The shaft in a machine tool is the axis of rotation in it, and most likely there will be a drive shaft in it.
What is the Spindle?
The spindle refers to the shaft that receives power from the engine or electric motor and transmits it to other parts. The main axis, also known as the optical axis, has a symmetrical diameter in the optical train. For example, the main axis of a spherical mirror is a straight line passing through the center of the mirror and perpendicular to the mirror. The spindle of the lens or the optical axis optical train is the line connecting the centers of the lens surfaces. There will be a layer of hardened layer on the surface of the spindle after quenching heat treatment. It is because of this hardened layer that the hardness, wear resistance, and service life of the spindle are improved.
The spindle is usually composed of the main shaft, bearings, and transmission parts (gears or pulleys). In the machine, it is mainly used to support transmission parts such as gears and pulleys, and transmit motion and torque, such as machine tool spindles. Some are used to clamp workpieces, such as mandrels. Except for machine tools whose main motion is linear motion, such as planers and broaching machines, most machine tools have spindle components. The motion accuracy and structural rigidity of the spindle components are important factors that determine the processing quality and cutting efficiency. The indicators to measure the performance of spindle components are mainly rotation accuracy, stiffness, and speed adaptability.
The machine tool spindle is the motion axis that drives the tool or workpiece to rotate on the machine tool to generate cutting motion. According to different spindle drive methods, machine tool spindles can be divided into mechanical spindles and electric spindles. The components of the machine spindle are usually composed of the spindle, bearings, and transmission devices. In the machine, the spindle motor and the transmission device in the middle drive the spindle to rotate. The output torque and power are large, but the speed, accuracy, and stability are relatively poor.
- Rotation accuracy: The radial and axial runouts that occur in the direction that affects the machining accuracy when the spindle rotates are mainly determined by the manufacturing and assembly quality of the spindle and bearings.
- Dynamic and static stiffness: Mainly determined by the bending stiffness of the main shaft, the stiffness, and the damping of the bearing.
- Speed adaptability: The allowable maximum speed and speed range are mainly determined by the structure and lubrication of the bearing, as well as the heat dissipation conditions.
What is an Electric Spindle?
The electric spindle is a new technology that integrates machine tool spindle and spindle motor in the field of CNC machine tools in recent years. Together with linear motor technology and high-speed tool technology, it will push high-speed machining into a new era. The electric spindle is a set of components, which includes the electric spindle itself and its subsidiary files: electric spindle, high-frequency frequency conversion device, oil mist lubricator, cooling device, built-in encoder, and tool changer.
The high-speed motor of the electric spindle is placed inside the spindle component, which is a new type of spindle component that integrates the functions of the machine tool spindle and the motor from the structure. The electric spindle is directly driven by the built-in motor, and through the control system, the spindle can obtain the required working speed and torque, so it is also called the built-in electric spindle. The electric spindle saves the transmission links of belts, gears, or couplings, shortens the length of the transmission chain of the machine tool to zero, realizes zero transmission of the machine tool, and effectively improves the overall performance of the spindle under high-speed conditions.
The electric spindle is a new technology that integrates the machine tool spindle and the spindle motor in the field of CNC machine tools. The spindle is a set of components, which includes the electric spindle itself and its accessories: electric spindle, high-frequency frequency conversion device, oil mist lubricator, cooling device, built-in encoder, tool changer, etc. This kind of transmission structure in which the spindle motor and the machine tool spindle combine two into one, makes the spindle components relatively independent from the transmission system and the overall structure of the machine tool, so it can be made into a spindle unit. It is characterized by high speed, high precision, and low noise, and the structure with a lock on the inner ring is more suitable for spray lubrication. Motorized spindles do not rely on an external motor for torque and power, the motor is an integral part of the spindle and housing assembly. This allows the spindle to rotate at higher speeds as a complete unit without the additional constraints of belts or gears.
Market Analysis of the Electric Spindle Industry:
The global demand for electric spindles is mainly concentrated in China, Japan, the United States, Switzerland, Italy, South Korea, and other countries and regions. China is the main electric spindle market in the world, and the localization rate continues to increase. In recent years, with the improvement of market share and the maturity of domestic enterprise technology, the scale of China's electric spindle market will reach 478 million US dollars in 2021, a year-on-year increase of 11.75%. It is expected that the market size will reach 774 million US dollars in 2028. The CAGR for 2022-2028 is 6.67%.
From the perspective of enterprises, the main manufacturers of electric spindles are in China, Europe, Japan, and China. Among them, foreign companies occupied the main market for high-end products in the past few years. In recent years, as domestic companies have increased their investment in research and development, their products have gained certain market competitiveness and have begun to seize more and more market shares. At the same time, with the vigorous promotion of domestic industrial upgrading policies, customers have put forward more quality requirements and higher technical indicators for electric spindles, and the specifications and functions have become more and more diverse. Electric spindle companies should actively improve product quality so that their products are attractive to the market.
