Machine tools are affected by changes in workshop ambient temperature, motor heating and mechanical friction heating, cutting heat and cooling medium, resulting in uneven temperature rise in various parts of the machine tool, which leads to changes in the machine tool's shape accuracy and machining accuracy.
For example, when machining a 70mm × 1650mm screw on a standard precision CNC milling machine, the cumulative error difference between a workpiece milled at 7:00 AM and one milled at 2:30 PM can reach 85µm. However, under constant temperature conditions, the error can be reduced to 40µm.
For example, a precision double-end grinding machine used for grinding thin steel sheets with a thickness of 0.6–3.5 mm achieved a dimensional accuracy of mm when processing a 200 mm × 25 mm × 1.08 mm steel sheet during acceptance testing, with a curvature of less than 5 m over the entire length. However, after 1 hour of continuous automatic grinding, the dimensional variation range increased to 12 m, and the coolant temperature rose from 17°C at startup to 45°C. Due to the grinding heat, the spindle journal elongated, and the clearance of the spindle front bearing increased. Therefore, adding a 5.5 kW chiller to the machine's coolant tank yielded excellent results.
Practice has proven that the deformation of machine tools after heating is a significant factor affecting machining accuracy. However, machine tools operate in environments where temperature changes constantly; during operation, they inevitably consume energy, a considerable portion of which is converted into heat in various ways, causing physical changes in the machine tool's components. These changes vary greatly due to differences in structural design and materials. Machine tool designers should understand the mechanisms of heat formation and temperature distribution patterns, and take corresponding measures to minimize the impact of thermal deformation on machining accuracy.
I. Temperature rise and temperature distribution of machine tools
1. Influence of natural climate
my country has a vast territory, with most areas located in the subtropical zone, resulting in significant temperature variations throughout the year and even within a single day. Consequently, the methods and extent to which people intervene in indoor (such as workshop) temperatures differ, leading to vastly different temperature environments around machine tools.
For example, in the Yangtze River Delta region, the seasonal temperature range is approximately 45℃, with diurnal temperature variations of about 5-12℃. Machining workshops generally do not have heating in winter or air conditioning in summer, but as long as ventilation is good, the temperature gradient in the machining workshop is not significant. However, in Northeast China, the seasonal temperature difference can reach 60℃, with diurnal variations of about 8-15℃. The heating season is from late October to early April of the following year. Machining workshops are designed with heating, but air circulation is insufficient. The temperature difference between the inside and outside of the workshop can reach 50℃. Therefore, the temperature gradient inside the workshop in winter is very complex. At the time of measurement, the outdoor temperature was 1.5℃, and the time was 8:35 AM; the temperature change inside the workshop was about 3.5℃. The machining accuracy of precision machine tools will be greatly affected by the ambient temperature in such a workshop.
2. The influence of the surrounding environment
The machine tool's surrounding environment refers to the thermal environment created by various layouts within a short distance of the machine tool. This includes the following three aspects.
1) Workshop microclimate: such as the temperature distribution within the workshop (vertical and horizontal directions). The workshop temperature will change slowly when day and night alternate or when the climate and ventilation change.
2) Workshop heat sources: such as sunlight, heating equipment, and radiation from high-power lighting, which can directly and for extended periods affect the temperature rise of the machine tool as a whole or some of its components when they are close to the machine tool. The heat generated by adjacent equipment during operation can also affect the temperature rise of the machine tool through radiation or airflow.
3) Heat dissipation: The foundation has a good heat dissipation effect. In particular, the foundation of precision machine tools should not be close to underground heating pipes. Once they break and leak, they may become a heat source that is difficult to find the cause. An open workshop will be a good "radiator" and will help to balance the temperature in the workshop.
4) Temperature control: Using temperature control facilities in the workshop is very effective in maintaining the accuracy and machining precision of precision machine tools, but it consumes a lot of energy.
