Abstract:
An impact drill minimizing transmission of vibration to a handle gripped by a user&#39;s hand. A spindle extends tends through a main frame and is movable in its axial direction rection and rotatablea about its axis. A first ratchet is rotatable and axially movable together with the spindle. A second ratchet is axially movable but unortatable. The first ratchet has a first serrated surface and the second ratchet has a second serrated surface in confrontation with the first serrated surface. In an impact drilling mode, the first serrated surfaces is brought into abutment with the second serrated surface so that the spindle is reciprocally moved in the axial direction. A spring is interposed between the main frame and the second ratchet to urge the second ratchet toward the first ratchet. In the impact drilling mode, the second ratchet is always out of contact from the main frame and is floatingly maintained within the main frame even if a force ranging from 15 to 25 kg is applied to the main frame toward a workpiece.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an impact drill for boring a hole in a concrete, mortar and tiles, and more particularly, to such impact drill providing a drilling mode in which a boring is performed by rotating a drill bit and a impact drilling mode in which boring is performed by rotating and impacting or vibrating the drill bit.  
         [0002]     A conventional impact drill of this type is shown in  FIGS. 15 through 18 . A main frame  401  includes a gear cover  417 , an inner cover  418 , an outer cover  419 , a housing  407 , and a handle portion  406  connected thereto, those defining an outer configuration of the drill and housing therein various components at given positions. A spindle  402  extends through the gear cover  417 , and a drill chuck  3  is attached to a front end of the spindle  402 . The spindle  402  has an intermediate portion provided with a rotatable ratchet  404  rotatable together with the rotation of the spindle  402  and movable together with an axial displacement of the spindle  402 . The rotatable ratchet  404  has one side  404   a  formed with a serration or alternating projections and recesses.  
         [0003]     A fixed ratchet  405  is disposed in confrontation with the rotatable ratchet  404 , and has a side  405   a  formed with a serration or alternating projections and recesses. The fixed ratchet  405  has a hollow cylindrical shape and is fixed at a position regardless of the rotation and axial displacement of the spindle  402 .  
         [0004]     Meanwhile, a motor  408  is disposed within the housing  407 . The rotational driving force of the motor  408  is transmitted through a rotary shaft  409  to a gear  410 . The gear  410  is force-fitted into a pinion  411 , so the aforementioned rotational driving force is transferred to the pinion  411 . The pinion  411  has two pinions  411   a  and  411   b  those having numbers of teeth different from each other and which are meshedly engaged with a low speed gear  412  and a high speed gear  413 , respectively. When the pinion  411  rotates, the gears  412  and  413  rotate as well. These gears  412  and  413  are formed with concave portions.  
         [0005]     A clutch disc  414  is disposed over and engages the spindle  402 , and is slidable in an axial direction thereof. As shown in  FIG. 1 , when the clutch disc  414  is slidingly moved and pressed into the concave portion of the low speed gear  412 , the rotation of the pinion  411  is transferred to the spindle  402  through the low speed gear  412  and the clutch disc  414 . On the other hand, if the clutch disc  414  slides rightward from the position in  FIG. 15 , and when inserted into the concave portion of the high speed gear  413 , the rotation of the pinion  411  is transferred to the spindle  402  through the high speed gear  413  and the clutch disc  414 . Consequently, the spindle  402  can be given low-speed rotation or high-speed rotation based on the movement of the clutch disc  414 .  
         [0006]     A change lever  415  is provided for changing operation mode of the impact drill between a drilling mode and an impact drilling mode. A change shaft  416  is force-fitted into the change lever  415 . By rotating the change lever  415  about its rotation axis, the change shaft  416  is rotated about its axis along with the change lever  415 . As shown in  FIGS. 16 through 18 , the change shaft  416  is formed with a notch  416   a . The impact drill operates in drilling mode when the notch  416   a  is in the position in  FIG. 16 , and operates in impact drilling mode when the notch  416   a  is in the position in  FIG. 17 .  
