The machine tool is affected by the change of workshop ambient temperature, motor heating, mechanical motion friction heating, cutting heat and cooling medium, resulting in uneven temperature rise of each part of the machine tool, resulting in the change of shape accuracy and machining accuracy of the machine tool.  
 
For example, 70mm is machined on a CNC milling machine with ordinary precision × For 1650mm screw, the cumulative error of the workpiece milled from 7:30 a.m. to 9:00 p.m. can reach 85m compared with the workpiece machined from 2:00 p.m. to 3:30 p.m. Under constant temperature, the error can be reduced to 40m.  
 
For another example, a precision double end grinder for double end grinding 0.6 ~ 3.5mm thick thin steel sheet workpiece shall process 200mm during acceptance × 25mm × The 1.08mm steel sheet workpiece can reach the dimensional accuracy of mm, and the bending degree is less than 5m in the whole length. However, after continuous automatic grinding for 1h, the size change range increases to 12M, and the coolant temperature increases from 17 ℃ at startup to 45 ℃. Due to the influence of grinding heat, the main shaft journal is elongated and the front bearing clearance of the main shaft is increased. Therefore, 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 machining accuracy. However, the machine tool is in an environment where the temperature changes everywhere at any time; The machine tool itself will inevitably consume energy when working. A considerable part of these energy will be converted into heat in various ways, resulting in physical changes in various components of the machine tool. This change varies greatly due to different structural forms, material differences and other reasons. 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 machining accuracy.  
 
 Temperature rise and temperature distribution of machine tool
 
 1. Natural climate impact  
China has a vast territory and most areas are in subtropical areas. The temperature changes greatly throughout the year and the temperature difference in a day is also different. thus, People are interested in indoors (e.g. workshops) have different ways and degrees of temperature intervention, and the temperature atmosphere around machine tools varies greatly. For example, the seasonal temperature variation range in the Yangtze River Delta is about 45 ℃, and the temperature variation between day and night is about 5 ~ 12 ℃. Machining workshops generally have no heating in winter and no air conditioning in summer, but as long as the workshop is well ventilated, the temperature gradient in machining workshops changes little. However, in Northeast China , the seasonal temperature difference can reach 60 ℃, and the diurnal variation is about 8 ~ 15 ℃. The heating period is from late October to early April of the next year. The machining workshop is designed with heating and insufficient air circulation. The temperature difference inside and outside the workshop can reach 50 ℃. Therefore, the temperature gradient in the workshop in winter is very complex. During the measurement, the outdoor temperature is 1.5 ℃, the time is 8:15-8:35 a.m., and the temperature change in the workshop is about 3.5 ℃. The machining accuracy of precision machine tools will be greatly affected by the ambient temperature in such a workshop.  
 
 2. Impact on surrounding environment  
The surrounding environment of the machine tool refers to the thermal environment formed by various layouts within the close range of the machine tool. They include the following three aspects.  
(1) Workshop microclimate: such as the temperature distribution in the workshop (vertical and horizontal). The workshop temperature will change slowly when day and night alternate or the climate and ventilation change.  
 
(2) Workshop heat source: such as the radiation of the sun, heating equipment and high-power lamps. 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. Do not be close to the underground heating pipeline. 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 in maintaining the accuracy and machining accuracy of precision machine tools, but the energy consumption is large.  
 
 3. Internal thermal factors of machine tool  
(1) Structural heat source of machine tool. Heat generated by motor, such as spindle motor, feed servo motor, cooling and lubrication pump motor and electric control box, can generate heat. These conditions are allowed for the motor itself, but they have significant adverse effects on spindle, ball screw and other components, and measures shall be taken to isolate them. When the motor is driven by input electric energy, except A small part (about 20%) will be transformed into motor heat energy, and most of it will be transformed into kinetic energy by moving mechanisms, such as spindle rotation, workbench movement, etc.; however, it is inevitable that a considerable part will be transformed into friction heating during movement, such as bearing, guide rail, ball screw and transmission box.  
 
(2) Cutting heat in the process. In the cutting process, part of the kinetic energy of the tool or workpiece is consumed in the cutting work, and a considerable part is transformed into the cutting deformation energy and the friction heat between the chip and the tool, forming the heating of the tool, spindle and workpiece, and a large amount of chip heat is transmitted to the worktable fixture and other parts of the machine tool. They will directly affect the relative position between the tool and workpiece.  
 
