Understanding the Impact of Temperature on CNC Machining Accuracy

Thermal deformation is one of the factors that affect machining accuracy.

Thermal deformation affects machining accuracy in several ways. Changes in workshop ambient temperature, frictional heating due to engine operation and mechanical movement, cutting heat and cooling medium can cause uneven temperature increases in various parts of the machine tool, leading to changes in shape accuracy and precision machine machining.

For example, when machining a 70mm x 1650mm screw on a common precision CNC milling machine, the cumulative error change between parts processed from 7:30am to 9:00am and those processed from 2:00pm to 3:30pm can reach up to 85m. However, under constant temperature conditions, the error can be reduced to 40m.

Another example is a precision double surface grinding machine used for double-sided grinding of thin steel sheet parts with a thickness of 0.6-3.5mm. After continuous automatic grinding for 1 hour, the dimensional change range increases to 12m and the coolant temperature rises from 17°C at startup to 45°C. This increase in temperature causes the spindle journal to lengthen and the bearing clearance in front of the spindle to increase. Adding a 5.5 kW cooler to the machine's coolant tank has proven effective in this situation.

In conclusion, thermal deformation is a significant factor that affects machining accuracy, especially in an environment where the temperature is constantly changing. The machine tool consumes energy during operation, and a significant portion of this energy is converted into heat, causing physical changes in various components of the machine tool. Machine tool designers must understand the heat formation mechanism and temperature distribution rules and take measures to minimize the impact of thermal deformation on machining accuracy.

Increased temperature and distribution of machine tools and the influence of natural climate

1. Natural climate impact

China is a large country, located mainly in the subtropics. The temperature varies greatly throughout the year and presents different temperature fluctuations during the day. As a result, people's interventions to regulate the temperature in the room, such as in the workshop, also vary, and the temperature around the machine tool is very different.

For example, in the Yangtze River Delta, the seasonal temperature variation is about 45°C, and the temperature change between day and night is about 5-12°C. The machining workshop generally does not have heating in winter or air conditioning in summer, but as long as the workshop is well ventilated, the temperature gradient in the workshop does not change much.

In Northeast China, the seasonal temperature difference can reach 60°C, and the day and night change is about 8-15°C. The heating period is from the end of October to the beginning of April of the following year, and the machining shop is designed to provide heating in case of insufficient air circulation. The temperature difference between the inside and outside of the workshop can reach 50°C, causing a complicated temperature gradient in winter. For example, when measured between 8:15 am and 8:35 am, the outside temperature is 1.5°C and the temperature change in the workshop is about 3.5°C.

The ambient temperature in such a workshop can greatly affect the machining accuracy of precision machine tools.

2. Influence of the surrounding environment

The surrounding environment of a machine tool refers to the thermal environment formed by various factors in the vicinity of the machine tool. These factors include:

(1) Workshop Microclimate: such as the temperature distribution in the workshop, which changes slowly with the changes of day and night, climate or ventilation.

(2) Workshop heat sources: such as solar radiation, heating equipment and high-power lighting. These sources, when close to the machine tool, can have a direct and lasting effect on increasing the temperature of all or part of the machine tool. 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 must be able to dissipate heat effectively, especially the foundation of precision machine tools, which should not be located near underground heating pipelines. If a pipe breaks and leaks, it can become a heat source that is difficult to locate, but an open workshop can serve as a good “radiator” and help equalize the temperature in the workshop.

(4) Constant Temperature: Maintaining a constant temperature in the workshop can effectively preserve the processing precision and accuracy of precision machine tools, but it can also lead to high energy consumption.

3.There are no thermal influence factors inside the machine tool

(1) Structural heat sources of machine tools

Heating of motors such as spindle motor, servo feed motor, cooling and lubrication pump motor, electrical control box, etc. may generate heat. While these conditions are acceptable for the motors themselves, they have a significant impact on components such as the screw and ball screw. Measures must be taken to isolate them.

When electrical energy runs the motor, most of it will be converted into kinetic energy by motion mechanisms such as spindle rotation and table movement, while a small part (about 20%) is converted into thermal energy from the motor. However, a considerable part is inevitably converted into frictional heat during movement. Components such as bearings, guide rails, ball screws and gearboxes also generate heat.

(2) Heat cut during process

During the cutting process, part of the kinetic energy of the tool or workpiece is consumed by the cutting work. A significant portion is converted into cutting deformation energy and frictional heat between the chip and the tool, which generates heat in the tool, spindle and workpiece. In addition, a large amount of heat from the chip is transmitted to the machine tool table support and other components, which will directly affect the relative position between the tool and the workpiece.

(3) Cooling

Cooling is a countermeasure against the temperature rise of machine tools such as cooling motors, spindle components and infrastructure. High-end machine tools often use coolers to cool their electronic control boxes.

4. The influence of machine tool structure on temperature rise

In the field of thermal deformation of machine tools, the structure of the machine tool is generally referred to in terms of its structural shape, mass distribution, material properties, and heat source distribution. The shape of the structure affects temperature distribution, heat conduction direction, thermal deformation direction, and machine tool matching, among other factors.