         [0007]     Drilling mode will be described. If the bit (not shown) attached to the drill chuck  403  is brought into contact with a workpiece (not shown), and the handle  406  is pressed in the direction of the arrow in  FIG. 15 , and if the notch  416   a  in the change shaft  416  is in the position shown in  FIG. 16 , an internal end of the spindle  402  will abut against the outer peripheral surface of the change shaft  416  and will not be able to move rightward any more. As a result, the contoured serrated surface  404   a  of the rotation ratchet  404  and the contoured serrated surface  405   a  of the fixed ratchet  405  will not come into contact. Consequently, the rotational driving force of the motor  408  is transferred through the low speed gear  412  or the high speed gear  413  to the spindle  402 , and only the rotational force is imparted to the bit.  
         [0008]     In case of the impact drilling mode, the change lever  415  is rotated about its axis so as to displace the position of the notch  416   a  in the change shaft  416  to the position shown in  FIG. 17 . In this state, if the bit attached to the drill chuck  403  is brought into contact with the workpiece, and if the handle  406  is pressed in the direction of the arrow in  FIG. 15 , the inner end of the spindle  402  will enter the notch  416   a  as shown in  FIG. 18 . In other words, since the spindle  402  can be moved rightward slightly, the contoured surface  404   a  of the rotation ratchet  404  resultantly comes into contact with the contoured surface  405   a  of the fixed ratchet  405 .  
         [0009]     When drilling into the workpiece, if the spindle  402  is rotated in the state shown in  FIG. 18 , the rotatable ratchet  404  engages the fixed ratchet  405 , so that vibration is generated by the pressure contact between the alternating projections and recesses of the serrated surfaces  404   a ,  405   a  of both of the ratchets  404  and  405 , and this vibration is transmitted through the spindle  202  to the bit (not shown).. In other words, rotational force and vibration are imparted to the bit, and drilling is performed by the combined rotational force and the vibration force.  
         [0010]     However, when the vibration drill described above is operated in the impact drilling mode, the vibration is transferred not only to the bit, but also to the handle  406  by way of the fixed ratchet  405 , the inner cover  418  and the housing  407 . This leads to the problem that a large amount of vibration is passed to users of the impact drill, thus causing discomfort. In particular, if the impact drill is used continuously for long periods of time, caution must be exercised such that there are no adverse effects on the health of users.  
         [0011]     Several proposals have been made for mechanisms to reduce the vibration passed to the users. For example, according to laid open Japanese utility model application publication No.S59-69808, as shown in  FIG. 19 , a spindle  520  is rotatably and axially movably supported to a housing through a bearing  511 . A rotation cam  521  is fixed to the spindle  520 , so that the rotation cam  521  is rotated together with the rotation of the spindle  520  and movable together with the spindle  520 . A serrated contour is formed on a cam surface  521   a  of the rotation cam  521 .  
         [0012]     A clutch cam  522  is supported on a spindle  520  and is slidably movable in the axial direction of the spindle  520 . The clutch cam  522  includes a hollow cylindrical section slidable with respect to the spindle  520 , and a flange section  522   b . A serrated contour is formed on a cam surface  522   c  of the flange section  522   b . Further, a regulation slot  522   a  is formed at an outer peripheral surface at a position near a rear end portion  522   d  of the hollow cylindrical section. A plate  524  extending perpendicular to the spindle  520  is engaged with the regulation slot  522   a . A spring  523  is interposed between the flange section  522   b  and the plate  524 .  
         [0013]     The spring  523  continuously urges the clutch cam  522  toward the rotation cam  521 , and the cam surfaces  521   a  and  522   c  are pressed together when the spindle  520  is retracted into the housing. Then, when the force applied to the spindle  520  surpasses the biasing force of the spring  523 , the spring  523  is compressed and the clutch cam  522  retracts (moves rightward in  FIG. 19 ). However, the displacement of the clutch cam  522  is limited within a length of the slot  522   a . When the clutch cam  522  moves forward from the retracted position by the biasing force of the spring  523 , the clutch cam  522  strikes against the rotation cam  521 , and the rotation cam  521  vibrates along with the spindle  520 .  
