How Does Temperature Affect Accuracy in CNC Machining?

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

Temperature Affect Accuracy in CNC Machining

When the machine is affected by changes in the ambient temperature of the workshop, friction heating due to motor heating and mechanical movement as well as cutting heat and cooling medium, all of these will cause uneven temperature rise in various parts of the machine tool, resulting in changes in the form accuracy and machining accuracy of the machine tool.

For example, when machining a 70mm×1650mm screw on a common-precision CNC milling machine, the workpieces milled at 7: 30-9: 00 am compared with the workpieces processed at 2: 00-3: 30 pm, the cumulative error change can reach up to 85m. Under constant temperature conditions, the error can be reduced to 40m.

For another example, a precision double-end surface grinding machine used for double-side grinding of thin steel sheet workpieces with a thickness of 0.6-3.5mm, when processing 200mm×25mm×1.08mm steel sheet workpieces during inspection and acceptance, the dimensional accuracy of mm can be achieved and the curvature is less than 5m in the whole length.

However, after continuous automatic grinding for 1 hour, the dimensional change range will increase to 12m, and the coolant temperature will rise from 17 °C at startup to 45 °C .

Due to the influence of grinding heat, the spindle journal is elongated, and the bearing clearance at the front of the spindle is increased.

Based on this, adding a 5.5kW refrigerator to the machine’s coolant tank will be very effective.

Practice has proved that the deformation of the machine tool after heating is an important reason that affects the machining accuracy but the machine is in an environment where the temperature changes at any time.

The machine tool itself will necessarily consume energy during work, 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.

These changes can be very different because of different structural forms and materials.

The machine tool designer should master the heat formation mechanism and temperature distribution rules, and take corresponding measures to minimize the impact of thermal deformation on machining accuracy.

Temperature rise and distribution of machine tools, and the influence of natural climate

Temperature Affect Accuracy in CNC Machining

1. Natural climate impact

China is a vast country, most of which are located in the subtropical region.

The temperature changes greatly in the four seasons of the year, and the temperature difference changes differently in a day.

As a result, people’s intervention methods and degrees of temperature in the room (such as the workshop) are also different, and the temperature atmosphere around the machine tool varies widely.

For example, the seasonal temperature range of the Yangtze River Delta is about 45 ° C and the temperature change between day and night is about 5 ~ 12 ℃.

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 of the machining 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 late October to early April of the following year. The machining workshop is designed to provide heating and there is insufficient air circulation.

The temperature difference between inside and outside the workshop can reach 50 ° C.

Therefore, the temperature gradient in winter in the workshop is very complicated.

When measuring from 8:15 to 8:35 am, the outdoor temperature is 1.5 ° C 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.

2. Influence of the surrounding environment

The surrounding environment of the machine tool refers to the thermal environment formed by various layouts within a close range of the machine tool.

They include the following 4 aspects:

1) Workshop microclimate

such as the temperature distribution in the workshop (vertical direction, horizontal direction). The temperature of the workshop changes slowly when the day and night are altered or the climate and ventilation change.

2) Workshop heat sources

such as solar radiation, heating equipment, and radiation from high-power lighting, etc.

When they are close to the machine tool, they can directly affect the temperature rise of the whole 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 by means of radiation or air flow.

3) Heat dissipation

The foundation has better heat dissipation, especially the foundation of precision machine tools should not be near the underground heating pipeline.

Once a pipe ruptures and leaks, it can become a heat source where it is difficult to find the cause. The open workshop will be a good “radiator”, which will help the workshop temperature equalization.

4) Constant temperature

It is effective to maintain the precision and processing accuracy of precision machine tools with constant temperature facilities in the workshop, but the energy consumption could be large.

3. The thermal influence factors inside the machine tool

1) Structural heat source of machine tools

Motor heating such as the spindle motor, servo feed motor, cooling and lubrication pump motor, electric control box, etc. All can generate heat.

These conditions are allowable for the motor itself, but have a significant adverse effect on components such as the spindle, ball screw, and so on.

Measures should be taken to isolate them.

