With the continuous development of high-tech industries such as defense, aerospace, automobiles, and microelectronics, higher demands are being placed on the manufacturing and processing industry. Ultra-high-speed machining and ultra-precision machining have become two themes for the future development of the machine tool industry. Traditional machine tool feed drive systems use a "rotary motor + ball screw" mechanism. This type of drive system involves many intermediate components, has a large moment of inertia, and the ball screw itself has physical limitations, resulting in limited linear speed, acceleration, and positioning accuracy, which cannot meet the needs of ultra-high-speed and high-precision machining. Therefore, linear motors have attracted attention. They directly generate linear motion, have a simple structure, low moment of inertia, high system rigidity, good fast response characteristics, can achieve precise positioning at high speeds, generate large thrust, and their motion speed and acceleration are several times higher than those of ball screws. Their working stroke can be infinitely long, requiring less maintenance and having a long service life. These advantages make them the ideal component for modern machine tool feed drives.
Key Technical Issues in Linear Motor Applications in Machine Tools The main linear motors used in machine tool feed servo systems are AC linear motors, which are further divided into synchronous and induction types. With the emergence and improved cost-effectiveness of rare-earth neodymium iron boron (NdFeB) permanent magnet materials, permanent magnet synchronous linear motors have become the mainstream and are the most widely used. This paper takes the application of this type of linear motor in high-speed, high-precision machine tools as an example to analyze the key issues that need to be overcome.
I. Insulation and Heat Dissipation Issues: When a permanent magnet linear motor is running, the coil will heat up due to copper and iron losses, which will bring several negative effects:
(1) It causes aging or damage to the coil insulation layer, making it difficult to pass a larger current into the coil, thus preventing the generation of a larger thrust.
(2) Increased temperature will change the operating point of the permanent magnet.
(3) If heat is transferred to the machine tool table or guide rails, thermal deformation will affect machining accuracy. Therefore, especially for flat, high-thrust linear motors, cooling is necessary. The magnet temperature should not exceed 70°C, and the coil temperature should not exceed 130°C. For moving-coil and general moving-magnet linear motors, cooling the coil is sufficient; however, for moving-magnet linear motors requiring ultra-precision operation, a double-layer water cooling system should be used, along with a temperature sensor monitoring system. Due to their structure, U-shaped linear motors generally do not require cooling measures.
II. Magnetic Isolation and Protection Issues: Machine tool cutting fluid, iron filings, dust, etc., can contaminate and corrode the motor, and even clog the air gap, so the motor must be enclosed. Permanent magnets have a strong attraction to ferromagnetic materials; for safety reasons, they should be isolated, which can be achieved by enclosing them with a stainless steel cover. Linear motors should have shock-absorbing devices and electronic limit switches at both ends to prevent collisions after the mover goes out of control. Cables should be protected with cable chains, and output signal lines should also be shielded.
III. Linear guides are required to withstand loads, adapt to high-speed motion, and ensure accuracy. When selecting a guide, factors such as stroke, mechanical characteristics, precision, and speed tolerance must be considered. Generally, rolling (ball or roller) linear guides are used. During installation, parallelism must be ensured. For ultra-precision applications, air hydrostatic guides can be used.
With continuous innovation in linear motor manufacturing processes, large-scale production, and declining prices of permanent magnet materials and electronic products, the cost of linear motors is decreasing at a rate of 20% annually, indicating a promising future for their application in machine tools. However, this application is still relatively new, and both the linear motor itself and the accompanying CNC technology have significant potential. As a major manufacturing country, my country faces a long and arduous road ahead in developing high-end CNC equipment.
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