Basic requirements for the feed drive of CNC machine tools
CNC machine tools are composed of two main parts: the CNC system and the machine tool itself. The CNC system is responsible for controlling various functions such as table positioning, spindle start/stop, direction changes, tool selection and replacement, as well as managing the hydraulic, cooling, and lubrication systems based on the input program. To complete the machining process, the machine tool must perform two essential movements: the main motion and the feed motion. In addition to being controlled by the CNC, these motions require a mechanical structure that ensures fast response, high precision, and strong stability.
This lecture will focus specifically on the mechanical characteristics of the feed system.
1. High Transmission Stiffness
A key factor in achieving accurate and stable movement is the stiffness of the feed drive system. This stiffness primarily comes from components like the ball screw nut assembly (for linear motion) or worm gear (for rotational motion), along with their supporting structures. If stiffness is inadequate, it can lead to issues like crawling or backlash, which negatively affect accuracy. To enhance stiffness, the transmission chain should be shortened, appropriate screw sizes selected, and the nut and support components pre-tightened.
2. High Resonance Frequency
To improve the vibration resistance of the feed system, the mechanical components must have a high natural frequency and sufficient damping. Ideally, the natural frequency of the mechanical transmission system should be 2 to 3 times higher than that of the servo drive system, ensuring better dynamic performance and reducing unwanted oscillations.
3. Low Friction
The feed system requires smooth and precise movement, as well as quick response. Reducing friction between moving parts and minimizing the difference between static and dynamic friction coefficients is essential. Ball screw nut assemblies are commonly used in feed systems due to their low friction and high efficiency.
4. Low Inertia
Since the feed system frequently starts, stops, reverses, or changes speed, high inertia can increase the load and reduce the system’s dynamic performance. Therefore, under conditions that maintain strength and rigidity, the weight of moving parts and the size of transmission elements should be minimized to improve responsiveness and control accuracy.
5. Zero Clearance
Clearance in the drive train—such as in gear pairs, screw nut assemblies, couplings, and other components—can cause backlash and dead zones in the feed system. To eliminate this, structural design measures must be implemented to ensure all components are tightly coupled and free of play.
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