Abstract:
An impact drill includes a first ratchet rotating along with a spindle and movable in an axial direction, a second ratchet engaged with the first ratchet and movable in a axial direction but unrotatable, and a spring provided between the second ratchet and a partial member of a housing. An amount of movement of the spindle in the axial direction is regulated, so that the pressing force is too excessive, the restoring force of the spring urging the second ratchet is controlled to maintain a state of generating a set stroke force.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an impact drill for use in a drilling operation on a concrete, mortar or tile, for example, and more particularly to an impact drill having a drill mode for performing a drilling operation by rotating a drill bit and an impact drill mode for performing a drilling operation by rotating and vibrating the drill bit.  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 1  shows a conventional example of the impact drill of this kind. In  FIG. 1 , reference numeral  1  denotes a main frame portion that forms an outer shell of the impact drill and has the self-contained parts at predetermined positions, comprising a gear cover  17 , an inner cover  18 , an outer cover  19 , a housing  7  and a handle portion  6 . Reference numeral  2  denotes a spindle inserted transversely through the gear cover  17 , and  3  denotes a drill chuck attached at the top end of the spindle. A rotational ratchet  4  is mounted near the central part of the spindle  2 . The rotational ratchet  4  is rotated along with the rotation of the spindle  2 , and moved along with the axial movement of the spindle  2 . The serrated irregularities are formed on one face  4   a  of the rotational ratchet  4 .  
         [0005]     Reference numeral  5  denotes a stationary ratchet disposed at a position opposed to the rotational ratchet  4 , in which the serrated irregularities are also formed on one face  5   a  of the stationary ratchet. The stationary ratchet  5  has a hollow cylindrical shape, and is fixed to the inner cover  18 , irrespective of the rotation and axial movement of the spindle  2 .  
         [0006]     On the other hand, a motor  8  is disposed inside the housing  7  linked to the handle portion  6 . A rotational driving force of the motor  8  is transmitted via a rotation shaft  9  to a gear  10 . Since the gear  10  is press fit into a second pinion  11 , the rotational driving force is transmitted to the second pinion  11 . The second pinion  11  has two pinion portions  11   a ,  11   b  having a different number of teeth, which are engaged with a low speed gear  12  and a high speed gear  13 , respectively. When the second pinion  11  is rotated, both the gears  12 ,  13  are also rotated.  
         [0007]     Reference numeral  14  denotes a clutch disk engaged with the spindle  2  and mounted to be slidable in the axial direction. If the clutch disk  14  is inserted into a concave portion of the low speed gear  12 , the rotation of the second pinion  11  is transmitted via the low speed gear  12  and the clutch disk  14  to the spindle  2 , as shown in  FIG. 1 . On the other hand, if the clutch disk  14  is slid to the right from the position of  FIG. 1 , and inserted into a concave portion of the high speed gear  13 , the rotation of the second pinion  11  is transmitted via the high speed gear  13  and the clutch disk  14  to the spindle  2 . Accordingly, the spindle  2  can be rotated at low speed or high speed by movement of the clutch disk  14 .  
         [0008]     Reference numeral  15  denotes a change lever for changing the operation mode of the impact drill, namely, between a drill mode and an impact drill mode. A change shaft  16  is press fit into the change lever  15 , whereby when the change lever  15  is rotated, the change shaft  16  is also rotated. The change shaft  16  has a notch portion  16   a , as shown in  FIGS. 2, 3  and  4 , whereby when the notch portion  16   a  is at the position of  FIG. 2 , the impact drill is operated in the drill mode, while when the notch portion  16   a  is at the position of  FIG. 3 , the impact drill is operated in the impact drill mode.  
         [0009]     (A) Drill Mode  
         [0010]     When a drill bit (not shown) attached in the drill chuck  3  is contacted with a machined surface and the handle portion  6  is pressed in a direction of the arrow in  FIG. 1 , an end part of the spindle  2  makes contact with the change shaft  16  to be immovable to the right, when the notch portion  16   a  of the change shaft  16  is at the position of  FIG. 2 . Accordingly, there is no contact between the irregular face  4   a  of the rotational ratchet  4  and the irregular face  5   a  of the stationary ratchet  5 . Accordingly, a rotational driving force of the motor  8  is transmitted via the low speed gear  12  or high speed gear  13  to the spindle, so that the drill bit is given a rotational force.  
