Abstract:
In order to provide an effective technique in simplifying the vibration-proof structure of a handle equipped in a rotary power tool, a representative rotary power tool  101  is provided with a tool body  103 , a rotary-disk shaped tool bit  115 , a handle  109 , a pivot region  121  and an elastic element  129 . The handle  109  coupled to the tool body  103  is disposed in a predetermined initial position in which the handle  109  longitudinally extends parallel to the longitudinal direction of the tool body  103 . The handle  109  can rotate with respect to the tool body  103  around the pivot region  121  in any direction crossing the longitudinal direction of the tool body  103 . The elastic element  129  is disposed between the tool body  103  and the handle  109  to apply a biasing force to the handle  109  such that the elastic element  129  biases the handle  109  to return to the initial position.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a rotary power tool that performs a predetermined operation on a workpiece by rotation of a tool bit. 
   2. Description of the Related Art 
   German Patent Application No. 10248866 discloses an electric disc grinder in which a grinding wheel as a tool bit is disposed on one end of a tool body in the longitudinal direction and a handle is disposed on the other end of the tool body. The handle is coupled to the rear end of the housing via a rubber isolator. The rubber isolator is configured as a unit of multilayer structure with a combination of a plurality of plates made of rigid materials and rubber plates and disposed between the rear end surface of the housing and the front end surface of the handle. With such construction, the rubber isolator can absorb vibration caused three-dimensionally in the housing when the disc grinder is driven. As a result, the vibration transmitted from the housing to the handle can be reduced. 
   However, further improvement to the rotary power tool such as an electric disc grinder is desired with respect to the rubber isolator with relatively many component parts so as to reduce manufacturing costs of the power tool. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the invention to provide an effective technique in simplifying the vibration-proof structure of a handle equipped in a rotary power tool. 
   The object of the invention as described above is achieved by a representative rotary power tool including a tool body, a tool bit, a handle, a pivot region and an elastic element. The tool bit has a rotary-disk shape and has a rotating surface. The tool bit performs a predetermined operation on a workpiece by a rotating movement around a rotating axis that extends perpendicular to the rotating surface. The tool bit is disposed on one tip end region of the tool body such that the rotating axis extends to cross the longitudinal direction of the tool body. The handle is coupled to the tool body at the end region of the tool body opposite to the region where the tool bit is mounted. The handle is disposed in a predetermined initial position in which the handle longitudinally extends parallel to the longitudinal direction of the tool body. The pivot region is provided between the tool body and the handle. The handle can rotate with respect to the tool body around the pivot region in any direction crossing the longitudinal direction of the tool body. The elastic element is disposed between the tool body and the handle. The elastic element applies a biasing force to the handle rotated around the pivot region with respect to the tool body such that the elastic element biases the handle to return to the initial position. 
   The “rotary power tool” according to the invention can be suitably applied to a grinder which performs grinding or cutting operation on a workpiece by rotating a grinding wheel or a polisher which performs polishing operation on a workpiece by rotating a pad. The manner in which the handle “extends in the longitudinal direction of the tool body” widely includes the manner in which the handle extends generally linearly in the longitudinal direction of the tool body, the manner in which the handle extends in a curved manner in the longitudinal direction of the tool body, as well as the manner in which the handle extends linearly with a slight inclination in the longitudinal direction of the tool body. 
   Further, “any direction” in which the handle may rotate according to the invention means any vertical or lateral direction as viewed from the longitudinal direction of the body and is also refereed to as all directions. Further, the “initial position” is a position in which the handle is standing still, and more specifically a position in which the biasing force of the elastic element does not act upon the handle as a force of rotating the handle. The “elastic element” may comprise a rubber or a spring. 
   According to the invention, vibration transmitted from the tool body to the handle can be efficiently reduced. Further, vibration-proof structure of the handle can be realized with a simple structure in which the elastic element is disposed between the tool body and the handle that is rotatably coupled to the tool body. Other objects, features and advantages of the present invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view, partially in section, showing an entire electric disc grinder according to an embodiment of the invention. 
       FIG. 2  is a plan view, partially in section, also showing the entire electric disc grinder. 
