1、 Introduction

Numerical control machine tools integrate high efficiency, high precision, and high flexibility. At the same time, in order to achieve high productivity and improve machining accuracy, high-speed machining technology is increasingly being valued by the industry. Ultra high speed CNC machine tools are the material basis for achieving ultra high speed machining, and high-speed spindles are the "core" components of ultra high speed CNC machine tools. They require high speed and accuracy, as well as continuous output of high torque capability and large-scale constant power operation. Therefore, CNC machine tool electric spindles with corresponding characteristics such as high speed, high precision, high-speed precision, and high efficiency have emerged.

Electric spindles were first used in grinding machines and later developed to include T-shaped centers. Due to the continuous demand of the powerful precision machinery industry, the power and quality of electric spindles are constantly improving. At present, the maximum speed of the electric spindle can reach 20000 r/min, with a diameter range of 33-300mm, a power range of 125w-80kw, and a torque range of 0.02-300nm. The electric spindle has a compact structure and light weight. It has the characteristics of low inertia, good dynamic characteristics, and can improve the dynamic balance of the machine tool. Avoiding vibration, pollution, and noise, it has been widely used in ultra high speed cutting machines.

2、 Working principle and typical structure of electric spindle

1. Working principle of electric spindle

The so-called electric spindle refers to the rotor of the motor being directly used as the spindle of the machine tool. The outer shell of the spindle unit is the motor seat, and other safety measures are taken to achieve the integration of the motor and the spindle of the machine tool. The electric spindle directly installs the hollow motor rotor on the spindle, and the stator is fixed in the spindle housing hole through a cooling sleeve, forming a complete spindle unit. After being powered on, the rotor directly drives the spindle to operate.

2. The basic components of an electric spindle

The basic structure of an electric spindle is usually composed of an electric spindle unit, bearings and their lubrication units, a spindle cooling unit, and a dynamic balancing unit. The typical structural layout of an electric spindle unit is that the motor is located between the front and rear bearings of the spindle. Its advantages are that the axial size of the spindle unit is shorter, the spindle stiffness is high, and the power is large, making it more suitable for large and medium-sized high-speed CNC machine tools; Its disadvantage is that the natural heat dissipation conditions of the motor inside the enclosed spindle box are poor, and the temperature rise is relatively high.

3. Typical structure of electric spindle

The torque transmission between the rotor of the motor and the spindle of the machine tool is achieved through the interference fit of the interference sleeve, and the interference amount is calculated based on the magnitude of the transmitted torque. Various forms of key and threaded connections have been eliminated on the spindle, facilitating precise dynamic balancing of the rotating parts of the spindle. Due to the large interference fit between the inner hole of the rotor and the mating surface of the main shaft, the rotor needs to be heated to around 200 ℃ in an oil bath during rapid hot pressing assembly. The stator of the motor is installed in the outer shell of the main shaft through a cooling sleeve.

3、 Key technologies of electric spindle

Electric spindle is a comprehensive application of high-speed bearing technology, lubrication technology, cooling technology, dynamic balancing technology, precision manufacturing and assembly technology, and high-speed motor drive technology.

1. High speed bearing technology for electric spindle

The key to achieving high speed and precision of electric spindles is the application of high-speed precision bearing technology. At present, the bearings used in electric spindles include precision rolling bearings, liquid hydrostatic bearings, air hydrostatic bearings, and magnetic bearings, but mainly precision angular contact ceramic ball bearings and precision cylindrical roller bearings. Angular contact ball bearings can not only withstand radial and axial loads simultaneously, but also have high stiffness, good high-speed performance, simple and compact structure, multiple varieties and specifications, and easy maintenance and replacement. They are widely used in electric spindles. With the development of ceramic bearing technology, the widely used electric spindle bearing is the hybrid ceramic ball bearing, which uses Si3N4 ceramic balls as rolling elements, adopts a "small bead dense ball" structure, and the bearing ring is GCrl5 steel ring. This hybrid bearing can reduce the friction between the ball and the channel by reducing centrifugal force and gyroscopic torque, thereby achieving lower temperature rise and better high-speed performance.

2. Lubrication technology for electric spindle

1) High speed electric spindles require a reasonable and controllable bearing lubrication method to control the temperature rise of the bearings, in order to ensure the accuracy and stability of the CNC machine tool process system. The lubrication methods for electric spindles mainly include grease lubrication, oil mist lubrication, and oil air lubrication.

2) Practice has shown that excessive or insufficient oil supply is harmful in lubrication, and the first two lubrication methods cannot accurately control the amount of oil supply, which is not conducive to improving the speed and service life of the spindle bearings. However, the newly developed oil air lubrication method can accurately control the amount of lubricating oil at each friction point, with high reliability.

