A dual-function electrical hand drill comprising:
- a casing defining therein an interior space;
- a motor securely fixed inside the interior space of the casing, the motor having an output spindle to which a speed reduction gear train is mounted to be rotatable therewith;
- a power source for driving the motor, the power source being controlled by a power switch;
- a driving shaft which is coupled to the speed reduction gear train to be driven thereby and releasably engageable by a chuck to transmit torque from the motor to the chuck;
- a setting ring which is rotatably fit over the driving shaft to be rotatable between a plurality of predetermined angular positions of which one corresponds to a vibration operation mode and the remaining positions correspond to different torque output levels of a rotation operation mode;
- speed control means which control rotational speed of the driving shaft in correspondence with the torque output levels set by the setting ring;
- and
- vibration means comprising:
- a vibration generating base which is securely
- fixed inside the casing, the vibration generating
- base comprising a cylindrical body having formed therein a recess with a circumferential step formed therein, a plurality of notches being formed on the step;
- a mode switching ring rotatably fit over the driving shaft and having an end received within the recess of the vibration generating base, comprising a circumferential shoulder on which a plurality of projections are formed, corresponding to the notches of the vibration generating base, to be receivable within the notches of the vibration generating means, the projections having lower ends to support the mode switching ring on the step of vibration generating base, the mode switching ring further comprising at least one slot to drivingly engage a rib formed inside the setting ring so that the rotation of the setting ring from the rotation operation mode into the vibration operation mode moves the projections of the mode switching ring into the notches of the vibration generating base and has the shoulder in direct contact engagement with the step to be directly supported thereby;
- a serration formed on a bottom of the recess of the vibration generating base;
- a ring member which is press fit over the driving shaft at a pre-determined position, the ring member having a serrated end surface facing the serration formed inside the recess of the vibration generating base to be engageable therewith when the projections of the mode switching ring are received within the notches of the vibration generating base.
- biasing means disposed between the driving shaft and the mode switching ring to bias the driving shaft away from the serrated bottom of the recess of the vibration generating base so that a reaction force that is caused by operating on a work piece moves the driving shaft against the biasing means to have the serrated end surface of the ring member engage the serration formed inside the vibration generating base and thus generate a to-and-fro vibration of the driving shaft relative to the casing by sliding movement occurring between the serrated end surface of the ring member and the serrated bottom of the recess of the vibration generating base and the biasing force provided by the biasing means.