(1) Structural form of machine tool: In terms of general structure, machine tools can be vertical, horizontal, gantry or cantilever types, which have great differences in thermal response and stability. For example, the temperature rise of a gear-shift lathe head can reach 35°C and it takes about 2 hours for thermal equilibrium to be reached when the spindle end is lifted. In contrast, the temperature rise for an inclined base type precision milling and turning machining center is generally less than 15°C as it has a stable base that improves the rigidity of the entire machine and a servo motor that drives the main axis.

(2) Influence of heat source distribution: Machine tools generally consider the electric motor as the heat source, such as spindle motor, feed motor, hydraulic system, etc. However, this is an incomplete view, as a considerable part of the energy is consumed by the heating caused by frictional work. of bearings, screw nuts, guide rails and chips. The motor can be considered a primary heat source, while the bearings, nuts, guide rails, and chips can be considered secondary heat sources, and thermal deformation is the result of their combined effects.

(3) Effect of mass distribution: The influence of mass distribution on thermal deformation has three aspects: (i) size and mass concentration, which affect the heat capacity and heat transfer speed, and the time to reach thermal balance, (ii) change in layout quality, such as adding multiple ribs to improve thermal stiffness, reduce thermal deformation, or keep relative deformation small under the same temperature rise, and (iii) reduce the temperature rise of components of the machine tool by changing the shape of the quality arrangement, such as adding heat dissipation ribs outside the structure.

(4) Influence of material properties: Different materials have different thermal performance parameters such as specific heat, thermal conductivity and linear expansion coefficient. Under the same heat, the temperature rise and deformation will be different.

Machine Tool Thermal Performance Test

1. Purpose of machine tool thermal performance test

The key to controlling thermal deformation in machine tools is a thorough understanding of changes in ambient temperature, heat sources and temperature changes within the machine tool, as well as the response of key points (deformation displacement) through thermal tests. By measuring the thermal characteristics of the machine tool, countermeasures can be taken to control thermal deformation and improve the machine's accuracy and efficiency.

The following objectives must be achieved through testing:

(1) Testing the machine environment: Measure the temperature in the workshop, the spatial temperature gradient, changes in temperature distribution throughout the day and night, and the impact of seasonal changes in temperature distribution around the machine tool .

(2) Machine tool thermal characteristics test: Eliminate environmental interference as much as possible and measure the temperature and displacement changes of important points on the machine tool during various operating states. Record temperature changes and displacements of key points over a sufficient period of time, using infrared thermal imaging instruments to capture the thermal distribution at each time period.

(3) Temperature rise and thermal deformation test during processing: Evaluate the impact of thermal deformation on processing accuracy by measuring the temperature rise and thermal deformation during processing.

(4) Accumulation of data and curves: Experiments can accumulate a large amount of data and curves, providing reliable criteria for machine tool design and thermal deformation control, and indicating the direction for effective measurements.

2. The principle of thermal deformation test of machine tools

The thermal deformation test begins by measuring the temperature of several relevant points, including:

(1) Heat source: such as feed motor, spindle motor, ball screw pair, guide rail and spindle bearings of each part.

(2) Auxiliary devices: including hydraulic system, cooler, refrigeration displacement detection system and lubrication.

(3) Mechanical structure: including the base, bed, slide, column and milling head box as well as the spindle. An indium steel probe is fixed between the spindle and the rotary table.

Five contact sensors are arranged in the X, Y and Z directions to measure comprehensive deformations in various states, simulating the relative displacement between the tool and the workpiece.

3. Test data processing and analysis

The thermal deformation test of the machine tool must be carried out over a long continuous period and continuous data recording must be done. After analysis and processing, the reliability of the reflected thermal deformation characteristics can be very high, and if error rejection is carried out through multiple experiments, the regularity shown will be reliable.

In the thermal deformation test of the spindle system, a total of five measuring points were defined, with point 1 being at the end of the spindle and point 2 close to the spindle bearing, and points 4 and 5 located close to the guide rail in the spindle. Z direction in milling head housing. The test lasted 14 hours, with the spindle speed changing alternately in the range of 0 to 9000 r/min during the first 10 hours, and then continuing to rotate at a high speed of 9000 r/min for the remaining time.

The following conclusions can be drawn from the test:

• The thermal equilibrium time of the spindle is approximately 1 hour, and the temperature rise after equilibrium is 1.5℃.
• The temperature rise mainly comes from the spindle bearing and spindle motor. The thermal performance of the bearing is good in the normal speed range.
• Thermal deformation has little effect in the X direction.
• The telescopic deformation in the Z direction is large, around 10m, due to the thermal extension of the main shaft and the increase in bearing clearance.
• When the rotation speed continues at 9000 r/min, the temperature increases sharply, about 7 ℃ in 2.5 hours, with a continuous upward trend. The deformations in the Y and Z directions reached 29m and 37m, indicating that the spindle can no longer work stably at a speed of 9000 rpm, but can work for a short period of time (20 minutes).