         [0011]     (B) Impact Drill Mode  
         [0012]     In an impact drill mode, the notch portion  16   a  of the change shaft  16  is brought into the position of  FIG. 3  by rotating the change lever  15 . Then, the drill bit attached in the drill chuck  3  is contacted with a machined surface. If the handle portion  6  is pushed in a direction of the arrow in  FIG. 1 , an end part of the spindle  2  enters the notch portion  16   a , as shown in  FIG. 4 . That is, the spindle  2  is slightly moved to the right, so that the irregular face  4   a  of the rotational ratchet  4  is contacted with the irregular face of the stationary ratchet  5 .  
         [0013]     In drilling the machined surface, if the spindle  2  is rotated in the state of  FIG. 4 , the rotational ratchet  4  is meshed and engaged with the stationary ratchet  5 , and rotated to cause vibration due to the irregular faces of both the ratchets  4  and  5 . This vibration is transmitted through the spindle  2  to the drill bit (not shown). That is, the drill bit is given a rotational force and vibration to perform a drilling operation.  
         [0014]     However, when the impact drill is operated in the impact drill mode, the vibration caused by rotation of the spindle in the state where the irregular faces of the ratchets  4  and  5  are contacted under pressure is transmitted not only to the drill bit, but also through the stationary ratchet  5  and the inner cover  18  from the housing  7  to the handle portion  6 . Therefore, there is a problem that the user of the impact drill undergoes a great vibration, and feels uncomfortable. Especially when the impact drill is continuously employed for a long time, care must be taken not to transmit the vibration to the user and cause adverse effect on the health of the user.  
         [0015]     Several proposals for reducing the vibration transmitted to the user have been made. For example, in JP-UM-B-2-30169, a structure was disclosed in which a clutch cam  22  is supported movably in the axial direction of the spindle  20 , and pressed and urged to a rotary cam  21  by a spring  23 , as shown in  FIG. 5 . In  FIG. 5 , reference numeral  21  denotes a rotary cam that is rotated along with the spindle  20 . A cam face  21   a  of the rotary cam  21  is formed with serrated irregularities.  
         [0016]     On the other hand, the clutch cam  22  is composed of a hollow cylindrical portion slidable in the axial direction of the spindle  20  and a flange portion  22   b . A cam face  22   c  of the flange portion  22   b  is formed with a serrated irregular face.  
         [0017]     The spring  23  is provided between the flange  22   b  of the clutch cam  22  and a plate  24   a  engaging a groove  22   a  of the clutch cam  22 , and always urges the clutch cam  22  toward the rotary cam  21 . Thus, when the spindle  20  is moved backward, the cam faces  21   a  and  22   c  are contacted under pressure. If a pressing force applied to the spindle  20  overcomes a resilient force of the spring  23 , the spring  23  is compressed, so that the clutch cam  22  is moved backward (to the right in the figure). When the clutch cam  22  is moved forward from the back position due to a resilient force of the spring  23 , it strikes against the rotary cam  21 , so that the rotary cam  21  is vibrated together with the spindle  20 .  
         [0018]     With this structure, since the vibration caused by contact between the cam faces  21   a  and  22   c  is relieved by the spring  23  and transmitted to the handle portion (not shown), there is the effect that the vibration transmitted to the user is reduced as compared with the structure in which the ratchet  5  is firmly disposed as shown in  FIG. 1 .  
         [0019]     On the other hand,  FIGS. 6 and 7  are schematic views showing the above structure in which the change shaft  16  and the change lever  15  as shown in  FIGS. 2, 3  and  4  are disposed at the right end portion of the spindle  20  as shown in  FIG. 5 . In  FIGS. 6 and 7 , a spring  25  is additionally inserted between the rotary cam  21  and the plate  24   a  to prevent the spindle  20  from being moved to the right.  
         [0020]     When the notch portion  16   a  of the change shaft  16  is at the position as shown in  FIG. 6 , the impact drill is operated in the drill mode in which the cam faces  21   a  and  22   c  are always out of contact. Also, when the notch portion  16   a  of the change shaft  16  is at the position as shown in  FIG. 7 , the impact drill is operated in the impact drill mode in which the cam faces  21   a  and  22   c  are contacted and collided.  