3. Internal thermal factors of machine tools
1) Structural heat sources in machine tools. Electric motors, such as spindle motors, feed servo motors, cooling and lubrication pump motors, and electrical control boxes, can all generate heat. While this is permissible for the motors themselves, it has a significant adverse impact on components such as the spindle and ball screw, and measures should be taken to isolate them. When electrical energy drives the motor, a small portion (approximately 20%) is converted into heat energy, while most is converted into kinetic energy by the moving mechanisms, such as spindle rotation and table movement. However, a considerable portion inevitably still generates frictional heat during operation, such as from bearings, guideways, ball screws, and transmission boxes.
2) Cutting heat during the manufacturing process. During the cutting process, part of the kinetic energy of the tool or workpiece is consumed as cutting work, while a considerable portion is converted into cutting deformation energy and frictional heat between the chips and the tool, generating heat in the tool, spindle, and workpiece. A large amount of chip heat is then conducted to components such as the machine tool's worktable and fixtures. These heats directly affect the relative position between the tool and the workpiece.
3) Cooling. Cooling is a countermeasure against rising machine tool temperatures, such as motor cooling, spindle component cooling, and basic structural component cooling. High-end machine tools often equip the electrical control box with a chiller for forced cooling.
4. The influence of machine tool structure on temperature rise
In the field of machine tool thermal deformation, discussions of machine tool structural morphology typically refer to issues such as structural form, mass distribution, material properties, and heat source distribution. Structural morphology influences the machine tool's temperature distribution, heat conduction direction, thermal deformation direction, and matching.
1) Machine Tool Structure. In terms of overall structure, machine tools include vertical, horizontal, gantry, and cantilever types, each with significant differences in thermal response and stability. For example, the temperature rise of the spindle box of a gear-driven lathe can reach as high as 35°C, requiring approximately 2 hours for thermal equilibrium due to spindle elevation. In contrast, a slant bed precision turning and milling machining center has a stable base, significantly improving overall machine rigidity. The spindle is driven by a servo motor, eliminating the gear transmission component, 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 considered to be the electric motor, such as the spindle motor, feed motor, and hydraulic system. However, this is not entirely accurate. The heat generated by the electric motor is only the energy consumed by the current in the armature impedance when under load. A significant portion of the energy is also consumed by the friction work of mechanisms such as bearings, lead screw nuts, and guide rails. Therefore, the electric motor can be considered the primary heat source, while the bearings, nuts, guide rails, and chips can be considered secondary heat sources. Thermal deformation is the result of the combined effect of all these heat sources.
Temperature rise and deformation of a vertical machining center with a moving column during Y-axis feed motion. The worktable does not move during Y-axis feed, so its impact on X-axis thermal deformation is minimal. On the column, the point farther from the Y-axis guide rail and leadscrew experiences the smallest temperature rise.
The situation when the machine moves along the Z-axis further illustrates the influence of heat source distribution on thermal deformation. The Z-axis feed is farther from the X-axis, so the impact of thermal deformation is smaller. The closer the nut on the column is to the Z-axis motor, the greater the temperature rise and deformation.
3) The Influence of Mass Distribution. The influence of mass distribution on the thermal deformation of machine tools is threefold. First, it refers to the size and concentration of mass, which usually means changing the heat capacity and the rate of heat transfer, thus altering the time to reach thermal equilibrium. Second, by changing the arrangement of mass, such as the arrangement of various stiffeners, the thermal stiffness of the structure can be increased, thereby reducing the impact of thermal deformation or maintaining relatively small deformation under the same temperature rise. Third, it refers to reducing the temperature rise of machine tool components by changing the form of mass arrangement, such as arranging heat dissipation fins on the outside of the structure.
4) Influence of material properties: Different materials have different thermal performance parameters (specific heat, thermal conductivity and coefficient of linear expansion). Under the influence of the same amount of heat, their temperature rise and deformation will be different.
II. Testing of Machine Tool Thermal Performance
1. Purpose of machine tool thermal performance testing
The key to controlling machine tool thermal deformation is to fully understand the changes in the ambient temperature, the machine tool's own heat sources and temperature changes, and the response (deformation displacement) at key points through thermal characteristic testing. Test data or curves describe the thermal characteristics of a machine tool, enabling countermeasures to be taken to control thermal deformation and improve the machine tool's machining accuracy and efficiency. Specifically, the following objectives should be achieved:
1) Machine tool surrounding environment testing. Measure the temperature environment within the workshop, its spatial temperature gradient, the changes in temperature distribution during day and night, and even the impact of seasonal changes on the temperature distribution around the machine tool.
