Abstract:
The present invention relates to an eccentric stroke adjusting mechanism for use in a power tool with a principle drive shaft comprising a first and a second eccentric member mounted on the principle drive shaft respectively and a coupling member for connecting the first and the second eccentric member. The principle drive shaft has a central axis. The first eccentric member has a first central axis and the second eccentric member has a second central axis. The eccentric stroke of the first central axis and the second central axis with respect to a central axis of the principle drive shaft is adjustable. The eccentric stroke adjusting mechanism of the invention is reliable and the adjusted eccentricity is non-displaceable. The eccentric stroke adjusting mechanism can be applied to adjust the eccentric stroke of single disk or multiple (e.g., double) disks.

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention relates to an eccentric stroke adjusting mechanism for use in a power tool and to a power tool per se. 
   2. Background Art 
   As illustrated in U.S. Pat. No. 4,744,177 and EP-A-0820838, known abrasive power tools such as sanders and grinders generally comprise a housing, a motor vertically located inside the housing, a principle drive shaft, a working (sanding) plate and an eccentric stroke adjusting mechanism. The eccentric stroke adjusting mechanism is used to adjust the vibration amplitude of the working (sanding) plate to meet different requirements of different workpieces. The eccentric stroke adjusting mechanism comprises an eccentric shaft securely attached to the principle drive shaft, an eccentric driving device and a bearing for connecting the eccentric driving device and the working (sanding) plate. However such an eccentric stroke adjusting mechanism is not easily and reliably balanced and the adjusted eccentricity is easy to displace. Furthermore the eccentric stroke adjusting mechanism can only be applied to adjust single disk working (sanding) plate. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a reliable and widely applicable eccentric stroke adjusting mechanism. 
   In an embodiment, the present invention provides an eccentric stroke adjusting mechanism for use in a power tool with a principle drive shaft. The eccentric stroke adjusting mechanism may comprise a first and a second eccentric member mounted on the principle drive shaft and a coupling member for coupling the first and the second eccentric members. The first eccentric member may have a first central axis and the second eccentric member may have a second central axis. The eccentricity of the first central axis and the second central axis with respect to a central axis of the principle drive shaft is adjustable. 
   The eccentric stroke adjusting mechanism of the invention advantageously prevents adjusted eccentricity from being easily displaced and is more reliable. More particularly, the relationship and co-action between the first eccentric member, the second eccentric member and the coupling member prevents the adjusted eccentricity from displacement. The eccentric stroke adjusting mechanism can be applied to adjust the eccentric stroke of a single disk or multiple (e.g., double) disks. 
   Viewed from a first aspect, the present invention provides an eccentric stroke adjusting mechanism for use in a power tool, the eccentric stroke adjusting mechanism comprising:
         a principle drive shaft with a central axis;   a first eccentric member mounted radially on the principle drive shaft, the first eccentric member having a first central axis;   a second eccentric member mounted radially on the principle drive shaft, the second eccentric member having a second central axis; and   a coupling member for coupling the first eccentric member to the second eccentric member, wherein the eccentricity of the first central axis and the eccentricity of the second central axis with respect to the central axis of the principle drive shaft are adjustable.       

   In a preferred embodiment, the first eccentric member has a first eccentric sleeve and the second eccentric member has a second eccentric sleeve. The first eccentric sleeve of the first eccentric member and the second eccentric sleeve of the second eccentric member are rotatable relative to the principle drive shaft, wherein a central axis of the first eccentric sleeve is the first central axis of the first eccentric member and a central axis of the second eccentric sleeve is the second central axis of the second eccentric member. 
   Particularly, the first eccentric member preferably has a first eccentric shaft, the second eccentric member has a second eccentric shaft and the first eccentric shaft and second eccentric shaft are securely mounted radially on the principle drive shaft, wherein the first eccentric sleeve and the second eccentric sleeve are rotatably mounted on the first eccentric shaft and the second eccentric shaft respectively, wherein a central axis of the first eccentric shaft and a central axis of the second eccentric shaft are eccentric with respect to the central axis of the principle drive shaft, wherein the central axis of the first eccentric sleeve is eccentric with respect to the central axis of the first eccentric shaft and to the central axis of the principle drive shaft and wherein the central axis of the second eccentric sleeve is eccentric with respect to the central axis of the second eccentric shaft and to the central axis of the principle drive shaft. 
