Abstract:
A trigger switch has an insulative enclosure, a movable contact element, and a sliding control device slidably mounted to the insulative enclosure. The insulative enclosure includes a case and a cover. The case has plural terminal subassemblies made of conductive metal members disposed therein. The case has an opening over which the cover is mounted. A fixed contact is mounted on one of the terminal subassemblies in the case. The movable contact element has a movable contact mounted at one end of the movable contact element and located opposite to the fixed contact. The movable contact element is swingably supported on another terminal subassembly. A trigger is mounted at one end of the sliding control device. An auxiliary brush is interposed in the movable contact element. A support member is provided with an opening portion in which the auxiliary brush is engaged such that the brush is swingably supported.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a trigger switch and, more particularly, to improvements in a switch structure inside a trigger switch used in an electric power tool. 
     2. Prior Art 
     A conventional and known trigger switch is disclosed, for example, in JP-A-2003-109451. This trigger switch has a control device that is mounted to a case by an improved mounting mechanism. An attempt has been made to reduce the thickness while securing good dustproofness. To achieve this, the state of the switch mechanism inside the case of the trigger switch is switched based on a triggering manipulation. The switch mechanism is incorporated in the internal space of the case. The opening at one side of the case that is opened in a corresponding manner to the internal space of the case is covered and closed by a cover. The control device is accommodated in a concave accommodation portion formed in a part of the other side surface of the case. The other side surface of the case including the outer surface of the control device is covered by a heat-dissipating plate to form an integrated structure. 
     However, in this conventional trigger switch, when a movable contact element moving swingably is mounted to a support member, the contact element is simply placed on the support member to permit the contact member to swing. Consequently, there is such a problem that the contact element comes off from the support member during use. Furthermore, when the movable contact element moves, the support member tends to bound, producing an arc across the contacted portion. This would produce poor contact. To prevent this, it is necessary to add a brush. However, to hold the brush, a process for tightening the brush is required. This produces a problem that the cost is increased. 
     Furthermore, as shown in  FIG. 26 , a trigger  120  is normally molded from resin. Therefore, opening portions are formed only from one direction. Utilizing the opening portions, trigger guide ribs  121  are attached from other components. Therefore, whenever the trigger  120  moves, dust  123  accumulates in the opening portions,  122 . The accumulated dust  123  is stored in the interiors of the opening portions  122 . The dust stays there without being discharged and interferes with the trigger guide ribs  121 . Under this condition, the trigger cannot be pulled to its full stroke, thus presenting a problem. 
     To permit the trigger to be mounted in an electric power tool easily, the switch itself is required to be integrated with the control device (FET). Therefore, an opening portion is formed in the cover of the switch, and the control device is disposed in the opening portion. In spite of this contrivance, a problem takes place. The movable contact element is received in a position adjacent to the opening portion. Because the opening portion is formed in a part of the cover, dust intruding from the opening portion reaches the chamber of the switch mechanism where the movable contact element is mounted. To prevent this problem, dustproof rubber is used in or around the opening portion in the cover as a dustproof countermeasure. Consequently, intrusion of dust can be prevented. However, there is the problem that dedicated packing or the like is necessary, increasing the cost. 
     Furthermore, the conventional product has the problem that heat from the FET stays inside the switch because the switch mechanism and the portion accommodating the FET are integrated. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a first object of the present invention to provide a trigger switch in which a movable contact element moving swingably does not easily come off from a support member. Further, it is a second object to provide a trigger switch that permits dust accumulated inside the trigger to be expelled to the outside easily. Further, it is a third object to provide a trigger switch having a cover opening portion to permit a control device to be mounted integrally with a switch, in which dust intruding from the opening portion does not easily intrude into the movable contact element side that is a switch mechanism. 
     In order to attain the above objects, the present invention provides a trigger switch including: an insulative enclosure including a case and a cover, the case having plural terminal subassemblies made of conductive metal members disposed therein, the case having an opening over which the cover is mounted; a fixed contact mounted on one of the terminal subassemblies in the case; a movable contact element which has a movable contact mounted at one end of the movable contact element and located opposite to the fixed contact and is swingably supported on another terminal subassembly; and a sliding control device which has a trigger mounted at one end of the sliding control device and is slidably mounted to the insulative enclosure, wherein the movable contact element is swingably supported on a support member in a state that an auxiliary brush interposed therebetween and the auxiliary brush is engaged with an opening provided on the support member. 
     Preferably, the trigger has trigger guide ribs for guiding sliding motion of the trigger, rib engagement portions in which the trigger guide ribs are inserted, and opening portions formed in the rib engagement portions to permit dust to be expelled. 
