Abstract:
An impact hammer drill capable of performing drilling operation at a high speed with low noise and without requiring a large thrust. The drill includes a main shaft rotatable by an output shaft of a motor, and a spindle having an impact-receiving section and disposed over the main shaft slidably in its axial direction and rotatable together with the rotation of the main shaft. A piston is reciprocatingly slidably disposed over the main shaft for impacting against the impact-receiving section. A piston drive unit is disposed for driving the piston with a compressed fluid. A compressed fluid supplying unit is disposed for supplying the compressed fluid to the piston drive unit. A drill bit is attachable to the spindle. When performing drilling operation, the drill bit is imparted with a combined rotational motion and the reciprocal impact motion.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a drilling machine, and more particularly, to a drilling machine that applies impacts on a target with a compressed air as a power source. 
   Conventionally, when drilling a concrete or the like, applying vibrational impacts, in addition to rotational motion, to a drill bit to crush a point of the concrete is known as the fastest drilling method. So as to apply such impacts to a drill bit, generally, part of rotational motion of a motor or the like that rotates a drill bit of a drilling machine is converted to reciprocating motion of a piston etc. arranged in the drilling machine. Then, impacts to be applied to the drill bit are generated from such reciprocating motion of the piston. 
   However, a drilling machine employing impacts cannot be used at places subject to noise regulation due to noise brought about when applying impacts. As a conventional drilling machine that is intended to be used at places subject to noise regulation with low noise, there is known a drilling machine for concrete structures disclosed in Laid-Open Japanese Utility Model Application Publication No. S62-201642. The drilling machine merely rotates a drill bit made mainly of diamond powder sintered metal, and the main body of the drilling machine is not provided with an impact mechanism for applying impacts to the drill bit. 
   However, when using a conventional drilling machine employing impacts, since part of motive energy to rotate a drill bit is used as motive energy to generate impacts, motive energy to rotate a drill bit is lowered, and intensity of thus generated impacts cannot be adjusted. 
   There is raised a problem that, when drilling concrete, the drilling speed of a drilling machine that employs only rotational motion and is not provided with an impact mechanism to apply impacts to a drill bit is extremely lowered when running into a high hardness aggregate such as a coarse aggregate. Furthermore, since the drilling operation is performed using friction generated between the leading end of a drill bit and a-concrete etc., the leading end of the drill bit has to be thrust against the concrete. Accordingly, when drilling a hard aggregate, an especially large thrust is required. In case the drilling operation is performed in a downward direction or in a transverse direction, a thrusting force can be obtained by employing the own weight of a drilling machine or the weight of a drilling worker. On the other hand, in case the drilling operation is performed in an upward direction, a drilling machine has to be uplifted and a load as a thrust has to be applied to the drill bit, which requires a hard labor. 
   As for drilling operation at places subject to noise regulation, the regulation may be varied depending on work time. Accordingly, for example, at least two drilling machines are required, one of which is for drilling operation employing impacts at a period of time with loosened noise regulation, while the other of which is for drilling operation employing only rotational motion at a period of time with tightened noise regulation. Furthermore, as for noise countermeasures, since a drilling machine has to be selected from only two drilling machines, that is, a drilling machine employing impacts with high noise and a drilling machine employing only rotational motion with low noise, there may be raised a case in which the drilling machine employing impacts cannot clear noise regulation while noise of the drilling machine employing only rotational motion is extremely low as compared with noise set down by noise regulation. In this case, the drilling machine employing only rotational motion alone can be used, which undesirably lowers working efficiency. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to overcome the above-mentioned drawbacks, and to provide a drilling machine capable of performing drilling operation at a high speed with low noise and without requiring a large thrust. 
