Temperature change and ultra precision machining o

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The temperature change of hydraulic spindle and the problem of ultra precision machining


during the process experiment of cjy-500 ultra precision grinder, a key subject of the eighth five year plan for ultra precision machining, it is required to turn f400mm plane with a flatness of about 1 m and a surface roughness of more than ra0.01. In the actual turning process, we measured a set of experimental data, and the results were unexpected. The experiment is to use the diamond turning tool to turn the tin grinding disc. The experimental situation is shown in Figure 1. The diamond turning tool is installed on the carriage, turning from the center to the outside. After turning, use the inductance micrometer to measure the straightness of a certain radius from the outside to the inside. The experimental result curve is shown in Figure 2

Fig. 1 experimental conditions

Fig. 2 straightness measurement result curve

according to the design index, the straightness of the tool carriage is 0.5 m in total, and the axial runout of the lower grinding disc is 0.2 M, Yaw 0.7 ", in a short period of two weeks, F4 confirmed that the alloy composition of the extra thin 3104 can body material and the preparation process plan were 0.8 m at 40mm, so the limit deviation of straightness should be

e=0.5+0.2+0.8=1.5 m

the measured and recorded data should be within e+0.5=2 m, but the actual value was 5.5 M. what is the problem?

2 cause analysis

according to the measurement curve analysis, the main factors affecting the turning accuracy are concentrated in the early stage of turning Within about 20min, the cutting error increases rapidly in one direction

during the experimental cutting, the room temperature changes little. The oil tank used in the spindle hydraulic system is a high-precision oil tank with a temperature control accuracy of less than 0.1 ℃. If the oil temperature is always controlled under this accuracy, it is impossible to produce such a large error. In addition, the motion accuracy of the tool rest can also be reliably guaranteed. Therefore, the analysis shows that the error mainly comes from the change of the position of the grinding disc relative to the tool during the cutting process. The factors affecting its position can be mechanical displacement, thermal deformation displacement and displacement caused by oil source change, such as oil pressure change, orifice blockage, etc. After careful inspection, it is finally determined that the thermal deformation displacement of the spindle causes the position change of the tool relative to the spindle

The thermal deformation of diamond cutting is very small and can be ignored. On the other hand, the extrusion, shearing and friction of the lubricating fluid in the clearance caused by the rotation of the main shaft will lead to the temperature rise of the "flow" body, which is further transmitted to the main shaft to cause the temperature rise of the main shaft. If the spindle temperature changes greatly, it may affect the final flatness and straightness obtained by cutting. Accordingly, the oil temperature of the spindle should be measured

the oil inlet temperature of the main shaft is the set temperature of the oil tank. The oil temperature at the oil outlet can be approximately regarded as the oil temperature of the main shaft bearing. When the main shaft of the footwall is supplied with oil, but the main shaft does not rotate, the inlet and outlet temperatures measured after 20min are 18.6 ℃ and 19.8 ℃ respectively. It can be seen that even if the main shaft does not rotate, there is a large temperature difference between the inlet and outlet of the main shaft bearing. As the main shaft absorbs heat and dissipates heat, the temperature difference will be greater. When the main shaft is opened and rotated at 250r/min, the measured oil temperature at the oil port is recorded in Table 1. Table 1 Relationship between oil temperature at oil outlet and time


t/min oil temperature

t/℃ 22.523.223.724.224.725.225.625.926.226.426.626.726.826.9

it can be seen from table 1 that in the first 25min, the oil temperature increased sharply, reaching 3.7 ℃, and in the next 15min, it increased only 0.7 ℃, which is basically consistent with the measurement curve of the front cutting line. It can be seen that the error is probably caused by temperature change. It takes about 40min for the diamond cutting tin plate to improve the overall development level of China's new material industry. During this process, the temperature difference between the oil inlet and outlet of the spindle can reach 4.4 ℃. If the set temperature of the oil tank is basically the same as the room temperature, the thermal deformation on the 100mm length of the main shaft is

