Abstract:
A power tool ( 1 ) and a method of producing such power tool capable of improving dust proof performance with respect to a circuit board ( 33 ) and a stator coil ( 31 B), and capable of securing cooling performance to the circuit board. The power tool includes a brushless motor ( 3 ) having a hollow stator ( 31 ), a rotor ( 32 ) positioned in an internal space of the stator, and a motor driver circuit connected to the stator. The stator includes a coil ( 31 B) electrically connected to the mo- for driver circuit. The coil is coated with a coating agent ( 31 D), and the motor driver circuit is covered with an insulation cover member ( 33 D).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a power tool, and more particularly to such power tool having a brushless motor, and a method for producing the power tool. 
       BACKGROUND ART 
       [0002]    Generally a brushless motor (DC motor) is downsized, and can provide a prolonged service life because electrical connection of a brush and a commutator to a rotor attached to a rotation shaft can be eliminated. The brushless motor is provided with an inverter circuit board (motor driver circuit board) which includes an output transistor as a switching element configured to supply drive signals of large electrical current to a stator coil provided around the rotor. 
         [0003]    Japanese Patent Application Publication No. 2011-062803 discloses a structure for protecting an inverter circuit board against rainwater and dust. The protective structure includes a heat radiation member for releasing heat from the switching element. Further, constituents of electronic circuit such as electronic components, coil connecting portion, control circuit portion connecting portion, a circuit board, and the heat radiation member are integrally covered with an insulating cover member by molding. 
       DISCLOSURE OF INVENTION 
     Solution to Problem 
       [0004]    According to a conventional power tool, cooling to the inverter circuit board and protection against rainwater and dust must be satisfied at the same time. Because the heat radiation member is integrally protected, an entire power tool must become bulky. Further, protection to the stator coil is insufficient because enamel coating and varnish coating over the stator coil are very thin, so that the coating may be peeled off as a result of long period of use in dusty environment. Thus short-circuiting may occur between windings of the stator coil, which may vary characteristic of the motor. 
         [0005]    It is therefore an object of the present invention to provide a power tool and a method for producing the power tool capable of improving dust-proof performance for the circuit board and the stator coil, and ensuring cooling performance to the circuit board. 
         [0006]    This and other object of the invention will be attained by a power tool including a brushless motor configured to drive an end bit. The brushless motor includes a stator, a rotor, and a circuit board. The stator has a hollow cylindrical shape providing an internal space and is provided with a coil. The rotor is positioned in the internal space. The circuit board is connected to the stator. The coil is electrically connected to the circuit board and the coil is covered with a first resin material. With this structure, the coil can be protected against dust and water droplet, since the coil is covered with the first resin material. 
         [0007]    Preferably, the circuit board in its entirety is covered with a second resin material. With this structure, the circuit board can be protected against dust, since the circuit board is covered with the second resin material. 
         [0008]    Preferably, the coil is connected to the circuit board such that a part of the first resin material is covered with the second resin material. The coil has a lead out portion electrically connected to the circuit board. With this arrangement, the lead out portion can be covered with the second resin material. Therefore, entire surface of the circuit board and the coil can be covered with at least one of the first resin material and the second resin material. Consequently, electrical current flowing portion in the brushless motor can be covered with the resin. 
         [0009]    Preferably, the first resin material is a thermosetting resin or a resin curable upon mixing a base resin with a curative agent. 
         [0010]    In another aspect of the invention, there is provided a power tool including a brushless motor configured to drive an end bit, the brushless motor including a stator, a rotor, and a circuit board. The stator has a hollow cylindrical shape providing an internal space. The rotor is positioned in the internal space. The circuit board is connected to the stator and is provided with a plurality of switching elements. The circuit board in its entirety is covered with a resin material. With this arrangement, the circuit board can be protected against dust, since the entire circuit board is covered with the resin material. 
         [0011]    Preferably, the resin material has a shape in conformance with a shape of the circuit board. With this arrangement, a thickness of the resin material covering over the circuit board can be thin, to ensure cooling to the circuit board. Further, a combination of the resin material and the circuit board can be compact because of the thin structure of the resin material. Therefore, a spatial tolerance can be increased within a housing of the power tool, thereby facilitating layout of components and mechanical parts such as male thread. 
