Thermal deformation is one of the reasons that affect machining accuracy. The machine tool is affected by changes in workshop ambient temperature, motor heating and friction heat of mechanical movement, cutting heat and cooling fluids, resulting in uneven temperature rise in various parts of the machine. the machine tool, resulting in machine tool shape accuracy and precision changes in processing. For example, when processing a 70 mm × 1650 mm screw on a CNC milling machine with ordinary precision, compared to the part milled between 7:30 a.m. and 9:00 a.m. and the part processed between 2:00 p.m. and 3:30 p.m., the change in L The cumulative error can be up to 85 m. Under constant temperature conditions, the error can be reduced to 40 m.
For another example, a double-end precision grinder used for double-end grinding of thin steel parts with a thickness of 0.6-3.5mm can achieve mm dimensional accuracy when processing steel parts of 200 mm × 25 mm × 1.08 mm, and the curvature is between Less than 5 m in total length. However, after 1 hour of continuous automatic grinding, the size change range increased to 12 m, and the coolant temperature increased from 17°C at start-up to 45°C. Due to the influence of grinding heat, the spindle journal lengthens and the clearance of the spindle front bearing increases. On this basis, a 5.5 kW refrigeration machine was added to the machine tool coolant tank, and the effect was very ideal.
Practice has proven that the deformation of machine tools after heating is an important reason that affects the precision of machining. However, machine tools are in an environment where the temperature changes anytime and anywhere; the machine tool itself will inevitably consume energy during its work, and a considerable part of this energy will be converted into heat in various ways, causing physical changes in the environment. machine tool components. This change is due to They vary greatly due to different structural shapes, differences in materials and other reasons. Machine tool designers should understand the heat formation mechanism and temperature distribution rules, and take corresponding measures to minimize the impact of thermal deformation on machining accuracy.
Temperature rise and temperature distribution of machine tools
1. Natural climatic impact
Our country has a vast territory and most regions are in subtropical zones. The temperature changes significantly throughout the year, and the temperature difference changes within a day. As a result, people have different means and degrees of intervention on indoor temperature (such as workshops), and the ambient temperature around machine tools varies greatly. For example, the seasonal temperature range in the Yangtze River Delta region is about 45°C, and the daytime and nighttime temperature changes are about 5°C to 12°C. Machine shops generally have no heating in the winter and no air conditioning in the summer. However, as long as the workshop is well ventilated, the temperature gradient in the machine shop will not change much. In the Northeast, the seasonal temperature difference can reach 60°C, and the day and night variation is about 8 to 15°C. The heating period extends from the end of October to the beginning of April of the following year. The machine shop is designed with insufficient heating and air circulation. The temperature difference between the inside and outside of the workshop can reach 50℃. Therefore, the temperature gradient in the workshop in winter is very complex. The outside temperature was 1.5°C when measured between 8:15 and 8:35 a.m., and the temperature in the workshop varied by approximately 3.5°C. The machining accuracy of precision machine tools will be greatly affected by the ambient temperature in such a workshop.
2. Influence of the surrounding environment
The environment around the machine tool refers to the thermal environment formed by various arrangements near the machine tool. They include the following 4 aspects.
(1) Workshop microclimate: such as the temperature distribution in the workshop (vertical direction, horizontal direction). Room temperature changes slowly when day and night change or when climate and ventilation change.
(2) Workshop heat sources: such as solar radiation, heating equipment and high-power lighting lamp radiation, when close to the machine tool, they can directly affect the heating of all or part of the machine tool. long lasting. Heat generated by adjacent equipment during operation will affect the temperature rise of the machine tool in the form of radiation or air flow.
(3) Heat dissipation: The foundation has good heat dissipation effect. In particular, the foundation of precision machine tools should not be close to underground heating pipes. Once it breaks and leaks, it can become a source of heat and it is difficult to find the cause. ; an open workshop will be a good “Radiator”, beneficial to the thermal balance in the workshop.
(4) Constant temperature: The use of constant temperature facilities in workshops is very effective in maintaining the precision and accuracy of precision machine tool processing, but it consumes a lot of energy.
3. Factors affecting internal heat of machine tools
1) Structural heat source of machine tools. Motors that generate heat, such as spindle motors, power servo motors, cooling and lubrication pump motors, electrical control boxes, etc., can all generate heat. These situations are allowed for the motor itself, but they have a significant negative impact on components such as the spindle and ballscrew, and measures must be taken to isolate them. When the input electrical energy drives the motor, except for a small part (about 20%) which is converted into thermal energy of the motor, most of it will be converted into kinetic energy by the movement mechanism, such as spindle rotation, workbench movement. , etc.; but inevitably there is still a considerable part of it which is converted into friction heat during movement, such as bearings, guide rails, ball screws, transmission boxes and other mechanisms.