     Another trigger switch according to the present invention switches the state of a switch mechanism mounted inside a case based on a sliding manipulation of a trigger. A control device is disposed under the condition where the control device is exposed from the outer wall surface of a cover that covers the case. Dustproof walls are mounted between a first chamber where the control device is disposed and a second chamber constituting the switch mechanism to keep out dust. 
     A further trigger switch according to the present invention switches the state of a switch mechanism mounted inside a case based on a sliding manipulation of a trigger. The trigger has trigger guide ribs for guiding sliding motion of the trigger, rib engagement portions in which the trigger guide ribs are inserted, and opening portions formed in the rib engagement portions to permit dust to be expelled. 
     In the present invention, the auxiliary brush is interposed in the movable contact element moving swingably. The auxiliary brush is engaged in the opening portion formed in the support member. The movable contact elements are supported swingably. The movable contact elements do not easily come off from the support member during assembly and during use. It is assured that the movable contact elements swing stably. During swinging motion, the movable contact elements are prevented from bounding; otherwise, an are would be induced across the contacted portion and poor contact would be made. 
     The trigger has a space in addition to the opening portions in which ribs are engaged. The space is formed in bottom and top portions and in communication with the opening portions. Therefore, dust produced when the ribs are engaged in the opening portions can be expelled to the outside via the space in communication with the opening portions. Consequently, accumulation of dust inside the bigger can be avoided. The problem that the trigger cannot be pulled to its full stroke can be prevented. 
     In addition, an opening portion is formed to permit a control device mounted in the cover to be opened. Dustproof walls are mounted at a position located on one side of the opening portion. In consequence, dust intruding from the opening portion can be prevented from intruding to the switch mechanism side. 
     Yet further, the switch mechanism and the FET accommodation portions are made independent of each other. Consequently, the switch mechanism can be prevented from being affected by heat generated from the FET. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a trigger switch according to one embodiment of the present invention, showing the appearance as viewed from the front side; 
         FIG. 2  is a perspective view of the trigger switch, showing the appearance as viewed from the rear side; 
         FIG. 3  is an exploded perspective view of the trigger switch; 
         FIG. 4  is a perspective view of a case of the trigger switch; 
         FIG. 5  is a perspective view of a cover of the bigger switch; 
         FIG. 6  is a perspective view of a sliding control device incorporated in the trigger switch; 
         FIG. 7  is a perspective view of the sliding control device, as viewed from the rear side; 
         FIG. 8  is a perspective view of the trigger switch, and in which the trigger has been separated from the bigger switch; 
         FIG. 9  is a vertical cross section of the trigger; 
         FIG. 10  is a partially enlarged vertical cross section of the trigger; 
         FIG. 11A  is a side elevation of the bigger switch, showing the manner in which terminal subassemblies are assembled; 
         FIG. 11B  is a partially enlarged vertical cross section of the trigger switch, illustrating operation of the sliding control device; 
         FIG. 12A  is a side elevation of the trigger switch, showing the manner in which the terminal subassemblies are assembled; 
         FIG. 12B  is a partially enlarged vertical cross section of the trigger switch, illustrating operation of the sliding control device; 
         FIG. 13  is a side elevation of a support portion of the trigger switch, and in which the support portion with which a second movable contact element engages has been extracted; 
         FIG. 14  is a side elevation of the trigger switch, and in which the second movable contact element is activated; 
         FIG. 15  is a plan view of a sliding circuit board of the bigger switch; 
         FIG. 16  is a perspective view of support portions of the trigger switch that support the first and second movable contact elements; 
         FIG. 17  is a perspective view of the first movable contact element; 
         FIG. 18  is a side elevation of the first movable contact element and its support portion; 
         FIG. 19  is a perspective view of parts of a support portion for the second movable contact element; 
         FIG. 20  is a perspective view of the second movable contact element of the trigger switch; 
         FIG. 21  is a perspective view of the auxiliary brush of the trigger switch; 
         FIG. 22  is a perspective view of the trigger switch, showing the manner in which the second movable contact element is engaged on its support portion; 
         FIG. 23  is an equivalent circuit diagram of a control system in the trigger switch; 
         FIG. 24A  is an exploded perspective view of a switching control portion of the trigger switch; 
         FIG. 24B  is a perspective view of the case of the trigger switch; 
         FIG. 25  is a plan view of the switching control portion; and 
         FIG. 26  is a vertical cross section of the prior-art trigger switch, and in which the trigger and sliding control device have been assembled. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Trigger switches according to embodiments of the present invention are hereinafter described in detail with reference to the drawings. 
