Thermal deformation is one of the reasons which affect the precision of treatment of the machine tool due to the modifications of the room temperature of the workshop, the heating of the engine and the friction of the mechanical movement, the heat of the heat of the heat and the cooling medium, resulting in an unequal temperature increase in each part of the tool machine, resulting in morphological precision and treatment accuracy.For example, if a 70mm × 1650 mm screw is treated on a CNC milling machine with ordinary precision, cumulative error changes can be made compared to the ground part from 7:30 am to 9:00 am and the parts processed from 2:00 to 2h00 at 3.30 p.m.Under constant temperature conditions, the error can be reduced to 40 m.
For example, a double -end double -end grinding machine for double -end grinding of the thin face in steel sheets with a thickness of 0.6 to 3.5 mm. And the degree of flexion is that the total length is less than 5 m. However, after continuous automatic grinding for 1 hour, the size change range increased to 12 m and the temperature of the cooling liquid increased from 17 ° C to 45 ° C at the time of starting. Due to the influence of grinding heat, the spindle newspaper extends and the clearance of the front bearing of the spindle increases. Consequently, adding a 5.5 kW refrigerator to the coolant tank of the machine tool is very ideal. The practice has proven that deformation of machine tools after heating is an important reason which affects the accuracy of machining. However, the machine tool is in an environment where the temperature changes at any time and anywhere; In the different components of the tool-machine. Machine tool designers must control the heat formation mechanism and temperature distribution rules and take corresponding measures to minimize the impact of thermal deformation on the accuracy of the treatment.
The increase in temperature, temperature distribution and the natural climate of machine tools affect the vast territory of my country. Consequently, the methods and degrees of intervention at interior temperatures (such as workshops) are also different, and the temperature atmosphere around the machine-tool varies considerably. For example, the seasonal temperature change range in the Yangtze River Delta region is around 45 ° C, and the day and night temperature change is around 5 to 12 ° C. The workshop machining has generally does not have a heating supply in winter and no air conditioning in summer, but as long as the workshop is well ventilated, the temperature gradient of the machining workshop does not change much. In the northeast, the seasonal temperature difference can reach 60 ℃ and day and night change from around 8 to 15 ℃. The heating period is from the end of October to early April of the following year. The temperature difference between the interior and the exterior of the workshop can reach 50 ℃. Therefore, the temperature gradient in winter in the workshop is very complicated. . The machining accuracy of precision machine tools will be greatly affected by room temperature in such a workshop.
The surrounding environment affects the surrounding environment of the machine tool refers to the thermal environment formed by various provisions in the short range of the tool-tool.
They include the following 4 aspects:
1) Atelier microclimate: such as temperature distribution in the workshop (vertical direction, horizontal direction). The workshop temperature changes slowly when day and night alternate, climate and ventilation change.
2) Atelier heat sources: such as sun exposure, radiation from heating equipment and high power lighting, etc. When they are close to the tooltip, they can directly affect the temperature of the whole or the part of the tool-tool for a long time. The heat generated by adjacent equipment during operation will affect the temperature of the tool-tool in a radiative or air flow.
3) Dissipation of heat: The foundation has a good heat dissipation effect, in particular the base of precision machines should not be close to the underground heating pipeline. The open workshop will be a good “heat dissipation”, which is conducive to the balance of the temperature of the workshop.
4) Constant temperature: The workshop adopts constant temperature installations to maintain the precision and machining accuracy of precision machines, but it consumes a lot of energy.
3. Factors affecting the internal heat of machine tools
1) Source of structural heat of machine tools. The heat generated by the engine heating such as spindle motors, power servomotors, coolant and lubrication pump motors, electric control boxes, etc. can generate heat. These situations are authorized for the engine itself, but they have a significant negative effect on components such as pins and ball screws, and measurements must be taken to isolate them. When the input electrical energy leads to the operation of the engine, with the exception of a small part (approximately 20%) converted into thermal energy of the engine, most of them will be converted from the mobile mechanism to kinetic energy, Like the rotation of the spindle, the movement of the table, etc.; But there are still a lot of games that are inevitable.
2) Cut the heat from the process. During the cutting process, part of the kinetic energy of the tool or part is consumed by cutting work, and a considerable part converts the deformation energy of the cut and the heat of friction between the shavings And the tool, forming the heat of the heat of the tool, the spindle and the room, and is transmitted to a large amount of heat from the chip with the workbench tweezer of the machine tool. They will directly affect the relative position between the tool and the part.
3) Cool. Cooling is a opposite measure for increasing the temperature of machine tools, such as engine cooling, cooling of spit components and infrastructure cooling. High-end machine tools often provide refrigerators for electric control boxes and cool them forcefully.
4. The influence of the structural form of the tool-tool on the rise in temperature. Distribution of the heat source. The structure affects the temperature distribution of the tool machine, the thermal conduction direction, the direction of thermal deformation and correspondence, etc.
1) Structural form of the machine tool. In terms of global structure, machine tools include vertical, horizontal, portico and cantilever, etc., and there are large differences in their response and their stability to heat. For example, the rise in the temperature of the tower spindle box with speed speed can reach 35 ℃, which will lift the end of the spindle and the thermal balance time will take approximately 2 hours. The machine tool has a stable base for the precision and machining center of the accuracy of the inclined bed. The rigidity of the whole machine is considerably improved.