(3) Cooling. Cooling is a reverse measure against the temperature rise of the machine tool, such as motor cooling, spindle component cooling and basic structure cooling. High end machine tools often prepare a refrigerator for the electric control box for forced cooling.  
 
 4. Influence of machine tool structure on temperature rise  
In the field of thermal deformation of machine tools, the structural form of machine tools is discussed, which usually refers to the structural form, mass distribution, material properties and heat source distribution. The structure shape affects the temperature distribution, heat conduction direction, thermal deformation direction and matching of the machine tool.  
(1) Structural form of the machine tool. In terms of the overall structure, the machine tool has vertical, horizontal, gantry and cantilever types, which have great differences in thermal response and stability. For example, the temperature rise of the spindle box of the gear variable speed lathe can be as high as 35 ℃, so that the spindle end can be lifted up, and the thermal balance time needs about 2H. While the inclined bed precision turning and milling machining center, the machine tool has a stable base. The rigidity of the whole machine is obviously improved. The main shaft is driven by servo motor, and the gear transmission part is removed. The temperature rise is generally less than 15 ℃.  
 
(2) Influence of heat source distribution. On machine tools, it is generally considered that heat source refers to motor, such as spindle motor, feed motor and hydraulic system. In fact, it is incomplete. The heating of motor is only the energy consumed by current on armature impedance when bearing load, and a considerable part of energy is consumed by the heating caused by friction work of bearing, lead screw nut and guide rail. Therefore, the motor can be called primary heat source, and the bearing, nut, guide rail and chip can be called secondary heat source. Thermal deformation is the result of the combined influence of all these heat sources.  
 
Temperature rise and deformation of a column mobile vertical machining center during Y-direction feeding movement. When feeding in Y direction, the workbench does not move, so it has little effect on the thermal deformation in X direction. On the column, the farther away from the y-axis guide screw, the smaller the temperature rise.  
 
The influence of heat source distribution on thermal deformation is further explained when the machine moves on the z-axis. The z-axis feed is farther away from the x-direction, so the thermal deformation has less influence. 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 tool has three aspects. First, it refers to the size and concentration of mass, which usually refers to changing the heat capacity and heat transfer speed to change the time to reach the thermal balance; second, by changing the layout of mass, such as the layout of various stiffeners, improve the thermal stiffness of the structure and reduce the thermal deformation under the same temperature rise Shape affects or maintains relatively small deformation; Third, it refers to reducing the temperature rise of machine tool parts by changing the form of mass layout, such as arranging heat dissipation ribs outside the structure.  
 
(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.  
 
 Testing of thermal performance of machine tools  
 
 1. Purpose of thermal performance test of machine tool  
The key to control the thermal deformation of the machine tool is to fully understand the change of the ambient temperature of the machine tool through the thermal characteristic test, The heat source and temperature changes of the machine tool itself and the response of key points (deformation displacement). The test data or curves describe the thermal characteristics of a machine tool, so as to take countermeasures to control thermal deformation and improve the machining accuracy and efficiency of the machine tool. Specifically, the following objectives should be achieved:  
(1) Surrounding environment test of machine tool: measure the temperature environment in the workshop, its spatial temperature gradient, the change of temperature distribution in the alternation of day and night, and even the influence of seasonal change on the surrounding temperature distribution of machine tool.  
 
(2) Thermal characteristic test of the machine tool itself. Under the condition of eliminating environmental interference as much as possible, keep the machine tool in various operating states, so as to measure the temperature change and displacement change of important points of the machine tool itself, record the temperature change and displacement of key points in a long enough period of time, or record the heat distribution in each period of time with an infrared thermal phasor.  
 
(3) The temperature rise and thermal deformation are tested in the machining process to judge the influence of the thermal deformation of the machine tool on the machining process accuracy.  
 
(4) The above tests can accumulate a large number of data and curves, which will provide reliable criteria for machine tool design and user control of thermal deformation, and point out the direction of effective measures.  
 