     A trigger switch according to an embodiment of the present invention is shown in  FIGS. 1 ,  2 , and  3 . The trigger switch has a vertically elongated box-like form, and has a case  13 , a cover  17 , a bigger  11  capable of being manipulated by hand fingers, a switching control portion  18  mounted on the top surface of the case  13  and acting to switch the direction of the rotation of a motor, and a heat-dissipating plate  19  disposed at the position of the outer periphery of the cover  17  mounted over the case  13 . A sliding control device  12  for transferring an external manipulating action from the trigger  11  is mounted at a higher position in the case  13 , which has an open side surface. A switch mechanism is incorporated in the case  13 . The cover  17  doses the open surface at the side of the case  13  and has an open portion  15  to cause a control device (FET)  14  to be exposed to the outside. 
     As shown in  FIGS. 3 and 4 , the case  13  has a switch chamber  20  as a second chamber and a device placement portion  22  as a first chamber on which the control device  14  is placed. The switch chamber  20  has one open side. The switch mechanism is mounted in the switch chamber  20 . Two linear protrusive dustproof walls, or first dustproof wall  23   a  and second dustproof wall  23   b , are mounted on the boundary line between the device placement portion  22  and the switch chamber  20 . The side surface in communication with the switch chamber  20  is provided with a coaxial engagement hole  28  in the form of an incomplete cylinder. The sliding control device  12  has a sliding shaft  21  engaged in the coaxial engagement hole  28 . Protrusive trigger guide ribs  29   a  and  29   b  are mounted at higher and lower positions, respectively, of the coaxial engagement hole  28 . A sliding circuit board  24  is placed from above the switch chamber  20 , and the control device (FET)  14  is placed on the device placement portion  22 . Thus, the bigger switch is assembled. 
     As shown in  FIGS. 3 and 5 , the cover  17  plugs the opening at the side face of the case  13  and has the open cutout portion  15  for exposing the control device (FET)  14  equipped in the case  13 . The cover  17  has a cover portion  25  covering over the sliding circuit board  24 . A third dustproof wall  23   c  inserted between the first dustproof wall  23   a  and the second dustproof wall  23   b  is formed on the boundary line between the open portion  15  and the cover portion  25 . The first and second walls  23   a  and  23   b  are present in the case  13 . 
     The cover  17  constructed in this way is placed over the above-described case  13  in such a way that the third dustproof wall  23   c  is inserted between the first dustproof wall  23   a  and the second dustproof wall  23   b . This can eliminate any gap in the region extending from the region of the placed FET  14  to the region of the switch chamber  20 . Consequently, a dustproof wall can be provided to prevent intrusion of dust into the FET  14  that is in an open state. Intrusion of dust into the switch chamber  20  can be prevented. Furthermore, heat generated by the FET  14  can be shielded by mounting the first through third dustproof wails  23   a ,  23   b ,  23   c  between the device placement portion  22  on which the FET  14  is placed and the switch chamber  20 . Effects of heat on the switch chamber  20  can be avoided. 
     A coaxial engagement hole  26  in the form of an incomplete cylinder is formed in an upper position within the cover portion  25 . The sliding shaft  21  of the sliding control device  12  is engaged in the engagement hole  26 . Protrusive trigger guide ribs  27   a  and  27   b  are mounted and arranged vertically symmetrically with respect to the coaxial engagement hole  26 . The surface of the FET  14  disposed so as to face the open portion  15  is made flush with the sidewall surface of the cover  17 . That is, when the heat-dissipating plate  19  is mounted while the FET  14  is made to face the open portion  15 , the surface of the FET  14  can be brought into direct contact with the inner wall surface of the heat-dissipating plate  19 . 
     The heat-dissipating plate  19  shown in  FIGS. 1 ,  2 , and  3  is shaped to cover both the sidewall surface of the cover  17  and the sidewall surface of the case  13 . One surface  19   b  connected to a connecting portion  19   a  is in direct contact with the surface of the device of the FET  14  accommodated in the case. The other surface  19   c  connected to the connecting portion  19   a  is sized to cover the sidewall surface of the case  13 . Heat from the surface  19   b  in direct contact with the FET  14  is directly diffused to the surface  19   b  that covers the cover  17  and, at the same time, is diffused via the connecting portion  19   a  to the surface  19   c  that covers the sidewall surface of the case  13 . Consequently, heat generated from the FET  14  can be dissipated away uniformly. 