   This and other objects of the present invention will be attained by a drilling machine including a frame, a motor, a rotation shaft, a piston, a piston drive unit, and a compressed fluid supplying unit. The motor is fixed within the frame and has an output shaft extending toward the one end of the frame. The rotation shaft is coupled to the output shaft to rotate about its axis, and extends toward the one end of the frame. The rotation shaft has a slidable section having one end provided with a drill bit attachment section and another end serving as an impact-receiving section. The piston extends in parallel with the axial direction, and is slidable in the reciprocatory manner in the axial direction to impact the impact-receiving section. The piston drive unit reciprocally drives the piston with a compressed fluid. The compressed fluid supplying unit is disposed within the frame for supplying the compressed fluid to the piston drive unit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a side cross-sectional view showing a drilling machine according to an embodiment of the present invention; 
       FIG. 2  is an enlarged side cross-sectional view showing an essential portion of the drilling machine according to the embodiment; 
       FIG. 3  is an exploded perspective view showing the relationship among a cylinder, a piston, a main shaft, and a spindle those being components of the drilling machine according to the embodiment; 
       FIG. 4  is a view for description of a pair of spindle protrusions protruding radially inwardly of the spindle; 
       FIG. 5  is a perspective view showing an engagement state between the spindle and the main shaft in the drilling machine according to the embodiment; 
       FIG. 6  is a view for description of the engagement between the spindle and the main shaft in the drilling machine according to the embodiment. 
       FIG. 7  is a side cross-sectional view showing a rearmost position of a piston of the drilling machine according to the embodiment; 
       FIG. 8  is a side cross-sectional view showing a moving state of the piston from its rearmost position toward its frontmost position in the drilling machine according to the embodiment; and 
       FIG. 9  is a side cross-sectional view showing the frontmost position, i.e., impact position of the piston of the drilling machine according to the embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A drilling machine according to an embodiment of the present invention will be described with reference to  FIG. 1  to  FIG. 9 . In the present embodiment, compressed air is used as a compressed fluid. One end of a drilling machine  1 , having a drill bit  50  to be described later, is set to be the front side, while the other end thereof is set to be the rear side. 
   The drilling machine  1  shown in  FIG. 1  includes a housing  2  as a main frame of the drilling machine  1 , a deceleration unit  10 , a cylinder unit  20 , a compressed air supplying unit  40 , and a drill bit  50 . The deceleration unit  10  is disposed at the front part of the housing  2 . The cylinder unit  20  accommodating therein a piston drive unit is disposed at the front side of the deceleration unit  10 . The compressed air supplying unit  40  is disposed at the front side of the housing  2  and below the cylinder unit  20 . The drill bit  50  is disposed at the front side of the cylinder unit  20 . 
   The housing  2 , which configures a first frame together with a gear cover  11  to be described later, accommodates therein a motor (not shown) serving as a driving source of the drilling machine  1 . An output shaft  6  extends from the motor toward the deceleration unit  10 , and a fan  5  is fixed to the output shaft  6  for cooling the motor. A handle  3  integrally extends downward from a rear lower side of the housing  2 . The handle  3  has a trigger  4 , and has built therein a switching circuit (not shown) operated upon manipulation of the trigger for controlling the rotation of the motor. 
   As best shown in  FIG. 2 , the deceleration unit  10  shown in  FIG. 2  includes a gear cover  11  that configures the first frame together with the housing  2 , and an inner cover  12 . The deceleration unit  10  further includes a first gear  13  and a second gear  14  those disposed between the gear cover  11  and the inner cover  12 . The inner cover  12  is in contact with the housing  2 , and is fixed to the housing  2  with screws (not shown). A front end of the output shaft  6  penetrates through the inner cover  12 , and has a pinion gear  7  attached thereto, so that the pinion gear  7  is disposed between the gear cover  11  and the inner cover  12 . A bearing  17  is fit at the inner cover  12  for rotatably supporting the output shaft  6 . In other words, the output shaft  6  extending from the motor is rotatably held by the inner cover  12  through the bearing  17 . 
   The first gear  13  includes a first gear  13   a  meshedly engaged with the pinion gear  7 , and a first pinion gear  13   b  integrally and coaxially disposed with the first gear  13   a . The first gear  13   a  and the first pinion gear  13   b  are rotatably supported by the gear cover  11  and the inner cover  12  through a bearing  15 A fit into the gear cover  11  and a bearing  15 B fit into the inner cover  12 . The second gear  14  is meshedly engaged with the first pinion gear  13   b  of the first gear  13 . A main shaft  23 (described later) has a rear end portion  23 D concentrically fit with the second gear  14 . Thus, the second gear  14  is coupled to the main shaft  23 . The rear end portion  23 D of the main shaft is rotatably held by the gear cover  11  and the inner cover  12  through a bearing  16 A fit into the gear cover  11  and a bearing  16 B fit into the inner cover  12 . 