         [0014]     Since the vibration arising from the contact between the cam surfaces  521   a  and  522   c  is alleviated by the spring  523  before being transmitted to a handle (not shown), the mechanism shown in  FIG. 19  is advantageous in reducing the transmission of vibration to the user in comparison with the mechanism shown in  FIG. 15  where the ratchet  405  is placed in a fixed position.  
       SUMMARY OF THE INVENTION  
       [0015]     However, the present inventors have found the drawbacks in the structure shown in  FIG. 19 . That is, since the clutch cam  522  moves backward and forward repeatedly across the length of the slot  522   a  engaged with the plate  524 , the rear end  522   d  of the clutch cam  522  repeatedly strikes against the plate  524 .  
         [0016]     Consequently, the problems arise that the transfer of the vibration arising in this part to the handle still cannot be avoided, and furthermore that the rear end  522   d  or the plate  524  will be prone to breaking due to mechanical fatigue. In addition, if the function of the spring  523  is insufficient, the spindle  520  or the clutch cam  522  would strike against the rear part, and the transfer of the vibration to the handle could not be avoided, if even slight pressing force is applied to the bit during drilling.  
         [0017]     It is therefore an object of the present invention to overcome the above-described problems and to provide an impact drill solving the problems described above.  
         [0018]     Specifically, an object of the present invention is to provide an impact drill capable of reducing transmission of the vibration to a user without causing a loss of drilling power.  
         [0019]     Another object of the present invention is to provide such an impact drill capable of generating a large amount of repeated impact force at a bit, yet minimizing transmission of a vibration to a handle.  
         [0020]     These and other objects of the present invention will be attained by an impact drill for boring a workpiece including a main frame, a motor, a spindle, a first ratchet, a second ratchet, and a first spring. The motor is housed in the main frame. The spindle is movably supported by the main frame and is rotatable by the motor and movable in its axial direction. The first ratchet is fixed to the spindle and has a first serrated surface and includes an alternating projections and recesses. The second ratchet has a second serrated surface and includes an alternating projections and recesses and in confrontation with the first serrated surface. The first spring biases the second ratchet toward the first ratchet. The first serrated surface is abuttable on the second serrated surface upon axial displacement of the spindle, and relative rotation between the first ratchet and the second ratchet causes 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 provides a spring constant capable of preventing 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.  
         [0021]     In another aspect of the invention there is provided an impact drill for boring a workpiece including a main frame, the motor, a spindle, a first ratchet, a second ratchet, and a first spring. The motor is housed in the main frame. The spindle is movably supported by the main frame and is rotatable by the motor and movable in its axial direction between a protruding position and a retracted position. The first ratchet is rotatable together with the rotation of the spindle and movable in the axial direction together with the spindle. The second ratchet is positioned in confrontation with the first ratchet and is movable in the axial direction but unrotatable about its axis. The first spring is interposed between the second ratchet and the main frame for biasing the second ratchet toward the first ratchet. The retracted position of the spindle causes abutment between the first ratchet and the second ratchet and relative rotation between the first ratchet and the second ratchet causes reciprocating motion of the spindle in the axial direction. The first spring has a biasing force capable of preventing the second ratchet and the spindle from abutting against the main frame when the spindle is moved to the retracted position. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     In the drawings:  
         [0023]      FIG. 1 ( a ) is a cross-sectional view showing an impact drill according to a first embodiment of the present invention;  
         [0024]      FIG. 1 ( b ) is a cross-sectional view taken along the line I-I of  FIG. 1 ( a );  
         [0025]      FIG. 2  is a cross-sectional view showing the impact drill and showing a situation where a small pressing force is applied to a bit;  
         [0026]      FIG. 3  is a cross-sectional view showing the impact drill and showing a situation where a greater pressing force is applied to the bit;  
         [0027]      FIG. 4  is a view for description of a transmission of vibration in the impact drill according to the embodiment;  
