Thermal deformation is one of the reasons that affect machining accuracy. The machine tool is affected by changes in the ambient temperature of the workshop, heating of the motor and mechanical movement friction, cutting heat and cooling medium, resulting in uneven temperature rise in various parts of the machine tool, leading to changes in the shape accuracy and machining accuracy of the machine tool. For example, when processing a 70mm×1650mm screw on a CNC milling machine with ordinary precision, the cumulative error of the workpiece milled at 7:30-9:00 in the morning can reach 85m compared with the workpiece processed at 2:00-3:30 in the afternoon. Under constant temperature conditions, the error can be reduced to 40m.
For example: A precision double-end grinder used for double-end grinding of thin steel sheet workpieces with a thickness of 0.6 to 3.5mm can achieve millimeter-level dimensional accuracy when processing 200mm×25mm×1.08mm steel sheet workpieces during acceptance, and the curvature is less than 5m in the entire length. However, after continuous automatic grinding for 1h, the size change range increases to 12m, and the coolant temperature rises from 17℃ at startup to 45℃. Due to the influence of grinding heat, the spindle journal is elongated and the front bearing clearance of the spindle is increased. Based on this, a 5.5kW refrigerator is added to the coolant tank of the machine tool, and the effect is very ideal. Practice has proved that the deformation of the machine tool after heating is an important reason affecting the processing accuracy. However, the machine tool is in an environment where the temperature changes anytime and anywhere; the machine tool itself will inevitably consume energy when working, and a considerable part of this energy will be converted into heat in various ways, causing physical changes in the components of the machine tool. This change varies greatly due to different structural forms and material differences. Machine tool designers should master the formation mechanism of heat and the law of temperature distribution, and take corresponding measures to minimize the impact of thermal deformation on processing accuracy.
People's ways and degrees of intervention in indoor (such as workshop) temperature are also different, and the temperature atmosphere around the machine tool varies greatly. The temperature gradient in winter is very complex. When measured, the outdoor temperature is 1.5℃, the time is 8:15-8:35 in the morning, and the temperature in the workshop changes by about 3.5℃. The processing accuracy of precision machine tools will be greatly affected by the ambient temperature in such a workshop.
The surrounding environment refers to the thermal environment formed by various layouts within the close range of the machine tool. They include the following 4 aspects:
1) Microclimate of the workshop: such as the distribution of temperature in the workshop (vertical direction, horizontal direction). When the day and night alternate or the climate and ventilation change, the temperature of the workshop will change slowly.
2) Heat source of the workshop: such as sunlight, heating equipment and radiation of high-power lighting lamps, etc., when they are close to the machine tool, they can directly affect the temperature rise of the whole or some parts of the machine tool for a long time. The 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 a good heat dissipation effect, especially the foundation of precision machine tools should not be close to underground heating pipes. Once it breaks and leaks, it may become a heat source that is difficult to find the cause; an open workshop will be a good "radiator", which is conducive to the temperature balance of the workshop.
4) Constant temperature: The constant temperature facilities adopted in the workshop are very effective for maintaining the accuracy and processing accuracy of precision machine tools, but the energy consumption is large.
1) Structural heat sources of machine tools. Heat from motors such as spindle motors, feed servo motors, cooling and lubrication pump motors, and electronic control boxes can all generate heat. These situations are permissible for the motor itself, but have a significant adverse effect on components such as the spindle and ball screw, and measures should be taken to isolate them. When the input electrical energy drives the motor to operate, except for a small part (about 20%) that is converted into motor heat energy, most of it will be converted into kinetic energy by the motion mechanism, such as spindle rotation, worktable movement, etc.; but inevitably, a considerable part is still converted into frictional heat during the movement process, such as heating of bearings, guide rails, ball screws, and transmission boxes.
2) Cutting heat during the process. During the cutting process, part of the kinetic energy of the tool or workpiece is consumed in cutting work, and a considerable part is converted into cutting deformation energy and friction heat between chips and tools, forming heating of the tool, spindle, and workpiece, and a large amount of chip heat is conducted to the machine tool's worktable fixture and other components. They will directly affect the relative position between the tool and the workpiece.
3) Cooling. Cooling is a reverse measure for the temperature rise of machine tools, such as cooling of motors, cooling of spindle components, and cooling of basic structural parts. High-end machine tools often equip the electric control box with a refrigerator for forced cooling.
In the field of machine tool thermal deformation, the discussion of the structural form of the machine tool usually refers to the structural form, mass distribution, material properties, and heat source distribution. The structural form affects the temperature distribution, heat conduction direction, thermal deformation direction, and matching of the machine tool.
1) The structural form of the machine tool. In terms of the overall structure, machine tools are vertical, horizontal, gantry, and cantilever, and the response and stability to heat are quite different. For example, the temperature rise of the spindle box of a gear-shifted lathe can be as high as 35°C, which causes the spindle end to lift up, and the thermal balance time takes about 2 hours. The inclined bed precision turning and milling machining center has a stable base. The rigidity of the whole machine is significantly improved. The spindle is driven by a servo motor, and the gear transmission part is removed. Its temperature rise is generally less than 15°C.
