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The influence of temperature on the machining accuracy of the machine tool

Thermal deformation is one of the reasons that affect the machining accuracy, the machine tool is affected by the change of the ambient temperature of the workshop, the heating of the motor and the friction of mechanical movement, the heat of cutting 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, if a 70mm×1650mm screw is machined on a CNC milling machine with ordinary precision, the cumulative error of the milled workpiece from 7:30 a.m. to 9:00 a.m. can vary by up to 85m compared with the workpiece processed from 2:00 p.m. to 3:30 p.m. At constant temperature, the error can be reduced to 40m.

For another example, a precision double-end grinding machine for double-end grinding of 0.6~3.5mm thick thin steel sheet workpieces, processing 200mm×25mm×1.08mm steel sheet workpieces can achieve mm dimensional accuracy, and the bending degree is less than 5m in the full length. However, after continuous automatic grinding for 1 hour, the size change range increased to 12m, and the coolant temperature increased from 17°C at start-up to 45°C. Due to the influence of grinding heat, the spindle journal elongates and the front bearing clearance of the spindle increases. Accordingly, the addition of a 5.5 kW chiller to the machine’s coolant tank was a perfect result.

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, and a considerable part of these energy will be converted into heat in various ways, causing physical changes in the components of the machine tool, which are very different because of the different structural forms, the differences in materials and other reasons. Machine tool designers should grasp 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 the machine tool

  1. Natural climate influences

China has a vast territory, most of which is in the subtropics, and the temperature varies greatly throughout the year, and the temperature difference varies differently in one day. As a result, people intervene in different ways and degrees of temperature in the room (e.g. on the shop floor), and the temperature atmosphere around the machine can vary greatly. For example, the seasonal temperature change range in the Yangtze River Delta region is about 45 °C, and the diurnal temperature change is about 5~12 °C. The machining workshop generally has no heating in winter and no air conditioning in summer, but as long as the workshop is well ventilated, the temperature gradient in the machining workshop does not change much. In the Northeast, the seasonal temperature difference can reach 60 °C, and the diurnal variation is about 8~15 °C. The heating period is from late October to early April of the following year, and the design of the machine shop is heated, and the air circulation is insufficient. The temperature difference between the inside and outside of the workshop can reach 50°C. Therefore, the temperature gradient in the workshop in winter is very complex, the outdoor temperature is 1.5 °C when the measurement is timed, the time is 8:15-8:35 in the morning, and the temperature change in the workshop is about 3.5 °C. The machining accuracy of precision machine tools will be greatly affected by the ambient temperature in such a workshop.

  1. The influence of the 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 4 aspects.

(1) Workshop microclimate: such as the distribution of temperature in the workshop (vertical direction, horizontal direction). When day and night alternate or when the climate and ventilation change, the temperature of the workshop changes slowly.

(2) Workshop heat source: such as sun exposure, heating equipment and high-power lighting radiation, etc., they can directly affect the temperature rise of the whole or part of the machine tool for a long time when they are close to the machine tool. The heat generated by adjacent equipment during operation can affect the temperature rise of the machine in the form of radiation or air movement.

(3) Heat dissipation: the foundation has a good heat dissipation effect, especially the foundation of the precision machine tool should not be close to the underground heating pipeline, once it is ruptured and leaked, 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 equalization of the workshop.

(4) Constant temperature: It is very effective for the workshop to maintain the accuracy and machining accuracy of precision machine tools by adopting constant temperature facilities, but the energy consumption is large.

  1. Thermal influencing factors inside the machine tool

1) Structural heat source of machine tool. Electric motor heating, such as spindle motors, feed servo motors, cooling and lubrication pump motors, electric control boxes, etc., can generate heat. These conditions are permissible for the motor itself, but have a significant adverse effect on components such as spindles and ball screws, 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%), it is converted into the heat energy of the motor, and most of it will be converted into kinetic energy by the moving mechanism, such as spindle rotation, worktable movement, etc.; However, it is inevitable that a considerable part of it is still converted into friction heat during movement, such as the heat generation of bearings, guide rails, ball screws and gearboxes.