     As shown in  FIGS. 3 ,  6 , and  7 , the sliding control device  12  has a so-called switch mechanism. The sliding control device can implement four functions of supplying electric power to the motor in response to a manipulation of the trigger  11 , controlling the speed of the motor according to the manipulating degree of the trigger  11 , electrically shorting the power supply for the motor and supplying electric power to the motor in response to the manipulating degree of the trigger  11 , and electrically shorting the motor when the motor is at rest, in one sliding operation. These structures will be described later. 
     As shown in  FIGS. 3 ,  8 ,  9 , and  10 , the trigger  11  is formed in a semi-elliptic columnar shape, and has a sidewall having a handle portion  11   a . The trigger has an axial engagement portion  31  on the opposite side of the handle portion  11   a . The sliding shaft  21  of the sliding control device  12  engages the axial engagement portion  31 . Rib engagement portions  32   a  and  32   b  are formed on the lower and upper sides, respectively, of the axial engagement portion  31 . The rib engagement portions  32   a  and  32   b  are made of hollow cavities in which the trigger guide ribs  29   a ,  27   a  and  29   b ,  27   b  for guiding motion of the trigger can be accommodated. The trigger has an opening portion  33  located on the side of the handle portion  11 a in communication with the rib engagement portions  32   a  and  32   b . The opening portion  33  has a hollow interior and an open upper end. A trigger stopper portion  45  shaped like a rectangular parallelepiped is formed at the top of the trigger  11 . When the switching control portion  18  is in its neutral position, the trigger stopper portion  45  inhibits the trigger  11  from being pulled in. 
     The trigger  11  constructed in this way has a function of guiding motion of the trigger  11  by bringing the front side of the sliding shaft  21  of the sliding control device  12  into fitting engagement with the axial engagement portion  31  and, at the same time, causes the trigger guide ribs  29   a ,  29   b  and  27   a ,  27   b  to be received in the rib engagement portions  32   a  and  32   b , respectively. The trigger guide ribs  29   a ,  29   b ,  27   a , and  27   b  are brought into engagement with the rib engagement portions  32   a ,  32   b  by placing the upper trigger guide ribs  27   b  and  29   b  above the upper end of the rib engagement portion  32   b  with gaps therebetween as shown in  FIG. 9  and placing the lower trigger guide ribs  27   a  and  29   a  under the lower end of the rib engagement portion  32   a  with gaps therebetween. 
     When the trigger  11  constructed in this way is brought into engagement with the sliding shaft  21  and a manipulation is performed while holding the handle portion  11   a  on one hand, dust  34  accumulated inside the trigger  11  is accumulated within the opening portion  33  when a machine tool is being used as shown in  FIG. 10 . When the trigger guide ribs  27   a  and  29   a  are moved rearward along the rib engagement portion  32   a  by manipulation of the trigger  11 , the dust  34  accumulated in the opening portion  33  is expelled to the outside from the opening position (in the direction indicated by the arrow A) at the upper end of the opening portion  33  or expelled toward the body side (in the direction indicated by the arrow B) along the gaps in the rib engagement portion  32   a  with which the trigger guide ribs  27   a  and  29   a  are in engagement. 
     Dust accumulated when the trigger  11  is being used is expelled to the outside of the trigger  11  by making use of motion of the trigger guide ribs  27   a  and  29   a  in this way. Consequently, incorrect operation of the trigger  11  due to dust accumulated in the trigger  11  can be prevented. 
     As shown in  FIGS. 3 ,  6 , and  7 , the sliding control device  12  has the sliding shaft  21  shaped like a rod, a velocity control portion  37  located on the base side of the sliding shaft  21  and acting to control the rotational velocity of a motor, a control device-shorting portion  39  mounted to the velocity control portion  37 , a power-supply control portion  42  having a sliding knob  41  sliding on the movable contact element for supplying electric power to the FET that controls the motor, the knob  41  being juxtaposed to the control device-shorting portion  39 , and a motor-shorting portion  43  (see  FIG. 7 ) on the base side of the sliding shaft  21  on the opposite side of the power-supply control portion  42 . The sliding shaft  21  has a free end to which the trigger  11  can be mounted. The velocity control portion  37  has two sliders  36   a  and  36   b  disposed parallel to the top surface of the trigger. The control device-shorting portion  39  has a sliding knob  38  sliding on a movable contact element for electrically shorting the control device to a sidewall. The motor-shorting portion  43  electrically shorts the motor to brake it. 
     The motor-shorting portion  43  has a sliding knob  50   d  and a motor-shorting terminal subassembly  50 . The knob  50   d  is engaged in an engagement hole  35  via a spring  50   e . The motor-shorting terminal subassembly  50  is cantilevered over the sliding knob  50   d.    