   In the cylinder unit  20 , an outer hull is configured by the gear cover  11  as a first wall, and a substantially cylindrical cylinder cover  21  abutting on the gear cover  11  with a packing  9  interposed therebetween. The cylinder cover  21  is fixed to the gear cover  11  with screws (not shown). A cylindrical cylinder holding portion  11 A protrudes from the wall of the gear cover  11  and extends in a direction perpendicular thereto. The cylindrical cylinder holding portion  11 A is located in an internal space of the cylinder cover  21 . A lower part of the cylinder cover  21  functions as an outer hull of the compressed air supplying unit  40  that is disposed at the lower part of the cylinder unit  20 . 
   A cylinder  22  that is a part of the piston drive unit is disposed in the internal space of the cylinder cover  21  that configures as a second frame. The cylinder  22  has a cylinder front end portion  22 A and a cylinder rear end portion  22 B as shown in  FIG. 3 . A spacer  26  which functions as a second wall is fitted into the inner space of the cylinder cover  21  as shown in  FIG. 2 , and a cylinder holding portion  26 A extends from the spacer  26 . The cylinder front end portion  22 A is fitted into the cylinder holding portion  26 A through a washer  27 . The spacer  26  has a tubular shape through which the main shaft  23  extends. A clearance defined by the spacer  26 , the rear end portion of the spindle  24 , and a front inner peripheral surface of the cylinder  22  configures a discharge outlet  62  to discharge compressed air that have been introduced into the cylinder  22 . 
   The cylinder rear end portion  22 B is fitted into the cylinder holding portion  11 A protruding from the gear cover  11  which functions as the first wall. A urethane washer  28  and a washer  29  are interposed between the rear end portion  22 B and the cylinder holding portion  11 A. Thus, the cylinder  22  is fixed in the inside the cylinder cover  21  at the cylinder front end portion  22 A and cylinder rear end portion  22 B. An O-ring  61  is interposed between the spacer  26  and the cylinder cover  21  for maintaining air-tightness between an anterior space and a posterior space of the spacer  26 . Annular cylindrical space  36  is defined by the cylinder cover  21 , the gear cover  11 , the cylinder  22 , and the spacer  26 . The space  36  is located immediately outside of the cylinder  22 . The housing  2 , the gear cover  11 , and the cylinder cover  21  form an outer frame of the drilling machine  1 . 
   At the front inner peripheral surface of the cylinder  22 , an annular inward projection  22 C that protrudes radially inwardly is provided. At part of a cylinder trunk  22 D located at the rear side vicinity of the annular inward projection  22 C, a plurality of vent holes  22   e  are formed allowing fluid communication between the internal space of the cylinder  22  and the annular cylindrical space  36 . 
   Inside the cylinder  22 , a cylindrical piston  25  is slidably disposed. As shown in  FIG. 3 , the piston  25  includes a piston trunk  25 A, and a piston rear end portion  25 B with its diameter greater than that of the piston trunk  25 A. The piston trunk  25 A extends through the annular inward projection  22 C at the front inner surface of the cylinder  22 , while the piston rear end portion  25 B extends through the cylinder trunk  22 D. The piston trunk  25 A has an outer diameter slightly smaller than an inner diameter of the annular inward projection  22 C, while the piston rear end portion  25 B has an outer diameter slightly smaller than an inner diameter of the cylinder trunk  22 D. Thus, clearances are formed between the piston trunk  25 A and the annular inward projection  22 C, and between the piston rear end portion  25 B and the cylinder trunk  22 D. Each clearance is filled with a lubricant to bring about sealing effect, which keeps air-tightness between the anterior space and the posterior space of the annular inward projection  22 C as well as between the anterior space and the posterior space of the piston rear end portion  25 B, and also improves sliding performance of the piston  25 . 
   Since the piston trunk  25 A and the annular inward projection  22 C are in hermetic relationship with each other while the piston rear end portion  25 B and the cylinder trunk  22 D are in hermetic relationship with each other, a space  37   a  is defined by the rear surface of the annular inward projection  22 C, the inner surface of the cylinder trunk  22 D located at the rear side of the annular inward projection  22 C, a front end surface of the piston rear end portion  25 B, and the outer surface of the piston trunk  25 A located in front of the piston rear end portion  25 B. The space  37   a  is in communication with the space  36  through the vent holes  22   e , and has its volume varied depending on the position of the piston  25  relative to the cylinder  22 . 