When the input electric energy drives the motor to run, except for a small part (about 20%), which is converted into motor thermal energy, most of it will be converted into kinetic energy by motion mechanisms, such as spindle rotation, table motion, and so on.

However, it is inevitable that a considerable part is converted into frictional heat during movement.

Mechanisms such as bearings, guide rails, ball screws, and gearboxes can generate heat.

2) Cutting heat during the process

During the cutting process, part of the kinetic energy of the tool or workpiece is consumed by the cutting work.

A considerable part is converted into the deformation energy of the cutting and the frictional heat between the chip and the tool, which generates heat in the tool, spindle and workpiece, and a large amount of chip heat is transmitted to the machine tool’s table fixture and other components.

They will directly affect the relative position between the tool and the workpiece.

3) Cooling

Cooling is a countermeasure against the temperature rise of the machine tool, such as motor cooling, spindle component cooling, and infrastructure cooling.

High-end machine tools are often forced to cool electronic control boxes with refrigerators.

4. The influence of the structure of the machine tool on the temperature rise

In the field of machine tool thermal deformation, the structure of the machine tool is generally referred to in terms of structural form, mass distribution, material properties and heat source distribution.

The structure shape affects the temperature distribution, heat conduction direction, thermal deformation direction, matching of the machine tool and so on.

1) Structural form of the machine tool

In terms of overall structure, machine tools include vertical, horizontal, gantry and cantilever types, which have large differences in thermal response and stability.

For example, the temperature rise of a gear-shift lathe headstock can be as high as 35 ° C.

When the spindle end is lifted, the thermal equilibrium time needs about 2h.

For the slant bed type precision turning and milling machining center, the machine tool has a stable base, this significantly improves the rigidity of the whole machine.

The main shaft is driven by a servo motor, and the gear transmission part is removed, and its temperature rise is generally less than 15 ° C.

2) Influence of heat source distribution

Machine tools usually consider the heat source to be the electric motor, such as spindle motor, feed motor, hydraulic system and etc., but it’s incomplete.

The heat of the electric motor is only the energy consumed by the armature resistance when the current is under load.

A considerable part of the energy is consumed by the heating caused by the frictional work of bearings, screw nuts and guide rails.

Therefore, the motor can be called a primary heat source, and the bearings, nuts, guide rails and chips can be called a secondary heat source.

Thermal deformation is the result of the combined effects of all these heat sources.

The temperature rise and deformation of a column mobile vertical machining center during the Y-feed movement.

The table does not move during Y-feed, so it has little effect on the thermal deformation in X direction.

On the column, the farther the point from the Y-axis guide screw, the smaller the temperature rise.

The situation of the machine when the Z axis moves is further, and the effect of heat source distribution on thermal deformation is explained.

The Z axis feed is farther from the X direction, so the thermal deformation is less affected.

The closer the column is to the Z axis motor nut, the greater the temperature rise and deformation.

3) The effect of mass distribution

The influence of mass distribution on the thermal deformation of machine tools has three aspects.

First, it refers to the size and concentration of mass, usually refers to changing the heat capacity and speed of heat transfer, and changing the time to reach thermal equilibrium.

Second, by changing the quality of the layout, such as the layout of various ribs.

Improve the thermal rigidity of the structure, reduce the effect of thermal deformation or keep the relative deformation small under the same temperature rise.

The third is to reduce the temperature rise of machine tool components by changing the quality arrangement form, such as arranging heat dissipation ribs outside the structure.

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.

Temperature Affect Accuracy in CNC Machining

Testing of thermal performance of machine tools

1. Purpose of machine tool thermal performance test

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 tool, and the response of key points (deformation displacement) through the thermal characteristics test.

Test data or curves describe the thermal characteristics of a machine tool in order to take countermeasures to control thermal deformation and improve the machining accuracy and efficiency of the machine tool.

Specifically, the following objectives should be achieved:

1) Testing of machine environment

Measure the temperature environment in the workshop, its spatial temperature gradient, the change in temperature distribution during the day and night alternation, and even the effect of seasonal changes on the temperature distribution around the machine tool.