2) Cut the heat in the process. During the cutting process, part of the kinetic energy of the tool or workpiece is consumed in the cutting work, and a considerable part is converted into cutting deformation energy and friction heat between the chip and the tool, causing the tool, spindle and workpiece to be heated, and a large amount of chip heat is transferred to the machine tool bench support and other parts. They will directly affect the relative position between the tool and the workpiece.
3) Cool. Cooling is a reverse measure to increasing machine tool temperature, such as motor cooling, spindle component cooling, and basic structural component cooling. High-end machine tools are often equipped with a refrigerator for the electrical control box to provide forced cooling.
4. The influence of the structural shape of machine tools on heating
Discussing the structural form of machine tools in the field of thermal deformation of machine tools generally refers to issues such as structural form, mass distribution, material properties, and heat source distribution. The structural shape affects the temperature distribution, heat conduction direction, thermal deformation direction and machine tool adaptation.
1) The structural form of the machine tool. In terms of overall structure, machine tools include vertical, horizontal, gantry and cantilever types, etc., and their thermal response and stability are very different. For example, the temperature increase of the spindle housing of a gear lathe can reach 35°C, causing the spindle end to lift, and the thermal equilibrium time takes about 2 hours. As for the precision inclined bed turning and milling machining center, the machine tool has a stable base. The rigidity of the whole machine is significantly improved. The main shaft is driven by a servo motor, and the gear transmission part is removed. The temperature rise is generally less than 15°C.
2) The influence of the distribution of heat sources. In machine tools, the heat source is generally considered to be the motor. Such as spindle motor, feed motor and hydraulic system etc. are actually incomplete. The heat generated by the motor is only the energy consumed by the current in the armature impedance when under load, and a considerable part of the energy is consumed by the heat caused by friction work bearings, nuts, guide rails and other mechanisms. Therefore, the motor can be called primary heat source and the bearings, nuts, guide rails and chips are called secondary heat sources. Thermal deformation is the result of the combined influence of all these heat sources. Temperature rise and deformation of a vertical moving column machining center when feeding in the Y direction. The worktable does not move when feeding in the Y direction, so it has little effect on thermal deformation in the X direction. On the column, the further away from the Y axis guide screw, the lower the heating. The situation where the machine moves on the Z axis further illustrates the influence of the heat source distribution on thermal deformation. The Z axis feed is further away from the X direction, so the impact of thermal deformation is small. The closer the column is to the Z-axis motor nut, the greater the temperature rise and deformation.
3) The influence of mass distribution. The influence of mass distribution on the thermal deformation of machine tools presents three aspects. First, it refers to the size and concentration of the mass, generally referring to the change in heat capacity and rate of heat transfer, as well as the change in the time required to reach thermal equilibrium. Second, improving the thermal rigidity of the structure by modifying the arrangement of the mass; , like the arrangement of various ribs. Under the same temperature rise, reduce the influence of thermal deformation or keep the relative deformation small, this refers to changing the shape of mass arrangement, such as the arrangement of heat dissipation ribs outside of the structure, to reduce the heating of the machine tool components.
4) Influence of material properties: different materials have different thermal performance parameters (specific heat, thermal conductivity and linear expansion coefficient). Under the influence of the same heat, their temperature rise and deformation are different.
Thermal performance testing of machine tools
1. Purpose of thermal performance testing of machine tools
The key to controlling thermal deformation of machine tools is to fully understand the ambient temperature changes of the machine tool, the heat source, and the temperature changes of the machine tool itself, as well as the response ( deformation displacement) of key points through thermal characteristic tests. Test data or curves describe the thermal characteristics of a machine tool so that countermeasures can be taken to control thermal deformation and improve the processing accuracy and efficiency of the machine tool. More specifically, the following objectives should be achieved:
1) Test the machine tool environment. Measure the thermal environment of the workshop, its spatial temperature gradient, changes in temperature distribution when alternating day and night, and even the impact of seasonal changes on the temperature distribution around the machine -tool.
2) Testing the thermal characteristics of the machine tool itself. Under the condition of eliminating environmental interference as much as possible, the machine tool is placed in various operating states to measure temperature changes and displacement changes of important points of the machine tool itself, and record the changes temperature and the movements of key points over a long period. sufficient period of time, and also an infrared thermal camera can be used to record the heat distribution during each period of time.
3) Test the temperature rise and thermal deformation during the machining process to determine the impact of thermal deformation of the machine tool on the accuracy of the machining process.