   The piston trunk  25 A is formed with first holes  25   c  that extend from the outer peripheral surface thereof to the center of the piston  25 . Further, the piston  25  is formed with second holes  25   d  extending in parallel with the axis of the piston  25  each second hole  25   d  has a front open end opened at the first hole  25   c , and a rear open end opened at the rear end surface of the piston rear end portion  25 B. 
   The main shaft  23  as a rotation shaft extends through the piston  25 . As described above, the main shaft rear end portion  23 D penetrates through the gear cover  11 , and is fixed to the second gear  14 . An oil seal  35  is provided between the gear cover  11  and the main shaft  23  for maintaining air-tightness between the main shaft  23  and the gear cover  11 . 
   A main shaft trunk  23 C has its outer diameter slightly smaller than the inner diameter of the piston  25 . Thus, a clearance is formed between the piston  25  and the main shaft trunk  23 C. The clearance is filled with a lubricant to bring about sealing effect, which keeps air-tightness between the anterior space and the posterior space of the piston  25 , and also secures slidability of the piston  25  as well as rotating ability of the main shaft  23 . Further, a space  37   b  is defined by the main shaft trunk  23 C, the rear end face of the piston rear end portion  25 B, the inner surface of the cylinder  22 , and the washer  29 . The space  37   b  is in communication with the first holes  25   c  through the second holes  25   d.    
   A main shaft  23  has a front end portion  23 A having a diameter slightly smaller than that of the main shaft trunk  23 C. A pair of grooves  23   b  are formed at the outer surface of the main shaft front end portion  23 A. The grooves  23   b  extend from the front end of the main shaft front end portion  23 A in parallel with the axis of the main shaft  23 . 
   A cylindrical spindle  24  is disposed over the main shaft front end portion  23 A such that the spindle  24  is slidable in an axial direction thereof relative to the main shaft  23 . The spindle  24  has a spindle front end portion  24 A protruding from the front end of the cylinder cover  21 . An internal female thread is formed at an inner peripheral surface of the spindle front end portion  24 A for threading engagement with a male thread formed at the drill bit  50  to be described later. A spindle rear end portion  24 B of the spindle  24  functions as an impact-receiving region to be impacted by the piston  25 . 
   The rear portion of the inner peripheral surface of the spindle  24  is provided with a pair of spindle protrusions  24 C that protrudes toward the center thereof, as shown in  FIG. 4 . The pair of the spindle protrusions  24 C is insertedly engaged with the pair of the grooves  23   b  formed at the main shaft front end portion  23 A of the main shaft  23 . Thus, the spindle  24  cannot be rotated relative to the main shaft  23 , but can slide in its axial direction relative to the main shaft  23 , as shown in  FIG. 5  and  FIG. 6 . 
   The spindle  24  has a second air path  39  formed therein, and is held by a metal piece  33  and a sleeve  30 . The metal piece  33  is fitted into the cylinder cover  21 , and has its inner diameter slightly larger than the outer diameter of the spindle  24 . Thus, a clearance is defined between the metal piece  33  and the spindle  24 . The clearance is filled with lubricant which enables the spindle  24  to rotate and slide relative to the metal piece  33 . The sleeve  30  is fitted into an inner race of a bearing  32  that is fitted into the cylinder cover  21 . Thus, the sleeve  30  is rotatable relative to the cylinder cover  21 . 
   The sleeve  30  is formed with a hole  30   a  in which a steel ball  31  is inserted such that a spherical part thereof protrudes from the inner peripheral surface of the sleeve  30 . A part of the outer peripheral surface of the spindle  24  over which the sleeve  30  is disposed is formed with an elongated groove  24   d  extending in parallel with the axis of the spindle  24 , so that the part of the steel ball  31  can be received in the elongated groove  24   d  as shown in  FIG. 2  and  FIG. 5 . The sleeve  30  has its inner diameter slightly larger than the outer diameter of the spindle  24 . However, the clearance between the sleeve  30  and the spindle  24  is sized to prevent the steel ball  31  from dropping out of the elongated groove  24 . Therefore, the steel ball  31  can move only within the groove  24   d . Accordingly, the spindle  24  can slide relative to the sleeve  30  corresponding to the length of the groove  24   d  within which the steel ball  31  can move. 
