1. Thermal deformation is one of the reasons that affect machining accuracy.
When the machine tool is affected by the change of ambient temperature in the workshop, motor heat and mechanical motion frictional heat, cutting heat and cooling medium, uneven temperature rise of all parts of the machine tool will happen, leading to changes in the shape of the machine tool accuracy and machining accuracy.
For example, when machining φ70mm×1650mm screw in an ordinary precision CNC milling machine, the cumulative error change of the workpiece milled at 7:30-9:00 a.m. compared with the workpiece machined at 2:00-3:30 p.m. can reach 85μm.
At constant temperature, the error can be reduced to 40μm.
Another example is a precision double face grinding machine used for double face grinding of 0.6 to 3.5mm thick thin steel workpieces, which was able to process 200mm x 25mm x 1.08mm steel workpieces with a dimensional accuracy of mm and a curvature of less than 5μm over its entire length.
However, after 1 hour of continuous automatic grinding, the dimensional variation increased to 12 μm, and the coolant temperature rose from 17°C at start-up to 45°C.
Due to the influence of grinding heat, the spindle journal elongates and the spindle front bearing clearance increases.
Accordingly, a 5.5kW chiller was added to the machine’s coolant tank, with ideal results.
It has been proved that the deformation of the machine tool after being heated is an important reason for the machining accuracy.
However, the machine is in an environment where the temperature is constantly changing.
The machine itself inevitably consumes energy in its operation, and a significant part of this energy is converted into heat in various ways, causing physical changes in the various components of the machine.
This variation varies greatly due to different structural forms, differences in materials, etc.
The machine tool designer should grasp the mechanism of heat formation and temperature distribution laws, and take appropriate measures to minimize the impact of thermal deformation on machining accuracy.
2. Temperature rise and temperature distribution of the machine tool
2.1 Natural Climate Impacts
China is a vast country, most of which is in the subtropical region, where the temperature varies greatly throughout the year, and the difference in temperature varies throughout the day.
As a result, people intervene in indoor (e.g. workshop) temperatures in different ways and to different degrees, and the temperature atmosphere around machine tools varies greatly.
For example, the Yangtze River Delta has a seasonal temperature variation range of about 45 ℃, and the temperature changes about 5 to 12 ℃ during the day and night.
Workshop generally has no heating in winter, no air conditioning in summer, but as long as the workshop ventilation is better, the temperature gradient of the machine shop does not change much.
In the northeast, the seasonal temperature difference is up to 60 ℃, day and night change is about 8 ~ 15 ℃.
From late October to early April of the next year is the heating period, the design of the machining workshop has heating, so the air circulation is insufficient.
The temperature difference between inside and outside of the workshop can reach 50℃.
Therefore, the temperature gradient in the workshop in winter is very complicated, the outdoor temperature at the time of measurement is 1.5℃, the time is 8:15-8:35am, 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.
2.2 Impact of the surrounding environment
The environment around the machine is the thermal environment created by the various layouts within close proximity of the machine.
They include the following three aspects:
(1) Workshop microclimate: Such as the distribution of temperature in the workshop (vertical, horizontal).
The temperature of the workshop changes slowly when there are changes in the day and night or in climate and ventilation.
(2) Workshop heat sources: such as solar radiation, heating equipment and high-power lighting radiation, etc..
Their proximity to the machine can have a direct and prolonged effect on the temperature rise of the machine or parts of the machine.
Neighboring equipment in operation during the heat generated by radiation or air flow will affect the way the machine temperature rise.
(3) Heat Dissipation.
Foundations provide good heat dissipation, especially for precision machine tools, and should not be located near underground heating pipes, which can be a difficult heat source to locate in the event of a rupture or leak.
An open workshop will be a good “radiator”, which is conducive to a balanced workshop temperature.
(4) Constant temperature.
The adoption of constant temperature facilities in the workshop is effective in maintaining precision and machining accuracy for precision machines, but consumes more energy.
2.3 Thermal influence factors inside the machine tool
(1) Machine tool structural heat source.
Electric motors generate heat – such as spindle motors, feed servo motors, cooling and lubrication pump motors, and electronic control boxes.
These conditions are permissible for the motor itself, but have a significant adverse effect on the spindle, ball screw and other components, and measures should be taken to isolate them.
When the input electrical energy drives the motor operation, except for a small part (about 20%) which is transformed into motor heat energy, most of it will be transformed into kinetic energy by the movement mechanism, such as spindle rotation, table movement, etc..
