This paper introduces the applications of linear motors in high-speed and ultra-high-speed machining, ultra-precision machining, and machining of irregular cross-sections.
1. Introduction
With the rapid development of electronic technology and the emergence of numerous high-speed, multi-functional, or dedicated microprocessors and signal processors (such as DSPs), the application of linear motors in the machine tool field has reached its peak. From the 1996 International Machine Tool Show (IMTS'96) in Chicago, the 18th International Machine Tool Show in Japan, to the 1999 EMO International Exhibition in Paris, a series of influential international exhibitions showcased the powerful appeal of DC motors in various machine tools. This heralded the arrival of a new era for linear motors.
2. Applications in the machine tool field
2.1 Applications in high-speed and ultra-high-speed precision machining
High-speed and ultra-high-speed machining, developed to improve production efficiency and part machining quality, has become a major trend in machine tool development. A responsive, high-speed, and lightweight drive system is required, with speeds exceeding 40-50 m/min and acceleration/deceleration reaching 25-50 m/s². The traditional "rotary motor + ball screw" transmission method is clearly unsuitable due to its inherent weaknesses. The presence of intermediate transmission links firstly reduces stiffness; elastic deformation can increase the system's order, thus reducing robustness and servo performance. Elastic deformation is also the root cause of mechanical resonance in CNC machine tools. Secondly, the presence of intermediate transmission links increases the inertia of the moving body, slowing down displacement and speed response. Furthermore, factors such as backlash, dead zones, friction, and error accumulation limit the maximum achievable feed rate of this traditional method to 30 m/min and acceleration to only 3 m/s².
The advantages of direct drive of DC motors can precisely make up for the shortcomings of traditional transmission methods. Its speed is 30 times that of ball screw pairs; its acceleration is 10 times that of ball screw pairs, up to 10g, and its stiffness is increased by 7 times. In addition, since the linear motor directly drives the worktable, there is no reverse dead zone. Due to the small armature inertia, the linear servo system constructed from it can achieve a high frequency response (such as 100Hz).
Based on the above comparison, linear motors have broad application prospects in high-speed and ultra-high-speed precision machining. Currently, AC linear motors are the main type that meet the requirements of high-thrust feed components in machine tools. Based on the excitation method, they can be divided into permanent magnet (synchronous) and induction (asynchronous) types. The secondary (stator) of the permanent magnet type is a permanent magnet. When used in machine tools, permanent magnets are laid on the machine bed, and three-phase energized windings are mounted in reverse under the worktable to form the primary (moving component) of the linear motor. The primary of the induction type is the same as that of the permanent magnet type, but its secondary uses electric grid bars instead of magnets, essentially unfolding the "squirrel cage" of an induction rotary motor along its circumference.
Permanent magnet linear motors are superior to induction motors in terms of thrust per unit area, power factor, and controllability, but they are more expensive and less convenient to install, debug, and dustproof. The HVM800 high-speed horizontal machining center manufactured by Ingersoll in the United States uses permanent magnet synchronous linear motors for its X, Y, and Z axes, with a maximum feed rate of 76.2 m/min and an acceleration α = 1~ 1.5g .
Induction linear motors have achieved performance levels approaching those of permanent magnet motors, and coupled with their inherent advantages, they are becoming increasingly popular. A typical example of their application is the XHC240 high-speed horizontal machining center developed by the German company Ex-cell-o. All three feed axes are directly driven by Indramat induction linear motors, achieving a maximum rapid traverse speed of 60 m/min and a maximum acceleration of 1g.
In the field of high-speed and ultra-high-speed machining, linear motors are widely used in high-speed milling machines, crankshaft lathes, ultra-precision lathes, grinding machines, and laser lathes. Currently, a popular research area is their application to high-speed parallel mechanisms, namely six-axis and three-axis parallel machine tools. These tools control the cutting tool through the extension and retraction of multiple sliding plungers, enabling high-speed machining of complex surfaces. The Swiss Federal Institute of Technology in Zurich has developed a novel six-slider machine tool that applies linear drive technology to its efficient milling operations. Furthermore, companies such as Lapik in Russia and Ingersoll in the United States have also conducted research in this area and have developed products in this field.