       FIG. 3  is an enlarged view of circled part A in  FIG. 1 . 
       FIG. 4  is an enlarged view of circled part B in  FIG. 2 . 
       FIG. 5  is a sectional plan view showing the vibration-proof structure of a main handle. 
       FIG. 6  is an enlarged view of circled part C in  FIG. 5 . 
       FIG. 7  is a sectional view taken along line VII—VII in  FIG. 1 . 
       FIG. 8  is a sectional view taken along line IIX—IIX in  FIG. 1 . 
       FIG. 9  is a sectional view showing a lock lever for locking the main handle against rotation. 
       FIG. 10  is a perspective view showing an entire motor housing. 
       FIG. 11  is a front view showing the entire motor housing. 
       FIG. 12  is an enlarged view of circled part D in  FIG. 11 . 
       FIG. 13  is a sectional view showing a rubber isolator. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Each of the additional features and method steps disclosed above and below may be utilized separately or in conjunction with other features and method steps to provide and manufacture improved rotary power tools and method for using such rotary power tools and devices utilized therein. Representative examples of the present invention, which examples utilized many of these additional features and method steps in conjunction, will now be described in detail with reference to the drawings. This detailed description is merely intended to teach a person skilled in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed within the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe some representative examples of the invention, which detailed description will now be given with reference to the accompanying drawings. 
   An embodiment of the present invention will now be described with reference to FIGS.  1  to  13 . The embodiment of the present invention will be explained as to an electric disc grinder  101  as a representative example of a rotary power tool.  FIGS. 1 and 2  show the entire disc grinder having a vibration-proof main handle.  FIGS. 3 to 9  show essential parts of the present invention in section.  FIGS. 10 and 11  show a motor housing in its entirety.  FIG. 12  is an enlarged view of circled part D in  FIG. 11 .  FIG. 13  shows a rubber isolator in section. As shown in  FIGS. 1 and 2 , the electric disc grinder  101  hereinafter referred to as grinder) has a body  103  that includes a motor housing  105  and a gear housing  107 . The body  103  is a feature that corresponds to the “tool body” in the present invention. The motor housing  105  is generally cylindrically formed (see  FIG. 10 ) and houses a driving motor  111 . The driving motor  111  is arranged such that its axis of rotation extends in the longitudinal direction of the grinder  101  or the longitudinal direction of the body  103 . 
   A power transmitting mechanism (not shown) is disposed within the gear housing  107  that is coupled to the front end of the motor housing  105  and serves to transmit the rotating output of the driving motor  111  to a grinding wheel  115 . The grinding wheel  115  is having a substantially rotary disk shape with rotating surface  115   a  and a rotating circumference  115   b . The grinding wheel  115  coupled to the gear housing  107  rotates around a rotating axis  116  such that rotating surface  115   a  performs a predetermined grinding operation to the work, otherwise the rotating circumference  115   b  performs a predetermined cutting operation to the work. 
   The grinding wheel  115  is a feature that corresponds to the “tool bit” in the present invention. The rotating output of the driving motor  111  is transmitted to the grinding wheel  115  as rotation in the circumferential direction via the power transmitting mechanism. The grinding wheel  115  is disposed on one end (the front end) of the disc grinder  101  in the longitudinal direction such that its axis of rotation is perpendicular to the longitudinal direction of the body  103  (the axis of rotation of the driving motor  111 ). Further, a main handle  109  is coupled to the other end (the rear end) of the motor housing  105 . The main handle  109  is a feature that corresponds to the “handle” in the present invention. 
   The main handle  109  is disposed such that its longitudinal direction coincides with the longitudinal direction of the body  103 . In other words, the main handle  109  extends generally linearly in the longitudinal direction of the body  103 . The main handle  109  as shown in  FIG. 1  is corresponding to “handle disposed in a predetermined initial position in which the handle longitudinally extends parallel to the longitudinal direction of the tool body” according to the invention. 
   Further, in the case of the grinder  101  of a large size (which is not shown), in addition to the main handle  109 , an auxiliary handle is provided which is removably mounted on the side or upper surface of the gear housing  107  such that its longitudinal direction is generally perpendicular to the longitudinal direction of the body  103 . User holds the main handle  109  and the auxiliary handle by hands when grinding or cutting a workpiece by rotation of the grinding wheel  115 . 