     The terminal subassemblies made of conductive metal members and activated by the speed control portion  37 , control device-shorting portion  39 , power-supply control portion  42 , and motor-shorting circuit  43  are composed of 7 contact components, i.e., a motor driver terminal subassembly  46 , a positive power supply terminal subassembly  47 , a control device-shorting terminal subassembly  48 , a negative power supply terminal subassembly  49 , the motor-shorting terminal subassembly  50 , a first movable contact element  51 , and a second movable contact element  52  as shown in  FIG. 3 . 
     As shown in  FIGS. 3 and 11A , the motor driver terminal subassembly  46  has a first contact  53  fabricated by bending a lower portion of a flat plate member. The first contact  53  makes contact with the contact portion of the first movable contact element  51 . The driver terminal subassembly  46  has an FET contact portion  54  at a higher position. The FET contact portion  54  is connected with the source side of the FET on the sliding circuit board  24 . 
     As shown in  FIGS. 3 and 11A , the positive power supply terminal subassembly  47  has a first switching contact  55 , out of contacts switched by the switching control portion  18 , which is made of an elongated conductive member having a top portion bent into a tongue-like shape. Furthermore, the terminal subassembly  47  has a first shorting portion  56 , which is bent at a position lower than the first switching contact  55 , and a convex portion  57 , which is located at a higher position of the first shorting portion  56  and engages the sliding circuit board  24 . In addition, the terminal subassembly  47  has a diode connection portion  58  for connection with a diode  16 . The diode connection portion  58  is split into two and made to protrude away from the convex portion  57  below the convex portion  57 . Additionally, the terminal subassembly  47  has a terminal portion  59  at its lower end, the terminal portion  59  being for use for connection with the positive power supply. 
     As shown in  FIGS. 3 and 11A , the control device-shorting terminal subassembly  48  has a tongue-like second switching contact  61  out of the contacts switched by the switching control portion  18 . The tongue-like second switching contact  61  is fabricated by bending a flat plate member at right angles and bending the bent end portion through about 180 degrees outwardly. In addition, the control device-shorting terminal subassembly  48  has a second shorting portion  62  in the form of a flat plate, a second contact element  63  making contact with the contact portion of the second movable contact element  52 , an FET contact portion  64  located below and on the upstream side of the second contact element  63 , and a diode connection portion  65  located on the opposite side of the FET contact portion  64 . The second shorting portion  62  is located on the base side and forms the second switching contact element  61 . The second contact element  63  is fabricated by bending a substantially central portion of the perpendicularly bent portion on the opposite side and cutting out the central portion. The second contact element  63  electrically shorts and energizes the drain and source of the FET. The FET connection portion  64  is connected with the drain of the FET  14 . The diode connection portion  65  is used for connection with the diode  16 . 
     As shown in  FIGS. 3 and 11A , the negative power supply terminal subassembly  49  has a first movable contact element support portion  66 , a second movable contact element support portion  67  disposed on the opposite side of the first movable contact element support portion  66  and spaced from it by a distance equal to the width of the flat plate member, and a negative terminal portion  68  for connection with a negative power supply. The first movable contact element support portion  66  is fabricated by bending an upper portion of an elongated flat plate member perpendicularly. The first movable contact element  51  is swingably placed on the bent flat plate member of the first movable contact element support portion  66 . The second movable contact element  52  is swingably placed on the second movable contact element support portion  67 . The negative terminal portion  68  is located at the opposite end. 
     As shown in  FIGS. 3 ,  7 , and  11 A, the motor-shorting terminal subassembly  50  is disposed on the opposite side of the velocity control portion  37  of the sliding control device  12 . When the control device  12  is biased outwardly by the biasing force of a return spring  44 , the shorting terminal subassembly  50  electrically connects the first shorting portion  56  of the positive power supply terminal subassembly  47  and the second shorting portion  62  of the control device-shorting terminal subassembly  48  so that the electrodes of the motor are electrically shorted to each other. Thus, the motor is braked. The shorting terminal subassembly  50  is made up of a first contact portion  50   a  fabricated by arcuately shaping one end portion of a metallic flat plate member, a second contact portion  50   b  fabricated by arcuately shaping other end portion, and an engagement portion  50   c  fabricated by bending an end portion of the arcuately shaped portion in an outward direction. 