Thermal deformation control of machine tools

From the analysis and discussion, it is evident that the increase in temperature and thermal deformation of machine tools can significantly impact their processing accuracy. When taking control measures, it is crucial to identify the main contributing factors and focus on a few effective measures to achieve optimal results.

In the design process, attention should be paid to reducing heat generation and increasing temperature, creating a balanced structure and providing efficient cooling.

1. Reduce the heat

Controlling heat sources is a fundamental measure to reduce temperature rise and thermal deformation of machine tools. To achieve this, the following steps must be taken in the design process:

(1) Reasonably select the rated power of the motor: The output power of the motor is proportional to voltage and current. In general, the voltage is constant and an increase in load leads to an increase in power and output current, resulting in increased heat consumed by the armature impedance. To minimize engine temperature rise, it is best to select a rated power approximately 25% higher than the calculated power.

(2) Reduction of heat generation from secondary heat sources: To minimize temperature rise from secondary heat sources, measures must be taken in the design of the machine structure. For example, improving the coaxiality of the front and rear bearings and using high-precision bearings can reduce friction and heat generation. Replacing slide guides with linear rolling guides or using a linear motor can also reduce heat generation.

(3) Using high-speed cutting in the machining process: High-speed cutting reduces heat generation during the cutting process. When the metal cutting linear speed is above a certain range, the metal does not have time to undergo plastic deformation and no deformation heat is generated in the chips. Most of the cutting energy is converted into chip kinetic energy and removed.

two. Structural balance to reduce thermal deformation

Controlling thermal deformation in machine tools requires attention to the direction and speed of heat transfer to reduce its effects. A symmetrical structure helps distribute heat evenly, reducing drift and warping.

(1) Prestressing and Thermal Deformation

In high-speed feed systems, ball screws are often pre-tensioned at both ends to reduce thermal deformation errors. The axial pretension structure reduces the cumulative error compared to a structure fixed at one end and free at the other. The main effect of increasing temperature in this structure is to change the tensile stress to zero or compression, having little effect on displacement accuracy.

(2) Change the structure and direction of deformation

The Z-axis spindle slip of a CNC needle slot milling machine with a different ball screw axial clamping structure requires a milling slot error of 0.05mm. The final floating structure ensures the change in groove depth during processing, while the axial floating structure results in gradual deepening of the groove.

(3) Symmetrical Geometry

A symmetrical machine tool structure minimizes thermal deformation and tool tip runout. The YMC430 micromachining center is an example of a machine that considered thermal performance in its design. It has a completely symmetrical layout, with integrated H-shaped columns and beams, circular spindle slide and linear motors for the three moving axes. The two rotating axes use direct drive, minimizing friction and mechanical transmission.

3. Reasonable cooling measures

(1) Refrigerant during processing directly affects processing accuracy.

A comparative test was carried out on a GRV450C double-sided grinding machine and showed that heat exchange treatment of the cooling liquid through a cooler greatly improves processing accuracy.

Traditional coolant delivery methods caused the part size to be out of tolerance after 30 minutes, while using a cooler allowed normal processing for more than 70 minutes. The excessive size of the part after 80 minutes was due to the need to trim the grinding wheel, which removed metal chips from the surface of the grinding wheel. The original machining precision was immediately restored after cutting, and the effect was very noticeable.

Likewise, forced cooling of the spindle can also result in very good results.

(2) Increase the natural cooling area.

For example, adding natural air cooling areas to the main axle box structure can also play an important role in heat dissipation in a workshop with good air circulation.

(3) Timely removal of chips.

Timely or real-time removal of high-temperature chips from the workpiece, table and tool greatly reduces the temperature rise and thermal deformation of critical parts.

Perspective and Vision

Controlling thermal deformation of machine tools is a crucial issue in modern precision machining and the factors that influence it are highly complex. The combination of high speed, efficiency and precision in modern cutting processing exacerbates the problem and has attracted significant attention from the machine tool manufacturing industry.

Researchers in the machine tool industry, both nationally and internationally, have made significant progress in understanding this issue through extensive research, making thermal deformation of machine tools a fundamental theory in the field.

This article examines the impact of design and application, measurement and analysis methods on the thermal performance of machine tools and proposes measures to improve the design.

To optimize the thermal performance of machine tools, the following measures must be taken:

• During the design phase of modern high-tech machine tools, consider the environmental conditions where the machine tool will be used.
• Control and configure the heat source. This involves managing energy consumption and source, adopting new structures, reducing secondary friction heat sources and improving energy efficiency.
• Reevaluate traditional thinking and elevate the importance of cooling, heat dissipation, lubrication and chip removal systems from auxiliary components to essential components.
• Consider the symmetry of the structure and the direction of thermal deformation in the design, to minimize the impact of thermal deformation on accuracy, mainly through research and application of mathematical models for the thermal deformation of structural parts, which can provide quantitative guidance for deformation thermal control design.