   Next, the vibration-proof structure of the main handle  109  will be explained with reference to  FIGS. 3 to 8 . The main handle  109  is a hollow cylindrical member, and its front end is coupled to the rear end of the motor housing  105  which forms the body  103 , via a spherical portion  123  and a spherical concave portion  125 . The concave portion  125  engages with the spherical portion  123  such that it can rotate with respect to the spherical portion  123 . The spherical portion  123  and the concave portion  125  form a coupling region  121  for coupling the main handle  109  to the body  103 . The spherical portion  123  and the concave portion  125  that form the coupling region  121  engage in sliding contact with each other, and the axial direction of the spherical portion  123  coincides with the longitudinal direction of the body  103 . 
   The spherical portion  123  is integrally formed with the motor housing  105 . Specifically, a hollow cylindrical portion  127  is integrally formed with the motor housing  105  and extends a predetermined length rearward from the rear end of the motor housing  105 . Further, the spherical portion  123  is contiguously formed with the rear end of the cylindrical portion  127  (see  FIG. 10 ). The outer surface of the spherical portion  123  comprises a spherical surface  123   a  having a radius R with its center P on the axis of the spherical portion  123 . Further, the outer diameter of the cylindrical portion  127  is smaller than the outer diameter of the motor housing  105 . Thus, a vertical end surface  105   a  is formed on the border between the motor housing  105  and the cylindrical portion  127  and extends perpendicularly to the motor housing  105 . 
   The concave portion  125  is integrally formed with the main handle  109 . Specifically, the main handle  109  includes a grip  109   a  to be held by the user and an enlarged portion  109   b . The enlarged portion  109   b  is formed forward of the grip  109   a  and enlarged forward in a generally flared manner. The concave portion  125  is integrally formed on the inside surface of the enlarged portion  109   b . In this embodiment, as shown in  FIG. 6 , the concave portion  125  comprises two annular ribs  125   a  that extend (protrude) a predetermined length inward from the inside surface of the enlarged portion  109   b . Each of the annular ribs  125   a  has an inner surface of a spherical shape which is complementary to the spherical surface  123   a  of the spherical portion  123 , and is slidably engaged with the spherical surface  123   a  of the spherical portion  123 . The two annular ribs  125   a  are disposed in parallel to each other with a predetermined spacing in the axial direction of the spherical portion  123 . A pocket  125   b  is defined between the annular ribs  125   a.    
   As shown in  FIGS. 7 and 8 , the main handle  109  has a two-part structure which is divided into halves along a vertical plane on which the axis of the main handle  109  runs. Specifically, the main handle  109  comprises halves  109 A and  109 B. The halves  109 A and  109 B are butted against each other in such a manner that the concave portion  125  covers the spherical portion  123 . In this state, the halves  109 A,  109 B are clamped together by through bolts  141  (see  FIG. 8 ) at several predetermined points in order to mount the main handle  109  to the motor housing  105 . The main handle  109  is connected to the motor housing  105  via the coupling region  121  and can rotate about the center P of the sphere of the spherical portion  123  in any vertical or lateral direction (all directions) as viewed from the longitudinal direction of the body  103 . As shown in  FIG. 6 , a projection  123   b  to define the range of relative rotation of the main handle  109  is formed on the spherical surface  123   a  of the spherical portion  123  such that it is located in the pocket  125   b  between the annular ribs  125   a . The projection  123   b  prevents the relative rotation of the main handle  109  by contact with the annular ribs  125   a.    