 2. Principle of thermal deformation test of machine tool  
The thermal deformation test first needs to measure the temperature of several relevant points, including the following aspects:  
(1) Heat source: including 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 main shaft.  
An indium steel measuring rod is clamped between the spindle and the rotary worktable. Five contact sensors are configured in the X, y and Z directions to measure the comprehensive deformation under 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 shall be carried out in a long continuous time, and the continuous data shall be recorded. After analysis and processing, the reliability of the thermal deformation characteristics reflected is very high. If the error is eliminated through multiple tests, the regularity shown is credible.  
 
Five measuring 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 near the spindle bearing, and point 4 and point 5 are respectively at the milling head shell near the z-direction guide rail. The test lasted for 14h, in which the spindle speed changed alternately in the range of 0 ~ 9000r / min in the first 10h. From the 10th h, the spindle continued to rotate at a high speed of 9000r / min. The following conclusions can be drawn:  
(1) The thermal balance time of the spindle is about 1H, and the temperature rise range after balance is 1.5 ℃;  
(2) The temperature rise mainly comes from the spindle bearing and spindle motor. Within the normal speed range, the thermal performance of the bearing is good;  
(3) Thermal deformation has little effect in X direction;  
(4) The z-direction telescopic deformation is large, about 10m, which is caused by the thermal elongation of the main shaft and the increase of bearing clearance;  
(5) When the rotating speed continues at 9000r / min, the temperature rise rises sharply, about 7 ℃ in 2.5h, and there is a trend of continuous rise. The deformation in Y direction and Z direction reaches 29m and 37m, indicating that the spindle can not operate stably at 9000r / min, but can operate in a short time (20min).  
 
 Control of thermal deformation of machine tool  
 
From the above analysis and discussion, the temperature rise and thermal deformation of the machine tool have a variety of influencing factors on the machining accuracy. When taking control measures, we should grasp the main contradiction and focus on taking one or two measures to achieve twice the result with half the effort. In the design, we should start from four directions: reducing heating, reducing temperature rise, structural balance and reasonable cooling.  
 
 1. Reduce fever  
Controlling the heat source is the fundamental measure. In the design, measures should be taken to effectively reduce the heating capacity of the heat source.  
(1) Reasonably select the rated power of the motor. The output power P of the motor is equal to the product of voltage V and current I. generally, voltage V is constant. Therefore, the increase of 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 design and select is close to or stable for a long time When the rated power is greatly exceeded, the temperature rise of the motor increases significantly. Therefore, a comparative test is carried out on the milling head of bk50 CNC needle slot milling machine (motor speed: 960r / min; ambient temperature: 12 ℃).  
 
The following concepts are obtained from the above tests: considering the performance of heat source, when selecting the rated power of spindle motor or feed motor, it is better to choose about 25% higher than the calculated power. In actual operation, the output power of motor matches the load, and increasing the rated power of motor has little impact on energy consumption. However, it can effectively reduce the temperature rise of the motor.  
 
(2) Appropriate measures shall be taken in the structure to reduce the heating capacity of the secondary heat source and reduce the temperature rise. For example, in the design of the main shaft structure, the coaxiality of the front and rear bearings shall be improved and high-precision bearings shall be used. If possible, the sliding guide rail shall be changed into a linear rolling guide rail or a linear electric motor shall be used. These new technologies can effectively reduce friction, heat and temperature rise. Metal processing wechat, good content, worthy of attention!  
 
(3) In terms of technology, high-speed cutting is adopted. Based on the mechanism of high-speed cutting, when the linear speed of metal cutting is higher than a certain range, the metal to be cut has no time to produce plastic deformation, no deformation heat is generated on the chip, and most of the cutting energy is transformed into chip kinetic energy and taken away.  
 
 2. Structural balance to reduce thermal deformation  
On the machine tool, the heat source always exists. What needs further attention is how to make the heat transfer direction and speed conducive to reducing thermal deformation. Or the structure has good symmetry, so that the heat transfer channel is along the symmetrical direction, the temperature distribution is uniform, and the deformation counteracts each other to become a thermal affinity structure.  
 