   A clearance or a first air path  38  is defined by the sleeve  30 , the bearing  32 , and the cylinder cover  21 . A part of the spindle  24  that always faces the first air path  38  is formed with an air hole  24   e  allowing fluid communication between the first air path  38  and the second air path  39 . 
   An oil seal  34  is fitted into a part of the cylinder cover  21 , the part being located ahead of the metal piece  33 . The oil seal  34  is adapted to prevent dust attached to the surface of the spindle  24  that protrudes from the cylinder cover  21  and is exposed to the atmosphere from entering into the inside of the cylinder cover  21  as well as to block off the inside of the cylinder cover  21  from the atmosphere. 
   The compressed air supplying unit  40  has an air chamber  43  defined by the cylinder cover  21  and the packing  9 . The compressed air supplying unit  40  mainly includes a coupling unit  42 , an impact cock portion  44 , and a cooling cock portion  47 . The coupling unit  42  is coupled to a compressor (not shown) for introducing compressed air into the air chamber  43 . The impact cock portion  44  is adapted to selectively shut off fluid communication between the air chamber  43  and the annular cylindrical space  36 . The cooling cock portion  47  is adapted to selectively shut off fluid communication between the air chamber  43  and the first air path  38 . 
   An impact air path  45  is formed in the impact cock portion  44  for providing fluid communication between the air chamber  43  and the annular cylindrical space  36 . A cooling air path  48  is formed in the cooling cock portion  47  for providing a fluid communication between the air chamber  43  and the first air path  38 . Compressed air is supplied from the compressor (not shown) to the air chamber  43 . In the midstream of the impact air path  45  and in the midstream of the cooling air path  48 , there are arranged an impact cock  46  and a cooling cock  49  for adjusting cross-sectional areas of these paths, respectively. 
   The drill bit  50  includes a stem section and a conical cutting edge section fixed to a front end of the stem section by brazing. The cutting edge is made from cemented carbide. The rear end portion of the stem section is formed with the male thread threadingly engaged with the female thread of the spindle  24  as described above. An air path  52  extends through the stem section. The air path  52  has a front open end serving as a discharge outlet  54  in the vicinity of the cutting edge  56  and a rear open end serving as an inlet  53  opened at the rear end surface of the drill bit  50  and is communicated with the second air path  39 . Furthermore, the stem section has an outer surface formed with a spiral flute  58  connecting with the cutting edge  56 . 
   Next, operation of the drilling machine  1  of the present embodiment will be described. When a drilling worker presses the drill bit  50  against an object to be drilled, not shown, such as a concrete wall, and pulls the trigger  4 , the output shaft  6  of the motor (not shown) rotates. At this time, the fan  5  fixed to the output shaft  6  is also rotated to suck air into the housing  2  through slits (not shown) formed at the housing  2 . 
   The first gear  13  is rotated, since the pinion gear  7  provided at the front end of the output shaft  6  is meshedly engaged with the first gear  13   a . The rotation of the first gear  13  is transmitted to the second gear  14 , since the first pinion gear  13   b  is meshedly engaged with the second gear  14 . The main shaft  23  and the second gear  14  rotate concurrently, since the main shaft rear end portion  23 D is concentrically connected to the second gear  14 . 
   As described above, the spindle  24  is disposed over the main shaft front end portion  23 A, and a pair of the spindle protrusions  24 C is inserted into and engaged with a pair of the grooves  23   b  formed at the main shaft front end portion  23 A. Thus, the spindle  24  can move freely along the axial direction thereof relative to the main shaft  23 , and is fixed in the rotational direction. Therefore, the spindle  24  and the main shaft  23  rotate together. Since the drill bit  50  is fixed to the front end portion of the spindle  24 , the drill bit  50  also rotates to drill a concrete wall etc. 
   When drilling a concrete wall, etc. by rotating the drill bit  50 , the cutting edge  56  is pressed against the concrete wall, etc. to crush the pressed portion of the wall. At this time, temperature of the cutting edge  56  becomes high temperature due to friction. When this state is left intact, drilling capability is lowered because of change in material characteristics, etc. due to high temperature. Furthermore, when performing the drilling operation, a great amount of concrete dust is brought about around the cutting edge  56 . When the concrete dust exists between the cutting edge  56  and the concrete wall, the cutting edge  56  cannot directly come into contact with the concrete wall, which lowers drilling capability. Therefore, the cutting edge  56  has to be cooled down and concrete dust has to be removed from the drilled hole. 