   As shown in  FIGS. 3 to 5 , a rubber isolator  129  is disposed between the main handle  109  and the motor housing  105  and applies a spring force to the main handle  109  against rotation of the main handle  109  in any direction with respect to the motor housing  105 . The rubber isolator  129  is disposed in a region closer to the motor housing than the spherical portion  123  in a longitudinal direction “L” of the grinder  101  (see  FIG. 1 ). The rubber isolator  129  is a feature that corresponds to the “elastic element” according to the invention. The rubber isolator  129  has a generally ring-like shape (see  FIG. 13 ) and is fitted on the cylindrical portion  127  of the motor housing  105  such that it can rotate in the circumferential direction with respect to the motor housing  105 . As shown in  FIG. 6 , one axial end (front end) of the rubber isolator  129  is in contact with the rear end surface  105   a  of the motor housing  105 , while the other axial end (rear end) is in contact with a front end surface  109   c  of the main handle  109 . Further, a flange  129   a  is formed on one axial end (front end) of the rubber isolator  129  and engages with an annular groove  127   a  of the cylindrical portion  127 . On the other axial end (rear end), the rubber isolator  129  has a flange  129   b  that engages with an annular groove  109   d  of the main handle  109 . In this manner, the rubber isolator  129  is securely fixed in the axial direction with respect to the motor housing  105  and the main handle  109 . 
   The rubber isolator  129  is mounted on the motor housing  105  prior to the process of coupling the main handle  109  to the motor housing  105 . The rear flange  129   b  is engaged with the annular groove  109   d  of the main handle  109  when the main handle  109  is coupled to the motor housing  105 . Further, an annular groove  129   c  is formed around the inner surface of the rubber isolator  129  and serves to control the coefficient of elasticity of the rubber isolator  129 . A projection  127   b  is formed on the outer surface of the cylindrical portion  127  and engages with the annular groove  129   c . Thus, the rubber isolator  129  is prevented from moving to the main handle  109  side. 
   Further, the spherical portion  123  is hollow having a through hole  123   c  that extends axially through the spherical portion  123 . The inner space of the motor housing  105  communicates with the inner space of the main handle  109  via the through hole  123   c  (see  FIGS. 3 to 5 ). Vents  109   e  for air intake are formed in the enlarged portion  109   b  of the main handle  109 . When the driving motor  111  is driven, air is taken in (sucked) through the vents  109   e  by a cooling fan (not shown). The intake air is then led into the motor housing  105  through the through hole  123   c  and cools the driving motor  111  within the motor housing  105 . Thereafter, the air is discharged from the gear housing  107  to the outside. Here, the through hole  123   c  of the spherical portion  123  serves as a ventilation passage for introducing cooling air into the motor accommodation space within the motor housing  105 . Further, as shown in  FIGS. 1 and 3 , a power switch  119  is disposed within the main handle  109  and actuated by the trigger  117  in order to start or stop the driving motor  111 . The power switch  119  is connected to the driving motor  111  by a wire (not shown) installed through the through hole  123   c . Thus, the through hole  123   c  also serves as a wiring passage for the wires that connect electrical components disposed within the motor housing  105  and electrical components disposed within the main handle  109 . 
   The main handle  109  is coupled to the motor housing  105  via the spherical portion  123  and the concave portion  125 . The main handle  109  can rotate around the axis of the spherical portion  123  in the circumferential direction with respect to the motor housing  105 . On the other hand, however, if the main handle  109  may freely rotate around the longitudinal axis of the body  103 , the orientation of the handle  109  (the direction of the grip) and the orientation of the grinding wheel  115  (the direction of the axis of rotation) will change and not accord with respect to each other, resulting that ease of use will be impaired. Therefore, in order to prevent such free rotation of the main handle  109  in the circumferential direction, as shown in  FIGS. 3 and 9 , a lock lever  131  is provided in the main handle  109 . The lock lever  131  is a feature that corresponds to the “rotation preventing member” according to the invention. The lock lever  131  is disposed in a region of the enlarged portion  109   b  which is located on the lower side when the user grips the main handle  109  with the longitudinal axis of the body  103  held in a horizontal position. Further, the lock lever  131  is vertically rotatably mounted to the main handle  109  via a support shaft  135  that extends in a horizontal direction crossing the axial direction of the main handle  109 . A generally rectangular engagement portion  133  is provided on one end portion (front end portion) of the lock lever  131  and protrudes inward. The engagement portion  133  can engage with an engagement groove  137  that is formed in the outer rear end of the spherical portion  123 , so that the main handle  109  is locked against rotation around the axis of the spherical portion  123 . Thus, the orientation of the handle  109  and the orientation of the grinding wheel  115  can be held constant with respect to each other. 