         [0012]    Preferably, the brushless motor further includes a holding member positioned between the stator and the circuit board for connecting the stator to the circuit board. The holding member is formed with at least two openings in communication with the internal space of the stator. With this structure, for forming the resin material over the circuit board in a state where the circuit board is assembled to the stator, the resin material can reach, through the openings, an inside area of the circuit board, the inside area being within an inside of the stator. Thus, entire surface of the circuit board can be covered with the resin material. 
         [0013]    Preferably, the resin material is a resin curable upon mixing a base resin with a curative agent, or the resin material is urethane resin. 
         [0014]    Preferably, the power tool further includes a housing accommodating therein the brushless motor. A cooling air passage is defined between the housing and the brushless motor, and the circuit board faces the cooling air passage. With this structure, stabilized cooling to the circuit board can be obtained. 
         [0015]    In another aspect of the invention, there is provided a method for producing a power tool including a brushless motor configured to drive an end bit, the brushless motor including a stator having a hollow cylindrical shape providing an internal space and provided with a coil having an lead out portion, a rotor positioned in the internal space, and a circuit board electrically connected to the lead out portion, the method including (a) coating the coil with a first resin material, (b) removing the first resin material at an end portion of the lead out portion, (c) electrically connecting the end portion of the lead out portion to the circuit board, and (d) coating the circuit board, the end portion of the lead out portion, and the lead out portion coated with the first resin material with a second resin material. With this method, the coil and the circuit board can be fully covered with at least one of the first resin material and the resin material. Therefore, electrical current flowing portion of the brushless motor can be covered with the resin material. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a cross-sectional side view of an impact driver according to one embodiment of the present invention. 
           [0017]      FIG. 2  is a partial perspective view of the impact driver according to the embodiment as viewed from an upper rear side of the impact driver. 
           [0018]      FIG. 3  is a perspective view of a stator in the impact driver according to the embodiment. 
           [0019]      FIG. 4  is a perspective view of the stator and a motor driver circuit device in the impact driver as viewed from an upper rear side of the stator. 
           [0020]      FIG. 5  is a side view, with partially cross-sectioned, of the stator and the motor driver circuit device in the impact driver according to the embodiment. 
           [0021]      FIG. 6  is a cross-sectional view of the stator and the motor driver circuit device in the impact driver according to the embodiment. 
           [0022]      FIG. 7  is a perspective view of the stator and the motor driver circuit device in the impact driver as viewed from a side and front of the stator. 
       
    
    
     REFERENCE SIGNS LIST 
       [0023]      1 : impact driver 
         [0024]      2 : housing 
         [0025]      2 A: body portion 
         [0026]      2 B: handle 
         [0027]      2   a : intake port 
         [0028]      2   c : cooling air passage 
         [0029]      3 : motor 
         [0030]      4 : gear mechanism 
         [0031]      4 A: frame body 
         [0032]      4 B: bearing 
         [0033]      5 : impact mechanism 
         [0034]      6 : battery 
         [0035]      21 : terminal portion 
         [0036]      23 A: trigger 
         [0037]      23 B: switching portion 
         [0038]      25 : light 
         [0039]      31 : stator 
         [0040]      31 A: winding portion 
         [0041]      31 B: coil 
         [0042]      31 C: lead out portion 
         [0043]      31 D: coating agent 
         [0044]      31   a : slot 
         [0045]      32 : rotor 
         [0046]      32 A: rotor shaft 
         [0047]      32 B: fan 
         [0048]      32 C: pinion gear 
         [0049]      32 D,  32 E: bearing 
         [0050]      33 : motor driver circuit 
         [0051]      33 A: large size switching element 
         [0052]      33 B: small size switching element 
         [0053]      33 C: solder portion 
         [0054]      33 D: insulation cover member 
         [0055]      33   a : through-hole 
         [0056]      34 : insulator 
         [0057]      34   a,    34   b : opening 
         [0058]      41 : spindle 
         [0059]      41 A: flange 
         [0060]      41 B: ball 
         [0061]      41   a : groove 
         [0062]      42 : ring gear 
         [0063]      43 : planetary gear 
         [0064]      51 : hammer case 
         [0065]      51 A: bearing 
         [0066]      52 : anvil 
         [0067]      52 A: end bit attachment portion 
         [0068]      52 C: ball 
         [0069]      52 D: operation portion 
         [0070]      52 E blade portion 
         [0071]      52   a : bore 
         [0072]      52   b : attachment hole 
         [0073]      53 : hammer 
         [0074]      53 A: pawl portion 
         [0075]      53   a : through-hole 
         [0076]      53   b : groove 
         [0077]      53   c : retaining portion 
         [0078]      54 A: first spring 
         [0079]      100 : control circuit portion 
       DESCRIPTION OF EMBODIMENTS 
       [0080]    An impact driver as a power tool according to one embodiment of the present invention will be described with reference to  FIGS. 1 through 7 .  FIG. 1  shows the impact driver  1  adapted to fasten a bolt, nut and a male thread, and mainly includes a housing  2 , a motor  3 , a gear mechanism  4 , and an impact mechanism  5 . A rechargeable battery  6  is used as a power source for driving the tool. 
