This paper develops a mechanical tool as well as its manipulation policies for 2-finger parallel robotic grippers. It primarily focuses on a mechanism that converts the gripping motion of 2-finger parallel grippers into a continuous rotation to realize tasks like fastening screws. The essential structure of the tool comprises a Scissor-Like Element (SLE) mechanism and a double-ratchet mechanism. They together convert repeated linear motion into continuous rotating motion. At the joints of the SLE mechanism, elastic elements are attached to provide resisting force for holding the tool as well as for producing torque output when a gripper releases the tool. The tool is entirely mechanical, allowing robots to use the tool without any peripherals and power supply. The paper presents the details of the tool design, optimizes its dimensions and effective stroke lengths, and studies the contacts and forces to achieve stable grasping and screwing. Besides the design, the paper develops manipulation policies for the tool. The policies include visual recognition, picking-up and manipulation, and exchanging tooltips. The developed tool produces clockwise rotation at the front end and counter-clockwise rotation at the back end. Various tooltips can be installed at both two ends. Robots may employ the developed manipulation policies to exchange the tooltips and rotating directions following the needs of specific fastening or loosening tasks. Robots can also reorient the tool using pick-and-place or handover, and move the tool to work poses using the policies. The designed tool, together with the developed manipulation policies, are analyzed and verified in several real-world applications. The tool is small, cordless, convenient, and has good robustness and adaptability.