   The engagement groove  137  is configured such that the engagement portion  133  is substantially in point contact with both circumferential side wall surfaces  137   a  and a bottom  137   b  of the engagement groove  137  when the main handle  109  is prevented from rotating in the circumferential direction by engagement of the engagement portion  133  with the engagement groove  137  (as shown in  FIG. 3 ). Specifically, as shown in  FIG. 12 , the engagement groove  137  extends a predetermined length in the axial direction of the spherical portion  123 . Each of the side wall surfaces  137   a  comprises an inclined surface having a predetermined inclination θ with respect to a line X orthogonal to the axis of the spherical portion  123  such that the width of the engagement groove  137  is at the minimum in the middle in the length direction and at the maximum on the both ends. Preferably, the inclination θ of the inclined surface may be about 1 to 5 degrees. Further, the bottom  137   b  of the engagement groove  137  comprises a spherical surface which is concentrically formed with the spherical surface of the spherical portion  123  around the center P. With such construction, the engagement portion  133  engages with the engagement groove  137  substantially in point contact with the both circumferential side wall surfaces  137   a  and the bottom  137   b  of the engagement groove  137 . As a result, the main handle  109  is allowed to rotate on the center P of the spherical portion  123  in all directions with respect to the motor housing  105  while being held prevented from rotating around the axis of the spherical portion  123  by the lock lever  131 . 
   Generally, the grinder  101  may be used not only for grinding but for cutting a workpiece. In this case, a flat surface region (rotating surface  115   a  as shown in  FIG. 1 ) of the grinding wheel  115  is mainly used to grind a workpiece, while a peripheral edge region (rotating circumference  115   b  as shown in  FIG. 1 ) of the grinding wheel  115  is used to cut a workpiece. Specifically, the grinding operation is performed with the grinding wheel  115  held in a position in which rotating axis  116  of the grinding wheel  115  crosses the work surface of the workpiece, while the cutting operation is performed with the grinding wheel  115  held in a position in which the rotating axis  116  is parallel to the work surface of the workpiece. Thus, the orientation of the grinding wheel  115  is changed about 90 degrees according to whether a grinding operation or a cutting operation is performed. At this time, according to this representative embodiment, the direction (orientation) in which the user grips the main handle  109  is also changed about 90 degrees, so that ease of use is impaired. 
   Therefore, in accordance with the operation mode, the user can rotate the main handle  109  in order to change orientation of the main handle  109  between a grinding position in which the user holds the main handle  109  (or applies a grip) in a direction parallel to the rotating axis  116  (see  FIG. 1 ) of the grinding wheel  115  and a cutting position in which the user holds the main handle  109  in a direction perpendicular to the rotating axis  116  ( FIG. 1 ) of the grinding wheel  115 , or a position in which the grinder is turned about 90 degrees clockwise or counterclockwise from the grinding position. The grinding position and the cutting position respectively correspond to the “rotating position” according to the invention. For the purpose of such change of orientation of the main handle  109 , as shown in  FIGS. 7 and 10 , three engagement grooves  137 ,  137 A,  137 B are provided in the spherical portion  123  and the lock lever  131  can engage with and disengage from each of the engagement grooves. Of the three engagement grooves, the engagement groove  137  is provided for grinding operation and located on the lower side (in the middle between the other two engagement grooves) when the body  103  is held in a horizontal position. The other two engagement grooves  137 A,  137 B are provided for cutting operation and spaced 90 degrees apart from the middle engagement groove  137  in the opposite circumferential directions. The side walls and the bottom of the engagement grooves  137 A,  137 B for cutting operation are identically configured with those of the engagement groove  137  for grinding operation. 
   Further, when the user rotates the main handle  109 , the rubber isolator  129  rotates together with the main handle  109 . Specifically, it is configured such that the engaging force between the front flange  129   a  of the rubber isolator  129  and the annular groove  127   a  of the cylindrical portion  127  is weaker than the engaging force between the rear flange  129   b  and the annular groove  109   d  of the main handle  109 . Thus, the rubber isolator  129  is caused to rotate with respect to the motor housing  105 . Further, the engagement portion  133  engages with the engagement groove  137  when the user presses one end portion (front end portion) of the lock lever  131 , while it disengages from the engagement groove  137  when the user presses the other end portion (rear end portion) of the lock lever  131 .  FIG. 3  shows the state of engagement between the engagement portion  133  and the engagement groove  137  and  FIG. 9  shows the state of disengagement. 
