Application and Common Problems of Linear Motor in CNC Machine Tools

With the continuous development of high-tech industries such as defense, aerospace, automotive, and microelectronics, higher demands are placed on the manufacturing industry. Ultra-high-speed machining and ultra-precision machining have become the two themes of the future development of the machine tool industry. The traditional machine feed drive system is the "rotary motor + ball screw" mechanism. This kind of drive system involves many intermediate parts, large motion inertia, and the ball screw itself has physical limitations, so the linear speed, acceleration and positioning accuracy are limited, which can not meet the needs of ultra-high speed and high precision machining; Linear motor has attracted people's attention. It directly produces linear motion, simple structure, small motion inertia, high system stiffness, good fast response characteristics, precise positioning at high speed, and high thrust, especially moving speed and acceleration higher than ball screw. Several times, the work schedule can be infinitely long, with less maintenance and long life. These advantages make it an ideal component for modern machine feed drives.

The key technical problems of linear motors in machine tool applications are mainly for AC linear motors in machine tool feed servo systems . They are also divided into synchronous and inductive types. With the advent of rare earth neodymium iron boron (NdFeB) permanent magnet materials and the improvement of cost performance, the development of permanent magnet synchronous linear motors has become the mainstream and the most widely used. Taking the application of such linear motors on high-speed and high-precision machine tools as an example, the key problems to be overcome are analyzed.

I. Adiabatic and heat dissipation problems When the permanent magnet linear motor is running, the coil will heat up due to copper loss and iron loss, which brings several negative effects:

(1) Old damage or damage to the coil insulation layer, making the coil inconvenient to pass a larger current, so that no greater thrust can be generated.

(2) An increase in temperature will change the operating point of the permanent magnet.

(3) If the heat is transferred to the machine table or the guide rail, the thermal deformation will affect the machining accuracy. Therefore, especially the flat-plate large thrust linear motor must be cooled, and the temperature of the magnetic steel is required to not exceed 70 ° C. The coil temperature is not More than 130 °C. For the moving coil and the general moving magnet linear motor, the coil part can be cooled; but the moving magnetic linear motor under the ultra-precision requirement should adopt double-layer water cooling method with temperature sensor monitoring system. . U-shaped linear motors generally do not require cooling measures due to structural causes.

Second, magnetic isolation and protection problems Machine cutting fluid, iron filings, dust, etc. can pollute the motor, and even block the air gap, so the motor must be closed. Permanent magnet steel has a strong attraction to ferromagnetic materials. For safety reasons, it should be magnetically shielded and closed with a stainless steel cover. There should be a shock-absorbing device and an electronic limit switch at both ends of the linear motor to prevent the collision of the mover out of control. To protect the cable from the towline, the output signal cable must be shielded.

Third, the linear guide rails are required to bear the load, adapt to high-speed motion and ensure accuracy. The guide rail should consider the stroke size, mechanical characteristics, precision and speed bearing capacity. Generally, rolling (ball or roller) linear guides are used to ensure parallelism during installation. For ultra-precision requirements, an aerostatic guide can be used.

With the continuous innovation of the linear motor manufacturing process, the scale of production, and the decline in the price of permanent magnet materials and electronic products, the cost of linear motors is decreasing at a rate of 20% per year, and the application prospects on machine tools are broad. But this application is a new thing after all, whether it is a linear motor itself or a matching CNC technology, the potential is great. China is a big manufacturing country and it has a long way to go to develop high-end CNC equipment.