Measurement and Data Processing of Rotational Precision of Ultra-precision Air Spindle
Ultra-precision air spindles (herein referred to as spindles) developed by us can be used as spindle systems for ultra-precision machining machines or high-precision measuring instruments. The spindle adopts advanced gas static pressure technology. The radial bearing and the bidirectional thrust bearing are working surfaces of static pressure air bearing and undergo strict dynamic balance, thus realizing non-contact high-precision rotation with no vibration and low. The characteristics of noise.
Due to the average effect of the gas film on the machining error of the shaft parts, the accuracy of rotation after assembly of the shaft system can be higher than that of the parts. The main technical index of the spindle - the spindle radial rotation accuracy requirements is less than 0.1μm. With such a high-precision index, even if it is processed, it is difficult to measure its accuracy, and this brings great difficulties to measurement. Through several years of exploration and research, the spindle radial rotation accuracy (0.07 μm) was finally measured using a two-way transposition method. Now introduce the measurement method to the reader.
1 Test
1.1 Test conditions <br> Ambient temperature: 20±1°C Change per hour: <0.1°C
Relative humidity: 30% to 60%
The fixed spindle temperature in the constant temperature room is not less than 24h;
The temperature difference between the spindle under test and the standard appliance is not more than 0.2 °C;
The spindle to be tested must be placed on the basis of vibration isolation, there is no severe vibration and impact around; the supply pressure is stable at 0.4±0.02MPaq
1.2 Testing Instruments and Apparatus
1 TESA inductance micrometer (Switzerland) (resolution 0.01μm),
2 Out of roundness of standard glass ball ≤ 0.05μm (UK roundness meter attachment)
1.3 Detection method The bidirectional measurement method is used. The measurement device is shown in Figure 1.
Figure 1 Measuring device
The standard glass ball is used as a measuring tool, and is fixed on the main shaft by means of adjustable eccentricity and capable of indexing tooling. Inductive micrometer probe is aligned with the ball in the vertical direction from the axis of rotation of the spindle and is perpendicular to the sphere. The adjustment ball is basically concentric with the axis of rotation of the shaft. The smaller the eccentricity, the better, generally about 0.5μm. In the first step, the probe starts measuring from the zero-degree direction and rotates the spindle evenly, with intervals of 10° from zero, θi=0°, 10°, and 20°-350°. Read the inductance micrometer reading, measure 3 to 5 circles, take the average value X1(θi) as the reading before the reversal, then the shaft is not moving, the ball and the probe each rotate 180° with respect to the shaft system, that is to say, In the opposite direction, the eccentricity of the ball will change, and 3 to 5 turns will be measured after the eccentricity is re-adjusted, and the average value is the reverse reading X2(θi). In the second step, measurement is started from the 90° direction, and the above measurement steps are repeated. The measurement process is shown in FIG. 2 . The Yi(θi) is read before the reversal, then the shaft is immobile and the ball and the probe are each rotated 180° relative to the shaft, ie from 90° to 270°. Y2(θi) is the reverse reading.
Figure 2 Measurement process
2 data processing
2.1 Reverse reading before and after reading minus 2
The resulting measurement data is an axis error that eliminates the effects of ball out-of-roundness.
2.2 Eliminating Constant Terms and Eccentricity
Residual:
A Fuli series coefficient:
The result of the calculation is expressed in terms of Fu Li's series:
The installation of the eccentricity of the standard ball causes a first harmonic component, and zero and first harmonics are removed to obtain the rotation error of each point of the spindle.
ΔVxi=Vxi-[axjcos(θi)+bxjsin(θi)]
ΔVyi=Vyi-[ayjcos(θi)+byjsin(θi)]
2.3 X, Y-axis error synthesis
ΔVimax is the maximum rotation error.