   A first sanding plate may be operably connected or coupled to the first eccentric sleeve so that in practice the central axis of the first eccentric sleeve is the central axis of the first eccentric member and of the first sanding plate. Similarly a second sanding plate may be operably connected or coupled to the second eccentric sleeve so that in practice the central axis of the second eccentric sleeve is the central axis of the second eccentric member and of the second sanding plate. 
   Preferably, the central axis of the first eccentric sleeve and the central axis of the second eccentric sleeve are on opposite sides of the central axis of the principle drive shaft. The central axis of the first eccentric sleeve and the central axis of the second eccentric sleeve may be parallel to the central axis of the principle drive shaft. The central axis of the first eccentric sleeve, the central axis of the second eccentric sleeve and the central axis of the principle drive shaft may be in a common plane. The central axis of the first eccentric sleeve and the central axis of the second eccentric sleeve may be equidistant from the central axis of the principle drive shaft. The central axis of the first eccentric sleeve and the central axis of the second eccentric sleeve may be angularly displaced relative to the central axis of the principle drive shaft by 180°. 
   Preferably, the central axis of the first eccentric shaft and the central axis of the second eccentric shaft are on opposite sides of the central axis of the principle drive shaft. The central axis of the first eccentric shaft and the central axis of the second eccentric shaft may be parallel to the central axis of the principle drive shaft. The central axis of the first eccentric shaft, the central axis of the second eccentric shaft and the central axis of the principle drive shaft may be in a common plane. The central axis of the first eccentric shaft and the central axis of the second eccentric shaft may be equidistant from the central axis of the principle drive shaft. The central axis of the first eccentric shaft and the central axis of the second eccentric shaft may be angularly displaced relative to the central axis of the principle drive shaft by 180°. 
   In an embodiment, the first eccentric sleeve and second eccentric sleeve may be substantially cylindrical and may terminate in a radial collar. 
   A bearing may be tightly mounted radially on the second eccentric sleeve. A bearing seat may be mounted around the perimeter of the bearing. A sanding plate may be fastened to the bearing seat. 
   Preferably, the coupling member is securely mounted on the first eccentric shaft and has a first pin on an upper surface and a second pin on a lower surface, wherein each of the first eccentric sleeve and the second eccentric sleeve has a radial slot to receive the first pin and second pin respectively, wherein the width of the slot approximates to the diameter of the pin. To mount the coupling member on the first eccentric shaft, the first eccentric shaft may comprise an annular protrusion on its lower face. 
   The coupling member preferably has an operating body rotatably mounted on the principle drive shaft and an extension pin extending axially from the operating body, wherein each of the first eccentric sleeve and the second eccentric sleeve has a receiving bore for receiving the extension pin, wherein the diameter of the receiving bore approximates to the diameter of the extension pin and the receiving bore is longer than the extension pin. 
   Preferably, a plurality of locating sockets are formed on the outer surface of the first eccentric shaft and of the second eccentric shaft and a plurality of receiving recesses are formed on the inner circumferential surface of the first eccentric sleeve and of the second eccentric sleeve, wherein in each receiving recess is seated an elastic element (e.g., a spring) connected to a locating post, wherein each locating post is selectively received in a locating socket so as to restrainedly couple the first and the second eccentric sleeve with the first and the second eccentric shaft respectively. 
   Viewed from a further aspect, the present invention provides a power tool comprising:
         a housing;   a rotary motor in the housing; and   an eccentric stroke adjusting mechanism as hereinbefore defined.       

   Preferably, the power tool further comprises a first sanding plate connected or coupled to the first eccentric member. The power tool may further comprise a second sanding plate connected or coupled to the second eccentric member. 
   The first sanding plate and second sanding plate may terminate at the base of housing. The first sanding plate and second sanding plate may be an outer sanding and an inner sanding plate. The or each sanding plate may be annular (e.g., stepped annular). 