△ L = a · L ·△ t = 11 × zero point one × 4.4 = 4.84 m

from this, it is possible that the main shaft floats and deforms due to bearing heating, resulting in the error of cutting straightness. According to the design parameters of the bearing, the temperature rise of the oil is relatively high, up to 5.40 ℃

the friction power of the bearing is divided into two parts, namely, the friction power of the radial bearing and the end thrust bearing. The calculation formula and process are as follows:

for radial bearings:

nf=hv2 × (al+ap) 10 200h0h0+z1

where H - dynamic viscosity of oil, 3\

v - velocity, cm/s

al - total oil seal area

A=6 × 53.2=319.2cm;

ap - area of all oil cavities

h0 - radius gap, cm

z1 - depth of oil chamber, cm

since the depth of oil cavity is much greater than the radius clearance, the latter item in brackets can be ignored. Substitute the data to get

nf=5.85 ×× (250/60 × p × 12)2 × 319.2=2.15 × kW10 2000.021 × 0.1

for thrust bearing:

nf=2 × sixteen point nine ×× hN2 × [(R44-R14)-F2 × (r34-r24)]htp

where n - speed

h - bearing clearance

r1, R2, R3, R4 - structural parameters

f2 - half of the opening angle of a single oil chamber; The bearing has 6 oil chambers, f2=30 °, i.e. 0.52rad. Substitute the data to get

nf=2 × sixteen point nine ×× five point eight five ×× [(17..64)-0.52 × ()]=0.466kW2 × P

at this time, the power consumed by the oil pump on the main shaft is:

np=p3q=50 × 5=0.233kW612h612 × 0.7

and oil temperature rise

dt=102 × (nf+np) 427c0gq

where nf=nr+nt is substituted into each value to obtain

dt=102 × (0.466+0.0215+0.233)=5.40℃427 × zero point four five × zero point zero zero zero eight five × five × 103/60

the calculated results are slightly different from the actual measurement results. The calculation formula is derived under the condition that there is no heat exchange between the oil and the outside world. In the actual process, the actual measured value is smaller than the calculated result because the oil inevitably has heat exchange with the surrounding objects. However, in the stage of inconvenient measurement or design, it can still be used as a reference through calculation

when the spindle speed is 500r/min and 1000r/min, the oil temperature rises to 15.9 ℃ and 57.8 ℃ respectively. This shows that the heating of the hydraulic bearing is very large. In the field of ultra precision machining, the accuracy is required to be in the order of 0.1 M, and there must be strict restrictions on bearing heating, otherwise the predetermined index cannot be reached. In contrast, the heat generated by air bearing is much smaller. However, in some cases, the hydraulic bearing must be used. At this time, efforts should be made to control its heating deformation

3 solution

combined with the previous analysis and error curve, it is considered that the error is mainly caused by the change of bearing oil temperature. At this time, the spindle speed is 250r/min. If the speed is high, the deformation of the spindle will be more unfavorable. It can be seen that the heating of the hydraulic spindle can not be ignored. In order to ensure the precision of ultra precision machining, the change of spindle oil temperature and the deformation and elongation of spindle should be strictly controlled. This has strict requirements for the working environment, such as temperature regulation accuracy. In addition, the structure of the machine tool and the hydraulic cooling system should also adapt to ultra precision machining. The following solutions are proposed:

1) adopt precision temperature control oil tank. The hydrostatic bearing in the field of ultra precision machining must use a cooling temperature control oil tank. Controlling the oil temperature is the basic means to limit the bearing heating and ensure the machining accuracy. The temperature change of the hydraulic spindle and ultra precision machining are the basic means.  At present, the temperature control accuracy of domestic temperature control oil tanks can usually reach 1 ℃ or 0.5 ℃. As an ultra precision machining, the higher the temperature control accuracy, the better. This machine adopts a precision oil tank with a temperature control accuracy of 20 ℃± 0.1 ℃, which better controls the heating of the machine tool's return oil and blocks the continuous temperature rise of the oil in the circulation process

however, as the oil in the bearing clearance still has the effects of friction, extrusion and shear, the oil will inevitably heat up, resulting in a large temperature difference between the inlet and outlet of the oil, which can still be transmitted to the spindle, resulting in the temperature rise of the spindle and seriously affecting the machining accuracy