         [0021]     In this impact drill mode, if a pressing force is applied to the main body (not shown), the spindle  20  is moved to the right. However, when the pressing force is weak, a right end portion of the spindle  20  slightly enters the notch portion  16   a , and the cam faces  21   a and  22   c  of  FIG. 7  are lightly contacted, so that the back movement amount of the clutch cam  22  is small, the restoring force of the spring  23  is small, and a stroke force from the clutch cam  22  to the rotary cam  21  is reduced.  
         [0022]     On the other hand, when the pressing force is strong, a right end portion of the spindle  20  deeply enters the notch portion  16   a , and the cam faces  21   a  and  22   c  are greatly engaged, so that the clutch cam  22  is greatly moved backward, whereby the restoring force of the spring  23  is great, and the stroke force from the clutch cam  22  to the rotary cam  21  is significant.  
         [0023]     Herein, when an object to be drilled which is hard and thin tile or concrete is positioned for drilling, or drilled prudently, it is necessary to sustain a state where the pressing force is weakened to suppress the stroke force, as described above. Several proposals have been conventionally made for the structure in which the magnitude of stroke force is adjustable.  
         [0024]     In Japanese Patent No. 3002284, the maximal movement amount of the rotational ratchet and the spindle is made larger than engageable with the stationary ratchet, in which the stationary ratchet is provided movably in the axial direction, and biased forward by the spring. A biasing force of the spring is adjusted by changing a force for pressing the main body.  
         [0025]     In JP-A-62-74582, there was described an impact drill in which the rotational ratchet and the spindle can not be moved in the axial direction, and the stationary ratchet is provided movably in the axial direction, and biased forward by the spring, whereby a member for regulating the axial movement of the stationary ratchet is provided adjustably from the outside. The stationary ratchet is regulated from moving forward beyond a predetermined position by adjusting the regulating member, so that the intermeshing depth of ratchets is adjusted.  
         [0026]     In Japanese Patent No. 2754047, there was described an impact drill in which the rotational ratchet and the spindle can not be moved in the axial direction, and the stationary ratchet is provided movably in the axial direction, and biased forward by the spring, whereby a second spring for adjusting the compression amount from the outside is provided, in addition to a first spring for always biasing the stationary ratchet. By adjusting the compression amount from the outside, a combination of the first spring and the second spring is varied to adjust the biasing force of the spring.  
         [0027]     In JP-A-3-178708, there was described an impact drill in which the rotational ratchet and the spindle are provided to be movable backward to the position at which they are engaged with the stationary ratchet, and the stationary ratchet is provided movably in the axial direction, and biased forward by the spring, whereby the axial position of a spring seat is provided adjustably from the outside. The biasing force of the spring is adjusted by moving the spring seat from the outside. Also, there was described a similar impact drill in which the length of an outer frame itself is provided adjustably. In this case, the biasing force of the spring is adjusted by changing the length of the outer frame itself.  
         [0028]     In JP-A-4-240010, there was described an impact drill in which the rotational ratchet and the spindle are provided to be movable backward to the position at which they are engaged with the stationary ratchet, and the stationary ratchet is provided movably in the axial direction, and biased forward by the spring, whereby the axial position of a seat accepting the spring from behind is provided adjustably from the outside. The biasing force of the spring is adjusted by changing the axial position of the seat accepting the spring from behind.  
       SUMMARY OF THE INVENTION  
       [0029]     In Japanese Patent No. 3,002,284, it is difficult to keep the pressing force constant, and particularly when a small stroke force is attained by the weak pressing force, the stroke force is too excessive if the pressing force is too strong, resulting in a problem that the fragile partner member is possibly broken.  
         [0030]     In JP-A-62-74582 the vibration transmitted from the spindle to the housing is not relieved, and the intermeshing depth of ratchets may be reduced but the relative position of the ratchet and the spring is invariable, resulting in a problem that the biasing force of the spring can not be weakened. Likewise, with this constitution, the intermeshing depth of ratchets may be increased, but the relative position of the ratchet and the spring is invariable, whereby the biasing force of the spring could not be increased. That is, with this constitution, the intermeshing depth of ratchets may be changed but the relative position of the ratchet and the spring is invariable, resulting in a problem that the adjustment width of the stroke force is small.  
         [0031]     In Japanese Patent No. 2754047, JP-A-3-178708, and JP-A-4-240010, the vibration transmitted from the spindle to the housing is not relieved, and the biasing force of the spring may be changed but the intermeshing depth of ratchets may not be changed, resulting in a problem that the adjustment width of the stroke force is small.  