2) Thermal characteristics test of the machine tool itself

Under the conditions of eliminating environmental interference as much as possible, the machine tool is in various operating states to measure the temperature change and displacement change of important points of the machine tool, and record the temperature change and key point displacement within a sufficiently long period of time.

Infrared thermal phase instrument can be used to record the thermal distribution of each time period.

3) Test the temperature rise thermal deformation during processing to judge the influence of machine tool thermal deformation on the accuracy of processing.

4) The above experiments can accumulate a large amount of data and curves, which will provide reliable criteria for machine tool design and user control of thermal deformation, and point out the direction to take effective measures.

2.The principle of machine tool thermal deformation test

The thermal deformation test first needs to measure the temperature of several relevant points, including the following aspects:

1) Heat source: including the feed motor, spindle motor, ball screw drive pair, guide rail, and spindle bearing of each part.

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

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

Five contact sensors are arranged 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.

3. Processing and analysis of test data

The thermal deformation test of the machine tool should be performed in a long continuous time, and continuous data recording is performed.

After analysis and processing, the reliability of the reflected thermal deformation characteristics can be very high.

If error rejection is performed through multiple experiments, the regularity shown is credible.

In the thermal deformation test of the spindle system, a total of 5 measurement points were set, of which point 1 is at the end of the spindle and point 2 is near the spindle bearing, points 4 and 5 are respectively located at the milling head housing near the Z-direction guide rail.

The test time lasted a total of 14 hours. The first 10 hours of the spindle speed were alternately changed in the range of 0 to 9000 r / min. From the 10th hour, the spindle continued to rotate at a high speed of 9000 r / min.

The following conclusions can be drawn:

1) The thermal equilibration time of this spindle is about 1h, and the temperature rise after the equilibration range is 1.5 ℃;

2) The temperature rise mainly comes from the spindle bearing and the spindle motor. In 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-direction telescopic deformation is large, about 10m, which is caused by the thermal extension of the main shaft and the increase of bearing clearance;

5) When the rotation speed continues at 9000r / min, the temperature rises sharply, about 7 ℃ in 2.5h, and there is a continued upward trend. The deformations in the Y and Z directions reached 29m and 37m, indicating that the spindle can no longer run stably at a speed of 9000r / min, but can run in a short time (20min).

Temperature Affect Accuracy in CNC Machining

Control of thermal deformation of machine tools

From the above analysis and discussion, the temperature rise and thermal deformation of machine tools have a variety of influencing factors on processing accuracy.

When taking control measures, we should seize the main contradictions and focus on one or two measures to achieve more results with less effort.

In design, we should start from 4 directions:

Reducing heat generation and temperature rise, balanced structure and reasonable cooling.

1. Reduce heat

Control heat source is a fundamental measure. Measures must be taken in the design to effectively reduce the amount of heat generated by 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.

In general, the voltage V is constant.

Therefore, the increase of the load means that the output power of the motor increases, that is, the corresponding current I also increases, and the heat consumed by the current in the armature impedance increases.

If the motor we designed and selected works for a long time near or greatly exceeds the rated power, the temperature rise of the motor will increase significantly.

To this end, a comparative test was performed on the milling head of the BK50 CNC needle slot milling machine (motor speed: 960r / min, ambient temperature: 12 ° C).

From the above test, the following concepts are obtained: Considering the performance of the heat source, whether it is the spindle motor or the feed motor, when selecting the rated power, it is better to choose about 25% greater than the calculated power.

In actual operation, the output power of the motor matches the load, and increasing the rated power of the motor has little effect on energy consumption.

However, it can effectively reduce the temperature rise of the motor.

2) Take appropriate measures on the structure to reduce the heat generation of the secondary heat source and reduce the temperature rise.

For example, when designing the structure of the main shaft, 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.

3) In the process, high-speed cutting is used. (Based on the mechanism of high-speed cutting.)

When the linear speed of metal cutting is higher than a certain range, the metal to be cut does not have time to produce plastic deformation, and no deformation heat is generated on the chips. Most of the cutting energy is converted into chip kinetic energy and taken away.

Temperature Affect Accuracy in CNC Machining

2. Structural balance to reduce thermal deformation

On the machine tool, the heat source is always present, and further attention needs to be paid to how to make the direction and speed of heat transfer beneficial to reduce thermal deformation.