4) The above-mentioned tests can accumulate a large amount of data and curves, which will provide reliable criteria for machine tool design and users to control thermal deformation, and indicate the direction for taking measurements effective.
2. Principle of thermal deformation test of machine tools
The thermal deformation test should first measure the temperature of several relevant points, including the following aspects:
1) Heat source: including each part of feed motor, spindle motor, ball screw transmission pair, guide rail and spindle bearing.
2) Auxiliary devices: including hydraulic system, refrigerator, cooling and lubrication displacement detection system.
3) Mechanical structure: including bed, base, sliding plate, column, milling head box and spindle. An indium steel measuring rod is clamped between the spindle and the rotary table, and five contact sensors are configured in the X, Y and Z directions to measure the overall deformation under different states to simulate the relative displacement between the tool and the part. .
3,Processing and analysis of test data
Thermal deformation test of machine tools should be carried out for a long continuous period, with continuous data recording after analysis and processing, the reflected thermal deformation characteristics are very reliable. If errors are eliminated through several tests, the regularity demonstrated is credible.
A total of 5 measuring points were set during the thermal deformation test of the spindle system, among which points 1 and 2 are at the end of the spindle and near the spindle bearing, and points 4 and 5 are located respectively at the level of the milling head housing. near the rail in the Z direction. The test lasted a total of 14 hours, of which the spindle speed was varied alternately in the range of 0 to 9,000 rpm during the first 10 hours. From the 10th hour, the spindle continued to rotate at a high speed of 9,000 rpm. . The following conclusions can be drawn:
1) The thermal equilibrium time of the spindle is about 1 hour, and the temperature rise change range after equilibrium is 1.5°C;
2) The temperature increase mainly comes from the spindle bearing and spindle motor. In the normal speed range, the thermal performance of the bearing is good;
3) Thermal deformation has little effect in the X direction;
4) The telescopic deformation in the Z direction is large, about 10 m, caused by the thermal elongation of the spindle and the increase in bearing clearance;
5) When the rotation speed continues to be 9000 rpm, the temperature rises sharply, rising sharply by about 7 ℃ in 2.5 hours, and shows a continuous upward trend. The deformation in the Y and Z directions reached 29 m and 37 m, which indicates. that the spindle rotates at 9000r/min. When it reaches 9000r/min, it can no longer work stably, but it can work in a short time (20min).
Control of thermal deformations of machine tools
According to the above analysis and discussion, the temperature increase and thermal deformation of the machine tool have various factors that affect the machining accuracy. When taking control measures, we must grasp the main contradiction and focus on taking one or two measures to achieve double the result. with half the effort. When designing, we should start from four directions: reduce heat generation, reduce temperature rise, structural balance and reasonable cooling.
1. Reduce fever
Controlling heat sources is a fundamental measure. During design, measures should be taken to effectively reduce the calorific value of the heat source.
1) Select the rated power of the motor reasonably. The output power P of the motor is equal to the product of voltage V and current I. Under normal circumstances, voltage V is constant. Therefore, an increase in load means that the power output of the motor increases, ie. , the corresponding current I also increases, and then the current The heat dissipated in the armature impedance increases. If the motor we design and select operates for a long time near or well above the rated power, the heating of the motor will increase significantly. For this purpose, a comparison test was carried out on the milling head of the BK50 CNC needle milling machine (motor speed: 960 rpm; ambient temperature: 12°C).
From the above experiments, the following concepts are obtained: From the performance point of view of the heat source, whether it is a spindle motor or a feed motor, when When selecting the rated power, it is best to choose a power that is approximately 25% higher than the calculated power. In actual operation, the motor output power is consistent with the load, increasing the motor rated power has little impact on power consumption. But it can effectively reduce engine heating.
2) Take appropriate structural measures to reduce the calorific value of the secondary heat source and reduce the temperature increase. For example: When designing the spindle structure, the coaxiality of the front and rear bearings should be improved, and high-precision bearings should be used. Where possible, replace the sliding guide rail with a bearing linear guide rail or use a linear motor. These new technologies can effectively reduce friction, heat generation and temperature rise. WeChat on metal processing has good content and deserves attention!
3) In terms of technology, high-speed cutting is adopted. Based on high speed cutting mechanism. When the metal cutting linear speed is above a certain range, the cut metal has no time to undergo plastic deformation, no deformation heat is generated on the chips, and most of the energy of cutting is converted into kinetic energy of the chips and is removed.
