The rotary motion is driven by the feed motor, into linear motion of the carriage or as a platform of ding. An arc spiral groove is disposed on the lead screw and the nut, and when they are sheathed together, a spiral raceway is formed and a ball bearing is arranged in the raceway. When the lead screw rotates relative to the nut, the nut produces an axial shift, while the ball rolls along the raceway. The two ends of the spiral groove of the nut are connected with a bead returning device, so that the ball can move round and round again, and the two ends of the pipe also play the role of blocking beads, so as to prevent the ball falling out along the raceway.