         [0028]      FIG. 5  is a graphical representation showing a characteristic of vibration transmission in the impact drill according to the embodiment;  
         [0029]      FIG. 6  is a cross-sectional view showing an impact drill according to a second embodiment of the present invention;  
         [0030]      FIG. 7  is a cross-sectional view showing the impact drill according to the second embodiment and showing a situation where a small pressing force is applied to a bit;  
         [0031]      FIG. 8  is a cross-sectional view showing the impact drill according to the second embodiment. and showing a situation where an intermediate pressing force greater than the pressing force in  FIG. 7  is applied to the bit;  
         [0032]      FIG. 9  is a cross-sectional view showing the impact drill according to the second embodiment and showing a situation where a greater pressing force greater than the intermediate pressing force in  FIG. 8  is applied to the bit;  
         [0033]      FIG. 10  is a cross-sectional view showing the impact drill according to a modification to the second embodiment and showing a situation where no pressing force is applied to the bit;  
         [0034]      FIG. 11 ( a ) is a cross-sectional view showing an impact drill according to a third embodiment of the present invention;  
         [0035]      FIG. 11 ( b ) is an enlarged cross-sectional view showing an essential portion in the impact drill according to the third embodiment;  
         [0036]      FIG. 12  is a cross-sectional view taken along the line XI-XI of  FIG. 11 ( a ) and showing a state where a ball is disengaged from a recess;  
         [0037]      FIG. 13  is a cross-sectional view taken along the line XI-XI of  FIG. 11 ( a ) and showing a state where the ball is engaged with the recess;  
         [0038]      FIG. 14 ( a ) is a cross-sectional view showing an impact drill according to a fourth embodiment of the present invention;  
         [0039]      FIG. 14 ( b ) is a cross-sectional view taken along the line XIV-XIV of  FIG. 14 ( a );  
         [0040]      FIG. 15  is a cross-sectional view showing a conventional impact drill;  
         [0041]      FIG. 16  is an enlarged cross-sectional view showing an essential portion of  FIG. 15  for description of a drilling mode;  
         [0042]      FIG. 17  is an enlarged cross-sectional view showing the essential portion of  FIG. 15  for description of a starting phase of an impact drilling mode;  
         [0043]      FIG. 18  is an enlarged cross-sectional view showing the essential portion of  FIG. 15  for description of the impact drilling mode; and  
         [0044]      FIG. 19  is a cross-sectional view showing an essential portion of another conventional impact drill. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]     An impact drill according to a first embodiment of the present invention will be described with reference to  FIGS. 1 through 5 . A main frame  1  supports a spindle  2  by a bearing  24  such that the spindle  2  is movable forward (leftward in the drawing) and backward (rightward in the drawing) with respect to a workpiece  19 . A chuck  3  for securing a bit  18  is disposed on a front tip end of the spindle  2 . A spindle spring  23  is interposed between the spindle  2  and an inner race of the bearing  24  for normally biasing the spindle frontward (leftward in  FIG. 1 ). An inner end portion of the spindle  2  is provided with a speed changing mechanism described later.  
         [0046]     A first ratchet  4  and a second ratchet  5  are provided substantially concentrically with the main frame  1 . The first ratchet  4  is rotatable and axially movable along with the rotation and axial displacement of the spindle  2 . The first ratchet  4  has one surface having a serrated contour or alternating projections and recesses. The main frame  1  is formed with an annular recess  1   a  in which a stop member  25  is provided. A front end of the stop member  25  is in contact with an outer race of the bearing  24 . The stop member  25  is sufficiently thick and provides no stress concentration. To this effect, the stop member  25  is preferably made from an elastic material such as a rubber. The outer peripheral surface of the first ratchet  4  is in sliding contact with the inner peripheral surface of the stop member  25 . Further, no impacting abutment occurs between the first ratchet  4  and the stop member  25 .  
         [0047]     The second ratchet  5  includes an inner cylinder  5   a , an outer cylinder  5   b  and a base wall  5   c  integrally connecting the inner and outer cylinders  5   a  and  5   b  together so as to configure a dual concentrically cylindrical shape. The base wall  5   c  is positioned to a front end of the inner and outer cylinders  5   a ,  5   b . The front surface of the base wall  5   c  is abuttable on a rear end face of the stop member  25 .  
