A chips dischargeable wiper is disposed at the end of a nut, the wiper is defined with a flange, a plurality of ribs integrally formed with the flange, and a plurality of through holes located beside the flange. The flange is used to mate with the helical groove of the ball screw in such a manner at least a rib tapered from outside in is disposed between the flange and the screw shaft, which not only can reduce the resistance of the wiper, but also can effectively prevent intrusion of foreign objects into the screw shaft and can prevent leakage of lubricating agent. Furthermore, the rib can collect dusts and the foreign objects and discharge them out of the through holes. Therefore, the structure of wiper is simplified and can be used for a long time without maintenance.