High speed electric spindle, also known as the spindle unit with built-in motor, is an important component of CNC machine tools. The spindle motor is installed inside the spindle unit of the machine tool to drive the spindle, thereby integrating the motor and spindle into a whole. In order to improve the operational efficiency of CNC machine tools, it is necessary to master the technical points of high-speed electric spindles, leverage their advantages, and promote the continuous improvement of electric spindle technology.

1. Advantages of high-speed electric spindle

The spindle operation of traditional CNC machine tools mainly drives the intermediate variable speed device and transmission device, such as gears, belts, and couplings, during the process of exerting the driving effect of the motor. This is called the "mechanical spindle", also known as the separated spindle and the direct coupled spindle. Compared to traditional spindles, electric spindles have the following advantages:

(1) The operation of the spindle is driven by an internally installed motor, without the need for intermediate speed changing devices and transmission devices. Its design structure is simple and compact, which can improve operating efficiency with high precision. During operation, there will be no significant noise or small vibrations.

(2) The electric spindle adopts AC frequency conversion technology, which can achieve continuously variable speed within the rated speed range. When the machine tool is running, regardless of any working conditions or load changes, the electric spindle has good adaptability.

(3) When the built-in motor is running, it can control the closed-loop vector, effectively regulate the power according to the control instructions, and flexibly control the operating speed, output torque, etc. of the driving device. Electric spindles can meet various high-power requirements, such as low-speed heavy cutting and high torque, or high-speed precision machining. Electric spindles can play a good role in achieving accurate stopping while meeting the C-axis transmission function.

(4) The electric spindle can operate at high speed, with good stability and high dynamic accuracy, making the cutting speed of CNC machine tools faster and the machining accuracy higher.

(5) Due to the fact that the operation of the electric spindle does not require intermediate transmission links, it has higher stability and will not be affected by external impacts. The bearings of the spindle do not need to bear large dynamic loads, which extends the precision life

(6) The electric spindle integrates the motor and spindle into a unit, enabling the series production of electric spindles and forming a certain scale, making production more specialized. As a functional component of CNC machine tools, the electric spindle has also entered the market as a commodity. Enterprises can choose electric spindles based on the operational requirements of CNC machine tools, which will promote the modular development of machine tools.

2. Key points of high-speed electric spindle technology

2.1 High speed precision bearing technology for high-speed electric spindles

In the electric spindle system, spindle bearing technology is key, mainly including three types: dynamic and static pressure bearings, angular contact ball bearings, and magnetic levitation bearings. Among them, the dynamic and static pressure bearings are a combination of static pressure bearings and dynamic pressure bearings, which combine the advantages of both and have good high-speed performance and a wide speed range. Angular contact ball bearings are commonly used in precision CNC machine tools to support the spindle and have a certain impact on high-speed performance. The main reason for this phenomenon is that the ball bearings are made of silicon nitride material, which increases centrifugal force and gyroscopic torque under high-speed action, allowing the high-speed performance of the bearings to be exerted and reducing the diameter of the rolling elements. Due to the high manufacturing cost and complex control system of bearings, it is not easy to solve the heating problem during bearing operation, so they will only be used in special occasions.

2.2 Dynamic balancing technology of high-speed electric spindle

High speed electric spindles have the characteristics of high operating speed, high-precision operation, and high machining efficiency, but these characteristics are all based on dynamic balance. The operating status of an electric spindle is affected by many factors, such as manufacturing factors, errors during spindle installation, or uneven materials, which inevitably lead to the problem of unbalanced operation. Usually, during the operation of the electric spindle, the maximum operating speed can exceed 10000r/min, and even reach 60000-100000 r/min. When the spindle is running, even if the unbalance is small, it may affect the accuracy of the spindle rotation and even affect the stability of the bearing support system operation. Therefore, it is necessary to strictly require the dynamic balance accuracy of high-speed electric spindles.

