Linear motion via ballscrew drives

Linear motion of machine tool axes is often achieved using screws and nuts. In a typical screw-and-ball-nut design, the nut is rigidly attached to the moving structure (table), and the screw is attached directly to the rotor of a servomotor. Via CNC, the rotor rotates the screw. The table cannot rotate because it is supported by the guide ways, so the nut moves along the axis of the screw as the screw rotates, carrying the table along (Figure 1).

Figure 1. A screw-and-ball-nut drive for a linear axis. Adapted from “Hybrid modeling of machine tool axis drives,” by R. Whalley, M. Ebrahimi, and A.A. Abdul-Ameer, International Journal of Machine Tools and Manufacture, Vol. 45, Issue 14, Nov. 2005, pp. 1560-1576.

To reduce friction between the threads on the screw and nut, balls are placed between them. The screw rotates and the balls roll similar to ball bearings. As the balls roll, they progress from one end of the nut to the other. As the balls reach one end of the nut, they are guided into tubes that carry them back to the other end.

Sometimes, several nuts are used together so they can be preloaded against each other to eliminate backlash. This kind of nut is called a “recirculating ball nut” (Figure 2 on page 24). A recirculating ball-nut drive is popular for NC machine tools. The friction is so low in the ball nut that it is possible to drive the screw backward.

In a vertical axis, the weight of the slide can be enough to rotate the screw, allowing the axis to drop if power to the servomotor or CNC is lost. Often a brake, which actuates if power is lost, is included in the design of vertical axes. In a horizontal axis, the cutting force can be enough to cause the screw to rotate unless the servomotor is actively preventing it. Therefore, screw-and-ball-nut designs are generally not suitable for manual machine tools.