What is the impact of temperature on accuracy in CNC machining

Thermal deformation in CNC machining is one of the reasons that affects machining accuracy. The machine tool is affected by changes in workshop environment temperature, electric motor heating, mechanical motion friction heating, cutting heat, and cooling medium, resulting in uneven temperature rise in various parts of the machine tool, leading to changes in the shape accuracy and processing accuracy of the machine tool. For example, machining 70mm on a regular precision CNC milling machine × Compared with the workpiece processed from 2:00 to 3:30 in the afternoon, the cumulative error change of the 1650mm screw can reach 85m. Under constant temperature conditions, the error can be reduced to 40m.

For example, a precision double end grinding machine used for grinding thin steel sheet workpieces with a thickness of 0.6-3.5mm on both ends, which processes 200mm during acceptance inspection × 25mm × The 1.08mm steel sheet workpiece can achieve a dimensional accuracy of mm, and the bending degree is less than 5m throughout the entire length. But after 1 hour of continuous automatic grinding, the range of size change increased to 12m, and the coolant temperature increased from 17 ℃ at startup to 45 ℃. Due to the influence of grinding heat, the spindle journal extends and the clearance between the front bearings of the spindle increases. Based on this, adding a 5.5kW refrigerator to the coolant tank of the machine tool has achieved very ideal results. Practice has proven that the deformation of machine tools after being heated is an important reason for affecting machining accuracy. But the machine tool is in an environment where the temperature changes anytime and anywhere; The machine tool itself will inevitably consume energy during operation, and a considerable portion of this energy will be converted into heat in various ways, causing physical changes in the various components of the machine tool. This change varies greatly due to differences in structural forms, materials, and other reasons. Machine tool designers should master the formation mechanism of heat and the temperature distribution law, and take corresponding measures to reduce the impact of thermal deformation on machining accuracy to Z small.

The temperature rise, temperature distribution and natural climate of machine tools affect China's vast territory. Most areas are located in the subtropical zone. The temperature changes greatly throughout the year, and the temperature difference changes differently within a day. As a result, people have different ways and degrees of intervention in indoor (such as workshop) temperature, and the temperature atmosphere around machine tools varies greatly. For example, the seasonal temperature variation range in the Yangtze River Delta region is about 45 ℃, and the diurnal temperature variation 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 has good ventilation, the temperature gradient in the machining workshop does not change much. In the Northeast region, the seasonal temperature difference can reach 60 ℃, with a diurnal variation of about 8-15 ℃. The heating period is from late October to early April of the following year, and the machining workshop is designed with heating, resulting in insufficient air circulation. The temperature difference inside and outside the workshop can reach 50 ℃. Therefore, the temperature gradient in the workshop during winter is very complex, with an outdoor temperature of 1.5 ℃ measured from 8:15 to 8:35 am, and a temperature change of approximately 3.5 ℃ in the workshop. The machining accuracy of precision machine tools will be greatly affected by environmental temperature in such a workshop.

 

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

They include the following four aspects:

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

2) Workshop heat source: such as solar radiation, heating equipment, and high-power lighting radiation, which can directly affect the temperature rise of the entire machine tool or some components 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 tool through radiation or air flow.

3) Heat dissipation: The foundation has a good heat dissipation effect, especially for precision machine tools. Do not approach the underground heating pipeline. Once it ruptures and leaks, 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 temperature balance in the workshop.

4) Constant temperature: Adopting constant temperature facilities in the workshop is very effective in maintaining precision and processing accuracy of precision machine tools, but it consumes a lot of energy.