Which Technologies are Integrated into the Electric Spindle?
- High-speed bearing technology: Electric spindles usually use composite ceramic bearings, which are wear-resistant and heat-resistant, and their service life is several times that of traditional bearings. Sometimes electromagnetic suspension bearings or static pressure bearings are also used, the inner and outer rings are not in contact, and the service life is theoretically unlimited.
- High-speed motor technology: The motorized spindle is the product of the integration of the motor and the spindle. The rotor of the motor is the rotating part of the spindle. Theoretically, the motorized spindle can be regarded as a high-speed motor. The key technology is dynamic balance at high speed.
- Lubrication: The lubrication of the electric spindle generally adopts regular and quantitative oil-air lubrication. Grease lubrication can be used. The timing means injecting oil at regular intervals. The so-called quantitative means to accurately control the amount of lubricating oil each time through a device called a quantitative valve. The oil-air lubrication refers to the fact that the lubricating oil is blown into the ceramic bearing under the pressure of compressed air. It is important to control the amount of oil. If it is too little, it will not be lubricated. If it is too much, it will generate heat due to the resistance of the oil when the bearing rotates at high speed.
- Cooling device: To dissipate heat for the high-speed electric spindle as soon as possible, the outer wall of the electric spindle is usually circulated with coolant, and the function of the cooling device is to maintain the temperature of the coolant.
- Built-in pulse encoder: To realize automatic tool change and rigid tapping, the electric spindle has a built-in pulse encoder to achieve accurate phase angle control and coordination with the feed.
- Automatic tool changer: To be applied to the machining center, the electric spindle is equipped with an automatic tool changer, including disc springs, broach cylinders, etc.
- The clamping method of high-speed cutting tools: The familiar BT and ISO cutting tools have been proven to be unsuitable for high-speed machining. In this case, high-speed tools such as HSK and SKI are extended.
- High-frequency frequency conversion device: To realize the speed of tens of thousands or even hundreds of thousands of revolutions per minute of the electric spindle. A high-frequency conversion device must be used to drive the built-in high-speed motor of the electric spindle, and the output frequency of the frequency converter must reach thousands or thousands of hertz.
Challenges Faced in the Development, Design, and Production of High-Speed Electric Spindles:
- Lubrication: The lubrication methods of electric spindles mainly include grease lubrication, oil mist lubrication, and oil-air lubrication. China mainly uses grease lubrication and oil mist lubrication. Although grease lubrication is easy to use and has strong versatility, neither of these two lubrication methods can accurately control the amount of oil supplied, which is not conducive to the improvement of the spindle bearing speed and life. Mist lubrication will pollute the environment to a certain extent. The oil-air lubrication can accurately control the amount of lubricating oil at each friction point, but the required equipment is complicated and the cost is high. At present, foreign high-speed electric spindle bearings have generally adopted advanced oil-air lubrication technology.
- In terms of function and performance: Foreign countries are already developing multifunctional and high-performance electric spindle products for CNC machine tools, while China still takes conventional products as the main development direction. Part of the reason is that foreign electric spindle manufacturers are generally only responsible for the overall design of electric spindles, technology research and development, and the assembly and testing of spare parts, and the rest of the key spare parts are purchased by the market. Due to the meticulous division of labor, each can carry out scientific and technological research on the key technologies of this specialty, so foreign electric spindle manufacturers have strong forward-looking and innovative capabilities in the research and development of key functional components.
- Basic industry: There is a certain gap between China and industrially developed countries in terms of materials and heat treatment, especially in the process of spindle processing and the manufacturing technology of key parts. Due to the limitation of machine tools, many parts are not as good as foreign advanced manufacturing levels in terms of surface roughness and shape and position tolerances.
The Overall Development Trend of the Electric Spindle:
- High power: The motorized spindle is developing towards high speed, high power, low speed, and high torque. According to the needs of practical applications, most CNC machine tools need to be able to meet the requirements of heavy cutting during low-speed rough machining and finishing during high-speed cutting. Therefore, the electric spindle should have the performance of low-speed, high-torque, high-speed, and high-power.
- High speed: The electric spindle continues to develop towards high speed and high rigidity. In the context of the continuous development of electric spindle bearings and their lubrication technology, precision machining technology, and precision dynamic balancing technology, the high-speed electric spindle of CNC machine tools has become a common trend in current development. In terms of rigidity, due to the development of bearings and their lubrication technology, the system rigidity of the electric spindle is increasing, which meets the needs of the development of high-speed, high-efficiency, and precision machining CNC machine tools.
- High precision: The electric spindle is further developed in the direction of high precision, high reliability, and long life. As end users put forward higher and higher requirements for the accuracy and reliability of CNC machine tools, correspondingly, the requirements for the accuracy and reliability of the electric spindle are also constantly increasing. At the same time, due to the use of special precision bearings, advanced lubrication methods, and special preload application methods, the life of the electric spindle is correspondingly extended, and its reliability is getting higher and higher.