         [0081]    The housing  2  is made from resin such as 6-nylon. The resin housing  2  includes a body portion  2 A in which the motor  3  is accommodated, and a handle  2 B extending from the body potion  2 A. The housing  2  is provided by two housing sections divided into two symmetrical sections complementary with each other. The two symmetrical sections are mated with each other in a mating plane extending in vertical direction and frontward/rearward direction to provide an accommodation space within the body portion  2 A and the handle portion  2 B. The above-described motor  3 , the gear mechanism  4  and the impact mechanism  5  are arrayed in line coaxially in the accommodation space of the body portion  2 A from one end to another end portion of the body portion  2 A. A frontward/rearward direction is coincident with the direction of this array in which the side of the motor  3  will be referred to as a rear side. Further, a direction from the body portion  2 A to the handle portion  2 B will be referred to as vertical direction perpendicular to the frontward/rearward direction. The side of the handle  2 B will be referred to as lower side. 
         [0082]    In the body portion  2 A, a discharge port (not shown) and an intake port  2   a  ( FIG. 2 ) are formed at each lateral side of the body portion  2 A and at positions frontward and rearward of the motor  3 . In the housing  2 , a terminal portion  21  is provided at a lower end portion of the handle  2 B. The battery  6  is detachably attached to the terminal portion  21  and is electrically connected to the terminal portion  21 . Further, a control circuit portion  100  is provided above the terminal portion  21  for controlling rotation of the motor  3 . The terminal portion  21  is electrically connected to the control circuit portion  100 . 
         [0083]    The handle  2 B has a base end portion provided with a trigger  23 A for user&#39;s manipulation and a switching portion  23 B connected to the trigger  23 A and the control circuit portion  100  and adapted to control conduction to the motor  3 . Further, a forward-reverse changeover lever (not shown) is provided at the base end portion of the handle  2 B and above the trigger  23 A for changing rotational direction of the motor  3 . 
         [0084]    An LED light  25  is provided at a front end of the housing  2  and below the impact mechanism  5 . The LED light  25  is connected to the control circuit portion  100  and is adapted to irradiate light frontward. Further, as shown in  FIGS. 1 and 2 , a cooling air passage  2   c  is formed within the housing  2 . The cooling air passage  2   c  is positioned around the motor  3  and is spanned between the intake port  2   a  and the discharge port (not shown). 
         [0085]    The motor  3  is DC brushless motor, and mainly includes a stator  31 , a rotor  32  and a motor driver circuit  33 . The stator  31  is hollow cylindrical shaped constituting an outer shell of the motor  3 , and has an outer peripheral surface held to the hosing  2 . As shown in  FIG. 3 , six slots  31   a  are formed inside the stator  31 . Neighboring slots  31   a  are spaced away from each other at a constant interval in a circumferential direction of the stator  31 , and each slot  31   a  extends in frontward/rearward direction. Further, a wire winding portion  31 A is provided between the neighboring slots  31   a  and  31   a.  A coil  31 B is provided by winding an insulated conductive wire over the winding portion  31 A. A lead out portion  31 C extends from the coil  31 B. The lead out portion  31 C is an end portion of the insulated conductive wire. The coil  31 B has an exposed portion. The lead out portion  31 C and at least the exposed portion are subjected to coating with a coating agent  31 D as a first resin material made from thermoplastic resin. 