2) increase the thermal rigidity of the spindle system. The heating of the spindle oil is inevitable, but if the material with low expansion coefficient is used, the heating deformation of the spindle can be reduced. At present, the known materials with low coefficient of thermal expansion include indium steel, quartz, glass ceramics and composites with coefficient of thermal expansion close to zero. Using super indium steel as spindle material, its thermal expansion coefficient is 6 ×/℃, which is 1/20 of ordinary steel. High thermal rigidity spindles made of new materials are being paid attention to

3) change the bearing arrangement. In the bearing design, the position of the end thrust bearing shall be reasonably arranged to make it as close to the machining surface as possible, because the deformation and elongation of the main shaft starts from the end thrust bearing. Shortening the distance between it and the machined surface can change the length factor in the deformation calculation formula, so the deformation is reduced. However, this improvement is limited by machine tool layout and bearing structure

4) adopt cooling system for main shaft. In order to control the heating deformation of the main shaft, another cooling system can be set on the main shaft. For example, a cooling groove is opened on the spindle to keep the spindle constant temperature through an external circulating cooling system. Figure 3 shows the grinding disc with cooling groove. When the coolant passes through, the heating of the grinding process and the heating of the spindle can be controlled. It can also be sprayed with constant temperature outside the spindle to directly ensure the constant temperature of the machining process, the spindle and even the whole machine tool. The diamond lathe dtm-3 developed by LLNL laboratory in the United States uses constant temperature oil spray. The change of oil temperature is controlled within 20 ℃± 0.0025 ℃, and the machine tool and workpiece are sprayed with a flow rate of 1.5m3/min, effectively eliminating the thermal impact

Fig. 3 cooling groove

5) in view of the fact that the spindle heating cannot be well controlled, it is possible to achieve high machining accuracy by adopting appropriate measures to control the process. In order to eliminate the influence of temperature on the machining accuracy, the machine tool can be operated at a constant temperature for a long time at the working speed, and then cut. After a long time of constant temperature, all parts reach thermal balance, and the deformation tends to be stable. Then higher accuracy can be obtained by machining. Figure 4 shows the data obtained by turning the machine tool after half an hour of constant temperature. The error is 1.2 m, and the cutting straightness is within the expected range, The constant temperature of machine tool is a common method to realize precision machining and ultra precision machining under the premise of low technical guarantee. Fig. 4 straightness measurement curve after constant temperature machining

Figure 4 straightness measurement curve after constant temperature machining

4 conclusion

hydraulic spindle has its specific advantages in application, but in ultra precision machining, even if the spindle does not rotate, its heating is enough to affect the machining accuracy. Therefore, enough attention must be paid to the temperature change of hydraulic spindle, which is sometimes the main factor limiting the further improvement of machining accuracy

in the field of ultra precision machining, the characteristics of hydraulic bearing and air bearing should be comprehensively considered to determine the structural form of the main shaft. The hydraulic main shaft is complex and huge because of its necessary cooling and temperature control oil tank and the structure or system to control the heating of the main shaft. In the case of using the hydraulic main shaft, how to solve the problem of temperature rise of the main shaft must be considered, Simply using the temperature control oil tank to control the oil temperature at the oil inlet of the main shaft can not effectively control the heating of the main shaft. To achieve high machining accuracy, in addition to the reasonable structure and layout, the control of the machining process can also be achieved. 1. High degree of automation: the full-automatic operation of the experimental machine according to the set experimental plan plays a positive role. (end)

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