         [0032]     It is an object of the invention to provide an impact drill that solves the above-mentioned problems associated with the prior art. It is a further object of the invention to provide an impact drill in which a state of generating a set stroke force is maintained even if the biasing force is excessive, the adjustment width of the stroke force is large, and the vibration transmitted to the user is reduced.  
         [0033]     According to one aspect of the invention, there is provided with an impact drill including: a spindle rotated by a motor and movable in an axial direction; a drill chuck fixed to the spindle and mountable with a drill bit; a first ratchet fixed to the spindle and having a face of an irregular portion; a second ratchet having a face of an irregular portion opposed to the face of the irregular portion of the first ratchet and movable in the axial direction but unrotatable; and a spring for urging the second ratchet in a direction of the first ratchet, in which the spindle is given an axial vibration by a contact and separation action between the irregular faces of the first and second ratchets due to a relative rotation of the first ratchet to the second ratchet, wherein a regulating member regulates an amount of movement of the spindle at a plurality of positions in a range where the first and second ratchets can be engaged.  
         [0034]     According to another aspect of the invention, the regulating member is movable relative to a main frame portion to come into contact with the spindle. The regulating member is formed to gradually change an interval between the spindle and the regulating member, when the regulating member is moved relative to the main frame portion.  
         [0035]     According to another aspect of the invention, the regulating member has a columnar shape. The regulating member has a plurality of notch portions having a different distance from a center of the regulating. The regulating member is rotatably provided in the main frame portion so as to make the notch portions contactable with the spindle.  
         [0036]     According to another aspect of the invention, the regulating member has a plate-like shape. The regulating member has a plurality of step portions having a different depth. The regulating member is movably provided in the main frame portion so as to make the step portions contactable with the spindle.  
         [0037]     According to another aspect of the invention, the movement amount of the spindle movable in the axial direction is regulated to be a minimum value (as a first mode). The movement amount of the spindle movable in the axial direction is regulated to be a middle value (as a second mode). The movement amount of the spindle movable in the axial direction is regulated to be a maximum value (as a third mode).  
         [0038]     According to another aspect of the invention, the first mode is a mode of regulating the movement amount of the spindle to an extent that the irregular portion of the first ratchet and the irregular portion of the second ratchet are contacted with each other. The second mode is a mode of regulating the movement amount of the spindle to an extent that the irregular portion of the first ratchet and the irregular portion of the second ratchet are engaged with each other at a bottom portion of the first ratchet and the second ratchet. The third mode is a mode of regulating the movement amount of the spindle to an extent that the irregular portion of the first ratchet and the irregular portion of the second ratchet are engaged with each other to a bottom portion of the first ratchet and the second ratchet. The second ratchet is further moved backward by pressing a main frame of the impact drill onto a workpiece.  
         [0039]     According to another aspect of the invention, a fourth mode of regulating the movement amount of the spindle to an extent that the irregular portion of the first ratchet and the irregular portion of the second ratchet are not contacted with each other.  
         [0040]     Since the amount of back movement of the spindle and the rotational ratchet is regulated, the work may be performed with such a pressing force that the spindle comes into contact with the regulating member, whereby even though the pressing force is further increased, the compression amount of the spring is not increased, and the biasing force of the spring is not increased, so that the stroke force does not become excessive to prevent the partner member from being broken.  
         [0041]     When the stroke force is weakened, the amount of back movement of the spindle and the rotational ratchet is regulated to be smaller, whereby the intermeshing depth of ratchets is not only shallower, but also the compression amount of the spring is reduced, so that the biasing force of the spring can be weakened. Accordingly, the stroke force can be weaker than conventionally, and therefore made adequate for the fragile partner member.  
         [0042]     Moreover, when the stroke force is intensified, the amount of back movement of the spindle and the rotational ratchet is regulated to be larger, whereby the intermeshing depth of ratchets is not only deeper, but also the compression amount of the spring is increased, so that the biasing force of the spring can be intensified. Accordingly, the stroke force can be stronger than conventionally, and therefore made adequate for the partner member difficult to be drilled.  
