What is the effect of temperature on accuracy in CNC machining?
Thermal deformation is one of the reasons that affect the machining accuracy. The machine tool is affected by the change of the workshop environment temperature, the heating of the motor and the friction of the mechanical movement, the cutting heat and the cooling medium, resulting in the uneven temperature rise of each part of the machine tool, resulting in the change of the shape accuracy and machining accuracy of the machine tool. For example, 70mm is processed on an ordinary precision CNC milling machine × For 1650mm screw, the cumulative error of workpieces milled from 7:30 to 9:00 in the morning can reach 85m compared with workpieces processed from 2:00 to 3:30 in the afternoon. But under constant temperature, the error can be reduced to 40m.
Another example is a precision double end grinding machine used for double end grinding of 0.6-3.5mm thick thin steel sheet workpieces, which can process 200mm at the time of 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 increased to 12M, and the coolant temperature increased from 17 ℃ at startup to 45 ℃. Due to the influence of grinding heat, the main shaft journal is elongated and the clearance of the front bearing 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. It has been proved that the deformation of machine tool after heating is an important factor affecting machining accuracy. However, the machine tool is in an environment where the temperature changes at any time; 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, resulting in physical changes of various components of the machine tool. Such changes vary greatly due to different structural forms and materials. Machine tool designers should master the formation mechanism and temperature distribution law of heat and take corresponding measures to reduce the influence of thermal deformation on machining accuracy to Z.
The temperature rise and temperature distribution of machine tools and the natural climate affect China's vast territory. Most of the areas are located in subtropical areas. The temperature varies greatly throughout the year and the temperature difference in a day is also different. Therefore, the way and degree of people's intervention on indoor (such as workshop) temperature are also different, and the temperature atmosphere around the machine tool varies greatly. For example, the seasonal temperature change range in the Yangtze River Delta is about 45 ℃, and the diurnal temperature change is about 5-12 ℃. Generally, the machining workshop has no heating in winter and no air conditioning in summer. However, as long as the workshop is well ventilated, the temperature gradient of the machining workshop does not change much. 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 following 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 in the morning, 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.
Influence of 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 four aspects:
1) Workshop microclimate: such as the temperature distribution in the workshop (vertical direction and horizontal direction). When the day and night alternate or the climate and ventilation change, the workshop temperature will change slowly.
2) Workshop heat sources: such as solar radiation, radiation of heating equipment and high-power lighting, etc. when they are close to the machine tool, they can directly affect the temperature rise of the whole or part of the machine tool for a long time. The heat generated by the 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 the underground heating pipe. Once it breaks and leaks, it may become a heat source that is difficult to find the cause; The open workshop will be a good "radiator", which is conducive to the temperature balance in the workshop.
4) Constant temperature: the constant temperature facilities adopted in the workshop are very effective in maintaining the accuracy and processing accuracy of precision machine tools, but the energy consumption is large.
3. Internal thermal influence factors of machine tool
1) Machine tool is a structural heat source. Motor heating such as spindle motor, feed servo motor, cooling and lubricating pump motor and electric control box can generate heat. These conditions are allowed for the motor itself, but they have significant adverse effects on the main shaft, ball screw and other components, and measures shall be taken to isolate them. When the input electric energy drives the motor to run, except that a small part (about 20%) will be converted into the thermal energy of the motor, most will be converted into kinetic energy by the motion mechanism, such as the rotation of the main shaft and the movement of the workbench; However, it is inevitable that a considerable part of the heat will be converted into friction heat during the movement, such as the heat of bearings, guide rails, ball screws and transmission boxes.
2) Cutting heat of the process. During the cutting process, part of the kinetic energy of the tool or workpiece is consumed by the cutting work, and a considerable part is converted into the deformation energy of the cutting and the friction heat between the chip and the tool, forming the heat of the tool, the spindle and the 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 the 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 structural component cooling. High end machine tools are often equipped with refrigerators for forced cooling.
4. The influence of the structural form of machine tool on the temperature rise in the field of thermal deformation of machine tool, the structural form of machine tool usually refers to the structural form, mass distribution, material performance and heat source distribution. The structure shape 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, the machine tools are vertical, horizontal, gantry and cantilever, etc., which have great differences in thermal response and stability. For example, the temperature rise of the main axle box of a gear speed lathe can be as high as 35 ℃, so that the main shaft end is lifted up, and the heat balance time needs about 2H. For the precision turning and milling machining center with inclined bed, the machine tool has a stable base. The rigidity of the whole machine is obviously 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 ℃.