2) Cutting heat of the process. In 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 cutting and the friction heat between the chip and the tool, forming the heating of the tool, the spindle and the workpiece, and the heat of a large number of chips is transmitted to the workbench fixture and other components of the machine tool. They will have a direct impact on the relative position of the tool and the workpiece.

3) Cooling. Cooling is a countermeasure to the temperature increase of the machine tool, such as the cooling of the electric motor, the cooling of the spindle components, and the cooling of the basic structural parts. High-end machine tools often equip the electric control box with a chiller and force cooling.

  1. The influence of the structural form of the machine tool on the temperature rise

In the field of thermal deformation of machine tools, the structural form of machine tools is discussed, usually referring to the structural form, mass distribution, material properties and heat source distribution. The structural shape affects the temperature distribution of the machine tool, the direction of heat conduction, the direction of thermal deformation and matching, etc.

1) The structural form of the machine tool. In terms of overall structure, the machine tool has vertical, horizontal, gantry and cantilever type, etc., and the thermal response and stability are quite different. For example, the temperature rise of the headstock of the lathe with variable gear speed can be as high as 35°C, so that the spindle end is raised, and the thermal equilibrium time is about 2h. In the case of 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 spindle is driven by a servo motor, and the gear transmission part is removed, and its temperature rise is generally less than 15 °C.

2) The influence of heat source distribution. On machine tools, the heat source is often thought of as an electric motor. Such as spindle motors, feed motors and hydraulic systems, etc., are actually incomplete. The heat generation 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 friction work of the bearing, screw nut and guide rail. Therefore, the motor can be called the primary heat source, and the bearing, nut, guide rail and chips can be called the secondary heat source. Thermal distortion is the result of the combined influence of all these heat sources. The temperature rise and deformation of a column mobile vertical machining center in the Y-direction feed movement. The table does not move during the Y-direction feed, so it has little effect on the thermal deformation in the X-direction. On the column, the farther the point from the guide screw of the Y-axis, the smaller its temperature rise. The effect of heat source distribution on thermal deformation is further illustrated when the machine is moved in the Z-axis. The Z-axis feed is farther away from the X-direction, so the influence of thermal deformation is smaller, and the closer the column is to the Z-axis motor nut, 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 mass size and concentration, 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 form of the mass, such as the arrangement of various ribs, the thermal stiffness of the structure is improved, and the influence of thermal deformation is reduced or the relative deformation is kept small under the same temperature rise; 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), and under the influence of the same heat, their temperature rise and deformation are different.

Testing of the thermal performance of machine tools

  1. The purpose of the thermal performance test of the machine tool

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 heat source and temperature changes of the machine itself, and the response (deformation displacement) of the key points through the thermal characteristics test. Test data or curves describe the thermal characteristics of a machine tool in order to take countermeasures, control 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, changes in the temperature distribution during the alternation of day and night, and even the influence of seasonal changes on the temperature distribution around the machine tool.

2) Test the thermal characteristics of the machine tool itself. Under the condition of eliminating environmental interference as much as possible, let the machine tool be in various operating states to measure the temperature change and displacement change of the important points of the machine tool itself, record the temperature change and the displacement of key points in a long enough period of time, and also use the infrared thermal phase to record the heat distribution in each time period.

3) Test the temperature rise thermal deformation in 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 test can accumulate a large amount of data and curves, which will provide a reliable criterion for the design of machine tools and the control of thermal deformation by users, and point out the direction of effective measures.

  1. The principle of thermal deformation test of machine tools

Thermal deflection testing begins with the measurement of temperature at several relevant points, including the following:

1) Heat source: including feed motor, spindle motor, ball screw transmission pair, guide rail, spindle bearing.