   Further, a rotation stopper  139  in the form of a projection is provided near each of the engagement grooves  137 A,  137 B for cutting operation (see  FIG. 10 ). The rotation stopper  139  contacts the lock lever  131  when the main handle  109  is rotated in the circumferential direction and thus prevents the main handle  109  from rotating over 360 degrees (in the neighborhood of about 290 degrees in this embodiment). 
   Operation and usage of the grinder  101  having the above-mentioned construction according to the representative embodiment will now be explained. During operation by the grinder  101 , vibration is caused in the body  103  in different directions. The vibration which has been caused in the body  103  is absorbed, when transmitted to the main handle  109 , by the construction in which the main handle  109  can rotate in all directions with respect to the motor housing  105  via the spherical portion  123  and the concave portion  125  and by elastic deformation of the rubber isolator  129  against such relative rotation of the main handle  109 . Thus, the vibration which is transmitted from the body  103  to the main handle  109  can be reduced, so that usability of the main handle  109  can be enhanced. 
   Further, when the engagement portion  133  is disengaged from the engagement groove  173 ,  173 A or  173 B by rotating the lock lever  131  around the support shaft  135 , rotation of the main handle  109  is allowed. In this state, the main handle  109  can be rotated on the axis of the spherical portion  123  in the circumferential direction. Therefore, the user can selectively change the position of the main handle  109  according to the working condition between a grinding position and a cutting position. Thereafter, the user can lock the main handle  109  in that selected position by engaging the engagement portion  133  of the lock lever  131  with one of the engagement grooves  173 ,  173 A,  173 B which is assigned to the selected position. As a result, the working operation can be performed with improved ease-of-use of the main handle  109 . 
   Thus, the function of isolating vibration of the main handle  109  can be obtained with a simple construction in which the main handle  109  is coupled to the motor housing  105  such that it can rotate in all directions with respect to the motor housing  105  via the spherical portion  123  and the concave portion  125  and in which the rubber isolator  129  is disposed between the motor housing  105  and the main handle  109 . Further, the function of adjusting the orientation of the main handle  109  with respect to the grinding wheel  115  can be obtained by selectively engaging the lock lever  131  with the engagement groove  137 . In this case, the coupling region  121  comprises the spherical portion  123  integrally formed with the motor housing  105  and the concave portion  125  that is integrally formed with the main handle  109 , so that the parts count can be reduced. 
   Further, the main handle  109  is prevented from moving in the longitudinal direction of the body  103  with respect to the body  103  because the main handle  109  is coupled to the motor housing  105  via the spherical portion  123  and the concave portion  125 . Therefore, when the user performs any working operation with moving the main handle  109  in the longitudinal direction of the body  103 , the body  103  can integrally move together with the main handle  109  in the longitudinal direction and thus, excellent feel of use can be obtained. 
   Further, with the construction in which the main handle  109  is prevented from rotating in the circumferential direction by the lock lever  131  while being allowed to rotate on the center P of the spherical portion  123  in all directions with respect to the motor housing  105 , the orientation of the main handle  109  and the orientation of the grinding wheel  115  can always be held in a fixed positional relationship with respect to each other. As a result, the vibration isolating effect of the main handle  109  can be obtained without impairing ease of use. 
   Further, the hollow configuration of the spherical portion  123  can effectively provide a passage of air for cooling the driving motor  111  and a passage for wiring. Further, with the construction in which the rubber isolator  129  is disposed on the cylindrical portion  127  that is contiguous to the spherical portion  123 , the distance from the center P of the spherical portion  123  to the rubber isolator  129  can be made longer. In other words, the rubber isolator  129  is disposed in a position in which the vibration amplitude of the main handle  109  increases, so that the rubber isolator  129  can efficiently absorb vibration. For example, if the rubber isolator  129  is placed nearer to the axis of the spherical portion  123  and at a longer distance from the center P (at a location in which the vibration amplitude is increased to a maximum), the trigger  117  or the power switch  119  will interfere with the rubber isolator  129 . Therefore, the main handle  109  must be elongated in the axial direction, which results in increase of the whole length of the grinder  191 . According to the representative embodiment, however, such a problem does not arise. 