     The five contact elements shaped as mentioned above are accommodated in the case  13 . First, as shown in  FIG. 11A , the motor driver terminal subassembly  46  is inserted and mounted in the central position of the bottom of the space forming a switch mechanism as viewed from the opening of the case  13 . Then, the positive power supply terminal subassembly  47  is mounted to the front wall surface on the side of the sliding shaft  21  mounted to the case  13 . The control device-shorting terminal subassembly  48  is mounted to the rear wall surface of the case  13 . Finally, the negative power supply terminal subassembly  49  to which the first and second movable contact elements  51  and  52  have been mounted is mounted substantially in the center position of the case  13 . 
     Referring back to  FIGS. 3 ,  4  and  5 , the sliding shaft  21  is engaged in the coaxial engagement holes  26  and  28  formed by the case  13  and cover  17 . A packing accommodation portion  70  in which packing  69  is accommodated is formed in the coaxial engagement holes  26  and  28 . 
     As shown in  FIG. 11A , in the power-supply control portion  42 , the sliding knob  41  slides on the surface of the first movable contact element  51  in response to the degree of pushing motion of the sliding shaft  21  of the sliding control device  12 . The contact on the sliding knob is brought into contact with the first contact  53 , thus supplying electric power to the motor. As shown in  FIGS. 11A ,  16 ,  17 , and  18 , the first movable contact element  51  is made of an elongated conductive flat plate member. A movable contact  71  for supplying electric power is formed at one end of the elongated conductive plate member. A concave engagement portion  72  is formed at a widthwise end in a substantially central position. The first movable contact element support portion  66  has a placement portion  66   a  with which the engagement portion  72  engages. An auxiliary brush engagement portion  74  with which an auxiliary brush  73  engages is formed behind the engagement portion  72 . 
     The first movable contact element  51  constructed as described so far does not easily come off when it engages the first movable contact element support portion  66  by attaching the auxiliary brush  73 . The rear surface of the first movable contact element  51  is aligned with the position of the placement portion  66   a  of the first movable contact element support portion  66  equipped in the negative power supply terminal subassembly  49 . The auxiliary brush  73  has a small spring  76  that is inserted and mounted in the opening portion  66   b . The auxiliary brush  73  inhibits the first movable contact element support portion  66  and first movable contact element  51  from bounding and prevents poor contact. In an OFF state, the movable contact  71  of the first movable contact element  51  is located opposite to the first contact  53  of the motor driver terminal subassembly  46  placed in the case  13  (see  FIG. 11A ). 
     The first movable contact element  51  is disposed in this way. The sliding knob  41  (see  FIGS. 11A and 11B ) of the sliding control device  12  is placed on the top surface of the disposed first movable contact element  51 . A spring is incorporated in the sliding knob  41  and so the knob is kept biased. That is, when the sliding knob  41  is placed on the top surface of the first movable contact element  51 , the knob  41  biases the top surface of the first movable contact element  51 . As shown in  FIGS. 11A and 11B , when the sliding control device  12  is not operated, the first movable contact element  51  is pushed in by the return spring  44 . Therefore, the position of the sliding knob  41  is at the rearward end as viewed in  FIG. 11A  on the right side of the first movable contact element support portion  66  about which the first movable contact element  51  is seesawed. The movable contact  71  is raised upward and spaced from the first contact  53 . 
     At this time, the first contact portion  50   a  of the motor-shorting terminal subassembly  50  of the motor-shorting portion  43  mounted at a lower position in the sliding control device  12  is in contact and connected with the first shorting portion  56  of the positive power supply terminal subassembly  47 . The second shorting portion  62  of the control device-shorting terminal subassembly  48  and the second contact portion  50   b  of the motor-shorting terminal subassembly  50  are connected. The motor is electrically shorted and thus supply of electric power to the motor is cut off. 
     Under this condition, if the sliding control device  12  is pulled in, the sliding shaft  21  operates to move the first contact portion  50   a  of the motor-shorting terminal subassembly  50  of the motor-shorting portion  43  mounted at a lower position in the sliding control device  12  away from the first shorting portion  56  of the positive power supply terminal subassembly  47  as shown in  FIGS. 12A and 12B . Both of the first contact portion  50   a  and the second contact portion  50   b  are in contact and connected with the second shorting portion  62  of the control device-shorting terminal subassembly  48 . Because the first contact portion  50   a  is moved away from the first shorting portion  56 , supply of electric power to the motor is enabled. The sliding knob  41  that is a push member interlocking with the sliding shaft  21  slides on the top surface of the first movable contact element  51  and moves toward the movable contact  71 . When the sliding knob  41  goes across the first movable contact element support portion  66  of the negative terminal  40 , the first movable contact element  51  is returned in the horizontal direction. The movable contact  71  is brought into contact with the first contact  53 . This makes preparations for supply of electric power to the motor (not shown). Then, the rotational speed of the motor is controlled under control of the velocity control portion  37 . 