But inevitably there is still a considerable part of the process of movement into frictional heat, such as bearings, rails, ball screws and transmission heat and other mechanisms.
(2) The cutting heat of the process.
Part of the kinetic energy of the tool or workpiece in the cutting process is consumed in cutting work.
A considerable part of it is converted into deformation energy of cutting and the frictional heat between the chips and the tool, resulting in heat generation of the tool, spindle and workpiece, and transferred by a large amount of chip heat to the machine tool’s table fixtures and other components.
They will have a direct impact on the cutting tool and the relative position of the workpiece.
Cooling is the reverse of measures taken in response to an increase in the temperature of the machine tool, such as motor cooling, spindle component cooling, and base structure component cooling.
High-end machines are often equipped with chillers for the electronic control box to force cooling.
2.4 Influence of the structural form of the machine on the 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 structural form affects the temperature distribution of the machine, the direction of heat conduction, the direction of thermal deformation and matching.
(1) Structure of machine tools
In terms of the overall structure, machine tools have vertical, horizontal, gantry and cantilever, etc., which have large differences in thermal response and stability.
For example, the temperature rise of the spindle box of a gear-shifting lathe can be as high as 35°C, which causes the spindle end to rise, and the thermal equilibrium time takes about 2h.
But for the inclined bed type precision milling machining center, the machine tool has a stable base, which significantly improves the rigidity of the machine.
The spindle is driven by a servo motor, with the removal of the gear transmission part, the temperature rise is generally less than 15 ℃.
(2) Effects of the heat source distribution
Heat sources are often thought of on machine tools as electric motors, such as spindle motors, feed motors and hydraulic systems, etc., which are actually incomplete.
The heat generation of the motor is only the energy consumed by the current on the armature impedance when the load is borne, and a considerable part of the energy is consumed by the heat generation caused by the frictional work of the bearing, screw nut and guide rail, etc.
Therefore, the motor can be called the primary heat source and the bearings, nuts, rails and chips are the secondary heat source.
Thermal deformation is the result of the combined effect of all these heat sources.
(3) Effect of mass distribution
The effect of mass distribution on the thermal deformation of machine tools is threefold.
First, it refers to the size and concentration of the mass, which usually means that it changes the heat capacity and the rate of heat transfer, changing the time it takes to reach thermal equilibrium.
Secondly, to increase the thermal stiffness of the structure by changing the arrangement of the mass, such as the arrangement of various ribs, to reduce the effect of thermal deformation or to keep the relative small deformation for the same temperature rise.
Thirdly, it refers to the reduction of the temperature rise of the machine component by changing the form of mass arrangement, for example, heat dissipation ribs can be placed on the outside of the structure to reduce the temperature rise of machine tool components.
(4) The 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 heat, their temperature rise and deformation are different.
3. Test of machine tools thermal properties
3.1 Purpose of thermal performance testing of machine tools
The key to controlling the thermal deformation of a machine is to fully understand the machine’s ambient temperature changes, the machine’s own heat source and temperature changes, and the response of critical points (deformation displacements) through thermal characteristic testing.
The test data or curve describes the thermal characteristics of a machine tool so that countermeasures can be taken to control thermal deformation and improve the machining accuracy and efficiency of the machine tool.
Specifically, the following four objectives should be met:
(1) Ambient test of machine tools
It can measure the temperature environment in the workshop, its spatial temperature gradient, the changes in temperature distribution in the day-night change, and even the effect of seasonal changes on the temperature distribution around the machine tool.
(2) Test the thermal characteristics of the machine tool itself.
Keeping the machine tool in various operating states under conditions that exclude environmental disturbances as much as possible to measure temperature changes, displacement changes, at important points in the machine tool itself.
It can record temperature changes and critical point displacements over a sufficiently long time period, and also record heat distribution overtime periods with an infrared thermal phase meter.
(3) The machining process tests the temperature rise and thermal deformation to determine the influence of the machine tool thermal deformation on the accuracy of the machining process.
(4) The above tests can accumulate a large amount of data and curves, which will provide reliable criteria for machine design and user control of thermal deformation, pointing out the direction of taking effective measures.
3.2 Principles of machine tool thermal deformation testing
(1) Heat source: It includes each part of feed motor, spindle motor, ball screw drive pair, guide rail, and spindle bearing.