2) Thermal characteristic testing of the machine tool itself. Under conditions that minimize environmental interference, allow the machine tool to operate in various states to measure the temperature and displacement changes at key points of the machine tool itself. Record the temperature changes and key point displacements over a sufficiently long period of time. Alternatively, an infrared thermal imager can be used to record the heat distribution over different time periods.
3) Test the temperature rise and thermal deformation during the machining process to determine the impact of machine tool thermal deformation on the machining accuracy.
4) The above experiments 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 for taking effective measures.
2. Principle of machine tool thermal deformation testing
Heat distortion testing first requires measuring the temperature at several relevant points, including the following aspects:
1) Heat source: including feed motors, spindle motors, ball screw drives, guide rails, and spindle bearings.
2) Auxiliary devices: including hydraulic system, refrigeration unit, cooling and lubrication displacement detection system.
3) Mechanical structure: including bed, base, slide, column, milling head housing and spindle.
An indium steel measuring rod is clamped between the spindle and the rotary table, and five contact sensors are configured in the X, Y, and Z directions to measure the overall deformation under various conditions in order to simulate the relative displacement between the tool and the workpiece.
3. Test data processing and analysis
Machine tool thermal deformation tests must be conducted over a relatively long and continuous period, with continuous data recording and analysis. The resulting thermal deformation characteristics are highly reliable. If errors are eliminated through multiple tests, the observed patterns are considered credible.
Five measurement points were set up for the thermal deformation test of the spindle system. Points 1 and 2 were located at the end of the spindle and near the spindle bearing, respectively, while points 4 and 5 were located on the milling head housing near the Z-guide rail. The test lasted for 14 hours. For the first 10 hours, the spindle speed alternated between 0 and 9000 r/min. From the 10th hour onwards, the spindle rotated continuously at a high speed of 9000 r/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 bearing and spindle motor. Within the normal speed range, the thermal performance of the bearing is good.
3) Thermal deformation has little effect in the X-direction;
4) The Z-axis expansion and contraction deformation is relatively large, about 10m, which is caused by the thermal expansion of the main shaft and the increase in bearing clearance;
5) When the rotation speed is maintained at 9000 r/min, the temperature rises sharply, increasing by about 7°C within 2.5 hours, and there is a continuing upward trend. The deformation in the Y and Z directions reaches 29m and 37m respectively, indicating that the spindle can no longer operate stably at a rotation speed of 9000 r/min, but can operate for a short time (20 min).
III. Control of machine tool thermal deformation
Based on the above analysis and discussion, the factors affecting the machining accuracy of machine tools' temperature rise and thermal deformation are diverse. When taking control measures, it is essential to focus on the main issues and implement one or two key measures to achieve optimal results. The design should address these issues from four perspectives: reducing heat generation, lowering temperature rise, ensuring structural balance, and implementing appropriate cooling.
1. Reduce fever
Controlling the heat source is the fundamental measure. The design should incorporate measures to effectively reduce the heat generated by the heat source.
1) Select the rated power of the motor appropriately.
The output power P of an electric motor is equal to the product of voltage V and current I. Under normal circumstances, voltage V is constant. Therefore, an increase in load means an increase in the motor's output power, which in turn increases the current I, leading to increased heat dissipation in the armature impedance. If the motor we designed and selected operates for a long time under conditions close to or significantly exceeding its rated power, the motor's temperature rise will increase significantly. Therefore, a comparative test was conducted on the milling head of a BK50 CNC pin slot milling machine (motor speed: 960 r/min; ambient temperature: 12℃).
The above experiments yielded the following conclusions: Considering heat source performance, when selecting the rated power of either the spindle motor or the feed motor, it is best to choose one that is approximately 25% larger than the calculated power. In actual operation, the motor's output power matches the load, and increasing the motor's rated power has little impact on energy consumption. However, it can effectively reduce the motor's temperature rise.
