Basic requirements for the feed drive of CNC machine tools
CNC machine tools are typically divided into two main components: 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 auxiliary systems like hydraulics, cooling, and lubrication based on the input program. To perform part machining, the machine tool must execute two essential movements: the main motion and the feed motion. In addition to being controlled by the CNC, these motions must be characterized by fast response, high precision, and strong stability in the mechanical structure.
This lecture focuses specifically on the mechanical characteristics of the feed system. One key aspect is **high transmission stiffness**, which largely depends on the rigidity of the screw-nut pair (for linear motion) or the worm gear (for rotational motion), along with their supporting structures. If stiffness is insufficient, combined with friction, it can lead to crawling of the table and create dead zones, which negatively impact accuracy. To enhance stiffness, the transmission chain should be shortened, appropriate screw sizes selected, and the nut pairs and supports pre-tightened.
Another important feature is **high natural frequency**. To improve vibration resistance, 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.
**Low friction** is also critical for the feed system. It requires smooth movement, accurate positioning, and quick response. This means reducing frictional resistance and minimizing the difference between static and dynamic friction. Ball screw-nut pairs are commonly used to achieve this.
Additionally, **low inertia** is essential because the feed system frequently starts, stops, reverses, or changes direction. High inertia increases the load and reduces dynamic performance. Therefore, under the condition of meeting strength and rigidity requirements, the weight of moving parts and the size of transmission elements should be minimized to improve the system's responsiveness.
Lastly, **no backlash** is crucial. Mechanical clearances in the drive chain—such as in gears, screw-nut pairs, couplings, and their supporting components—can cause reverse dead zones. Structural design measures must be implemented to eliminate all such gaps.
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