Or the structure has good symmetry, so that the heat transfer passes along the symmetrical direction, so that the temperature distribution is uniform, the deformations cancel each other out, and become a thermal affinity structure.

(1) Prestressing and thermal deformation.

In higher-speed feed systems, ball screws are often fixed axially at both ends to form pre-tensioning stress.

In addition to improving dynamic and static stability, this structure has a significant effect on reducing thermal deformation errors for high-speed feed.

The axially fixed structure that is pre-stretched 35m in a total length of 600mm is relatively close to the temperature rise at different feed rates.

The cumulative error of the pre-stretched structure fixed at both ends is significantly smaller than that of the structure that is fixed at one end and freely extended at the other end.

In the axially fixed pre-tensioned structure at both ends, the temperature rise caused by heat is mainly to change the stress state inside the lead screw from tensile stress to zero stress or compressive stress.

Therefore, the effect on displacement accuracy is small.

(2)Change the structure and change the direction of thermal deformation.

Z axis spindle slide of CNC needle slot milling machine with different ball screw axial fixing structure requires milling slot error of 0.05mm during processing.

It adopts the axial floating structure at the lower end of the screw. Within 2 hours of processing, the groove depth gradually deepens from 0 to 0.045mm.

On the contrary, the floating end structure of the screw can ensure the change of the groove depth.

(3)The symmetrical geometry of the machine tool structure can make the thermal deformation uniform and minimize the drift of the tool point.

For example, the YMC430 micro-machining center produced by Yasda Precision Tool Company of Japan is a sub-micron high-speed machining machine. The machine design has fully considered the thermal performance.

First of all, a completely symmetrical layout is adopted on the machine tool structure. The columns and beams are integrated structures, which are H-shaped, which is equivalent to a double-column structure and has good symmetry. The approximately circular spindle slide is symmetrical in both longitudinal and lateral directions.

The feed drive of the three moving shafts uses linear motors, which makes it easier to achieve symmetry in the structure. The two rotary shafts use direct drive to minimize frictional losses and mechanical transmission.

3. Reasonable cooling measures

(1)The influence of the coolant during processing on the accuracy of processing is direct.

A comparative test was carried out on the GRV450C double-face grinder.

Tests show that the heat exchange treatment of the cooling liquid by means of a refrigerator is very effective for improving the processing accuracy.

With the traditional coolant supply method, after 30 minutes, the workpiece size will be out of tolerance. After using the refrigerator, it can be processed normally to more than 70min.

The main reason for the excessive workpiece size at 80min is that the grinding wheel needs to be trimmed (removing metal shavings on the wheel surface), and the original machining accuracy can be restored immediately after trimming. The effect is very obvious.

Similarly, very good results can be expected for forced cooling of the spindle.

(2)Increase natural cooling area.

For example, adding natural air cooling area to the structure of the main shaft box can also play a good role in heat dissipation in a workshop with good air circulation.

(3)Automatic chip removal in time.

The timely or real-time discharge of high-temperature chips from the workpiece, table, and tool will greatly reduce the temperature rise and thermal deformation of key parts.

Temperature Affect Accuracy in CNC Machining

Outlook and Vision

Controlling the thermal deformation of the machine tool is an important issue in the field of modern precision machining, and the factors affecting the thermal deformation of the machine tool are very complicated.

Furthermore, the combination of high speed, high efficiency and precision in modern cutting processing makes the thermal deformation problem of the machine tool more prominent. Aroused widespread attention in the machine tool manufacturing industry.

Scholars in the machine tool industry at home and abroad have done a lot of research for this, and have made considerable progress in theory.

Machine tool thermal deformation has become one of the basic theories in machine tool research.

This article analyzes the influencing factors of thermal performance of machine tools from the perspective of machine tool design and application, measurement and analysis methods, and proposes improved design measures.

Therefore, we believe that the optimal design of the thermal performance of the machine tool should start from the following aspects:

(1) At the design stage of modern high-end machine tools, we should pay attention to the environmental conditions for the future application of the designed machine tools.

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

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

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

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