High speed, precision, and modularity are the development directions of modern manufacturing technology. A new cutting theory suggests that once the cutting speed reaches a certain level (approximately 500 m/min), the temperature in the cutting zone stops rising, and the cutting force actually decreases, reducing tool wear. This improves productivity while also enhancing the surface quality and machining accuracy of the parts.
Generally speaking, the cutting speed and feed rate of high-speed machining are an order of magnitude higher than those of conventional machining. Therefore, high-speed spindles and rapid feed systems are two key technologies for achieving high-speed machining. The following new requirements are put forward for the feed system: (1) The feed rate must be matched with the high-speed spindle, reaching 60 m/min or higher; (2) The acceleration must be large so that the required high speed can be achieved in the shortest time and stroke, at least 1 to 2 g; (3) The dynamic performance must be good, and it can realize rapid servo control and error compensation, with high positioning accuracy and rigidity.
For a long time, the feed system of CNC machine tools has mainly consisted of a rotary servo motor and a ball screw. The highest feed speed achievable with this system is 90–120 m/min, and the maximum acceleration is only 1.5g . Furthermore, because there are a series of intermediate components such as couplings, ball screws, nuts, bearings, and supports between the motor spindle and the worktable, the elastic deformation, friction, and backlash generated by these mechanical components when the feed components need to perform actions such as starting, accelerating/decelerating, reversing, and stopping will cause…
The hysteresis of the feed motion and many other nonlinear errors: these intermediate links also increase the system's inertial mass, affecting the rapid response to motion commands. In addition, the leadscrew is a slender rod, which will deform under the action of force and heat, affecting machining accuracy.
To overcome the shortcomings of traditional feed systems, simplify machine tool structures, and meet the requirements of high-speed precision machining, people began to research new feed systems. Linear motors are the most promising rapid feed systems. They eliminate all intermediate transmission links between the power source and the worktable components, making the length of the machine tool feed transmission chain zero. This is the so-called "direct drive" or "zero transmission".
The principle and classification of linear motors
A linear motor is a device that uses the principle of electromagnetic interaction to directly convert electrical energy into linear motion kinetic energy. In practical applications, to ensure that the coupling between the primary and secondary windings remains constant throughout the entire stroke, the primary and secondary windings are generally manufactured with different lengths. Similar to rotary motors, when three-phase current is applied to a linear motor, a magnetic field is generated in the air gap. If end effects are ignored, the magnetic field has a sinusoidal distribution in the linear direction; however, this magnetic field translates rather than rotates, hence it is called a traveling wave magnetic field. The interaction between the traveling wave magnetic field and the secondary winding generates electromagnetic thrust, which is the basic principle of linear motor operation.
Because of the above correspondence between linear motors and rotary motors, each type of rotary motor has a corresponding linear motor. However, the structural forms of linear motors are more flexible than those of rotary motors. Linear motors can be classified according to their working principle as: linear DC motors, linear induction motors, linear synchronous motors, linear stepper motors, linear piezoelectric motors, and linear reluctance motors; and according to their structural form as: flat plate type, U-shaped type, and cylindrical type. (Edited by Wu Peng)
