Patent ID: 7428934

Claim:
An impact fastening tool comprising: a motor for generating a driving force; an output shaft for fastening an object to be fastened; a strike mechanism including a hammer and an anvil integrally rotated with the output shaft, generating impact force by striking the anvil by the hammer and transmitting the driving force generated by the motor to the output shaft with the impact force; a rotation sensor that outputs a predetermined number of pulses by one rotation of a shaft of the motor; a rotation speed detector that detects rotational speed of a shaft of the motor by measuring a pulse width of pulses outputted from the rotation sensor; a strike detector that detects occurrence of strikes of the anvil by the hammer based on variation of the pulse width of the pulses measured by the rotation speed detector; a current detector for detecting current information in an interval of the strikes; a strike judger that judges whether detection of the strike by the strike detector is real or unreal by comparing current information detected by the current detector with a predetermined threshold; a fastening torque calculator for calculating a fastening torque equivalent to a fastening torque generated by the impact forces by ignoring the strike judged unreal by the strike judger; and a motor controller for stopping driving of the motor at a time when the calculated fastening torque reaches a predetermined reference value.