2) The influence of heat source distribution. The heat source on machine tools is usually considered to be the motor. Such as spindle motor, feed motor and hydraulic system, etc., which is actually incomplete. The heat generated by the motor is only the energy consumed by the current on the armature impedance when it is under load, and a considerable part of the energy is consumed by the friction work of the bearings, screw nuts and guide rails. Therefore, the motor can be called a primary heat source, and the bearings, nuts, guide rails and chips can be called secondary heat sources. Thermal deformation is the result of the combined influence of all these heat sources. The temperature rise and deformation of a column-moving vertical machining center during the Y-axis feed movement. The worktable does not move during the Y-axis feed, so the influence on the thermal deformation in the X-axis is very small. On the column, the farther the point is from the Y-axis guide screw, the smaller its temperature rise. The situation of the machine when the Z-axis moves further illustrates the influence of heat source distribution on thermal deformation. The Z-axis feed is farther from the X-axis, so the thermal deformation effect is smaller. The closer the Z-axis motor nut on the column is to the Z-axis, the greater the temperature rise and deformation.
3) The influence of mass distribution. There are three aspects of the influence of mass distribution on the thermal deformation of machine tools. First, it refers to the size and concentration of mass, which usually refers to changing the heat capacity and the speed of heat transfer, and changing the time to reach thermal equilibrium; second, by changing the layout of mass, such as the layout of various ribs, the thermal stiffness of the structure is improved, and under the same temperature rise, the influence of thermal deformation is reduced or the relative deformation is kept small; third, it refers to changing the layout of mass, such as arranging heat dissipation ribs outside the structure to reduce the temperature rise of 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 amount of heat, their temperature rise and deformation are different.
(1) Purpose of machine tool thermal performance testing The key to controlling the thermal deformation of machine tools is to fully understand the changes in the ambient temperature of the machine tool, the heat source and temperature changes of the machine tool itself, and the response (deformation displacement) of key points through thermal characteristic testing. 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.
Specifically, the following objectives should be achieved:
1) Testing the surrounding environment of the machine tool. Measure the temperature environment in the workshop, its spatial temperature gradient, the changes in temperature distribution during the alternation of day and night, and even measure 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, put the machine tool in various operating states to measure the temperature changes and displacement changes of important points of the machine tool itself, record the temperature changes and key point displacements in a sufficiently long period of time, and use an infrared thermal camera to record the thermal distribution of each time period.
3) Testing the temperature rise and thermal deformation during the processing process to determine the impact of the thermal deformation of the machine tool on the accuracy of the processing process.
4) The above 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 point out the direction of taking effective measures.
(2) Principle of machine tool thermal deformation test The thermal deformation test first needs to measure the temperature of several related points, including the following aspects:
1) Heat source: including the feed motor of each part, 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, slide, column, milling head box and spindle. An indium steel measuring rod is clamped between the spindle and the rotary table, and 5 contact sensors are configured in the X, Y and Z directions to measure the comprehensive deformation in various states to simulate the relative displacement between the tool and the workpiece.
(3) Test data processing and analysis The thermal deformation test of the machine tool should be carried out in a long continuous time, and continuous data recording is performed. After analysis and processing, the thermal deformation characteristics reflected are highly reliable. If the error is eliminated through multiple tests, the regularity shown is credible. A total of 5 measurement points are set in the thermal deformation test of the spindle system, of which point 1 and point 2 are at the end of the spindle and close to the spindle bearing, and point 4 and point 5 are respectively at the milling head housing close to the Z guide rail. The test lasted for 14 hours. In the first 10 hours, the spindle speed was changed alternately in the range of 0 to 9000r/min. From the 10th hour, the spindle continued to rotate at a high speed of 9000r/min. The following conclusions can be drawn:
1) The thermal equilibrium time of the spindle is about 1 hour, and the temperature rise after equilibrium varies by 1.5℃.
2) The temperature rise mainly comes from the spindle bearings and the spindle motor. Within the normal speed range, the thermal performance of the bearings is good.
3) The thermal deformation has little effect in the X direction.
4) The Z-direction expansion deformation is large, about 10m, which is caused by the thermal elongation of the spindle and the increase in the bearing clearance.
5) When the speed is continuously at 9000r/min, the temperature rise rises sharply, rising by about 7℃ in 2.5h, and there is a trend of continued increase. The deformation in the Y and Z directions reaches 29m and 37m, indicating that the spindle can no longer run stably at a speed of 9000r/min, but can run in a short time (20min).
The control of thermal deformation of machine tools is discussed in the above analysis. There are many factors that affect the temperature rise and thermal deformation of machine tools on the processing accuracy. When taking control measures, we should grasp the main contradictions and take corresponding measures to achieve twice the result with half the effort.
In the design, we should start from four directions: reduce heat generation, reduce temperature rise, structural balance, and reasonable cooling. Reducing heat generation and controlling heat sources are fundamental measures. In the design, measures should be taken to effectively reduce the heat generation of heat sources. Reasonably select the rated power of the motor. The output power P of the motor is equal to the product of the voltage V and the current I. Under normal circumstances, the voltage V is constant. Therefore, the increase in load means that the output power of the motor increases, that is, the corresponding current I also increases, and the heat consumed by the current in the armature impedance increases. If the motor we designed and selected works for a long time under conditions close to or greatly exceeding the rated power, the temperature rise of the motor will increase significantly. For this reason, a comparative test was conducted on the milling head of the BK50 CNC needle slot milling machine (motor speed: 960r/min; ambient temperature: 12℃). The following concepts are obtained from the above tests: Considering the heat source performance, whether it is the spindle motor or the feed motor, when selecting the rated power, it is best to choose one that is about 25% larger than the calculated power. In actual operation, the output power of the motor matches the load. Increasing the rated power of the motor has little effect on energy consumption, but can effectively reduce the temperature rise of the motor.
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