With the continuous advancement of high-tech industries such as defense, aerospace, automotive, and microelectronics, the manufacturing sector faces increasingly demanding requirements. Ultra-high-speed and ultra-precision machining have emerged as key trends in the evolution of the machine tool industry. Traditionally, machine feed drive systems have relied on the "rotary motor + ball screw" mechanism. However, this setup involves multiple intermediate components, leading to significant motion inertia and physical limitations in terms of speed, acceleration, and positioning accuracy. These constraints make it unsuitable for modern high-speed and high-precision machining needs. In contrast, linear motors have gained attention due to their direct linear motion capability, compact design, low inertia, high stiffness, fast response, and precise high-speed positioning. They offer higher speeds and accelerations compared to ball screws, with longer travel distances, lower maintenance, and a longer lifespan, making them an ideal choice for modern machine tool drives.
The main technical challenges in applying linear motors to machine tools are primarily related to AC linear motors used in servo systems. These can be categorized into synchronous and induction types. With the development of rare earth neodymium iron boron (NdFeB) permanent magnet materials and improved cost-performance ratios, permanent magnet synchronous linear motors have become the dominant and most widely used type. For example, in high-speed and high-precision machine tools, several critical issues must be addressed.
First, thermal management and heat dissipation remain a major concern. When a permanent magnet linear motor is in operation, the coil heats up due to copper and iron losses, which can lead to several negative consequences. The insulation layer of the coil may degrade, limiting the current it can carry and thus reducing thrust. Increased temperature can also shift the operating point of the permanent magnet, and if heat transfers to the machine table or guide rail, it may cause thermal deformation that affects machining accuracy. Therefore, flat-plate large-thrust linear motors require cooling, with magnetic steel temperatures not exceeding 70°C and coil temperatures below 130°C. While the coil in moving-coil or general moving-magnet motors can be cooled, ultra-precision applications may require a dual-layer water-cooling system with temperature monitoring. U-shaped linear motors typically do not need cooling due to their structural design.
Second, magnetic isolation and protection are essential. Cutting fluids, metal shavings, and dust can contaminate the motor and even block the air gap, so the motor must be enclosed. Permanent magnets have strong attraction to ferromagnetic materials, necessitating magnetic shielding and a stainless steel cover for safety. Shock-absorbing devices and electronic limit switches should be installed at both ends of the motor to prevent uncontrolled collisions. Additionally, output signal cables must be shielded to protect against cable dragging.
Third, the linear guide rails must support the load, accommodate high-speed movement, and maintain precision. The design should consider stroke length, mechanical properties, accuracy, and speed capacity. Rolling (ball or roller) linear guides are commonly used to ensure parallelism during installation. For ultra-precision applications, aerostatic guides are preferred.
As the manufacturing process of linear motors continues to improve, production scales expand, and the costs of permanent magnets and electronic components decrease, the price of linear motors is dropping by about 20% annually. This trend opens up broad application prospects in machine tools. Although still a relatively new technology, both the linear motor itself and the associated CNC technology hold great potential. As a major manufacturing country, China has a long way to go in developing advanced CNC equipment.
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