(1) Prestress and thermal deformation. In high-speed feeding systems, ball screws are often fixed axially at both ends to form pre tensile stress. This structure plays an obvious role in reducing thermal deformation error in addition to improving dynamic and static stability for high-speed feeding.  
The temperature rise of the axial fixed structure pre stretched for 35m within 600mm is close under different feed speeds. The cumulative error of the pre stretched structure with two ends fixed is significantly less than that of the structure with one end fixed and the other end freely extended. In the axially fixed pretensioned structure at both ends, the temperature rise caused by heating is mainly to change the stress state inside the lead screw from tensile stress to zero stress or compressive stress. Therefore, it has little influence on the displacement accuracy.  
 
(2) Change the structure and change the direction of thermal deformation. The z-axis spindle sliding seat of CNC needle groove milling machine with different ball screw axial fixed structure requires a milling groove depth error of 5m in processing. The axial floating structure at the lower end of the lead screw is adopted, and the groove depth gradually deepens from 0 to 0.045mm within 2h of processing. On the contrary, the floating structure at the upper end of the lead screw can ensure the change of groove depth.  
 
(3) The symmetry of the structural geometry of the machine tool can make the thermal deformation consistent and minimize the drift of the tool tip. For example, the ymc430 micro machining center launched by yasda precision tool company in Japan is a submicron high-speed machining machine tool, and the thermal performance is fully considered in the design of the machine tool.  
 
Firstly, the machine tool structure adopts a completely symmetrical layout. The column and beam are an integrated structure, which is H-shaped, equivalent to double column structure, with good symmetry. The approximate circular spindle sliding seat is symmetrical in both longitudinal and transverse directions.  
 
The feed drive of the three moving shafts adopts linear motor, which is easier to realize symmetry in structure. The two rotating shafts adopt direct drive to minimize the friction loss and loss of mechanical transmission.  
 
 3. Reasonable cooling measures  
(1) The influence of coolant on machining accuracy is direct. The comparative test of grv450c double face grinder is carried out. The test shows that the heat exchange treatment of coolant with the help of refrigerator is very effective to improve machining accuracy.  
Using the traditional coolant supply mode, the workpiece size exceeds the tolerance after 30min. After using the refrigerator, it can be processed normally for more than 70min. The main reason for the out of tolerance of workpiece size at 80min is that the grinding wheel needs to be trimmed (remove the metal chips on the grinding wheel surface), and the original machining accuracy can be restored immediately after trimming. The effect is very obvious. Similarly, a very good effect can be expected for the forced cooling of the spindle.  
 
(2) Increase the natural cooling area. For example, adding natural air cooling area to the spindle box structure can also play a good heat dissipation effect in the workshop with good air circulation.  
 
(3) Timely and automatic chip removal. Timely or real-time discharge of high-temperature chips from the workpiece, worktable and tool parts will be very helpful to reduce the temperature rise and thermal deformation of key parts.  
 
 Outlook and vision  
 
Controlling the thermal deformation of machine tools is an important subject in the field of modern precision machining, and the factors affecting the thermal deformation of machine tools are very complex. Moreover, the simultaneous development of high speed, high efficiency and precision in modern cutting makes the thermal deformation of machine tools more prominent. It has attracted extensive attention in the field of machine tool manufacturing. Scholars in machine tool circles at home and abroad have done a lot of research and made considerable progress in theory. Thermal deformation of machine tools has become one of the basic theories in machine tool research.  
 
From the point of view of machine tool design and application, this paper analyzes the influencing factors, measurement and analysis methods of machine tool thermal performance, and puts forward the improvement design measures. Therefore, we believe that the optimal design of thermal performance of machine tool should start from the following aspects:  
(1) In the design stage of modern high-end machine tools, we should pay attention to the environmental conditions for the future application of the designed machine tools.  
(2) Controlling and configuring heat source is the key. Controlling heat source mainly refers to controlling the matching of energy consumption and power source, adopting new structure, reducing secondary friction heat source and improving energy utilization.  
(3) Change the traditional thinking and upgrade the cooling, heat dissipation, lubrication, chip removal and other devices from the status of "auxiliary" parts of machine tools to the status of "important" parts, which can not be despised.  
(4) Pay attention to the design of structural symmetry and the direction of thermal deformation to minimize the impact of thermal deformation on accuracy, especially pay attention to the research and application of thermal deformation mathematical model of structural parts, so as to provide qualitative and quantitative instructions for thermal deformation control design.