3 Measurement Accuracy Analysis
3.1 Bidirectional measurement method is used to retain the depolarization of the primary linear harmonic motion of the shaft system. <br> When the shaft system rotates, the response due to dynamic factors such as vibration or the structural factors of the shaft system, including the first harmonic component, is The components of the shafting error motion have an influence on the precision and performance of the shafting system and should therefore not be eliminated. To distinguish the first harmonics of different properties, it is necessary to measure the rotation of the shafting system by bidirectional measurement. The error, that is, the sensor is installed at an interval of 90°, is observed and discriminated from two mutually perpendicular components of X and Y, and it cannot be judged by using a one-way measurement method.
3.2 Using Average Method to Reduce the Influence of Occasional Error <br><br> When the shaft system rotates, it is impossible to have no error. To reduce the influence of accidental error, measure 3 to 5 cycles before or after the reverse direction, and then take the average.
3.3 Eliminating Installation Error Caused by Eccentricity of Measuring Tool Installation by Calculating Decentering Method <br> When measuring the accuracy of shafting rotation, even if the standard ball is adjusted to be very concentric with the center of rotation of the shafting system, there is inevitably a different degree of misalignment. , The misalignment of the misalignment is received by the displacement sensor and mixed into the measurement value. This is due to the adjustment, not the shafting error, which must be eliminated during the data processing. The principle of elimination is the installation of misalignment of the ball. Quantity is a harmonic. Therefore, the first harmonic component is eliminated by the calculation method, which eliminates the influence of different installation positions of the standard ball.
3.4 Application of Error Separation Technology, EST, Elimination of the Effect of Standard Ball Outburst <br> When measuring the radial error of a shafting system, a standard glass ball is used as a tool ball, so the measurement data contains the out-of-roundness of the tool ball. In degrees, if the roundness of the tool ball is less than one-third of the radial error of the shaft system, then the out-of-roundness effect of the tool ball is negligible. However, due to the improvement of the accuracy of shaft rotation, it has reached the same or even higher data as the standard ball out-of-roundness, and it is difficult to make a more accurate standard ball. The solution is to use the error separation technique. With proper measurement methods and data processing methods, the radial error of the shaft system after eliminating the influence of tool ball out-of-roundness is separated from the measurement data. At the same time, the out-of-roundness of the tool ball, which eliminates the influence of radial errors (mainly systematic error components) in the shafting system, can also be separated. If the out-of-roundness of the ball is basically in agreement with other measurement results (eg, roundness meter measurements), the measurement equipment and measurement method are correct and reliable. The radial error of the shafting separated by the same measured raw data is also accurate. Instead, you can find questions and find problems. We can see from the example of the measuring spindle that the out-of-roundness of the separated ball is 0.057 μm, which is basically consistent with the roundness of the ball measured by the roundness meter, which is basically equal to 0.05 μm. It can be said that our measuring method is correct and reliable. Its shafting radial error is also accurate.
4 Measurement results
The measurement result is shown in Figure 3. After many measurements, the spindle radial rotary precision is 0.07μm.
Figure 3 Measurement results
5 Precautions
1 When adjusting the standard ball and the axis of rotation of the shaft system concentric. The eccentricity can not be too large, so as to avoid the increase of the moving amount of the probe, which causes the measuring instrument's use scope to be too large and increases the instrument error. We have done experiments, when the eccentricity is adjusted to 3 ~ 1.5μm, the measured shaft rotation accuracy is 0.25μm. When the eccentricity was adjusted to 1.5 to 1 μm, the shafting rotation accuracy was measured to be 0.17 μm.
When the eccentricity was adjusted to 1 to 0.5 μm, the shafting rotation accuracy was measured to be 0.07 μm. Therefore, for high-precision shafting tests, the eccentricity must be adjusted to 0.5 μm.
2 The measuring force should not be too large to avoid the effects of force on the workpiece and the mechanical inertia of the measuring head when it is telescopic.
3 Spindle rotation must be uniform, and the sampling position of each circle should be coincident.
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