2) Influence of heat source distribution. It is generally considered that the heat source refers to the motor on the machine tool. For example, the spindle motor, feed motor and hydraulic system are not complete. The heating of the motor is only the energy consumed by the current on the armature impedance when bearing the load, and a considerable part of the energy is consumed by the heating caused by the friction work of the bearing, screw nut, guide rail and other mechanisms. Therefore, the motor can be called the primary heat source, and the bearing, nut, guide rail and chip can be called the secondary heat source. Thermal deformation is the result of the comprehensive influence of all these heat sources. The temperature rise and deformation of a vertical machining center with movable columns during the y-direction feeding movement. The workbench does not move when feeding in the Y direction, so it has little influence on the thermal deformation in the X direction. On the column, the farther away from the y-axis guide screw, the smaller the temperature rise. When the machine moves along the z-axis, the influence of heat source distribution on thermal deformation is further explained. The z-axis feed is farther away from the x-direction, so the thermal deformation has less influence. The closer the z-axis motor nut is to the column, the greater the temperature rise and deformation.
3) Influence of mass distribution. The influence of mass distribution on the thermal deformation of machine tools has three aspects. First, it refers to the size and concentration of mass, usually refers to changing the heat capacity and the speed of heat transfer, and changing the time to reach the heat balance
2、 By changing the arrangement form of mass, such as the arrangement of various ribs, the thermal stiffness of the structure can be improved, and under the same temperature rise, the influence of thermal deformation can be reduced or the relative deformation can be kept small;
Third, it means to reduce the temperature rise of machine tool parts by changing the form of mass arrangement, such as arranging heat dissipation ribs outside the structure.
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. The purpose of the thermal performance test of the machine tool is to control the thermal deformation of the machine tool. The key is to fully understand the change of the ambient temperature of the machine tool, the heat source and temperature change of the machine tool itself and the response (deformation displacement) of the key points through the thermal characteristic test. The test data or curves describe the thermal characteristics of a machine tool, so that countermeasures can be taken to control the thermal deformation and improve the machining accuracy and efficiency of the machine tool.
Specifically, the following objectives should be achieved:
1) Test the surrounding environment of the 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 temperature distribution around the machine tool.
2) Thermal characteristic test of the machine tool itself. Under the condition of eliminating environmental interference as much as possible, the machine tool shall be kept in various operating states to measure the temperature change and displacement change of the important points of the machine tool itself, and record the temperature change and displacement of the key points within a long enough period of time. The infrared thermal phase meter can also be used to record the thermal distribution of each time period.
3) The temperature rise and thermal deformation are measured during the machining process to judge the influence of the thermal deformation of the machine tool on the accuracy of the machining process.
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 taking effective measures.
2. Principle of thermal deformation test of machine tool thermal deformation test first needs to measure the temperature of several relevant points, including the following aspects:
1) Heat source: including feed motor of each part, spindle motor, ball screw drive pair, guide rail and spindle bearing.
2) Auxiliary devices: including hydraulic system, refrigerator, cooling and lubrication displacement detection system.
3) Mechanical structure: including machine bed, base, slide plate, column, milling head box and spindle. An indium steel measuring rod is clamped between the spindle and the rotary table. Five contact sensors are arranged in the X, y and Z directions to measure the comprehensive deformation under various conditions 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 continuous data recording shall be carried out. After analysis and processing, the thermal deformation characteristics reflected are highly reliable. If the error is eliminated through multiple tests, the displayed regularity is credible. There are 5 measuring points 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 housing near the z-direction guide rail. The test time lasted for 14h, in which the rotation speed of the main shaft in the first 10h was alternated within the range of 0-9000r / min. from the 10th h, the main shaft 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 main shaft bearing and the main shaft motor. Within the normal speed range, the bearing has good thermal performance;
3) Thermal deformation has little influence on X direction;
4) The z-direction expansion deformation is large, about 10m, which is caused by the thermal extension of the main shaft and the increase of the bearing clearance;
5) When the rotation speed is kept at 9000r / min, the temperature rise rises sharply, rising sharply by about 7 ℃ within 2.5h, and there is a tendency to continue to rise. The deformation in Y direction and Z direction reaches 29m and 37m, indicating that the main shaft can no longer operate stably at the rotation speed of 9000r / min, but can operate in a short time (20min). The control of thermal deformation of the machine tool is analyzed and discussed above. The temperature rise and thermal deformation of the machine tool have various influence 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. The design should start from four directions: reducing heat generation, reducing temperature rise, balancing structure and reasonable cooling.
1. Reducing heat generation and controlling heat source are fundamental measures. In the design, measures shall be taken to effectively reduce the heat generation 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 the voltage V and the current I. generally, the voltage V is constant. Therefore, the increase of the 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 near or greatly exceeds the rated power for a long time, the temperature rise of the motor will obviously increase. Therefore, a comparative test was carried out on the milling head of bk50 numerical control 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, when selecting the rated power of the spindle motor or the feed motor, it is appropriate to select about 25% higher than the calculated power. In actual operation, the output power of the motor matches the load, and increasing the rated power of the motor has little impact on the energy consumption. But the temperature rise of the motor can be effectively reduced.