Machine tools are affected by changes in the ambient temperature of the workshop, motor heating and mechanical motion friction heat, cutting heat and cooling media, resulting in uneven temperature rise in various parts of the machine tool, resulting in changes in the shape accuracy and machining accuracy of the machine tool.
Case 1: A 70mm × 1650mm screw is processed on an ordinary precision CNC milling machine. Compared with the workpiece milled between 7:30-9:00 am and the workpiece processed between 2:00-3:30 pm, the change in cumulative error can be Up to 85m. Under constant temperature conditions, the error can be reduced to 40m.
Case 2: A precision double-end face grinder used for double-end grinding of thin steel workpieces with a thickness of 0.6 to 3.5mm. During acceptance, the 200mm×25mm×1.08mm steel workpiece can be processed with a dimensional accuracy of mm, and the curvature is within Less than 5m in total length. However, after 1 hour of continuous automatic grinding, the size change range increased to 12m, and the coolant temperature increased from 17°C at startup to 45°C. Due to the influence of grinding heat, the spindle journal elongates and the spindle front bearing clearance increases. Based on this, a 5.5kW refrigerator was added to the machine tool coolant tank, and the effect was very ideal.
Practice has proved that the deformation of machine tools after heating is an important reason that affects machining accuracy. However, machine tools are in an environment where the temperature changes at any time 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 various components of the machine tool. This change is due to They vary greatly due to different structural forms, material differences 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.
Our country has a vast territory, and most areas are in subtropical areas. The temperature changes greatly throughout the year, and the temperature difference changes within a day. As a result, people have different ways and degrees of intervention in indoor (such as workshops) temperature, and the temperature atmosphere around machine tools varies widely.
For example, the seasonal temperature range in the Yangtze River Delta region is about 45°C, and the day and night temperature changes are about 5°C to 12°C. Machine shops generally have no heating in winter and no air conditioning in 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 is from late October to early April of the following year. The machine shop is designed with heating and insufficient air circulation. The temperature difference between inside and outside the workshop can reach 50℃. Therefore, the temperature gradient in the workshop in winter is very complicated. The outdoor temperature was 1.5°C during the measurement, and the time was from 8:15 to 8:35 am. The temperature change in the workshop was about 3.5°C. The machining accuracy of precision machine tools will be greatly affected by the ambient temperature in such a workshop.
The environment around the machine tool refers to the thermal environment formed by various layouts within the close range of the machine tool. They include the following 3 aspects.
1) Workshop microclimate: such as the distribution of temperature in the workshop (vertical direction, horizontal direction). The room temperature will change slowly when day and night change or when climate and ventilation change.
2) Workshop heat sources: such as solar radiation, heating equipment and radiation from high-power lighting lamps. When they are close to the machine tool, they can directly affect the temperature rise of the entire or part 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 "heat dissipation" device", which is beneficial to the temperature balance in the workshop.
4) Constant temperature: The use of constant temperature facilities in workshops is very effective in maintaining precision and processing accuracy of precision machine tools, but it consumes a lot of energy.
1) Structural heat source of machine tools. Motors that generate heat, such as spindle motors, feed servo motors, cooling and lubricating pump motors, electric control boxes, etc., can all generate heat. These situations are allowed for the motor itself, but they have a significant adverse impact on components such as the spindle and ball screw, and measures should be taken to isolate them. When the input electric energy drives the motor to run, 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, workbench movement, etc.; but inevitably there is still a considerable part It is converted into frictional heat during movement, such as bearings, guide rails, ball screws, transmission boxes and other mechanisms.
2) Cutting heat in 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 the deformation energy of the cutting and the friction heat between the chip and the tool, causing the tool, spindle and workpiece to heat up, and a large amount of chip heat is transferred to the workbench fixture of the machine tool. and other parts. They will directly affect the relative position between the tool and the workpiece.
3) Cool down. Cooling is a reverse measure against the increase in 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 electric control box to provide forced cooling.
Discussing the structural form of machine tools in the field of machine tool thermal deformation usually refers to issues such as 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 overall structure, machine tools include vertical, horizontal, gantry and cantilever types, etc., and their thermal response and stability are quite different. For example, the temperature rise of the spindle box of a gear-speed lathe can be as high as 35°C, causing the spindle end to lift up, and the thermal equilibrium time takes about 2 hours. As for the inclined bed type precision 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 heat source distribution. In machine tools, the heat source is usually 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 it is under load, and a considerable part of the energy is consumed by the heat caused by the friction work of bearings, screw nuts, guide rails and other mechanisms. Therefore, the motor can be called a 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 column-mobile vertical machining center during Y-direction feed movement. 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 farther away from the Y-axis guide screw, the smaller the temperature rise.