         [0043]     If the work is performed with such a pressing force that the spindle does not come into contact with the regulating member, the vibration of the spindle in the axial direction is relieved and transmitted via the ratchet and the spring to the outer frame portion, whereby the operator performs the work comfortably with less vibration transmitted to the operator. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]      FIG. 1  is a cross-sectional view showing one example of the conventional impact drill;  
         [0045]      FIG. 2  is an explanatory view of the impact drill in a drill mode;  
         [0046]      FIG. 3  is an explanatory view of the impact drill in an impact drill mode;  
         [0047]      FIG. 4  is an explanatory view of the impact drill in the impact drill mode;  
         [0048]      FIG. 5  is a partial constitutional view showing another example of the conventional impact drill;  
         [0049]      FIG. 6  is an explanatory view of another example of the conventional impact drill in the drill mode;  
         [0050]      FIG. 7  is an explanatory view of another example of the conventional impact drill in the impact drill mode;  
         [0051]      FIG. 8  is a cross-sectional view showing an impact drill according to a first embodiment of the invention, in the drill mode;  
         [0052]      FIG. 9  is a cross-sectional view showing the impact drill according to the first embodiment of the invention, in a weak stroke mode;  
         [0053]      FIG. 10  is a cross-sectional view showing the impact drill according to the first embodiment of the invention, in a strong stroke mode;  
         [0054]      FIG. 11  is a cross-sectional view showing the impact drill according to the first embodiment of the invention, in a stroke force variable mode;  
         [0055]      FIG. 12  is a cross-sectional view showing an impact drill according to a second embodiment of the invention, in the drill mode;  
         [0056]      FIG. 13  is a cross-sectional view showing the impact drill according to the second embodiment of the invention, in the weak stroke mode;  
         [0057]      FIG. 14  is a cross-sectional view showing the impact drill according to the second embodiment of the invention, in the strong stroke mode;  
         [0058]      FIG. 15  is a cross-sectional view showing the impact drill according to the second embodiment of the invention, in the stroke force variable mode;  
         [0059]      FIG. 16  is an explanatory view of a change shaft of the impact drill according to the first embodiment of the invention; and  
         [0060]      FIG. 17  is an explanatory view of a plate-like change lever of the impact drill according to the second embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0061]     The preferred embodiments of the present invention will be described below in detail.  
         [0062]     First Embodiment  
         [0063]      FIGS. 8, 9 ,  10  and  11  are constitutional views of a main portion of an impact drill according to a first embodiment of the invention. Firstly, referring to  FIG. 8 , the constitution of each part will be described below.  
         [0064]     A spindle  102  is provided in a main frame portion  101  and moved forward (to the left in the figure) or backward (to the right in the figure) relative to a workpiece  119 . A chuck  103  for mounting a drill bit  118  is provided at the top end of the spindle  102 . A first ratchet  104  and a second ratchet  105  are provided in the almost central part of the main frame portion  101 . The first ratchet  104  is rotated along with the spindle  102  and moved axially, and has serrated irregularities  104   a  on one face. The second ratchet  105  is formed with serrated irregularities  105   d  on a bottom portion  105   c . Also, the second ratchet  105  has a dual cylindrical shape, in which an inner cylindrical portion  105   a  slides on the spindle  102  and an outer cylindrical portion  105   b  slides in the axial direction of the spindle  102  along an inner wall of the main frame portion  101 , but has a notch portion in a part on the circumferential face to prevent rotational motion.  
         [0065]     Moreover, a side wall portion  122  extends in a direction of the spindle inside the main frame portion  101 , and a spring  120  is provided between the side wall portion  122  and the cylindrical bottom portion  105   c . Reference numeral  109  denotes a rotation shaft to which a rotational driving force is transmitted from a motor (not shown), in which its rotational driving force is transmitted via a gear  110  to a second pinion  111 . Reference numeral  112  denotes a low speed gear,  113  denotes a high speed gear, and  114  denotes a clutch disk, in which when the clutch disk  114  is at the position as shown, a rotational force is transmitted via the low speed gear  112  to the spindle  102 .  
         [0066]     On the other hand, if the clutch disk  114  is rotated to the position where the high speed gear  113  and the spindle  102  are engaged by rotating a change lever  117 , a rotational force of the second pinion  111  is transmitted via the high speed gear  113  to the spindle  102 . Accordingly, the spindle  102  can be rotated at low speed or high speed depending on the rotated position of the change lever  117 .  