         [0086]    As shown in  FIG. 1 , the rotor  32  is rotatably disposed in the stator  31 . The rotor  32  has a rotor shaft  32 A integral with a rotor body and extending in the frontward/rearward direction. An axis of the rotor shaft  32 A defines a rotation axis of the rotor  32 . The rotor shaft  32 A has a front end portion provided with a fan  32 B and a pinion gear  32 C, those being coaxially and integrally rotatable with the rotor shaft  32 A. Further, the front end portion of the rotor shaft  32 A is rotatably supported to a frame body  4 A described later through a bearing  32 D. The rotor shaft  32 A has a rear end portion rotatably supported to the body portion  2 A through a bearing  32 E. 
         [0087]    Thus, the rotor shaft  32 A is rotatably supported to the body portion  2 A through the bearings  32 D,  32 E. Upon rotation of the rotor shaft  32 A, the fan  32 B is rotated integrally with the rotor shaft  32 A to generate air flow in which air is introduced through the intake port  2   a  into the accommodation space of the body portion  2 A and is discharged through the discharge port (not shown). 
         [0088]    As shown in  FIG. 4 , the motor driver circuit  33  as a circuit board is flat plate shaped and has a center region formed with a through-hole  33   a  through which a rear end portion of the rotor shaft  32 A extends. As shown in  FIG. 5 , the motor driver circuit  33  is positioned at a rear side of the stator  31  and extends in a direction perpendicular to the frontward/rearward direction. The motor driver circuit  33  is fixed to the stator  31  through an insulator  34  as a holding member. As shown in  FIG. 6 , the lead out portion  31 C is electrically connected to the motor driver circuit  33  by soldering. 
         [0089]    As shown in  FIG. 5 , a large size switching element  33 A, a small size switching element  33 B, and a plurality of solder portion  33 C protrude rearward from the rear surface of the motor driver circuit  33 . The small size switching element  33 B has a protruding amount smaller than that of the large size switching element  33 A. The solder portion  33 C is placed at end portion of the conductive wire connected to the control circuit portion  100 . 
         [0090]    The motor driver circuit  33  is covered with an insulation cover member  33 D as a second resin material which is a curable two-component type resin containing urethane resin as a main component and a curing agent mixed therewith. As shown in  FIGS. 5 and 6 , the insulation cover member  33 D covers the entire motor driver circuit  33 . The motor driver circuit  33  has an irregular surface (concave-convex surface) because the circuit  33  has the large size switching element  33 A, the small size switching element  33 B and the plurality of solder portions  33 C. The insulation cover member  33 D covers the motor driver circuit  33  in conformance with the surface irregularities thereof. In other words, the insulation cover member  33 D has a uniformly thin thickness without any excessively thick region. Further, as shown in  FIG. 6 , the insulation cover member  33 D is configured to overlap with the coating agent  31 D protecting the lead out portion  31 C connected to the motor driver circuit  33  to cover the coating agent  31 D. 
         [0091]    The insulator  34  is tubular shaped and is positioned between the stator  31  and the motor driver circuit  33 . As shown in  FIGS. 4 and 7 , two openings  34   a,    34   b  are formed at a rear end of the insulator  34  and at a position confronting the front surface of the motor driver circuit  33 . The openings  34   a  and  34   b  are positioned at upper and lower portions of the insulator  34 , respectively, and are in communication with an interior of the stator  31  through an interior of the insulator  34 . As a modification, more than two openings can be formed in the insulator  34 . 
         [0092]    Air flow is also generated in the internal space of the stator  31  since the fan  32 B is coaxially and integrally rotatable with the rotor  32 . Since the rear side of the stator  31  is physically blocked by the motor driver circuit  33 , external air will be introduced into the internal space of the stator  31  through the openings  34   a,    34   b  by the air flow generated in the internal space of the stator  31 . The openings  34   a,    34   b  are positioned at the front surface of the motor driver circuit  33 , and the openings  34   a,    34   b  are positioned in the cooling air passage  2   c  open to the intake port  2   a.  Therefore, the external air introduced into the stator  31  is the external air outside of the housing  2  and passing through the cooling air passage  2   c  via the intake port  2   a.    