2) Auxiliary devices: including hydraulic system, refrigerator, cooling and lubrication displacement detection system.

3) Mechanical structure: including bed, base, slide plate, column, milling head box and spindle. An indium steel probe is held between the spindle and the rotary table, and five 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.

  1. Test data processing and analysis

The thermal deformation test of the machine tool should be carried out in a long continuous time, continuous data recording, and after analysis and processing, the thermal deformation characteristics reflected are highly reliable. If error culling is carried out through multiple trials, the regularity shown is credible.

A total of 5 measuring points are set up in the thermal deformation test of the spindle system, among which points 1 and 2 are at the end of the spindle and close to the spindle bearing, and points 4 and 5 are respectively at the milling head housing near the Z guide rail. The test time lasted for 14 hours, of which the spindle speed in the first 10 hours was alternately changed in the range of 0~9000r/min, and 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 1h, and the temperature rise after equilibrium changes in the range of 1.5°C;

2) The temperature rise mainly comes from the spindle bearing and the spindle motor, and the thermal performance of the bearing is good in the normal speed range;

3) The effect of thermal deformation in the X direction is very small;

4) The Z-direction expansion and contraction deformation is large, about 10m, which is caused by the thermal elongation of the spindle and the increase of the bearing clearance;

5) When the speed continues at 9000r/min, the temperature rise rises sharply, rises sharply about 7°C within 2.5h, and there is a tendency to continue to rise, and the deformation in the Y and Z directions reaches 29m and 37m, indicating that the spindle can not run stably when the speed is 9000r/min, but it can run in a short time (20min).

Control of thermal deformation of machine tools

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, the main contradiction should be grasped, and one or two measures should be taken to achieve twice the result with half the effort. In the design, we should start from four directions: reduce heat generation, reduce temperature rise, balance structure, and reasonable cooling.  

  1. Reduce fever

Controlling the heat source is a fundamental measure. In the design, measures should 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 voltage V and current I, in general, the 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, then the heat dissipated in the armature impedance increases. If the motor we designed and selected works close to or greatly exceeds the rated power for a long time, the temperature rise of the motor will increase significantly. To this end, a comparative test was carried out on the milling head of BK50 CNC needle groove milling machine (motor speed: 960r/min; Ambient temperature: 12°C).

The following concepts are obtained from the above test: from the heat source performance consideration, whether the spindle motor or the feed motor, when selecting the rated power, it is best to choose about 25% greater than the calculated power, in the actual operation, the output power of the motor matches the load, and the impact of increasing the rated power of the motor on energy consumption is very small. However, it can effectively reduce the temperature rise of the motor.

2) Appropriate measures should be taken to reduce the heat generation 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 to a linear rolling guide, or use a linear motor. These new technologies can effectively reduce friction, reduce heat generation, and reduce temperature rise. Metal processing WeChat, the content is good, it is worth paying attention to!

3) In terms of process, high-speed cutting is adopted. Based on the mechanism of high-speed cutting. When the linear velocity 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 converted into chip kinetic energy and is taken away.

  1. Structural balance to reduce thermal deformation

In machine tools, where the heat source is always present, further attention needs to be paid to how the direction and speed of heat transfer can help reduce heat distortion. Or the structure has good symmetry, so that the heat transfer is along the symmetrical direction, so that the temperature distribution is uniform, and the deformation cancels each other out, becoming a thermal affinity structure.

1) Prestress and thermal deformation. In the higher-speed feed system, the ball screw is often fixed axially at both ends to form a pre-tensile stress. This structure has an obvious effect on reducing the thermal deformation error in addition to improving the dynamic and static stability of high-speed feed.

The axial fixed structure with a total length of 600 mm pre-stretched for 35 m is relatively close to the temperature rise under different feed rates. The cumulative error of the fixed pre-tensioned structure at both ends is significantly smaller than that of the structure with a single end fixed and the free elongation of the other end. In the axially fixed pre-tension structure at both ends, the temperature rise caused by heat generation mainly changes the stress state inside the screw from tensile stress to zero stress or compressive stress. Therefore, it has little effect on the displacement accuracy.