2. Structural balance to reduce thermal deformation
In machine tools, heat sources are always present, and special attention should be paid to how to make the direction and speed of heat transfer conducive to reducing thermal deformation. Or the structure has good symmetry, so that the heat transfer is in the symmetrical direction, the temperature distribution is uniform and the deformations cancel each other out, forming a thermal affinity structure.
1) Prestressing and thermal deformation. In higher speed power systems, both ends of the ball screw are often clamped axially to form a pre-tension constraint. This structure not only improves dynamic and static stability for high-speed feeding, but also plays an important role in reducing thermal deformation errors.
The heating of the axially fixed and pre-stretched structure of 35 m over a total length of 600 mm is relatively similar at different feeding speeds. The cumulative error of the pre-stretched structure with two fixed ends is significantly lower than that of the structure with one end fixed and the other end free to extend. In the prestressed structure fixed axially at both ends, the temperature increase caused by heating mainly changes the stress state inside the screw from tensile stress to zero stress or compressive stress. This therefore has little impact on the precision of the movement.
2) Change the structure and change the direction of thermal deformation. The spindle slide on the Z axis of a needle groove CNC milling machine using different ball screw axial fixing structures requires a milling groove depth error of 5m during processing. Through an axial floating structure at the lower end of the screw, the groove depth gradually deepens from 0 to 0.045mm within 2 hours after treatment. On the contrary, using a structure with the upper end of the screw floating can ensure that the depth of the groove changes.
3) The symmetry of the geometric shape of the machine tool structure can make the thermal deformation trend consistent and minimize the drift of the tool tip. For example, the YMC430 micromachining center launched by Yasda Precision Tools Company of Japan is a submicron high-speed machining machine tool. The design of the machine tool takes full account of thermal performance.
Firstly, a completely symmetrical layout is adopted in the structure of the machine tool. The columns and beams are H-shaped integrated structures, which is equivalent to a double column structure and has good symmetry. The approximately circular spindle slide is also symmetrical both longitudinally and transversely.
The power drives of the three moving axes all use linear motors, which makes it easier to achieve structural symmetry. Both rotary axes use direct drives to minimize friction losses and mechanical transmission.
3. Reasonable cooling measures
1) The coolant during processing has a direct impact on the processing accuracy. A comparative test was carried out on the GRV450C double-sided grinder. Tests show that heat exchange of coolant using a refrigerator is very effective in improving machining accuracy.
Using the traditional coolant supply method, the part size will be out of tolerance after 30 minutes. After using the refrigeration machine, it can be processed normally for more than 70 minutes. The main reason why the workpiece size is out of tolerance at 80 minutes is that the grinding wheel needs to be dressed (to remove metal chips on the surface of the grinding wheel), and the original machining accuracy can be restored immediately after training. The effect is very obvious. Likewise, very good results can be expected from forced cooling of the spindle.
2) Increase the natural cooling zone. For example, adding a natural air cooling area to the spindle housing structure can also achieve a good heat dissipation effect in a workshop with good air circulation.
3) Automatic chip removal in a timely manner. Ejecting high temperature chips from workpiece, bench and tool parts timely or in real time will be very helpful to reduce temperature rise and thermal deformation of key parts.
Perspectives and vision
Controlling the thermal deformation of machine tools is an important issue in the field of modern precision machining, and the factors that affect the thermal deformation of machine tools are very complex. In addition, the simultaneous development of high speed, high efficiency and precision in modern cutting processing makes the problem of thermal deformation of machine tools even more significant. It attracted the attention of the machine tool manufacturing industry. Machine tool specialists in Germany and abroad have carried out extensive research on this subject and made considerable theoretical progress. The thermal deformation problem of machine tools has become one of the fundamental theories of machine tool research.
This article analyzes the influencing factors, measurement and analysis methods of machine tool thermal performance from the perspectives of machine tool design and application, and proposes improved design measures. Therefore, we believe that the optimized thermal performance design of machine tools should start from the following aspects:
1) During the design stage of modern high-end machine tools, attention should be paid to the environmental conditions for future applications of the designed machine tools.
2) Control and configuration of the heat source is essential. Heat source control mainly refers to controlling the adequacy of energy consumption and energy source, adopting new structures, reducing secondary frictional heat sources, and improving energy use.
3) Change traditional thinking and elevate cooling, heat dissipation, lubrication, chip removal and other devices from the status of “auxiliary” components of machine tools to the status of “important” components, which cannot be underestimated.
4) Pay attention to the design of structure symmetry and thermal deformation direction to minimize the impact of thermal deformation on accuracy. Particular attention should be paid to the research and application of mathematical models of thermal deformation of structural parts. provide qualitative and quantitative design of thermal deformation control instructions.
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