         [0093]    As shown in  FIG. 1 , the gear mechanism  4  is positioned at a front side of the motor  3  in the body portion  2 A. The gear mechanism  4  is a planetary gear mechanism whose sun gear is the pinion gear  32 C, and whose outer shell is the frame body  4 A supported to the housing  2 . The planetary gear mechanism also includes a spindle  41 , a ring gear  42 , and a plurality of planetary gears  43 . The spindle  41  functions as a carrier for supporting the plurality of planetary gears  43 . The spindle  41  has a front end portion coaxially and rotatably supporting an anvil  52  (described later), and has a rear end portion rotatably supported to the frame body  4 A through a bearing  4 B. 
         [0094]    A flange  41 A is provided at a position adjacent to the rear end portion of the spindle  41  for rotatably supporting the planetary gears  43 . Further, a first spring  54 A (described later) is seated on the flange  41 A. A hammer  53  is axially movably disposed over the spindle  41 . The spindle  41  is formed with a pair of grooves  41   a,    41   a  extending diagonally relative to the axial direction. A ball  41 B is inserted in each groove  41   a  so that the spindle  41  and the hammer  53  are connected with each other through the balls  41 B. 
         [0095]    The ring gar  42  is fixed to the frame body  4 A such that the ring gear  41  is positioned at radially outer side and concentrically with the spindle  41 . Each planetary gear  43  is rotatably supported to the spindle  41  and is in meshing engagement with the ring gear  42  and the pinion gear  32 C. With this structure, rotation of the pinion gear  32 C is deceleratingly transmitted to the spindle  41 . 
         [0096]    As shown in  FIG. 1 , the impact mechanism  5  includes a hammer case  51 , the anvil  52 , the hammer  53 , and the first spring  54 A. The hammer case  51  is hollow cylindridal shaped and has a front end portion diametrically reduced. The hammer case  51  has a rear end portion connected to the body portion  2 A and concentric with the motor  3 , and has a front end portion provided with a bearing  51 A for rotatably supporting the anvil  52 . 
         [0097]    The anvil  52  is cylindrical shaped extending in the frontward/rearward direction, and is rotatably supported to the hammer case  51  through the bearing  51 A. The anvil  52  has a rear end portion formed with a bore  52   a  with which a front end portion of the spindle  41  is loosely fitted, so that the anvil  52  is rotatably supported to the spindle  41 . The anvil  52  has a front end portion formed with an attachment hole  52   b  and provided with an end bit attachment portion  52 A for attaching a socket (not shown). 
         [0098]    The end bit attachment portion  52 A includes a plurality of balls  52 C protrudable into the attachment hole  52   b,  and an operation portion  52 D biased rearward by a spring and abuttable on the balls  52 C to push the balls  52 C in a direction for permitting the balls  52 C to protrude into the attachment hole  52   b  in the rearwardly biased state. The rear portion of the anvil  52  is integrally provided with a pair of blade portions  52 E as engaged portions extending radially outwardly and diametrically opposite sides to each other. 
         [0099]    The hammer  53  is cylindrical shaped and is formed with a through-hole  53   a  through which the spindle  41  extends. The hammer  53  has a front end portion provided with a pair of pawl portions  53 A as engagement portions engageable with the blade portions  52 E. Each pawl portion  53 A protrudes frontward from a front end of the hammer  53 , and is angularly spaced away from each other by 180 degrees. The pair of pawl portions  53 A are shaped into a rotational symmetry, and each pawl portion  53 A has a side surface extending in a direction perpendicular to the circumferential direction of the pawl portion  53 A, and the side surface is inclined to make the pawl portion to be taper shaped. 