(2) Auxiliary devices: It includes hydraulic system, chiller, cooling and lubrication displacement detection system.
(3) Mechanical structure: It includes bed, base, slide, column, milling head box and spindle.
An indium steel bar is clamped between the spindle and the rotary table, and five contact sensors are installed in the X, Y and Z directions to measure the combined deformation in various states to simulate the relative displacement between the tool and the workpiece.
3.3 Test data processing and analysis
The machine tool thermal deformation test is performed over a long continuous period of time for continuous data recording.
After analysis and processing, the reflected heat deformation characteristics are highly reliable.
The regularity shown is plausible if error elimination is carried out through multiple tests.
There are 5 measuring points in the test, including point 1 and point 2 at the spindle end and near the spindle bearing, and point 4 and point 5 at the milling head shell near the Z-rail respectively.
The test time lasted for 14h in total, in which the spindle speed in the first 10h alternated between 0 and 9000r/min.
From the 10th hour, the spindle continues to rotate at a high speed of 9000r/min.
(1) The thermal equilibrium time of this spindle is about 1h, and the range of temperature rise change after equilibrium is 1.5°C.
(2) The temperature rise mainly comes from the main shaft bearing and the main shaft motor, the bearing’s thermal state performance is good in the normal speed range.
(3) Thermal deformation in the X-direction is very small, about <0.005mm, and the Y-direction deformation is much larger than the X-direction.
As the temperature rise of point 4, point 5 is <1℃, the main deformation of the spindle Y comes from the structure of the milling head box, with no correlation with the screw and guide rail, so it should improve the casting structure of the milling head.
(4) Z-directional expansion and contraction are large, about 10 μm, which is caused by the thermal elongation of the spindle and the increase in bearing clearance.
(5) When the rotational speed is maintained at 9000r/min, the temperature rises sharply, rising sharply by about 7℃ in 2.5h, and there is a tendency to continue to rise.
The deformations in the Y and Z directions reached 29μm and 37μm, indicating that the spindle is no longer stable at a speed of 9000r/min, but it can be run for a short time (≤20min).
4. Control of machine tools thermal deformation
From the above analysis and discussion, the temperature rise and thermal deformation of machine tools affect machining accuracy by a variety of factors.
When taking control measures, it should grasp the main contradiction and focus on taking one or two measures to get yield twice the result with half the effort.
In the design should start from four aspects: heat reduction, temperature rise reduction, structural balance and reasonable cooling.
4.1 Reduction of 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) Reasonable selection of 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, the voltage V is constant.
Therefore, an increase in load means an increase in motor output power, i.e., the corresponding current I also increases, while the heat consumed by the current in the armature impedance increases.
If the motor we designed and selected works for a long time under conditions close to or greatly exceeding the rated power, the temperature rise of the motor will increase significantly.
For this reason, a comparative test was conducted on the milling head of BK50 CNC pin slot milling machine (motor speed: 960r/min; ambient temperature: 12℃).
From the above tests, the following concept has been obtained: when selecting the rated power for both the spindle motor and the feed motor, it is best to select a power that is about 25% larger than the calculated power, considering the heat source performance.
In actual operation, the motor’s output power matches the load, so increasing the motor’s rated power has little effect on energy consumption.
However, it can effectively reduce the temperature rise of the motor.
(2) It should take appropriate structural measures 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.
Under possible conditions, the sliding guide rail should be changed to linear rolling guide rail, or linear motor should be adopted.
These new technologies can all effectively reduce friction, heat generation and temperature rise.
(3) In the process, high speed cutting based on the mechanism of high speed cutting is used.
When the linear speed of metal cutting is higher than a certain range, the metal to be cut will not produce plastic deformation in time.
No heat of deformation is generated on the chips, and most of the cutting energy is converted to chip kinetic energy and removed.
4.2 Structural balance to reduce thermal deformation
On a machine tool, the heat source is always there.
A further concern is how to make the direction and speed of heat transfer conducive to reducing thermal deformation.
Alternatively, the structure may be well symmetrical so that the heat transfer is in the symmetrical direction, which makes the temperature distribution uniform, and the deformations cancel each other out, becoming a thermal affinity structure.
(1) Pre-stress and thermal deformation.
In the higher speed feed system, it often uses ball screw axially fixed at both ends, forming a pre-tensile stress.
This structure for high-speed feed, in addition to improving the dynamic and static stability, it has an obvious role in reducing the thermal deformation error.