2) Effect of the distribution of heat sources. It is generally believed that heat sources refer to electric motors on machine tools. Such as spindle engines, power engines and hydraulic systems, etc., are in fact incomplete. The heating of the engine is only the energy consumed by the current on the impedance of the reinforcement when loading the load, and a considerable part of the energy is consumed by the heating caused by the friction work Mechanisms such as bearings, screw nuts and guide rails. Therefore, the engine can be called a primary heat source, and the bearings, nuts, guides and shavings can be called a secondary heat source. Thermal deformation is the result of the combined effects of all these heat sources. The increase in temperature and the deformation of a mobile vertical column machining center during the feeding movement of the Y. The establishment did not move during the power supply to the Y direction, so that the effect on the thermal deformation of the X direction is very low. On the column, the more the guide screw is far from the Y axis, the smaller the increase in the temperature. The situation of the machine when the Z axis moves further illustrates the impact of the distribution of heat sources on thermal deformation. The food of the Z axis is further away from direction X, so the thermal deformation has less impact.
3) Effect of mass distribution. There are three aspects of the impact of mass distribution on the thermal deformation of machine tools. First, it refers to the size of the mass and the level of concentration, generally refers to the modification of the thermal capacity and the speed of the heat transfer, and the modification of time to reach the thermal balance.
2. smaller temperature;
Third, this means that by modifying the form of mass arrangement, such as the laying of heat dissipation hillock plates outside the structure, to reduce the temperature of the components of the machine tool.
Influence of the properties of materials: Different materials have different thermal performance parameters (specific heat, thermal conductivity and linear expansion coefficient). Test of thermal performance of machine tools
1. The goal of the machine-test machine test the thermal performance The key to controlling the thermal deformation of the machine tool is to fully understand the changes in the ambient temperature of the machine tool, the changes in the heat source and The temperature of the machine tool, and the response (deformation displacement) of the key points by thermal characteristic tests. The 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 accuracy and efficiency of the machine tool.
More specifically, the following objectives should be achieved:
1) Test the surrounding environment of the tooltip. To measure the temperature environment in the workshop, its spatial temperature gradient, temperature distribution changes during alternating day and night, we must even measure the impact of seasonal changes on temperature distribution Around the tooltip.
2) Test of the thermal characteristics of the machine tool itself. Under the conditions for the elimination of environmental interference as much as possible, the tool-tool is in various operating states to measure temperature changes and changes in the displacement of the important points of the tool-tool itself, and record Temperature changes and movement of key points over a long period of time.
3) Test the rise in temperature and thermal deformation during the processing process to judge the impact of the thermal deformation of the machine tool on the precision of the processing process.
4) The above tests can accumulate a large amount of data and curves, which will provide a reliable criterion for the design of machine tools and users to control thermal deformation and indicate the direction of effective measurements.
2. Principles of the tool-tool thermal deformation test The thermal deformation tests first require to measure the temperature of several related points, including the following aspects:
1) Heat source: including different parts of the power engine, pin engine, the transmission pair of ball screws, the guidance rail and the spindle bearing.
2) Auxiliary devices: including the hydraulic system, the refrigerator, the cooling and lubrication displacement detection system.
3) Mechanical structure: including the bed, the base, the roller board, the column, the crusher head box and the spindle. The steel measuring steel of the Indium is tight between the pin and the rotary, and 5 contact sensors are arranged in the instructions x, y and z to measure the complete deformation in various states to simulate the relative displacement between the tool and the part.3. Data processing and analyzes The thermal deformation test of machine tools must be carried out within a long continuous delay and continuous recording of the data is carried out. If the withdrawal of errors is carried out by several tests, the regularity indicated is credible. A total of 5 measurement points have been set up in the thermal deformation test of the pin system, whose points 1 and 2 are at the end of the spindle and near the spindle bearing, and points 4 and 5 Are respectively at the housing of the grinding head near the Z-Réctional guide rail. The test time lasted 14 hours and the speeds of the spindle in the first 10 hours alternately changed in the beach from 0 to 9000r / min.The following conclusions can be drawn:1) The thermal balance time of the pin is approximately 1 h, and the temperature elevation change range after the balance is 1.5 ℃;2) The increase in temperature comes mainly from spindle bearings and spit.3) The influence of thermal deformation in direction X is very low;4) The expansion of the Z and the contraction deformation are relatively important, about 10 m, which is caused by the thermal extension of the spindle and the increase in the clearance of the bearings;5) When the speed of continuous rotation of 9000r / min, the rise in temperature increases sharply, and within 2.5 hours, it increases sharply by about 7 ℃, and there is a trend to the trend to The increase continues. -The directions reach 29 m and 37 m, indicating that the spindle is at the speed of rotation. Control of the thermal deformation of machine tools is discussed above. Reach the result twice with half of the effort. In the design, we must start from four directions: reduce heat generation, reduce temperature elevation, balanced structure and reasonable cooling.1. Reducing heat generation and controlling heat sources is the fundamental measure. Measures must be taken in the design to effectively reduce the heat generation of the heat source.1) Reasonably select the nominal power of the engine. The output power P of the engine is equal to the product of voltage V and current I. In general, voltage V is constant. The corresponding current I also increases, so the current of heat consumed in the impedance of the interface increases. If the engine that we have designed and selected works for a long time when it is close or considerably exceeds nominal power, the increase in engine temperature will increase considerably. To this end, a comparison test was carried out on the BK50 BK50 CNC -type BK50 CHR -Type head (engine speed: 960r / min; room temperature: 12 ℃). According to the above tests, the following concept is obtained: from the consideration of the performance of the heat source, be it the spitting engine or the power engine, when choosing the nominal power, It is best to choose about 25% more than the computing power. , the engine output power is comparable to the load. But it can effectively reduce engine temperature.
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