         [0100]    With this structure, when load from the hammer  53  is imparted on the anvil  52 , the anvil  52  is urged to be moved frontward relative to the hammer  53  along the side surface, so that the blade portions  52 E,  52 E ride over or moves past the pawl portions  53 A,  53 A, thereby permitting the hammer  53  to rotate with respect to the anvil  52 . In an actual operation, because the anvil  52  is immovable frontward relative to the housing  2 , the hammer  53  is moved rearward relative to the anvil  52 , so that the blade portions  52 E,  52 E move past the pawl portions  53 A,  53 A, thereby permitting the hammer  53  to rotate with respect to the anvil  52 . 
         [0101]    The through-hole  53   a  of the hammer  53  has an inner peripheral surface formed with a pair of grooves  53   b,    53   b  extending in frontward/rearward direction into which the pair of balls  41 B are inserted. Since the pair of balls  41 B,  41 B are inserted into the pair of grooves  53   b,    53   b  of the hammer  53  and into the pair of grooves  41   a,    41   a  of the spindle  41 , the hammer  53  and the spindle  41  can be coaxially rotated together. The hammer  53  has a rear end portion provided with a spring retaining portion  53   c  to which one end portion of the first spring  54 A is seated. The spring retaining portion  53   c  is positioned around a sleeve-like wall that defines the through-hole  53   a.    
         [0102]    A rear end of the first spring  54 A is seated on the flange  41 A of the spindle  41  through a washer. A front portion of the spindle  41  frontward of the flange  41 A extends through an internal space of the first spring  54 A. The front end portion of the first spring  54 A is inserted into the spring retaining portion  53   c  so as to urge the hammer  53  frontward in the axial direction relative to the spindle  41 . Thus, urging force of the first spring  54 A is in conformance with the axial direction and frontward direction. The pawl portion  53 A of the hammer  53  can be engaged with the blade portion  52 E of the anvil  52  because of the biasing force of the spring  54 A applied to the hammer  53  in the forward direction. 
         [0103]    Even if the hammer  53  is moved rearward relative to the anvil  52  during application of the load, the hammer  53  can be moved forward toward the anvil  52  by the biasing force of the first spring  54 A at the timing when the blade portion  52 E has moved past the pawl portion  53 A, so that abutment between the pawl portion  53 A and the blade portion  52 E can be provided. In this way, impacting force in the rotational direction can be applied to the anvil  52  by the rotation of the hammer  53  relative to the anvil  52  and by the abutment of the blade portion  53 A onto the blade portion  52 E. 
         [0104]    A process for providing the coating agent  31 D and the insulation cover member  33 D to the motor  3  will next be described for the production of the above-described impact driver  1 . First, coil coating process is performed. That is, in the stator  31 , the insulated wire is wound over the winding portion  31 A to provide the coil  31 B. In this state, slurry containing a powdered thermosetting resin is coated over the stator  31  including the coil  31 B and the lead out portion  31 C, and then, the coil  31 B and the lead out portion  31 C is energized. As a result of energization, the coil  31 B and the lead out portion  31 C produce a heat, so that melting occurs only the thermosetting resin adhered onto the coil  31 B and the lead out portion  31 C. 
         [0105]    Then, thermosetting resin adhered onto a portion other than the coil  31 B and the lead out portion  31 C is removed, and the coil  31 B and the lead out portion  31 C is again energized at an output higher than that of the first energization. Upon energization, curing occurs in the thermosetting resin adhered onto the coil  31 B and the lead out portion  31 C, to thereby forming the coating agent  31 D 
         [0106]    Next, lead out portion peel off process will be performed. In this process, the coating agent  31 D and an insulation coating formed over the conductive wire at an end portion of the lead out portion  31 C is removed or peeled off. 
         [0107]    Then, connection process is performed. In this process, the motor driver circuit  33  is assembled to the stator  31  through the insulator  34 , and during this assembly an exposed or bare wire at the end portion of the lead out portion  31 C is electrically connected to a pattern on the motor driver circuit  33  by soldering. By this soldering, other conductive wires constituting the solder portion  33 C are also electrically connected to the pattern. 