2.2 Applications in Ultra-Precision Machining Ultra-precision machining is a cutting-edge science in manufacturing, playing a crucial role in various industries, especially defense. Gyroscopes in the inertial guidance systems of cruise missiles, waveguides (key components of radar), precision bearings in satellite instruments, and large-scale integrated circuits all involve ultra-precision machining. In ultra-precision machine tools, micro-feed devices are needed for online compensation of machining errors to improve shape accuracy and for machining certain special non-axisymmetric surfaces. High-precision micro-feed devices have become crucial components of ultra-precision machine tools.
Currently, piezoelectric ceramic linear motors are widely used in precision micro-feed devices, and their principle is based on the electrostrictive effect. The deformation of the electrostrictive effect is proportional to the square of the electric field strength. It can achieve high-stiffness, gapless displacement; the resolution can reach 1.0 ~ 2.5nm ; and the deformation coefficient is large; it has a high frequency, and its response time reaches 100μs. To increase the stroke, general motors are made of multiple crystals stacked together and bonded together. The inchworm-type piezoelectric ceramic motor is composed of three separately controlled tubular ceramic piezoelectric devices. Devices A and B perform radial extension and contraction to clamp and release the motor shaft, while device C performs axial extension and contraction to generate axial displacement of the motor shaft, realizing stepping linear motion.
The DTM-3 large diamond lathe and LODTM large optical diamond lathe at LLL National Laboratory in the United States, and the OAGM2500 large precision machine tool from Cranfieid in the United Kingdom, have all adopted electrostrictive micro-feed devices.
Ultrasonic motors (USMs) are a new type of direct-drive motor that is receiving increasing attention both domestically and internationally. Broadly speaking, they are also a type of piezoelectric ceramic motor. They utilize the inverse piezoelectric effect of piezoelectric ceramics to convert the microscopic deformation of materials into macroscopic motion of a rotor or slider through resonant amplification and frictional coupling. Under appropriate voltage, piezoelectric ceramics can form a traveling wave moving in a single direction. When the rotor is subjected to appropriate pressure on the surface of the elastic body, it will generate motion under the drive of point friction. Changing the direction of the traveling wave reverses the rotor's direction.
In ultra-precision machining, to produce aspherical surfaces with high shape accuracy, the feed drive system of ultra-precision machine tools requires high resolution, achieving a movement of 0.01 μm per pulse . Several well-known foreign companies possess this product, but it is embargoed to China. Currently, domestic institutions such as the National University of Defense Technology, Harbin Institute of Technology, and Tsinghua University are conducting research in this area. Ultrasonic motors are characterized by their small size, light weight, fast response speed, lack of electromagnetic interference, and high torque at low speeds. With a stroke within 10 cm, they can replace traditional electromagnetic motors. Stepper-driven ultrasonic motors with feedback closed-loop control have a step resolution of approximately 0.01 μm and are expected to replace mechanical friction drive methods. The surface acoustic wave linear USM developed by the University of Tokyo in Japan has a step resolution as high as 5 nm . 2.3 Application in the field of irregular cross-section machining
The linear motion mechanism, utilizing linear motors, has been successfully applied to computer-controlled precision turning and grinding of irregularly shaped workpieces (such as automotive engine pistons, wave-shaped bearing outer ring raceways, piston rings, and cams) due to its fast response and high precision. Compared with the traditional method of machining irregularly shaped inner and outer cylindrical contours using templates, it offers flexible programming modifications and high machining accuracy, making it highly suitable for processing diverse, small-batch products.
3. Application Examples of Linear Motors in Machine Tools
The following explanation uses the machining of engine pistons as an example.
The piston is one of the most critical components of an engine. During reciprocating operation, the temperature rises differently in the piston head and skirt, resulting in severely uneven deformation. Therefore, at room temperature, the piston cross-section is designed with an irregular shape (approximately elliptical). (Edited by Wu Peng)