     As shown in  FIGS. 3 ,  6 ,  7 ,  13 , and  15 , the velocity control portion  37  consists roughly of two juxtaposed sliders  36   a  and  36   b  connected to the sliding control device  12  and a sliding circuit board  24  (see  FIG. 15 ) having sliding contact elements  81 ,  82 ,  83 , and  84  making resilient contact with the sliders  36   a  and  36   b  that interlock with the sliding control device  12 . 
     The sliding circuit board  24  has circuit elements on its front surface. The sliding circuit board  24  has sliding contact elements  81 ,  82 ,  83 , and  84  on its rear surface. The contact elements  81 - 84  make sliding contact with the sliders  36   a  and  36   b . Each of the sliders  36   a  and  36   b  is a conductive and elongated flat plate member and bifurcated on each side to form side end portions each of which is shaped arcuately as a whole. A front-end portion of each side end portion is bent upward and then bent downward to form a contact. A hole is formed in the center of the contact. A boss protruding from the base portion is engaged in this hole. 
     In the velocity control portion  37  constructed in this way, when the sliding control device  12  is manipulated by the trigger  11  against the action of the return spring  44 , the sliders  36   a  and  36   b  come into contact with the sliding contact elements  81 ,  82 ,  83 , and  84  of the sliding circuit board  24 . The degree of contact is controlled such that the rate of rotation of the motor is controlled from 0% to about 100% in relation to the state of the power switch (i.e., ON or OFF) of the power-supply control portion  42 . When the rate of rotation of the motor is about 100%, the control device-shorting portion  39  operates to control the motor to its shorted state. Consequently, about 100% of electric power is supplied to the motor. 
     As shown in  FIGS. 3 ,  13 , and  14 , the control device-shorting portion  39  activates the contact by causing the sliding knob  38  to slide on the second movable contact element  52  in the same way as the sliding knob  41  of the power-supply control portion  42 . As shown in  FIGS. 19-22 , the second movable contact element  52  is made of an elongated conductive plate member. A movable contact  85  for electrically shorting the control device is mounted at one end of the plate member. A concave engagement portion  87  is formed at a widthwise end in a substantially intermediate position. The second movable contact element support portion  67  has a placement portion  67   a  engaged in the engagement portion  87 . An auxiliary brush engagement portion  88  in which an auxiliary brush  91  is engaged is formed behind the engagement portion  87 . 
     The auxiliary brush  91  is similar in shape to the auxiliary brush  73  mounted to the first movable contact element  51  but opposite in mounting direction. The auxiliary brush  91  is mounted to prevent the second movable contact element  52  from coming off easily when it engages the second movable contact element support portion  67 . The rear surface of the second movable contact element  52  is aligned with the placement portion  67   a  of the second movable contact element support portion  67  equipped in the negative power supply terminal subassembly  49 . The auxiliary brush  91  has a small spring  92  inserted in the opening portion  67   b . The auxiliary brush  91  inhibits the second movable contact element support portion  67  and second movable contact element  52  from bounding and prevents poor contact. In an OFF state, the movable contact  85  of the second movable contact element  52  is located opposite to the second contact  63  (see  FIG. 13 ) of the control device-shorting terminal subassembly  48  disposed in the case  13 . 
     The second movable contact element  52  is disposed in this way. The sliding knob  38  of the sliding control device  12  is placed on the top surface of the disposed second movable contact element  52 . A spring is incorporated in the sliding knob  38  and so the knob can be kept biased. That is, when the sliding knob  38  is placed on the top surface of the second movable contact element  52 , the sliding knob  38  biases the top surface of the second movable contact element  52 . When the sliding control device  12  is not operated, the second movable contact element  52  is pushed in by the spring. Therefore, the position of the sliding knob  38  is at the rearward end as viewed in  FIG. 13  on the right side of the second movable contact element support portion  67  about which the second movable contact element  52  is seesawed. The movable contact  85  is raised and spaced from the second contact  63  (see  FIG. 13 ). 
     In the control device-shorting portion  39  constructed in this way, if the sliding control device  12  is first pushed under the condition shown in  FIG. 13 , the sliding knob  38  of the connected control device-shorting portion  39  moves in the same direction while sliding on the top surface of the second movable contact element  52 . Then, as shown in  FIG. 14 , if the sliding control device  12  is pushed, the sliding knob  38  passes across the position of the second movable contact element support portion  67  while sliding on the top surface of the second movable contact element  52 . Consequently, the movable contact  85  moves toward the second contact  63 . When the movable contact  85  makes contact with the second contact  63 , the control device is electrically shorted. As a result, the motor can be rotated about 100%. 