         [0048]     The outer cylinder  5   b  has an axial length greater than that of the inner cylinder  5   a , and the outer cylinder  5   a  has an inner end face  5   d . The inner cylinder  5   a  is slidable over the spindle  2 . The outer cylinder  5   b  is movable in the axial direction of the spindle  2  and is slidable with respect to an inner peripheral surface of the main frame  1 . As shown in  FIG. 1 ( b ), the outer cylinder  2  is formed with a pair of cut away portions, and the inner peripheral surface of the main frame  1  is provided with a pair of complementary increased thickness portions. Thus, the second ratchet  5  is axially movable but non-rotatable about its axis. A cam surface having a serrated contour or alternating projections and recesses is provided at the base wall  5   c.    
         [0049]     A seat wall  22  radially inwardly protrudes from the main frame  1  toward the spindle  2 , and a coil spring  20  is interposed between the seat wall  22  and the base wall  5   c . The spring  20  provides a specific spring constant, so that the inner end face  5   d  of the second ratchet  5  will not come into contact with the seat wall  22  even when the bit  18  is pressed against the workpiece  19 .  
         [0050]     The speed changing mechanism will be described. A rotary shaft  9  having an output gear  10  is provided to which a rotational driving force from a motor (not shown) is transmitted. A pinion  11  is rotatable about its axis and is supported to the main frame  1  by bearings. A gear  32  is coaxially ally fixed to the pinion  11  and is meshingly engaged with the output gear  10 . The pinion  11  includes a first pinion  11 A and a second pinion  11 B. A low speed gear  12  in meshing engagement with the first pinion  11 A and a high speed gear  13  in meshing engagement with the second pinion  11 B are coaxially mounted on the spindle  2 . A clutch disc  14  is movably mounted on the spindle  2  and at a position between the low speed gear  12  and the high speed gear  13 . The clutch disc  14  is selectively engageable with one of the low speed gear  12  and the high speed gear  13 . A change lever  17  is disposed to move the clutch disc  14  to engage one of the low speed gear  12  and the high speed gear  13 .  
         [0051]     When the change lever  17  moves the clutch disc  14  into the position at which the low speed gear  12  and the spindle  2  engage with each other, the rotational force of the pinion  11  is transmitted to the spindle  2  through the low speed gear  12 . As a result, the spindle  2  is rotated at low speed. On the other hand, when the change lever  17  moves the clutch disc  14  into the position at which the high speed gear  13  and the spindle  2  engage with each other, the rotational force of the pinion  11  is transmitted to the spindle  2  through the high speed gear  13 . As a result, the spindle  2  is rotated at high speed.  
         [0052]     Next, the spring  20  will be described in detail. The present inventors found that ordinarily, a person using an impact drill presses the main frame  1  of the impact drill at a force ranging from 15 to 25 kgf so as to press the bit against the workpiece, despite variations from person to person. In the present embodiment, the spring  20  provides the spring constant capable of avoiding direct contact of the rear end face  5   d  of the second ratchet  105  with the seat wall  22  of the main frame  1  when 15 to 25 kgf of pressing force is applied to the main frame  1 . In other words, if the pressing force is within the range of 15 to 25 kgf, the second ratchet  5  is floated away from the main frame  1  by the specific spring constant of the spring  20 . Thus, the vibration which will be transmitted to the user as described above can be reduced even during impact drilling mode.  
         [0053]     Next, the reasons for the reduction in the vibration passed to the user will be described in detail. In the first embodiment, the second ratchet  5  is in contact with one end of the spring  20 , and components other than the second ratchet  5  (hereinafter simply referred to as “a main body”) is in contact with the other end of the spring  20 . This structure can be expressed as a simple model shown in  FIG. 4  in which M represents the main body. If the displacement due to the vibration of the second ratchet  5  is represented as “Zr”, and if the displacement of the main body M arising from the vibration of the second ratchet  5  is represented as “Zb”, the vibration transmission rate “T” can be expressed as follows.
