Why do high-end 5-axis CNC machines choose DD motors instead of servo motors?

DD motors are widely used in high-end equipment such as 5-axis CNC machines because of their high precision, fast dynamic response, low maintenance requirements, and long service life.

What is a DD motor?

DD stands for Direct Drive, which includes torque motors and linear motors. When combined with a motor, it is referred to as a DD direct drive motor, also known as a direct drive motor. Due to its high torque output, some companies also call it a torque servo motor. Unlike traditional motors, the high torque of a DD motor allows it to be directly connected to the motion system, eliminating the need for transmission components such as reducers, gearboxes, and pulleys. This is why it is called a direct drive motor.

Why use a DD motor?

DD motors are typically equipped with high-resolution encoders, enabling them to achieve a higher level of precision than conventional servo systems. In addition, the direct connection structure reduces positioning errors caused by mechanical transmission, ensuring machining accuracy. For certain cam-driven control methods, it not only reduces dimensional errors caused by mechanical friction but also significantly lowers noise during installation and operation.

What is the difference between a DD motor and a servo motor?

A DD motor, also known as a torque motor, features high output torque and an integrated structure that allows the load to be directly mounted on the motor surface. There is no loss of precision between the motor and the working platform, which enhances overall system accuracy. Without a gearbox or mechanical transmission, there is no energy loss associated with such components. The direct drive structure also makes it easier to change indexing positions through programming, while reducing operational noise. With high-resolution encoders, DD motors can achieve arcsecond-level precision and are therefore referred to as direct drive motors in the industry.

A conventional servo motor, also known as an actuator motor, is used in automatic control systems to convert electrical signals into angular displacement or angular velocity. It includes both AC and DC servo motors. Its working principle is that when the signal voltage is zero, the motor does not rotate, and the speed decreases uniformly as torque increases. Servo motors can operate in torque, position, and speed control modes and are typically used in closed-loop control systems with high precision. They generally have low inertia and are suitable for positioning applications.

Why are DD motors more suitable for high-end 5-axis CNC machines?

DD motors use a direct drive structure without a gearbox, achieving zero backlash, which is critical because backlash error is one of the biggest challenges in 5-axis rotary tables. Their extremely high precision and repeatability, combined with high-end encoders such as optical scales, can reach sub-arcsecond accuracy, which is especially advantageous in machining aerospace components and complex parts like impellers. In addition, because there is no mechanical transmission chain, DD motors offer extremely fast dynamic response, making them ideal for high-speed 5-axis simultaneous machining. Although the initial cost of DD motors is higher than that of servo motor and gearbox combinations, they require very little maintenance and provide a much longer service life, resulting in better cost performance over time.

Leading manufacturers such as DMG Mori and Mazak have widely adopted DD direct drive motors in their high-end 5-axis machines, demonstrating that DD motors are the optimal solution for high-end 5-axis CNC machining.