In order to improve the balance of the spindle, a symmetrical structure needs to be adopted in the design of the electric spindle, and precision should be improved during machining and assembly. When the spindle is recycled from the factory, the initial dynamic balance is adjusted to improve the balance of the spindle, but the spindle tools still have slight asymmetry problems, and even tool wear or chip sticking problems, the original dynamic balance will still be broken. Due to the complex working conditions, the spindle tool system is subject to interference, such as cutting force excitation, centrifugal force, and thermal deformation, which are important influencing factors that can damage the spindle system and prevent it from maintaining a stable operating state. In order to ensure the high-speed operation of the electric spindle and the efficient and continuous stability of the CNC machine tool, it is necessary to design a dynamic balancing system for online and automated operation, and continuously improve it according to practical application needs to maximize its value.

2.3 High speed electric spindle lubrication technology

Lubricating high-speed electric spindles mainly involves lubricating the spindle bearings. Therefore, it is necessary to adopt a scientifically effective lubrication system to control the temperature rise of bearings, thereby ensuring the accuracy and stability of the machine tool process system. When choosing the lubrication method for high-speed electric spindles, it is necessary to consider the type of bearing, operating speed, and load. Oil lubrication, oil mist lubrication, or spray lubrication can be selected according to the needs.

2.4 High speed electric spindle cooling technology

The stator of the electric motor is installed inside the electric spindle housing, and the electric spindle is designed to be enclosed. When it is running at high speed, due to the lack of heat dissipation conditions, the housing may overheat. There are two heat sources inside the electric spindle: the first heat source is the heat generated by the motor during operation due to losses; The second heat source is the heat generated by friction during the operation of the bearing. When heat is generated during the operation of the motor, the main heat dissipation function is the spindle housing. Part of the heat is transferred to the bearings through the spindle, causing a rapid increase in bearing temperature and affecting their service life. The manufacturing accuracy of the spindle is affected by thermal elongation, and the stability and reliability of the spindle system operation cannot be guaranteed. The method of cooling the electric spindle is to install a circulating cooling water jacket at the connection between the stator and the housing, lubricate the bearings to reduce heat generation. Reasonable lubrication not only serves the purpose of lubrication, but also serves the purpose of cooling.

2.5 High speed electric spindle spindle motor technology

Most of the electric motors used in electric spindles are AC asynchronous induction motors. Due to the improved performance of permanent magnet motors, AC permanent magnet synchronous motors have been widely used. The advantages are that the electric spindle rotor no longer heats up, the electric spindle no longer undergoes thermal deformation, the rotor has no losses, the operating efficiency of the electric spindle is improved, the moment of inertia is reduced, and the starting and stopping speeds are accelerated. When the electric spindle runs at low speed, it can maintain good performance.

2.6 High speed electric spindle precision machining and assembly technology

When the electric spindle is running at high speed, in order to ensure its stiffness and rotation accuracy, it is necessary to perform precision machining, even ultra precision machining, on important parts to improve the precision of spindle component assembly. In the spindle unit, all workpieces require precision machining, including shell machining, bearing seat machining, etc., with high precision. The bearing spacer will rotate at high speed with the spindle, which also requires high machining accuracy. In addition, advanced process technology should be adopted during the assembly of cutting tools on high-speed electric spindles, and the assembly precision should meet the requirements.

2.7 Temperature protection technology for high-speed electric spindle

When designing a high-speed CNC milling machine, it is necessary to activate the cooling system to cool the spindle. There are three bearings inside the spindle, namely the front bearing, the middle bearing, and the rear bearing. The spindle has different operating speeds and powers. If the temperature sensor is installed near the bearing, it can monitor the temperature of the spindle bearing in real time. When collecting signals, temperature sensors need to function as input modules for programmable logic controllers. Different types of sensors divide signals into 4-20mA. When designing machine tool control programs, it is necessary to calculate and calibrate temperature based on sensor signals. When implementing bearing temperature protection, it is also necessary to set threshold limit values in the program to protect the bearings.

3. Conclusion

Through the above research, it can be clarified that the application of high-speed machining technology can solve many problems in the manufacturing process of mechanical products, with high machining accuracy and guaranteed quality. Therefore, this technology has been widely applied in the manufacturing industry and has become a mainstream technology. At present, high-speed electric spindle technology is developing rapidly, and various industries have conducted in-depth research on this technology according to their own needs. While improving the efficiency of technology application, it is also necessary to leverage its energy-saving and environmental protection characteristics to achieve intelligent development.