Impact drill

1. An impact drill for boring a workpiece comprising:

a main frame;

a motor housed in the main frame;

a spindle movably supported by the main frame and rotatable by the motor and movable in its axial direction;

a first ratchet fixed to the spindle and having a first serrated surface including alternating projections and recesses;

a second ratchet having a second serrated surface including alternating projections and recesses and in confrontation with the first serrated surface; and

a first spring biasing the second ratchet toward the first ratchet, the first serrated surface being abuttable on the second serrated surface upon axial displacement of the spindle, and relative rotation between the first ratchet and the second ratchet causing alternating abutment between the projection and the recess and between the projection and the projection for reciprocating the spindle along its axis;

the first spring having a spring constant which maintains the second ratchet floatingly with respect to the main frame and prevents the second ratchet and the spindle from abutting against the main frame when a force ranging from 15 to 25 kg is applied to the main frame for boring the workpiece; and

wherein the first ratchet and the second ratchet are movable together in the main frame in a direction opposite to the biased direction of the second ratchet, the biased direction being given by the first spring.

2. The impact drill as claimed in claim 1 , wherein f/fc is not less than 2, in which f represents a vibration frequency of the second ratchet, and fc represents a normal frequency determined by components including the main frame and excluding the second ratchet and by the spring constant of the first spring.

3. The impact drill as claimed in claim 2 , wherein f/fc is not less than 3.

4. The impact drill as claimed in claim 1 , wherein the second ratchet is slidable with respect to the main frame in the axial direction, and

wherein the second ratchet is unrotatably supported to the main frame.

5. The impact drill as claimed in claim 4 , wherein the second ratchet has a generally cylindrical shape and formed with a cut away portion; and

wherein the main frame has a generally cylindrical space provided with a large thickness region complementary to the cut away portion.

6. The impact drill as claimed in claim 4 , further comprising:

a spindle spring interposed between the main frame and the spindle for normally urging the spindle in a direction to protrude out of the main frame; and

a stop member supported within the main frame for restricting the movement of the second ratchet toward the first ratchet, the first ratchet being movable through the stop member.

7. The impact drill as claimed in claim 4 , further comprising:

a spindle spring interposed between the main frame and the spindle for normally urging the spindle in a direction to protrude out of the main frame;

a washer movable within the main frame in the axial direction, the second ratchet being abuttable on the washer; and

a second spring interposed between the main frame and the washer for biasing the second ratchet in a direction away from the first ratchet.

8. The impact drill as claimed in claim 7 , further comprising a damper member disposed between the main frame and the second ratchet.

9. An impact drill for boring a workpiece comprising:

a main frame;

a motor housed in the main frame;

a spindle movably supported by the main frame and rotatable by the motor and movable in its axial direction;

a first ratchet positioned fixedly with respect to the spindle;

a second ratchet positioned with respect to the first ratchet; and

a spring biasing the second ratchet toward the first ratchet;

wherein the spring has at least one of a spring constant and a biasing force which maintains the second ratchet floatingly with respect to the main frame and prevents the second ratchet and the spindle from abutting against the main frame at least one of when a force ranging from 15 to 25 kg is applied to the main frame for boring the workpiece and when the spindle is moved to a retracted position; and

wherein the first ratchet and the second ratchet are movable together in the main frame in a direction opposite to the biased direction of the second ratchet, the biased direction being given by the spring.