     The aforementioned switch mechanism is further described by referring to the equivalent circuit shown in  FIG. 23 . When the trigger  11  is not manipulated, the restoring force of the return spring  44  pushes the sliding control device  12 , connecting the first contact portion  50   a  and second contact portion  50   b  of the motor-shorting terminal subassembly  50  with the first shorting portion  56  and second shorting portion  62 , respectively, thus electrically shorting the electrodes of the motor. Consequently, supply of electric power to the motor is cut off. When the trigger  11  is pushed in, the sliding control device  12  connected to the trigger  11  also moves. The motor-shorting terminal subassembly  50  also moves. The first contact portion  50   a  moves away from the first shorting portion  56 . This permits supply of electric power to the motor. Furthermore, if the trigger  11  is pushed, the first movable contact element  51  is activated. This permits control of the control device. When the trigger is manipulated by a certain amount, the second movable contact element  52  is activated. About 100% of the power-supply voltage can be applied to the motor. 
     As shown in  FIGS. 24A ,  246  and  3 , the switching control portion  18  has a sectorially shaped lever  95  having a front-end portion from which a knob  96  protrudes. Furthermore, the switching control portion  18  has a substantially trapezoidal switching terminal portion  97  at a distance from the knob  96 . The terminal portion  97  is continuous with the lever  95  but recessed a certain distance from the lever  95 . In addition, the switching control portion  18  has a lever center shaft  98  protruding downward from the junction between the lever  95  and the switching terminal portion  97 . A protrusive lever  99  having a round front end is mounted on the front-end side of the lever  95  and on the opposite side of the knob  96 . 
     The switching terminal portion  97  switches the connection of the contact by causing two connection elements  101   a  and  101   b  to engage each other obliquely and rotating them. The motor is controllably rotated forward or rearward by switching the two connection elements  101   a  and  101   b  to five contacts: (1) second switching contact  61  equipped to the top portion of the control device-shorting terminal subassembly  48 , (2) first switching contact  55  equipped to the top portion of the positive power supply terminal subassembly  47 , (3) third switching contact  103  equipped to the first switching terminal subassembly  102 , (4) fourth switching contact  104  equipped to the first switching terminal subassembly  102 , and (5) fifth switching contact  106  equipped to the second switching terminal subassembly  105 . 
     The center shaft  98  of the lever equipped at the junction between the lever  95  and the switching terminal portion  97  is engaged in the center hole  107  in the case  13  and forms the center of rotation of the switching terminal portion  97 . The switching terminal portion  97  has holes  108   a ,  108   b  and grooves  109   a ,  109   b  in which the obliquely arranged connection elements  101   a  and  101   b  are engaged. Springs  110  are engaged in holes  100   a  and  100   b  formed in center positions connecting the holes  108   a ,  108   b  and grooves  109   a ,  109   b , thus biasing the center positions of the connection elements  101   a  and  101   b  toward the contact. 
     The two connection elements  101   a  and  101   b  have engagement convex portions  111   a  and  111   b  fabricated by bending both elongated end portions in the same direction almost perpendicularly. The surfaces on the opposite sides of the engagement convex portions  111   a  and  111   b  form contact surfaces which make contact with the contacts (i.e., the four contacts consisting of the fourth switching contact  104 , first switching contact  55 , filth switching contact  106 , and second switching contact  61  or the four contacts consisting of the first switching contact  55 , fifth switching contact  106 , third switching contact  103 , and second switching contact  61 ). The center positions of the engagement convex portions  111   a  and  111   b  formed on both sides are biased by the springs  110 . Consequently, the contact surfaces are kept pushed toward the contacts at all times. 
     With the switching control portion  18  constructed in this way, the connection element  101   b  is connected with the first switching contact  55  and fourth switching contact  104  by moving the knob  96  of the lever  95  with a hand in a given direction. Also, the connection element  101   a  is connected with the fifth switching contact  106  and second switching contact  61 . The connection element  101   b  is connected with the first switching contact  55  and fifth switching contact  106 , and the connection element  101   a  is connected with the third switching contact  103  and second switching contact  61  by moving the knob  96  in the opposite direction. 
     Referring to  FIG. 25 , when the lever  95  is in its neutral state, if the control portion (trigger)  11  is moved in the direction indicated by the arrow A such that the trigger is pulled in, the front end of the trigger stopper portion  45  is stopped by the protrusive lever  99 . Consequently, the trigger  11  is hindered from being pulled in.