 
 T=|Zb/Zr|   (1)
 
         [0054]     In addition, if the vibration frequency of the second ratchet  5  is taken to be “f”, and the natural frequency determined from the spring constant and the main body M is taken to be “fc”, the transmission rate “T” can be expressed by the following formula.
 
 T=|Zb/Zr|= 1/|1−( f/fc ) 2 |  (2)
 
         [0055]     Here, if the rotational frequency of the first ratchet  4  is taken to be “N”, and the number of projections on each of the first and second ratchets is taken to be “A”, then the vibration frequency of the second ratchet  5  can be expressed as N X A. For example, if N=36.7 r.p.s. and A=13, then f is approximately 480 Hz. As is understood from the formula (2), transmission rate of the vibration of the second ratchet  5  to the main body M is reduced if a rate of the vibration frequency f of the second ratchet  5  to the natural frequency fc of the main body M is greater than 1.  
         [0056]      FIG. 5  shows a logarithmic graph of formula (2). When f/fc=1, T is infinite, and this is a dangerous region in which resonance occurs. However, it can be seen from formula (2) that if f/fc={square root}2 then T=1. If f/fc becomes not less than {square root}2 and increased more and more, the smaller the vibration transmission rate T becomes. Experiments have shown that the effects of vibration reduction are sufficient if the vibration transmission rate T is not more than about 0.5. To meet with the vibration transmission rate, f/fc should be larger than approximately  2 . Furthermore, if f/fc is larger than 3, then T becomes about 0.1, and the effect is even more obvious.  
         [0057]     In operation,  FIG. 1  shows the situation in which the pressing force imparted to the main frame  1  is zero, and the first ratchet  4  and the second ratchet  5  are separated from each other. More specifically, when the bit  18  is out of contact from the workpiece  19 , the spindle spring  23  interposed between the spindle  2  and the bearing  24  biases the spindle  2  forward (leftward in  FIG. 1 ), and accordingly, the first ratchet  4  moves forward as well. Further, the second ratchet  5  is in abutment with the stop member  25  and maintains its stop position. Meanwhile, the spindle  2  and the first ratchet  4  move forward even further by the biasing force of the spindle spring  23 , and move to a position at which the ratchets do not engage with each other. When the pressing force is zero, rotation alone is transmitted to the spindle  2  without generating vibration.  
         [0058]     If a small pressing force arises then, the spindle  2  is slightly moved rightward, so that the first ratchet  4  and the second ratchet  5  come into contact with each other, as shown in  FIG. 2 . Further, in this case, the second ratchet  5  collides against the stop member  25  when there is a relatively small amount of pressing force, and there is a probability that vibration may be transmitted to the main frame  1  through the stop member  25 . However, as described above, since the stop member  25  is sufficiently thick and provides no stress concentration and is made from the elastic material, the transmission of vibration can be reduced or dampened by the elastic force and damping effect of the rubber.  
         [0059]     If an even larger pressing force such as ranging from 15 to 25 kg arises, then the spring  20  is compressed, as shown in  FIG. 3 . Even when a large pressing force arises, the second ratchet  5  nevertheless remains in the floating state, as shown in  FIG. 3 , since the spring constant of the spring  20  is set at the specific range as described above. In addition, as can be ascertained from  FIG. 3 , the spindle  2  does not abut against the main frame  1  either.  
         [0060]     Because the second ratchet  5  is maintained in its floating phase with respect to the main frame  1  even during the impact drilling mode, transmission of vibration caused from the first and second ratchets  4 , 5  to the main frame  1  can be reduced. As a result, there is no discomfort imparted on the user of the impact drill, and there is also no need for concern regarding detrimental health effects.  
         [0061]     Although the description assumes that the impact drill is turned off, it has been confirmed experimentally that, even during actual drilling, the vibration passed to the hands can be reduced as long as the pressing force is in the range of 15 to 25 kgf.  