3) Oil air lubrication technology uses compressed air to continuously and accurately supply a small amount of lubricating oil to each set of spindle bearings. The tiny oil droplets form an elastic dynamic pressure oil film between the rolling bearings and the inner and outer race raceways, and the compressed air can take away some of the heat generated by the bearing operation.

4) Practice has proven that oil air lubrication is an ideal lubrication method for high-speed and high-power electric spindle bearings, but it requires complex equipment and high costs. Oil air lubrication has become a popular lubrication method in the world due to its ideal lubrication effect.

3. Cooling technology for electric spindle

1) The electric spindle has two main internal heat sources: the heat generated by the built-in motor and the heat generated by the spindle bearings. If not controlled, the resulting thermal deformation will seriously reduce the machining accuracy of the machine tool and the service life of the bearings, leading to a shortened service life of the electric spindle.

2) The electric spindle adopts a concealed spindle structure, and the motor located inside the spindle unit cannot use a fan for heat dissipation, resulting in poor natural heat dissipation conditions. During the energy conversion process, power loss occurs inside the motor, causing it to generate heat. Research has shown that at high speeds, nearly one-third of the heat generated by the motor is generated by the rotor, and most of the heat generated by the rotor is transferred to the stator through the air gap between the rotor and the stator; The remaining 2/3 of the heat is generated in the stator of the motor, so the main solution to heat the motor is to use the circulating flow of coolant to forcibly cool the stator of the motor. A typical cooling system uses an external circulating water cooling device to cool the motor stator and remove the heat from the motor.

3) The heating of angular contact ball bearings is mainly caused by rolling friction between rollers and raceways, sliding friction caused by gyroscopic torque at high speeds, and viscous friction of lubricating oil. The main measures to reduce the heat generation of bearings include: ① appropriately reducing the diameter of the ball bearings to reduce friction and heat generation. ② Adopting a reasonable lubrication method, the oil air lubrication method not only has a lubricating effect on the bearings, but also has a certain cooling effect.

4. Dynamic balancing technology of electric spindle

When the spindle rotates at high speed, any small unbalanced mass can cause large high-frequency vibrations in the electric spindle, so the dynamic balance accuracy of precision electric spindles needs to reach G1-G0.4 level. For this level of dynamic balancing, conventional methods such as separately balancing each component on the spindle before assembly are far from sufficient. After assembly, the entire spindle needs to be dynamically balanced, and even a specialized automatic balancing system can be designed to achieve online dynamic balancing of the spindle. In addition, when designing an electric spindle, it is necessary to strictly follow the principle of structural symmetry. Key connections and threaded connections are prohibited on the electric spindle, but interference fit connections are generally used to achieve torque transmission.

5. Motion Control Technology of Electric Spindle

In CNC machine tools, the electric spindle usually adopts the method of variable frequency speed regulation. There are currently three main control methods: ordinary variable frequency drive and control, vector control drive and control, and direct torque control.

1) Conventional frequency conversion is a scalar drive and control system, characterized by constant torque drive and output power proportional to speed. The dynamic performance of ordinary variable frequency control is not ideal, and the control performance is poor at low speeds. The output power is not stable enough and does not have C-axis function. However, it is cheap and has a simple structure. It is generally used for grinding machines and ordinary high-speed milling machines.

2) Vector Control Technology Model

Imitate the control method of DC motors, orient the rotor magnetic field, and use vector transformation to achieve driving and control, with good dynamic performance. The vector control driver has a large torque value in the initial start-up stage, and the electric spindle itself has a simple structure and low inertia, so the start-up acceleration is large. After start-up, the allowable maximum speed can be reached instantly.

3) Direct torque control is a new high-performance AC speed regulation technology developed after vector control technology. Its control idea is novel, the system structure is simple and clear, and it is more suitable for driving high-speed electric spindles. It can better meet the dynamic and static characteristics requirements of high-speed electric spindles with high speed, wide speed range, and high-speed instantaneous stop. It has become a popular technology in the field of AC transmission.

4、 Conclusion

At present, electric spindles are widely used, and high-speed electric spindle technology was researched earlier abroad. The development of electric spindle units is rapid, and the technical level is in a leading position. With the development of frequency conversion technology and digital technology becoming increasingly perfect, a series of standard products are gradually forming. High speed electric spindles are widely used in the machine tool industry and industrial manufacturing. In the future, we should focus on developing digital high standard electric spindle units with high power, high torque, wide speed range, fast start, accurate positioning, and automatic tool alignment.