An apparatus for grinding a threaded portion of a workpiece, is provided. The threaded portion has a spiral groove formed therein. The apparatus includes a motor, a grinding cord, a detector and a controller. The motor has a rotary portion for rotating the threaded portion of the workpiece about an axis. The grinding cord is configured for being looped around the threaded portion of the workpiece with a middle part thereof engagingly received in a given portion of turns of the spiral groove for grinding the thread portion of the workpiece. The detector is for detecting a rotating cycle of the rotary portion. The controller is communicatively coupled to the detector for controlling the rotary portion to selectively rotate in a first rotating direction or in a reverse second rotating direction based on the received signal from the detector.