         [0067]     As a result of the experiment, it has been confirmed that the vibration transmitted to the user in the drilling operation, namely, the vibration of an impact drill main body, is reduced owing to the above configuration.  
         [0068]     According to first embodiment of the invention, a steel ball  125  is provided at a rear end of the spindle  102 , and contacted with a columnar change shaft  141  having a plurality of notch portions different in the depth.  FIG. 16  shows a sectional face of the change shaft  141  taken along the A-A plane of  FIG. 8 . In this example, there are a face  141   a  having the largest notch depth W 3 , a face  141   b  having the next largest notch depth W 2 , a face  141   c  having the smallest notch depth W 1 , and a columnar face  141   d  without notch. This change shaft  141  is engaged with a change lever  140 , and the contact face with the steel ball  125  is changed in the order of  141   a ,  141   b ,  141   c  and  141   d  by rotating the change lever  140 .  
         [0069]     The operation of the impact drill with the above constitution will be described below.  
         [0070]     (a) Drill Mode  
         [0071]     A drill mode is shown in  FIG. 8 . That is, the change shaft  141  is rotated by turning the change lever  140 , so that the steel ball  125  disposed at the rear end of the spindle  102  is contacted with a part of the change shaft  141  without notch portion  142 , namely, the face  141   d  of  FIG. 16 . In this positional relation, even when the main frame portion  101  is pressed in the direction of the arrow, a serrated irregular portion  104   a  of the first ratchet  104  and a serrated irregular portion  105   d  of the second ratchet  105  are not engaged, causing no vibration, whereby the impact drill is operated as the normal drill mode.  
         [0072]     (b) Weak Stroke Impact Drill Mode  
         [0073]      FIG. 9  shows a weak stroke mode of the impact drill. By turning the change lever  140  from the state of  FIG. 8 , the steel ball  125  at the rear end of the spindle  102  is contacted with the face  141   c  of the change shaft  141  having the smallest notch depth W 1 . This notch depth W 1  is regulating the movement of the spindle  102  to the extent that the serrated irregular portion  104   a  of the first ratchet  104  and the serrated irregular portion  105   d  of the second ratchet  105  are lightly contacted at the tip. In this positional relation, even when the main frame portion  101  is pressed with a great force in the direction of the arrow, the restoring force of the spring  120  is small, and the impact force occurring between the first ratchet  104  and the second ratchet  105  is small. Accordingly, when this small impact force is sustained, this weak stroke mode is advantageous in prudently drilling the hard, thin tile or the like.  
         [0074]     (c) Strong Stroke Impact Drill Mode  
         [0075]      FIG. 10  shows a strong stroke mode of the impact drill. By further turning the change lever  140  from the state of  FIG. 9 , the steel ball  125  at the rear end of the spindle  102  is contacted with the face  141   b  of the change shaft  141  having the larger notch depth W 2 . This notch depth W 2  is regulating the movement of the spindle  102  to the extent that the serrated irregular portion  104   a  of the first ratchet  104  and the serrated irregular portion  105   d  of the second ratchet  105  are engaged to the bottom. Thus, the second ratchet  105  is moved further backward from the position of  FIG. 9 , the restoring force of the spring  120  is great, and the impact force occurring between the first ratchet  104  and the second ratchet  105  is great. Accordingly, when this great impact force is sustained, this strong stroke mode is optimal in prudently drilling the mortar wall or the like at high drilling speed.  
         [0076]     (d) Stroke Force Variable Impact Drill Mode  
         [0077]      FIG. 11  shows a stroke force variable mode of the impact drill. By further turning the change lever  140  from the state of  FIG. 10 , the steel ball  125  at the rear end of the spindle  102  is opposed to the face  141   a  of the change shaft  141  having the largest notch depth W 3 . This notch depth W 3  is regulating the movement of the spindle  102  to the extent that the serrated irregular portion  104   a  of the first ratchet  104  and the serrated irregular portion  105   d  of the second ratchet  105  are engaged to the bottom, the main frame portion  101  is further pressed in a direction of the arrow, and the rear end  105   e  of the second ratchet  105  does not abut against a main extension frame  122  even when the second ratchet  105  is moved backward. In this positional relation, when the main frame portion  101  is pressed according to the feeling of the operator himself or herself, the restoring force of the spring  120  is similarly changed depending on the magnitude of pressing force, whereby the operator can perform the operation by adjusting the magnitude of stroke force according to the force of pressing the main frame portion  101 .  