The ball screw fixing method has an influence on the thermal deformation of the feed drive system.
The temperature rise of an axially fixed structure pre-stretched to 35 μm over its entire length of 600 mm is comparable at different feed rates.
The cumulative error of the two-sided fixed pre-stretched structure is significantly smaller than that of a single-sided fixed structure with free extension at the other end.
In both ends of the axially fixed pre-stretched structure, the temperature rise caused by heat is mainly to change the internal stress state of the screw – from tensile stress to zero or compressive stress.
Therefore, it has less influence on the displacement accuracy.
(2) Change the structure, change the direction of thermal deformation.
It requires to use different ball screw axially fixed structure of CNC needle groove milling machine Z-axis spindle slides.
In the processing of the requirements of milling depth error is ≤ 5μm.
When using the lower end of the screw axial floating structure, within 2h of machining, the groove depth is gradually deepened from 0 to 0.045mm.
On the other hand, the upper end of the screw is floating, which ensures that the depth of groove changes by <5μm.
The symmetry of the machine structure geometry can make the thermal deformation direction consistent, so that the drift of the tool tip point is minimized.
For example, the YMC430 micro machining center launched by Japan’s Yasuda Precision Tools Corporation is a sub-micron high-speed machining center, the machine is designed with full consideration of the thermal properties and its overall structural configuration.
Firstly, the machine structure takes a completely symmetrical layout, the column and crossrail are integrated structure with H-shaped, which is equivalent to a double-column structure, with good symmetry.
The nearly circular spindle slide is also symmetrical in both longitudinal and transverse directions.
The feed drives of the 3 moving axes are all linear motors, making it easier to achieve symmetry in structure.
The 2 rotary shafts are directly driven to minimize mechanical drive friction losses and.
(1) The influence of coolant on machining accuracy during processing is direct.
A comparative test was carried out on the GRV450C double-end grinding machine, and the data are shown in Table 2.
The test shows that the heat exchange processing of the coolant with the aid of a chiller is very effective in improving machining accuracy.
With the conventional coolant supply method, the dimensions of the workpiece were extremely poor after 30 minutes.
With the use of a chiller, normal machining is possible for more than 70 min.
At 80min, the main reason for the workpiece size difference is that the grinding wheel needs to be trimmed (remove the metal chips on the surface of the grinding wheel), and the original machining accuracy can be restored immediately after trimming.
The effect is obvious.
Similarly, it is expected that the forced cooling of the spindle will be very effective.
(2) Increase the natural cooling area.
For example, adding the natural air cooling area to the spindle case structure can also have a good heat dissipation effect in the workshop where the air circulation is better.
(3)Automatic chip removal in time.
Timely or real-time high-temperature chips will be discharged from the workpiece, table and tool parts, which will be very helpful to reduce the temperature rise and thermal deformation of the key parts.
5. Outlook and vision
The control of thermal deformation of machine tools is an important issue in the field of modern precision machining, and the factors affecting the thermal deformation of machine tools are very complex.
Furthermore, the high-speed, high efficiency and precision in the modern cutting process develop simultaneously, which makes the thermal deformation of machine tools more prominent, causing a big concern from the machine tool manufacturing community.
Domestic and foreign machine tool industry scholars have made a lot of research, and made considerable progress in theory.
The problem of thermal deformation in machine tools has become one of the basic theories in machine tool research.
In this article, it analyzes the factors influencing the thermal performance of machine tools from the perspective of machine design and application, and the measurement and analysis methods, and design improvement measures are proposed.
So, we believe that the optimal design of the thermal properties of machine tools should start from the following aspects:
(1) In the design phase of modern high-end machine tools, attention should be paid to the environmental conditions of the future application of the machine tool designed.
(2) Control and configuration of the heat source is the key.
Control of the heat source mainly refers to the control of energy consumption and power source matching and the use of new structures to reduce the secondary friction heat source, which helps to improve energy efficiency.
(3) It should change the traditional thinking, which requires the cooling, heat dissipation, lubrication, chip removal and other devices from the status of “auxiliary” parts of the machine tool, elevated to the status of “important” components, without underestimate.
(4) It needs to pay attention to the symmetry of the structure and the design of thermal deformation directions, which helps to minimize the influence of thermal deformation on the accuracy, especially the study and application of mathematical models of thermal deformation of structural components to provide qualitative and quantitative indications for the design of thermal deformation control.