   Preferably, the power tool further comprises a principle drive shaft locking device. In an embodiment, the principle drive shaft locking device may comprise:
         a chuck mounted radially on the principle drive shaft, wherein the chuck has a skirt extending axially downwardly from its circumferential edge, wherein a plurality of recesses are located around the skirt; and   a locking member attached to the housing, wherein the locking member is selectively insertable into a recess to lock the chuck and prevent the principle drive shaft from rotating.
 
More preferably, the power tool may comprise: a balancing drum, wherein the chuck has a central aperture surrounded by an eccentric hub and the balancing drum is securely mounted on the eccentric hub.
       

   A plurality of spaced apart location holes may be formed around the eccentric hub. The balancing drum may comprise a central aperture bound by a hub. 
   In an embodiment of the present invention, the power tool further comprises:
         a clutch securely mounted radially on the first eccentric member (e.g., the first eccentric sleeve), wherein the clutch comprises an annular main body, wherein the annular main body has an inner circumferential surface with a plurality of first truncated conical recesses formed thereon and an outer circumferential surface with a plurality of second truncated conical recesses formed thereon, wherein in each first truncated conical recess and second truncated conical recess is seated an elastic element (e.g., a spring) connected to a roller whereby the elastic element urges the roller outwardly, wherein when in use the principle drive shaft rotates the rollers in the inner circumferential surface of the clutch securely engage the principle drive shaft and the rollers in the outer circumferential surface of the clutch disengage the first sanding plate.       

   The annular main body may abut an end face of the balancing drum (e.g., the hub). The balancing drum may have a part radial recess extending from the hub. A spring and a ball head locating post connected to the spring may be disposed in the recess. The ball head locating post may be urged partly into a location hole on the eccentric hub to restrainedly couple the balancing drum and the chuck. 
   The annular main body may abut an end face of the chuck (e,g, the eccentric hub). The annular main body may have a radial recess. A spring and a ball head locating post connected to the spring may be disposed in the recess. The ball head locating post may be urged partly into a location hole on the eccentric hub to restrainedly couple the clutch and the chuck. 
   In the base of each truncated conical recess, there may be a narrow receiving bore. The elastic element may be received in the receiving bore. An axial projection may extend from the lower face of the annular main body. The axial projection may engage a recess in the upper end of the first eccentric sleeve to securely connect the clutch and the first eccentric sleeve. 
   Preferably, the principle drive shaft is locked by the principle drive shaft locking device to adjust the eccentric stroke of the sanding plate, the first sanding plate securely engages the rollers seated in the outer circumferential surface of the clutch and the rollers seated in the inner circumferential surface of the clutch disengage the principle drive shaft. Particularly preferably the power tool further comprises: an outer race mounted radially on the annular main body between the clutch and the first sanding plate. A sanding plate may be mounted radially on the outer race. A support bearing may be radially mounted on the first eccentric sleeve. The support bearing may be substantially axially aligned with the outer race. The sanding plate may be radially mounted on the outer race and the bearing. 
   The power tool may further comprise: a balancing block connected or coupled to the second eccentric member. 
   The present invention will now be described in a non-limitative sense with reference to the accompanying Figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of a sander according to a first embodiment of the present invention; 
       FIG. 2  is a partially exploded view of the sander of  FIG. 1 ; 
       FIG. 3  is a sectional view taken along line M-M shown in  FIG. 1 ; 
       FIG. 4  is an exploded perspective view of the eccentric stroke adjusting mechanism shown in  FIG. 1 ; 
       FIG. 5  is a front plan view of  FIG. 4 ; 
       FIG. 6  is a sectional view of a sander according to a second embodiment of the present invention; 
       FIG. 7  is a sectional view of a sander according to a third embodiment of the present invention; 
       FIG. 8  is a sectional view from another direction of the sander according to the third embodiment of the present invention; and 
       FIG. 9  is a sectional view taken along line N-N shown in  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a first embodiment of the present invention is a rotary sander. The sander comprises generally a housing  1 , a motor  2  vertically disposed inside the housing  1 , a principle drive shaft  3 , a sanding plate  4  at the base of the housing and an eccentric stroke adjusting mechanism  5 . 