         [0062]     An impact drill according to a second embodiment of the present invention will next be described with reference to FIGS.  6  to  9  wherein like parts and components are designated by reference numerals added with  100  to those shown in  FIGS. 1 through 5  to avoid duplicating description.  
         [0063]     In the second embodiment, a member corresponding to the stop member  25  of the first embodiment is dispensed with. Instead, a washer  128  is provided slidably movably along the annular recess  101   a  of the main frame  101  at a position corresponding to the stop member  25 . The annular recess  101   a  defines an abutment face  101   b  at its rear end. The washer  128  has an inner diameter greater than an outer diameter of the first ratchet  104  for allowing the first ratchet  104  to enter the washer  128 .  
         [0064]     The front end of the second ratchet  105  is abuttable on a rear face of the washer  128 . Further, a second spring  121  is interposed between the outer race of the bearing  124  and a front face of the washer  128  for biasing the second ratchet  105  away from the first ratchet  104  against the biasing force of the first spring  120 . Furthermore, the washer  128  is abuttable on the abutment face  101   b  of the annular recess  101   a.    
         [0065]     With this arrangement, when the pressing force imparted to the main frame  101  is zero as shown in  FIG. 6 , the spindle  102  moves forward because of the biasing force of the spindle spring  123 , and consequently the first ratchet  104  moves forward as well. Further, the second ratchet  105  moves forward to the position at which the force of the first spring  120  and that of the second spring  121  are in equilibrium. The first ratchet  104  and the second ratchet  105  are placed in a separated position from each other by appropriately choosing the spring constants for the springs  120  and  121 .  
         [0066]     Then, as shown in  FIG. 7 , when a pressure lower than 15 kgf is applied to the main frame  101 , extremely small pressing force acts on the spindle  102 , and the first ratchet  104  and the second ratchet  105  assume positions in which they are lightly engaged. In this case, the washer  128  is separated from the abutment face  101   b , and the second ratchet  105  floats completely apart from the main body of the impact drill. As a result, the vibration which is passed to the user is extremely small since the vibration of the second ratchet  105  is not transmitted to the main frame  101  because of the floating. Furthermore, a boring location in the workpiece  19  can be easily set since the fluctuation of the main frame  101  is extremely small.  
         [0067]     As shown in  FIG. 8 , proceeding to press slightly more strongly on the main frame  101 , the washer  128  is brought into contact with the abutment face  101   b  in the main frame  101 . However, this abutment does not cause a significant problem in terms of the impact imparted to the main frame  101 . This is mainly because the weight of the washer  128  is extremely light in comparison with the second ratchet  105 , and partly because the biasing force of the second spring  121  does not serve as an external force to move the main frame  101 , but serves as an internal force on the main frame  101 . This has been confirmed experimentally as well.  
         [0068]     As shown in  FIG. 9 , if the main frame  101  is pressed further strongly with a force ranging from 15 to 25 kfg, the spindle  102  and the first ratchet  104  move backward (rightward in the drawing), while the washer  128  is in abutment with the abutment face  101   b . If the first ratchet  104  moves even farther backward from this position, then the first ratchet  104  will move backward interlocked together with the second ratchet  105 . However, in the same manner as in the first embodiment, with the pressing force ranging from 15 to 25 kgf, the second ratchet  105  still maintains its floating position, i.e., the second ratchet  105  does not abut against the spring seat  122 , since the first spring  120  provides the specific spring constant which is large enough that a gap is provided between the second ratchet  105  and the spring seat  122 . As a result, the vibration of the second ratchet  105  does not readily pass to the main frame  101 , and no discomfort is imparted on the user.  
         [0069]      FIG. 10  shows a modification to the second embodiment. In the second embodiment, when the pressing force is zero, the second ratchet  105  is held at a given floating position at which the force of the first spring  120  and that of the second spring  121  are balanced with each other as shown in  FIG. 6 . According to the modification shown in  FIG. 10 , the second ratchet  105  is held at the position at which the washer  128  is in contact with the abutment face  101   b  when the pressing force is zero. With this arrangement, the stationary position of the second ratchet  105  can be accurately determined. Further, and even with this structure, significant vibration does not occur due to the abutment relation between the washer  128  and the abutment face  101   b  because of the reason described above.  