3. Internal thermal influencing factors of machine tools

1) Structural heat source for machine tools. Electric motors that generate heat, such as spindle motors, feed servo motors, cooling and lubrication pump motors, and electric control boxes, can all generate heat. These situations are allowed for the motor itself, but have significant adverse effects on components such as the spindle and ball screw, and measures should be taken to isolate them. When the input electric energy drives the motor to operate, except for a small portion (about 20%) that is converted into motor thermal energy, most of it will be converted into kinetic energy by the motion mechanism, such as spindle rotation, workbench movement, etc; However, it is inevitable that a considerable portion of it will still be converted into frictional heat during movement, such as heat generated by mechanisms such as bearings, guide rails, ball screws, and transmission boxes.

2) The cutting heat of the process. During the cutting process, a portion of the kinetic energy of the tool or workpiece is consumed by the cutting work, while a considerable portion is converted into the deformation energy of the cutting and the friction heat between the chip and the tool, forming the heat generated by the tool, spindle, and workpiece. A large amount of chip heat is transmitted to the worktable fixtures and other components of the machine tool. They will directly affect the relative position between the tool and the workpiece.

3) Cooling. Cooling is a reverse measure aimed at increasing the temperature of machine tools, such as motor cooling, spindle component cooling, and basic structural component cooling. High end machine tools often have cold machines equipped with electric control boxes for forced cooling.

4. The influence of machine tool's structural form on temperature rise In the field of machine tool thermal deformation, the structural form of machine tool is discussed, which usually refers to structural form, Mass distribution, material properties, heat source distribution and other issues. The structure shape affects the temperature distribution, heat conduction direction, thermal deformation direction and matching of the machine tool.

 

1) The structural form of the machine tool. In terms of overall structure, machine tools include vertical, horizontal, gantry, and cantilever types, with significant differences in thermal response and stability. For example, the temperature rise of the main axle box of a gear variable speed lathe can reach up to 35 ℃, causing the spindle end to lift upwards, and the thermal balance time needs about 2 hours. The inclined bed precision turning and milling machining center has a stable base for the machine tool. Significantly improving the overall stiffness of the machine, the main shaft is driven by a servo motor, and the gear transmission part is removed. Its temperature rise is generally less than 15 ℃.

2) The influence of heat source distribution. On machine tools, it is commonly believed that a heat source refers to an electric motor. The spindle motor, feed motor, and hydraulic system are actually incomplete. The heating of an electric motor is only the energy consumed by the current on the impedance of the armature when bearing the load, and a considerable portion of the energy is consumed by the friction work caused by mechanisms such as bearings, screw nuts, and guide rails. So the electric motor can be referred to as the primary heat source, and the bearings, nuts, guide rails, and chips can be referred to as the secondary heat source. Thermal deformation is the result of the combined influence of all these heat sources. The temperature rise and deformation of a vertical machining center with a movable column during Y-axis feed motion. When feeding in the Y direction, the workbench does not move, so it has little effect on the thermal deformation in the X direction. On the pillar, the farther away the guide screw from the Y-axis is, the smaller the temperature rise. The situation of the machine moving on the Z-axis further illustrates the influence of heat source distribution on thermal deformation. The Z-axis feed is further away from the X-axis, so the impact of thermal deformation is smaller. The closer the column is to the Z-axis motor nut, 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 has three aspects.

 

Firstly, it refers to the size and concentration of mass, usually referring to changes in heat capacity and heat transfer rate, as well as changes in the time to reach thermal equilibrium.

Secondly, By changing the arrangement form of the quality, such as the arrangement of various ribs and plates, the thermal stiffness of the structure can be improved, reducing the influence of thermal deformation or maintaining relatively small deformation under the same temperature rise.

Thirdly, it refers to reducing the temperature rise of machine tool components by changing the form of quality arrangement, such as arranging heat dissipation ribs outside the structure.

 

The impact of material properties: Different materials have different thermal performance parameters (specific heat, thermal conductivity, and linear expansion coefficient), and under the same heat influence, their temperature rise and deformation are all different.