         [0108]    Next, a process of coating the circuit board will be performed. First, the motor driver circuit  33  and the insulator  34  assembled thereto are positioned within a frame body (not shown) in a state that the insulator  34  and the motor driver circuit  33  are vertically arranged in this order, and a liquidized resin containing a mixture of base resin and a curative agent is flowed into the frame body. The frame body has an internal configuration in conformance with an outer configuration of the insulation cover member  33 D. Since the insulator  34  is assembled to an upper surface (front side surface in  FIG. 7 ) of the motor driver circuit  33 , the liquidized resin may not sufficiently reach the upper surface of the motor driver circuit  33 . However, since the openings  34   a,    34   a  are formed in the insulator  34 , the openings  34   a,    34   a  can allow the liquidized resin to pass therethrough, so that the resin can reach the upper surface of the motor driver circuit  33 , a part of the upper surface being positioned inside the insulator  34 . Thus, an entire peripheral surface of the motor driver circuit  33  can be covered with the resin. 
         [0109]    In the resin flowing state, the connecting portion between the lead out portion  31 C and the motor driver circuit  33 , and a portion adjacent to the connecting portion are also covered with the resin. In the lead out portion  31 C, a portion other than the connecting portion connected to the motor driver circuit  33  is coated by the coating agent  31 D. Therefore, the portion coated by the coating agent  31 D is also covered with the liquidized resin. The insulation cover member  33 D can be formed upon curing the resin. Thus, the motor  3  can be protected by the coating agent  31 D and the insulation cover member  33 D. 
         [0110]    According to the above-described impact driver  1 , any damage to the coil  31 B and the motor driver circuit  33  can be restrained even if foreign material such as water droplet and dust is entered into the housing  2 , since the coil  31 B which is an electrically connecting portion and the motor driver circuit  33  are covered with the coating agent  31 D and the insulation cover member  33 D. Thus, change in characteristic of the motor due to short-circuit at the coil  31 B and the motor driver circuit  33  can be restrained. 
         [0111]    Further, since the motor driver circuit  33  faces the cooling air passage  2   c,  cooling to the motor driver circuit  33  can be effectively performed by introducing an external air into the motor  3  by the rotation of the fan  32 B. In particular, the insulation cover member  33 D does not affect cooling to the motor driver circuit  33 , because the insulation cover member  33 D is configured to be sufficiently thin conforming with the outer profile of the motor driver circuit  33 . Further, the thin structure of the insulation cover member  33 D can make the entire device compact. Therefore, spatial allowance within the housing  2  can be increased, which facilitates layout of components and mechanical parts required in the impact driver  1 . 
         [0112]    Further, each intake port  2   a  for introducing external air into the housing  2  are positioned at each lateral side of the housing  2 , whereas openings  34   a,    34   b  for introducing external air into the motor  3  are respectively positioned at upper and lower sides of the insulator  34 . With this arrangement, any foreign materials introduced into the housing  2  through the intake port  2   a  cannot reach to the openings  34   a,    34   b.  Thus, accidental locking of the rotation of the motor  3  due to the foreign materials can be restrained. 
         [0113]    Further, according to the above-described production method, secure resin coating can be performed with respect to the coil  31 B, the motor driver circuit  33 , and the connecting portion therebetween. In particular, thermosetting resin is used for coating the coil  31 B. Therefore, an entire surface of the coil wound over the winding portion  31 A and exposed to the outside can be covered with the resin, and the lead out portion  31 C extending from the coil  31 B can also be covered with the resin. 
         [0114]    Further, the resin which is a mixture of the base resin and the curative agent is used for coating the motor driver circuit  33 . This resin will be cured after elapse of predetermined time period from the timing of mixing the base resin with the curative agent. In other words, the resin can maintain its fluidity for the predetermined time period. Therefore, complicated irregular shape of the insulation cover member  33 D can be formed. Further, because of this fluidity, the resin can surely cover the motor driver circuit  33  in its entirety, because the resin can be easily entered into the interior of the insulator  34  through the openings  34   a,    34   b.  Further, the insulation cover member  33 D is made from urethane resin. Therefore, the insulation cover member  33 D can be produced at a low cost. 
         [0115]    The power tool according to the present invention is not limited to the impact driver, but is available for any type of power tool provided with the brushless motor. 
         [0116]    While the invention has been described in detail and with reference to the 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.