   Referring to  FIGS. 2-5 , the housing  1  comprises an upper housing part  12  and a lower housing part  14  securely connected to each other. A fan  16  is securely attached to the principle drive shaft  3 . The principle drive shaft  3  comprises an armature shaft  30  and a connecting shaft  32  connected to the lower end of the armature shaft  30 . The connecting shaft  32  and the armature shaft  30  have a common axis X 0 . The connecting shaft  32  has an irregular cross-section. The sanding plate  4  has an annular inner plate  42  and an annular outer plate  44 . A braking system  6  is disposed between the lower housing part  14  and the annular outer plate  44 . 
   The eccentric stroke adjusting mechanism  5  comprises a principle drive shaft locking device  8 , a first eccentric member  56 , a second eccentric member  58  and a coupling member  77  for coupling the first eccentric member  56  and the second eccentric member  58 . The first eccentric member  56 , the coupling member  77  and the second eccentric member  58  are radially mounted on the connecting shaft  32  in sequence downwardly. 
   The principle drive shaft locking device  8  comprises a chuck  52  mounted radially on an upper part of the connecting shaft  32  and a bolt member  50 . The chuck  52  has a skirt  520  extending axially downwardly from its circumferential edge. A plurality of recesses  524  are distributed around the skirt  520 . The bolt member  50  is attached to the lower housing part  14  and can be selectively inserted into a corresponding recess  524  to lock the chuck  52  during adjustment of the eccentric stroke of the sanding plate  4  (as described below). The chuck  52  has a substantially central aperture  51  surrounded by an eccentric hub  53 . A plurality of spaced apart location holes  525  are formed in the eccentric hub  53 . 
   A balancing drum  55  is mounted on the eccentric hub  53  so as to cooperate with the annular inner plate  42  whereby to balance the weight of the annular outer plate  44 . The balancing drum  55  comprises a central aperture  57   a  bound by a hub  57 . The balancing drum  55  has a part radial recess  59  extending from the hub  57 . A spring  590  and a ball head locating post  592  connected to the spring  590  are disposed in the recess  59 . The ball head locating post  592  extends partially into a location hole  525  under the force of the spring  590  to restrainedly couple the balancing drum  55  and the chuck  52 . 
   The rotary sander of the first embodiment of the present invention further comprises an overrun clutch  54 . The overrun clutch  54  is a one way rotation clutch with a self-locking function. The overrun clutch  54  has an annular main body  60  abutting an end face of the hub  57 . An outer race  64  is mounted radially on the annular main body  60  between the overrun clutch  54  and the annular outer plate  44 . A locking ring  594  is interposed axially between the outer race  64  and the balancing drum  55 . The annular main body  60  has an inner circumferential surface with three first truncated conical recesses  662  formed thereon and an outer circumferential surface with three second truncated conical recesses  66  formed thereon. In the base of each of the first truncated conical recess  662  and second truncated conical recess  66  is a narrow receiving bore  61 . A loaded spring  68  is disposed in each narrow receiving bore  61  and individually connects to a roller  63  seated in the conical recess  66 ,  662 . The loaded spring  68  urges the roller  63  away from the narrow receiving bore  61 . An axial projection  602  extends from the lower surface of the annular main body  60  adjacent to the inner circumferential surface. 
   The first eccentric member  56  is radially mounted on the principle drive shaft  3 . The first eccentric member  56  comprises a first eccentric shaft  62  having a first bore  65  formed along an axis parallel to its central axis X 3 . The shape of the first bore  65  matches the shape of the connecting shaft  32  so that the first eccentric shaft  62  can be securely mounted radially on the connecting shaft  32 . The central axis X 3  is eccentric with respect to the central axis X 0 . On the upper end of the first eccentric shaft  62  is radially mounted the overrun clutch  54 . The first eccentric member  56  further comprises a first eccentric sleeve  69  which is rotatably mounted on a lower end of the first eccentric shaft  62 . The first eccentric sleeve  69  has a central axis X 1  and the eccentricity of the central axis X 1  with respect to the central axis X 0  of the principle drive shaft  3  is adjustable. The central axis X 1  is eccentric with respect to the central axis X 3 . 