The situation when the machine is moving in the Z-axis further illustrates the influence of heat source distribution on thermal deformation. The Z-axis feed is further away from the X direction, so the impact of thermal deformation is smaller. 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 thermal deformation of machine tools has three aspects. First, it refers to the size and concentration of the mass, usually referring to changing the heat capacity and heat transfer speed, and changing the time to reach thermal equilibrium; second, improving the thermal stiffness of the structure by changing the arrangement of the mass, such as the arrangement of various ribs. Under the same temperature rise, reduce the influence of thermal deformation or keep the relative deformation small; third, it refers to changing the form of mass arrangement, 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 heat, their temperature rise and deformation are different.
From the above analysis and discussion, the temperature rise and thermal deformation of the machine tool have various factors that affect 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 heat generation, reducing temperature rise, structural balance, and reasonable cooling.
Controlling heat sources is a fundamental measure. In the design, measures should be taken to effectively reduce the calorific value 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. Under normal circumstances, the voltage V is constant. Therefore, an increase in the load means that the output power of the motor increases, that is, the corresponding current I also increases, then the current The heat dissipated in the armature impedance increases. If the motor we design and select works for a long time close to or greatly exceeds the rated power, the temperature rise of the motor will increase significantly. To this end, a comparative test was conducted on the milling head of the BK50 CNC needle groove milling machine (motor speed: 960r/min; ambient temperature: 12°C).
From the above experiments, the following concepts are obtained: From the perspective of heat source performance, whether it is a spindle motor or a feed motor, when selecting the rated power, it is best to choose a power that is about 25% larger than the calculated power. In actual operation, the output power of the motor is consistent with the load. Matching, increasing the rated power of the motor has little impact on energy consumption. But it can effectively reduce the temperature rise of the motor.
2) Take appropriate structural measures to reduce the calorific value of the secondary heat source and reduce the temperature rise.
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, change the sliding guide rail to a linear rolling guide rail, or use a linear motor. These new technologies can effectively reduce friction, heat generation, and temperature rise.
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 being cut has no time to undergo plastic deformation, no deformation heat is generated on the chips, and most of the cutting energy is converted into kinetic energy of the chips and is taken away.
In machine tools, heat sources are always present, and further attention needs to 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 along 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 feed systems, the two ends of the ball screw are often fixed axially to form pre-tension stress. This structure not only improves dynamic and static stability for high-speed feed, but also plays a significant role in reducing thermal deformation errors.
The temperature rise of the axially fixed structure pre-stretched by 35m within the total length of 600mm is relatively similar at different feed speeds. The cumulative error of the pre-stretched structure with two ends fixed is significantly smaller than that of the structure with one end fixed and the other end free to extend. In the axially fixed pretensioned structure at both ends, the temperature rise caused by heating mainly changes the stress state inside the screw from tensile stress to zero stress or compressive stress. Therefore, it has little impact on the displacement accuracy.
2) Change the structure and change the direction of thermal deformation.
The Z-axis spindle slide of a CNC needle groove milling machine using different ball screw axial fixation structures requires a milling groove depth error of 5m during processing. Using an axial floating structure at the lower end of the screw, the groove depth gradually deepens from 0 to 0.045mm within 2 hours of processing. On the contrary, using a structure with a floating upper end of the screw can ensure that the groove depth 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 point.
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 fully considers the thermal performance.
First of all, a completely symmetrical layout is adopted in the machine tool structure. The columns and beams are integrated structures in an H shape, 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 feed drives of the three moving axes all use linear motors, which makes it easier to achieve symmetry in structure. The two rotary axes use direct drives to minimize friction losses and mechanical transmission.
1) The coolant during processing has a direct impact on the processing accuracy.
A comparative test was conducted on the GRV450C double-end grinder. Tests show that heat exchange of coolant with the help of a refrigerator is very effective in improving machining accuracy.
Using the traditional coolant supply method, the workpiece size will be out of tolerance after 30 minutes. After using a refrigerator, normal processing can last 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 grinding wheel surface), and the original machining accuracy can be restored immediately after dressing. The effect is very obvious. Likewise, very good results can be expected from forced cooling of the spindle.
2) Increase the natural cooling area.
For example, adding a natural air cooling area to the spindle box 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 the workpiece, workbench and tool parts in a timely or real-time manner will be very helpful in reducing the temperature rise and thermal deformation of key parts.
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