         [0070]     As described above, in the second embodiment and its modified embodiment, since the second spring  121  is provided in addition to the first spring  120 , the second ratchet  105  is always maintained in its floating phase with respect to the main frame  101 . Consequently, transmission of vibration caused from the first and second ratchets  104 ,  105  to the main frame  101  can further be reduced. As a result, there is no discomfort imparted on the user of the impact drill, and there is also no need for concern regarding detrimental health effects.  
         [0071]     An impact drill according to a third embodiment of the present invention will be described with reference to FIGS.  11 ( a ) through  13 , wherein like parts and components are designated by reference numerals added with  200  to the reference numerals of the first embodiment.  
         [0072]     The third embodiment pertains to a modification to the second embodiment in that a recess  201   a  is formed at a center portion of the main frame  201  in its longitudinal direction. The recess  201   a  is formed with a through hole at its bottom, and a ball member  229  is provided in the recess  201   a . The ball member  229  can be passed through the through hole. Further, a change-lever  226  is movably disposed over the recess  201   a  and at a position radially outwardly from the ball member  229 .  
         [0073]     The outer cylinder  205   b  is formed with a groove  205   e  at its outer peripheral surface for receiving the ball member  229 . The change-lever  226  has an excitable magnet for attracting the ball member  229 . That is, the change-lever  226  is movable to a first position shown in  FIG. 11 ( b ) where the ball member  229  is attracted to the change lever  226  because of the excitation of the change lever  226  and the ball member  229  is disengaged from the groove  205   e  as shown in  FIG. 12  In this state, the second ratchet  205  is separated from the main frame  201 . Accordingly, when the spindle  202  rotates, the first ratchet  204  and the second ratchet  205  both rotate, and the impact drill is operated in the drill mode.  
         [0074]     On the other hand, if the change-lever  226  is switched to non-excited phase while moving to a second position shown in  FIG. 11 ( a ), the ball member  229  is pressed radially inwardly by the change-lever  226  to engage the groove  205   e  as shown in  FIG. 13 . In this state, the second ratchet  205  is coupled to the main frame  201 . As a result, when the spindle  202  rotates, the first ratchet  204  rotates together with the rotation of the spindle  202 , whereas the second ratchet  205  does not rotate. Therefore, due to the serrated contoured surfaces between the first and second ratchets  204  and  205 , a repeated striking force is generated, and the impact drill operates in impact drilling mode.  
         [0075]     In the third embodiment, the second ratchet  205  maintains its floating position in drilling mode as well as impact drilling mode. Furthermore, the vibration passed to the user can be reduced since the vibration caused by the first and second ratchets  204  and  205  is not readily transferred to the main frame  201 . In addition, the frictional force acting between the second ratchet  205  and the outer cylinder  205   b  can be reduced by the rolling of the ball member  229 . Therefore, friction loss can be reduced.  
         [0076]     FIGS.  14 ( a ) and  14 ( b ) show an impact drill according to a fourth embodiment of the present invention, wherein like parts and components are designated by reference numerals added with  300  to those of the first embodiment.  
         [0077]     In the fourth embodiment, an elastic sleeve member  331  is disposed at an inner peripheral surface of the main frame  301  at a position in confrontation with the outer cylinder  305   b . Further, a ratchet holder  330  is disposed at an inner peripheral surface of the elastic sleeve member  331  for surrounding the outer cylinder  305   b . The ratchet holder  330  is adapted for preventing the second ratchet  305  from rotating about its axis.  
         [0078]     Similar to the foregoing embodiments, the vibration of the second ratchet  305  become less readily passed to the user because the first spring  320  is interposed between the second ratchet  305  and the main frame  301  so as to floatingly maintain the second ratchet  305 . Further, because the elastic sleeve member  331  is interposed between the ratchet holder  330  and the main frame  301 , the vibration passed to the user can be reduced even further because of the buffering function of the elastic sleeve member  331 .  
         [0079]     While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention.