Traditional ball screw drive mechanisms can achieve a maximum feed rate of around 60 m/min, while linear motor drive systems in machine tools can reach feed rates exceeding 100 m/min. With the development of modern cutting technology, high-speed and ultra-high-speed cutting technologies for CNC machine tools have become increasingly mature. Traditional ball screws can no longer meet the requirements of high speed and high acceleration.
A linear motor is an electric motor that directly generates linear motion using electrical energy. It can be used as a feed drive component. Machine tool feed systems using linear motors for direct drive eliminate the intermediate mechanical transmission link between the motor and the worktable (slide), achieving "direct drive" and avoiding the disadvantages of lead screw drives such as backlash, inertia, friction, and insufficient rigidity. This speeds up system response, improves system accuracy, and allows for unrestricted stroke, leading to its widespread application. The basic structure of a linear motor is similar to that of a conventional rotary electric motor.
1. Working principle of a linear motor
The principle of a linear motor is not fundamentally different from that of a rotary motor. Imagine cutting a rotary motor in half along its radius and flattening it out; you get a linear motor. In a linear motor, the part that acts like the stator of a rotary motor is called the primary; the part that acts like the rotor is called the secondary. When alternating current is applied to the primary, the secondary moves linearly along the primary under the influence of electromagnetic force.
2. Types of Linear Motors
Linear motors come in various types. In CNC machine tools, they can be categorized by structure into flat, tubular, disc, and arc types; and by working principle into AC linear induction motors, AC linear synchronous motors, linear DC motors, and linear stepper motors. AC linear motors are primarily used in machine tools. Structurally, they can be either short-primary (i.e., primary is fixed, secondary moves) or short-primary (i.e., secondary is fixed, primary moves). To reduce heat generation and cost, linear motors used in high-speed machine tools generally employ a short-primary structure.
3. Advantages of linear motors
1) Excellent response and very high movement speed.
2) Excellent precision (nanometer level).
3) Simple installation.
4) It has no mechanical friction, simple structure, and small size.
5) The stroke is theoretically unlimited, and the performance of the CNC machine tool will not be affected by changes in the stroke.
6) It can provide a wide speed range and has great acceleration.
7) Smooth movement; there are only linear guides between the primary and secondary stages, without any other mechanical transmission links, so the machine tool electrical system is simple to maintain and maintains accuracy for a long time.
8) Easy to seal, not afraid of pollution, highly adaptable, with great thrust and good follow-up.
Therefore, using linear motors can achieve higher efficiency, higher precision in CNC machine tools, and greater practicality. However, linear motor systems are currently in the early stages of application. Issues such as anti-magnetic properties, heat dissipation, lightweighting of the worktable, selection of high-speed guide rail structure for machine tool electrical systems, and the linear motor drive and closed-loop control system all require further research and solutions. Furthermore, production volumes are small, and costs are relatively high. Nevertheless, the application of linear motors in high-speed machining centers has already demonstrated numerous advantages over ball screws in CNC machine tools, and machine tool electrical systems are expected to become the basic transmission method for feed systems in high-speed machining centers this century.
