Patent ID: 6978846

Claim:
A power tool for fastening a screw or bolt comprising: a motor; a motor controller for controlling operation of the motor; a power transmitter for transmitting driving force of the motor to a bit for fastening or loosening a screw or bolt; a torque estimator for estimating a value of a torque necessary for fastening the screw or the bolt; and a fastening judger for judging whether the screw or bolt is fastened completely or not; the fastening judger being configured to: calculate a torque variation quantity which is a ratio of torque variation with respect to a rotation angle of a shaft of the motor, and a torque variation ratio value which is a ratio of the torque variation quantity with respect to the rotation angle of the shaft of the motor; calculate a reference index using the torque variation ratio value, for judging whether or not an increase of the value of the torque is a temporary increase due to trouble in a fastening operation; judge that the fastening operation is completed at least when the torque variation ratio value becomes substantially zero and the reference index becomes equal or larger than a trouble threshold value which indicates whether or not trouble has occurred in the fastening operation; and outputs a signal to stop driving the motor to the motor controller when the fastening operation is completed.