   A support bearing  71  is tightly mounted on the first eccentric sleeve  69  and its outer surface is substantially aligned with the outer surface of the outer race  64 . The annular outer plate  44  is tightly engaged with the outer surface of the outer race  64  and of the support bearing  71 . The central axis of the annular outer plate  44  is coincident with the central axis X 1  of the first eccentric sleeve  69  and so the central axis X 1  defines in practice the central axis of the first eccentric member  56  as a whole. A washer  70  is disposed axially between the annular main body  60  and the support bearing  71 . The end of the first eccentric sleeve  69  abuts the lower face of the washer  70  and has a recess  622  formed on its upper end to receive the projection  602  so that the first eccentric sleeve  69  is securely connected to the overrun clutch  54 . The first eccentric shaft  62  has a plurality of bores  67  extending parallel to its central axis X 3  to reduce its weight. 
   The second eccentric member  58  is constructed substantially symmetrically to the first eccentric member  56  with respect to the principle drive shaft  3 . The second eccentric member  58  comprises a second eccentric shaft  73  securely mounted radially on the connecting shaft  32  and a second eccentric sleeve  75  rotatably mounted on the second eccentric shaft  73 . A central axis X 4  of the second eccentric shaft  73  and the central axis X 3  of the first eccentric shaft  62  are symmetrically distributed around the central axis X 0 . A central axis X 2  of the second eccentric sleeve  75  and the central axis X 1  of the first eccentric sleeve  69  are symmetrically distributed around the central axis X 0 . The central axis X 2  defines in operation the central axis of the second eccentric member  58  as a whole. 
   The coupling member  77  is mounted on an annular protrusion  624  formed on the bottom face of the first eccentric shaft  62 . The coupling member  77  has a first and second pin  79  formed respectively on its top surface and bottom surface. The first and second pin  79  are symmetrically distributed with respect to the central axis of the coupling member  77 . The first eccentric sleeve  69  and the second eccentric sleeve  75  each has a radial slot  80  to receive a corresponding pin  79 . The width of the slot  80  approximates to the diameter of the pin  79 . 
   A bearing  71  is tightly mounted radially on the second eccentric sleeve  75 . A bearing seat  82  is mounted around the perimeter of the bearing  71 . A plurality of bolts fasten the annular inner plate  42  to the bearing seat  82 . The annular inner plate  42  and the second eccentric sleeve  75  are coaxial. A guard  91  is mounted on the bottom end of the connecting shaft  32  to retain the second eccentric member  58  and a bolt  93  is fastened tightly thereto. 
   When the eccentric stroke of the sanding plate  4  is to be adjusted, the bolt member  50  is inserted into a corresponding recess  524  of the chuck  52  so as to prevent the principle drive shaft  3  from rotating. The annular outer plate  44  is rotated in the direction indicated by an arrow a in  FIG. 3 . The outer race  64  rotates together with the annular outer plate  44 . Friction between the outer race  64  and the roller  63  of the overrun clutch  54  causes the first eccentric sleeve  69 , the second eccentric sleeve  75  and the annular inner plate  42  to rotate accordingly. By virtue of the fact that the annular outer plate  44  is securely coupled to and coaxial with the first eccentric sleeve  69 , the central axis X 1  of the annular outer plate  44  rotates around the central axis X 3  of the first eccentric shaft  62 . Since the central axis X 0  of the principle drive shaft  3  is fixed, the distance between the central axis X 1  and X 0  (i.e., the eccentric stroke of the annular outer plate  44 ) changes. The eccentric stroke of the central axis X 2  of the annular inner plate  42  with respect to the central axis X 0  also changes and approximates to the eccentric stroke of the annular outer plate  44 . The overrun clutch  54 , the spring  590  and the ball head locating post  592  disposed between the balancing drum  55  and the chuck  52  prevent the adjusted eccentric stroke from displacement. According to the requirements of the workpiece, the eccentric stroke adjusting mechanism  5  can adjust the eccentric stroke of more than one sanding plate  42 ,  44  at the same time and can ensure that the sander is balanced during operation. 