2) Change the structure and change the direction of thermal deformation. The Z-axis spindle slide of the CNC needle groove milling machine with different ball screw axial fixed structures requires a milling groove depth error of 5m in the machining. The axial floating structure at the lower end of the lead screw is adopted, and the groove depth is gradually deepened from 0 to 0.045mm within 2 hours of processing. On the contrary, the structure of floating at the upper end of the lead screw can ensure that the groove depth changes.

3) The symmetry of the geometry of the machine tool structure can make the thermal deformation trend consistent, so that the drift of the tool tip point can be reduced as much as possible. For example, the YMC430 micromachining center launched by Yasda Precision Tools in Japan is a sub-micron high-speed machining machine, and the design of the machine tool is fully considered for thermal performance.

First of all, a completely symmetrical layout is adopted in the structure of the machine tool, and the column and beam are integrated structures, which are H-shaped, which is equivalent to a double column structure and has good symmetry. The approximately circular spindle slides are also symmetrical both longitudinally and transversely.

The feed drive of the three moving axes adopts linear motors, which makes it easier to achieve symmetry in the structure, and the two rotary axes are directly driven to minimize the friction loss of mechanical transmission.

  1. Reasonable cooling measures

1) The influence of coolant in machining on machining accuracy is direct. A comparative test was carried out on the GRV450C-type double-end grinding machine. The test shows that the heat exchange treatment of the coolant with the help of the refrigerator is very effective in improving the machining accuracy.

With the traditional coolant supply method, after 30 minutes, the size of the workpiece is out of tolerance. After using the refrigerator, it can be processed normally to more than 70min. The main reason for the deviation of the workpiece size at 80 minutes is that the grinding wheel needs to be trimmed (to remove metal chips on the grinding wheel face), and the original machining accuracy can be restored immediately after trimming. The effect is very noticeable. Similarly, very good results can be expected for 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 have a good heat dissipation effect in the workshop with good air circulation.

3) Automatic chip removal in time. Timely or real-time evacuation of high-temperature chips out of the workpiece, table and tool section will be very beneficial in reducing the temperature rise and thermal distortion of critical parts. 

Vision & Vision

Controlling the thermal deformation of machine tools is an important topic in the field of modern precision machining, and the factors affecting the thermal deformation of machine tools are very complex. In addition, the simultaneous development of high speed, high efficiency and precision in modern cutting makes the thermal deformation problem of machine tools more prominent. It has attracted wide attention from the machine tool manufacturing community. Scholars in the machine tool industry at home and abroad have done a lot of research on this and have made considerable progress in theory. The problem of thermal deformation of machine tools has become one of the basic theories in machine tool research.

From the perspective of machine tool design and application, this paper analyzes the influencing factors, measurement and analysis methods of machine tool thermal performance, and proposes improvement design measures. Therefore, we believe that the optimal design of the thermal performance of the machine tool should start from the following aspects:

1) In the design stage of modern high-end machine tools, attention should be paid to the environmental conditions of the future application of the designed machine tools.

2) Controlling and configuring the heat source is key. Controlling the heat source mainly refers to the matching of the control energy consumption and the power source, adopting a new structure, reducing the secondary friction heat source, and improving the utilization rate of energy.  

3) Change the traditional thinking, and promote the cooling, heat dissipation, lubrication, chip removal and other devices from the status of “auxiliary” components of the machine tool to the status of “important” components, which cannot be underestimated.

4) Pay attention to the symmetry of the structure and the design of the direction of thermal deformation, so that the influence of thermal deformation on the accuracy is minimized, especially the research and application of the mathematical model of thermal deformation of structural parts, so as to provide qualitative and quantitative instructions for the design of thermal deformation control. 

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