   Further, the configuration of the concave portion  125  that comprises the plurality of annular ribs  125   a  can ensure a necessity minimum of the area of contact with the spherical portion  123  and permit reduction of the wall thickness of the main handle  109 . Moreover, rotation of the main handle  109  with respect to the motor housing  105  can be stabilized. Furthermore, the contact surface of the concave portion  125  with respect to the spherical portion  123  can be narrowed in the axial direction, so that dusts which have entered between the contact surfaces of the spherical portion  123  and the concave portion  125  can be easily let out. 
   Further, the rubber isolator  129  is secured to the main handle  109  and the motor housing  105  by engagement between the flanges  129   a ,  129   b  and the annular grooves  109   d ,  127   a . With this construction, the numbers of parts and manufacturing man-hours can be reduced, compared with a known construction in which a resin fixing component is adhered to the rubber isolator and then fixed to the main handle and the motor housing. 
   Further, because the coefficient of elasticity of the rubber isolator  129  can be appropriately adjusted by the annular groove  129   c  and the annular groove  129   c  is formed on the inner surface of the rubber isolator  129 , the freedom of design of the outer surface of the rubber isolator  129  can be increased. Further, the rubber isolator  129  rotates together with the main handle  109  when the user rotates the main handle  109  around the axis of the spherical portion  123  in order to change orientation of the main handle  109 . With this construction, the distance from the contact surfaces of the rubber isolator  129  and the motor housing  105  in the axial direction to the sliding contact surfaces of the spherical portion  123  and the concave portion  125  can be gained. As a result, even if dusts enter through a clearance between the contact surfaces, the dusts do not easily reach as far as the sliding contact surfaces. 
   Further, while the structure for engagement between the spherical portion  123  and the concave portion  125  is described as a spherical surface sliding structure having a sliding contact surface, it may be a spherical surface rolling structure having a rolling contact surface. Further, while the spherical portion  123  of the coupling region  121  is described as being formed on the motor housing  105  and the concave portion  125  on the main handle  109 , the spherical portion  123  may be formed on the main handle  109  and the concave portion  125  on the motor housing  105 . Further, the coupling region  121  may be formed separately from the motor housing  105  and the main handle  109 . 
   Further, the invention may be applied to any other rotary power tool which performs an operation on a workpiece by rotation of a tool bit such as a polisher to perform a polishing operation. 
   It is explicitly stated that all features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original disclosure as well as for the purpose of restricting the claimed invention independent of the composition of the features in the embodiments and/or the claims. It is explicitly stated that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure as well as for the purpose of restricting the claimed invention, in particular as limits of value ranges. 
   DESCRIPTION OF NUMERALS 
   
       
         101  electric disc grinder (rotary power tool) 
         103  body (tool body) 
         105  motor housing 
         105   a  end surface 
         107  gear housing 
         109  main handle 
         109   a  grip 
         109   b  enlarged portion 
         109   c  end surface 
         109   d  annular groove 
         109   e  vent 
         111  driving motor (motor) 
         115  grinding wheel (tool bit) 
         117  trigger 
         119  power switch 
         121  coupling region 
         123  spherical portion 
         123   a  spherical surface 
         123   b  projection 
         123   c  through hole 
         125  concave portion 
         125   a  annular rib 
         125   b  pocket 
         127  cylindrical portion 
         127   a  annular groove 
         127   b  projection 
         129  rubber isolator (elastic element) 
         129   a  front flange 
         129   b  rear flange 
         129   c  annular groove 
         131  lock lever (rotation preventing member) 
         133  engagement portion 
         135  support shaft 
         137  engagement groove 
         137 A engagement groove 
         137 B engagement groove 
         137   a  side wall surface 
         137   b  bottom 
         139  rotation stopper 
         141  through bolt