2) Take appropriate structural measures to reduce the heat generation of the secondary heat source and lower the temperature rise.
For example, in the design of the spindle structure, the coaxiality of the front and rear bearings should be improved, and high-precision bearings should be used. Where possible, sliding guides should be replaced with linear rolling guides, or a linear motor should be used. These new technologies can effectively reduce friction, reduce heat generation, and lower temperature rise.
3) In terms of process, high-speed cutting is adopted. This is 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 does not have time to undergo plastic deformation, and no deformation heat is generated on the chips. Most of the cutting energy is converted into chip kinetic energy and carried away.
2. Structural balance to reduce thermal deformation.
In machine tools, heat sources are always present. Further attention needs to be paid to how to make the direction and speed of heat transfer conducive to reducing thermal deformation. Alternatively, the structure can have good symmetry, ensuring heat transfer along symmetrical directions, resulting in uniform temperature distribution and mutual cancellation of deformation, thus creating a thermally compatible structure.
1) Prestressing and thermal deformation.
In high-speed feed systems, ball screws are often axially fixed at both ends to create pre-tension stress. This structure not only improves dynamic and static stability for high-speed feeds but also significantly reduces thermal deformation errors.
The temperature rise of the axially fixed structure with a pre-tensioned length of 35m over a total length of 600mm was relatively similar under different feed rates. The cumulative error of the pre-tensioned structure with fixed ends was significantly smaller than that of the structure with one end fixed and the other end freely extended. In the pre-tensioned structure with axial fixation at both ends, the temperature rise caused by heat mainly changes the stress state inside the lead screw from tensile stress to zero stress or compressive stress. Therefore, it has a relatively small impact on displacement accuracy.
2) Change the structure to change the direction of thermal deformation.
The Z-axis spindle slide of a CNC pin groove milling machine employing different ball screw axial fixing structures requires a milling groove depth error of 5mm during machining. Using an axially floating structure at the lower end of the ball screw, the groove depth gradually increases from 0 to 0.045mm within 2 hours of machining. Conversely, using a structure with a floating upper end of the ball screw ensures consistent groove depth variation.
3) The symmetry of the machine tool structure geometry can make the thermal deformation direction consistent, thus minimizing the drift of the tool tip.
For example, the YMC430 micromachining center launched by Yasda Precision Tools Co., Ltd. of Japan is a submicron high-speed machining center, and the design of the machine tool has taken thermal performance into full consideration.
Firstly, the machine tool adopts a completely symmetrical layout in its structure. The column and crossbeam are an integrated structure, H-shaped, equivalent to a double-column structure, which has good symmetry. The near-circular spindle slide is also symmetrical in both the longitudinal and transverse directions.
The feed drives of the three moving axes all use linear motors, which makes it easier to achieve symmetry in the structure. The two rotary axes use direct drive to minimize friction loss in mechanical transmission.
3. Reasonable cooling measures
1) The coolant used in machining has a direct impact on machining accuracy.
A comparative experiment was conducted on the GRV450C double-end face grinder. The experiment showed that using a refrigeration unit to exchange heat with the coolant is very effective in improving machining accuracy.
Using a traditional coolant supply method, the workpiece dimensions become out of tolerance after 30 minutes. With a refrigeration unit, machining can proceed normally for over 70 minutes. The main reason for the workpiece dimensions becoming out of tolerance after 80 minutes is that the grinding wheel needs dressing (removing metal shavings from its surface). After dressing, the original machining accuracy is immediately restored. The effect is very noticeable. Similarly, forced cooling of the spindle can also be expected to achieve excellent results.
2) Increase the area for natural cooling.
For example, adding natural air cooling area to the spindle box structure can also achieve a good heat dissipation effect in workshops with good air circulation.
3) Timely and automatic chip removal.
Timely or real-time removal of high-temperature chips from the workpiece, worktable, and tooling section will greatly help reduce temperature rise and thermal deformation in critical parts.