   To avoid this, compressed air supplied from the compressor is directed to and accumulated in the air chamber  43  through the coupling unit  42  in the compressed air supplying unit  40 . The air chamber  43  communicates with the cooling air path  48 , while the cooling air path  48  communicates with the first air path  38 . Furthermore, the first air path  38  communicates with the second air path  39  through the air hole  24   e . The front end of the second air path  39  faces the inflow inlet  53  that is formed at the rear end surface of the drill bit  50 . Thus the compressed air in the air chamber  43  flows through the cooling air path  48 ,the first air path  38 , the second air path  39  and the air path  52 . 
   Accordingly, compressed air accumulated in the air chamber  43  is discharged out of the discharge outlet  54  formed in the vicinity of the cutting edge  56 . When compressed air is discharged, the heat of the cutting edge  56  is removed and the cutting edge  56  is cooled down. In the drilled hole, compressed air discharged from the discharge outlet  54  is directed along the spiral flute  58  to the outside. Thus, concrete dust brought about around the cutting edge  56  is also discharged. 
   Since the cooling cock  49  is provided at the midstream of the cooling air path  48 , amount of compressed air to be discharged from the discharge outlet  54  can be adjusted arbitrarily. Thus, amount of compressed air to be discharged can be adjusted depending on operating condition such as the number of revolutions of the drill bit  50 . 
   As described above, a concrete wall, etc. can be drilled by rotational motion alone of the drill bit  50 . In this case, since only rotational motion of the drill bit  50  occurs, noise brought about by the drilling operation is small. On the other hand, when the drill bit  50  abuts a coarse aggregate or a hard concrete such as a high strength concrete, drilling operation only with rotational motion of the drill bit  50  lowers working efficiency. Therefore, in this case, impacts are additionally applied to the drill bit  50 . 
   The piston  25  impacts the spindle rear end portion  24 B for applying impacts to the drill bit  50 . Specifically, in the state shown in  FIG. 2 , compressed air is directed from the air chamber  43  to the space  37   a  through the impact air path  45 , the space  36 , and the vent holes  22   e . In the state shown in  FIG. 2 , the piston  25  is located at the front end side, and the first hole  25   c  is located at the front side of the annular inward projection  22 C and is opened only to the discharge outlet  62 . Thus, the fluid communication between the space  37   a  and the space  37   b  is shut off. Accordingly, compressed air is accumulated in the space  37   a  and internal pressure thereof is increased, and thus internal pressure difference is established between the space  37   a  and the space  37   b , which enlarges the space  37   a.    
   Because of the pressure increase in the space  37   a , the piston  25  is moved toward the rear end side. Then, as shown in  FIG. 7  when the piston  25  is moved to the rearmost position, the first holes  25   c  have moved past the annular inward projection  22 C and are positioned at the rear side of the annular inward projection  22 C. At this time, the space  37   a  communicates with the space  37   b  through the first holes  25   c  and the second holes  25   d . Thus, internal pressure of the space  37   a  becomes equal to that of the space  37   b . Further, since the discharge outlet  62  positioned at the front side of the piston  25  is in communication with the first air path  38  that communicates with the atmosphere through the discharge outlet  54 , the pressure in the discharge outlet  62  is substantially equal to the atmospheric pressure. On the other hand, the space  37   b  located at the rear side of the piston  25 , has its internal pressure substantially equalized with pressure of compressed air. As a result, pressure difference is established between the front side and the rear side of the piston  25 . Thus, the piston  25  is moved toward the front side as shown in  FIG. 8 . 
   During this frontward movement of the piston  25 , the first holes  25   c  are moved past the annular inward projection  22 C and are positioned at the front side of the annular inward projection  22 C. Thus, the space  37   b  is brought into communication with the discharge outlet  62 , so that the pressure in the space  37   b  becomes substantially equal to that of the discharge outlet  62 . However, the piston  25  keeps moving forward due to inertial force, and then, as shown in  FIG. 9 , the piston  25  collides against the spindle rear end portion  24 B to apply impacts to the drill bit  50  fixed to the spindle  24 . At this time, the central axis of the piston  25  at which center of gravity thereof is located and the central axis of the spindle  24  at which center of gravity thereof is located are coaxial with each other, momentum of the piston  25  can be desirably transmitted to the spindle without dispersion of force. 
