well designed, the rotor residual imbalance is extremely small and can be well When the external disturbance is removed, the vibration of the spindle is extremely small. 05 μ。 The amplitude of the spindle end of the H I type ultra-precision lathe is less than 0. 05 μ.
3 Conclusions (1) Due to the use of a separate foundation, the machine bed is made of granite material and is isolated by air springs, thus effectively eliminating ground vibration interference.
(2) Since the bearings are made of aerostatic bearings, the vibration of the main shaft is extremely small and the rotation accuracy is extremely high.
(3) The anti-vibration measures of the whole machine tool are reasonable and the vibration is extremely small.
2 Ding Weigang and other translations. Aerostatic bearing design. Beijing: National Defense Industry Press, 1978.
The first author: Wang Chun, Harbin Harbin Institute of Technology Precision (editor Jiang Xuewen)
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