 

Testing of thermal performance of machine tools

1. The purpose of machine tool thermal performance testing is to control the thermal deformation of the machine tool. The key is to fully understand the changes in environmental temperature, the heat source and temperature changes of the machine tool itself, and the response of key points (deformation displacement) through thermal characteristic testing. 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, the change of temperature distribution in the alternation of day and night, and even the impact 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, keep the machine tool in various operating states to measure the temperature and displacement changes at important points of the machine tool itself, record the temperature changes and key point displacements over a sufficiently long period of time, and also record the thermal distribution of each time period using an infrared thermograph.

3) Test the temperature rise and thermal deformation during the machining process to determine the impact of machine tool thermal deformation on the accuracy of the machining process.

4) The above experiments can accumulate a large amount of data and curves, providing reliable criteria for machine tool design and user control of thermal deformation, and pointing out the direction for taking effective measures.

2. The principle of machine tool thermal deformation testing Thermal deformation testing first requires measuring the temperature of several relevant points, including the following aspects:

1) Heat source: including various feed motors, spindle motors, ball screw drive pairs, guide rails, and spindle bearings.

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

3) Mechanical structure: including the bed, base, sliding plate, column, milling head box, and spindle. An indium steel measuring rod is clamped between the spindle and the rotary workbench, and five contact sensors are configured in the X, Y, and Z directions to measure the comprehensive deformation under various states to simulate the relative displacement between the tool and the workpiece.

3. Test data processing and analysis: The machine tool thermal deformation test should be conducted over a long continuous period of time, with continuous data recording. After analysis and processing, the reflected thermal deformation characteristics have high reliability. If error elimination is carried out through multiple experiments, the displayed regularity is reliable. A total of 5 measurement points were set for the thermal deformation test of the spindle system, with points 1 and 2 located at the end of the spindle and near the spindle bearing, and points 4 and 5 located near the Z-direction guide rail of the milling head shell, respectively. The testing time lasted for a total of 14 hours, with the spindle speed alternating between 0 and 9000r/min for the first 10 hours. Starting 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 balance time of the spindle is about 1 hour, and the temperature rise range after balance is 1.5 ℃.

2) The temperature rise mainly comes from the spindle bearings and spindle motors, and the thermal performance of the bearings is good within the normal speed range.

3) Thermal deformation has little effect in the X-direction.

4) The Z-direction expansion deformation is relatively large, about 10m, which is caused by the thermal elongation of the main shaft and the increase in bearing clearance.

5) When the speed continues to be at 9000r/min, the temperature rise sharply increases, rising by about 7 ℃ within 2.5h, and there is a trend of further increase. The deformation in the Y and Z directions reaches 29m and 37m, indicating that the spindle can no longer operate stably at a speed of 9000r/min, but can operate for a short period of time (20min). The control of thermal deformation of machine tools is discussed through the above analysis. The temperature rise and thermal deformation of machine tools have various factors that affect machining accuracy. When taking control measures, the main contradiction should be grasped, and the first and second measures should be taken with emphasis to achieve twice the result with half the effort. In design, we should start from four directions: reducing heat generation, reducing temperature rise, structural balance, and reasonable cooling.

1. Reducing heat generation and controlling heat sources is the fundamental measure. Measures should be taken in the design to effectively reduce the heat generation of the heat source.

1) Reasonably select the rated power of the motor. The output power P of an electric motor is equal to the product of voltage V and current I. Generally, voltage V is constant. Therefore, an increase in load means that the output power of the motor increases, and the corresponding current I also increases. As a result, the heat consumed by the current in the armature impedance increases. If the motor we design and select operates for a long time under conditions close to or significantly exceeding the rated power, the temperature rise of the motor will significantly increase. For this purpose, a comparative test was conducted on the milling head of the BK50 CNC needle slot milling machine (motor speed: 960r/min; ambient temperature: 12 ℃). From the above experiments, the following concepts are obtained: considering the performance of the heat source, whether it is the spindle motor or the feed motor, it is advisable to choose a rated power that is about 25% higher 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 impact on energy consumption. But it can effectively reduce the temperature rise of the motor.