   Since the spindle  24  can slide freely along its axial direction independently of the main shaft  23 , the spindle  24  and the drill bit  50  alone are moved when the piston  25  impacts the spindle  24 . Since inertial masses of the spindle  24  and the drill bit  50  are small, impacts by the piston  25  can be desirably transmitted to the cutting edge  56 . Further, since the spindle  24  can move freely relative to the main shaft  23 , impacts transmitted to the spindle  24  are not transmitted to the main shaft  23 . Accordingly, impacts are not transmitted to the second gear  14  fixed to the main shaft rear end portion  23 D. 
   Then, the piston  25  moves backward due to reaction force of the collision, and returns to the initial position shown in  FIG. 2 . Then, a sequence of the operation is repeated, which consecutively impacts the spindle  24 . 
   The motion of the piston  25  can be controlled by varying the pressure of the compressed air. Specifically, flow channel area of the impact air path  45  is varied by operating the impact cock  46  disposed at the midstream of the impact air path  45 . Thus, amount of compressed air to be directed to the space  37   a  is varied, and accordingly, the expanding speed of the space  37   a  is varied. Consequently, the moving speed of the piston  25  is varied, and the impact intensity is also varied. When drilling operation with the impacts is desired at places where noise generation is restricted due to noise regulation or the like, the impact cock  46  is operated to adjust amount of compressed air so that drilling operation employing impacts can be performed under the noise regulation. 
   Further, compressed air directed to the space  37   a  which becomes motive energy to move the piston  25  is directed to the space  37   b  through the first holes  25   c  and the second holes  25   d , and is then discharged from the discharge outlet  62  through the second holes  25   d  and the first holes  25   c . Since the discharge outlet  62  communicates with the first air path  38 , compressed air having been passed through the impact air path  45  is discharged from the discharge outlet  54  formed at the drill bit  50  to the atmosphere through the second air path  39  similar to compressed air passing through the cooling air path  48 . This implies that the compressed air for the motive energy of the piston is also utilized for cooling purpose to the drill bit  50 . The part where the spindle  24  is disposed over the main shaft  23  is not provided with sealing effect. Thus, compressed air discharged from the discharge outlet  62  can be directed to the second air path  39  through the minute clearance between the spindle  24  and the main shaft  23 . 
   Accordingly, when large impacts are required, the cooling cock  49  is closed to shut off the cooling air path  48 , whereas the impact cock  46  is open to direct the compressed air to the impact air path  45  to move the piston  25 . In this case, entire compressed air can be exclusively used as motive energy source for the impact operation of the piston  25 . Even in this case, since compressed air for applying impacts flows from the discharge outlet  62  to the first air path  38  and the second air path  39 , compressed air can be discharged through the discharge outlet  54  for cooling down the drill bit  50  as well as for discharging concrete dust to the outside of the drilled hole through the flute  58 . 
   As described above, since the driving power for rotating the drill bit  50  is exclusively provided by the motor, and the driving power for reciprocating the drill bit  50  is exclusively provided by the compressed air. In other words, power source for the rotational motion and the power source for reciprocating motion is independent of each other. Therefore, each motion can be controlled independently without mutual compensation. 
   While the invention has been described in detail and with reference to the specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the sprit and scope of the invention. For example, the cooling air path  48  can be dispensed with. Even in the latter case, cooling down the drill bit  50  as well as discharging concrete dust to the outside of the drilled hole through the flute  58  can be achieved, since compressed air from the impact air path  45  can be discharged to the discharge outlet  54  through the discharge outlet  62 , the first and second air paths  38 ,  39  and the air path  52  as described above. 
   As another modification, if cooling to the drill bit  50  with the compressed air is not required, the cooling air path  48  can be dispensed with and further, a discharge outlet corresponding to the discharge outlet  62  can be formed at the cylinder cover  21  or the like to directly discharge compressed air as the motive power source for the piston  25  to the atmosphere. In the latter case, compressed air can be smoothly discharged outside, which can enhance driving efficiency of the piston  25 .