         [0078]     As described above, with the first embodiment, the change lever  140  is rotated by changing the face of the change shaft  141  in contact with the steel ball  125 , whereby vibration modes for various stroke forces can be implemented.  
         [0079]     Second Embodiment  
         [0080]      FIG. 12  shows a second embodiment of the invention, which has one feature in that the steel ball  125  provided at a rear end of the spindle  102  is contacted with a plate-like change lever  143  having the step portions different in the depth.  
         [0081]     That is,  FIG. 17  shows the plate-like change lever  143  in enlargement, which has a face  143   a  having the largest step W 3 , a face  143   b  having the next largest step W 2 , a face  143   c  having the smallest step W 1 , and a face  143   d  without step. This plate-like change lever  143  is provided movably in the vertical direction, whereby the contact face with the steel ball  125  is changed in accordance with its position. FIGS.  12  to  15  are cross-sectional views of the impact drill as looked from the above (opposite to the side where the handle portion  6  is provided in  FIG. 1 ). Accordingly, since the change lever  143  is provided movably in the left-to-right direction of the impact drill, one end of the change lever  143  can be pressed by a forefinger, and the other end pressed by a thumb, when the handle portion  6  is grasped, whereby the operability is excellent.  
         [0082]     (a) Drill Mode  
         [0083]     A drill mode is shown in  FIG. 12 . That is, the face  143   d  without step of the plate-like change lever  143  is contacted with the steel ball  125 . In this positional relation, even when the main frame portion  101  is pressed in the direction of the arrow, a serrated irregular portion  104   a  of the first ratchet  104  and a serrated irregular portion  105   d  of the second ratchet  105  are not engaged, causing no vibration, whereby the impact drill is operated as the normal drill mode.  
         [0084]     (b) Weak Stroke Impact Drill Mode  
         [0085]      FIG. 13  shows a weak stroke mode of the impact drill. By pressing down the plate-like change lever  143  from the state of  FIG. 12 , the steel ball  125  is contacted with the face  143   c  having the smallest step W 1 . This step W 1  has the depth of regulating the movement of the spindle  102  to the extent that the serrated irregular portion  104   a  of the first ratchet  104  and the serrated irregular portion  105   d  of the second ratchet  105  are lightly contacted at the tip. In this positional relation, even when the main frame portion  101  is pressed with a great force in the direction of the arrow, the restoring force of the spring  120  is small, and the impact force occurring between the first ratchet  104  and the second ratchet  105  is small.  
         [0086]     (c) Strong Stroke Impact Drill Mode  
         [0087]      FIG. 14  shows a strong stroke mode of the impact drill. By further pressing down the plate-like change lever  143  from the state of  FIG. 13 , the steel ball  125  is contacted with the face  143   b  having the step W 2 . This step W 2  has the depth of regulating the movement of the spindle  102  to the extent that the serrated irregular portion  104   a  of the first ratchet  104  and the serrated irregular portion  105   d  of the second ratchet  105  are engaged to the bottom. Thus, the second ratchet  105  is moved further backward from the position of  FIG. 9 , the restoring force of the spring  120  is great, and the impact force occurring between the first ratchet  104  and the second ratchet  105  is great.  
         [0088]     (d) Stroke Force Variable Impact Drill Mode  
         [0089]      FIG. 15  shows a stroke force variable mode of the impact drill. By further pressing down the plate-like change lever  143  from the state of  FIG. 14 , the steel ball  125  is contacted with the face  143   a  having the largest step W 3 . This step W 3  has the depth of regulating the movement of the spindle  102  to the extent that the serrated irregular portion  104   a  of the first ratchet  104  and the serrated irregular portion  105   d  of the second ratchet  105  are engaged to the bottom, the main frame portion  101  is further pressed in a direction of the arrow, and the rear end  105   e  of the second ratchet  105  does not abut against a main extension frame  122  even when the second ratchet  105  is moved backward. In this positional relation, when the main frame portion  101  is pressed according to the feeling of the operator himself or herself, the restoring force of the spring  120  is similarly changed depending on the magnitude of pressing force, whereby the operator can perform the operation by adjusting the magnitude of stroke force according to the force of pressing the main frame portion  101 .