     FIG. 6  illustrates a rotary sander of a second embodiment of the present invention. The parts in the second embodiment which are the same as or similar to the parts in the first embodiment will not be described in detail and will adopt the same numeral. The rotary sander of the second embodiment comprises an upper housing part  12 , a lower housing part  14 , a motor  2  vertically disposed inside the housing  1 , a principle drive shaft  3 , a sanding plate  4  and an eccentric stroke adjusting mechanism  5 . The eccentric stroke adjusting mechanism  5  comprises an overrun clutch  54 , a first eccentric member  56  and a second eccentric member  58 . A coupling member  77  couples the first eccentric member  56  and the second eccentric member  58 . The sanding plate  4  of the second embodiment is a single disk. A balancing block  9  is directly attached to a second eccentric sleeve  75  of the second eccentric member  58  (in place of the annular inner plate  44  and the bearing of the first embodiment). 
   In the second embodiment, no balancing drum  55  is present. Instead the overrun clutch  54  engages the chuck  52  directly and a recess  59  is present on the upper surface of an annular main body  60  of the overrun clutch  54 . A spring  590  and a ball head locating post  592  connected to the spring  590  are disposed in the recess  59 . The ball head locating post  592  extends partially into a location hole  525  under the force of the spring  590  to restrainedly couple the overrun clutch  54  and the chuck  52 . 
     FIGS. 7-9  illustrate an eccentric stroke adjusting mechanism  5 ′ of a third embodiment of the present invention (similar to the eccentric stroke adjusting mechanism  5  of the first and second embodiment described above) for use in a power tool having a principle drive shaft  3 ′. The eccentric stroke adjusting mechanism  5 ′ comprises a first eccentric member  56 ′ mounted on the principle drive shaft  3 ′, a second eccentric member  58 ′ mounted on the principle drive shaft  3 ′ and a coupling member  77 ′ for coupling the first eccentric member  56 ′ and the second eccentric member  58 ′. 
   The first eccentric member  56 ′ has a first central axis X 1 ′. The second eccentric member  58 ′ has a second central axis X 2 ′. The eccentric stroke of the first central axis X 1 ′ and the second central axis X 2 ′ with respect to a central axis X 0 ′ of the principle drive shaft  3 ′ is adjustable. The first eccentric member  56 ′ comprises a first eccentric shaft  62 ′ mounted radially on the principle drive shaft  3 ′ and a first eccentric sleeve  69 ′ rotatable with respect to the principle drive shaft  3 ′. The second eccentric member  58 ′ comprises a second eccentric shaft  73 ′ mounted on the principle drive shaft  3 ′ and a second eccentric sleeve  75 ′ rotatable with respect to the principle drive shaft  3 ′. A central axis X 3 ′ of the first eccentric shaft  62 ′ and a central axis X 4 ′ of the second eccentric shaft  73 ′ are eccentric with respect to the central axis X 0  of the principle drive shaft  3 ′ and on opposite sides thereof. The first eccentric sleeve  69 ′ and the second eccentric sleeve  75 ′ are separately rotatably mounted on the first eccentric shaft  62 ′ and the second eccentric shaft  73 ′. The central axis of the first eccentric sleeve  69 ′ defines in practice the central axis X 1 ′ of the first eccentric member  56 ′. The central axis of the second eccentric sleeve  75 ′ defines in practice the central axis X 2 ′ of the second eccentric member  58 ′. The central axis X 1 ′ of the first eccentric sleeve  69 ′ is eccentric with respect to the central axis X 3 ′ of the first eccentric shaft  62 ′ and the central axis X 0 ′ of the principle drive shaft  3 ′. The central axis X 2 ′ of the second eccentric sleeve  75 ′ is eccentric with respect to the central axis X 4 ′ of the second eccentric shaft  73 ′ and the central axis X 0 ′ of the principle drive shaft  3 ′. The central axes X 1 ′ and X 2 ′ are on opposite sides of the central axis X 0 ′. 
   A plurality of locating sockets  83  are formed on the outer surface of the first eccentric shaft  62 ′ and the second eccentric shaft  73 ′. A plurality of radial receiving recesses  85  are formed on the inner circumferential surface of the first eccentric sleeve  69 ′ and of the second eccentric sleeve  75 ′. The receiving recesses  85  each have an elastic element  87  seated therein and a locating post  89  is connected to the elastic element  87 . The locating post  89  can be selectively received in one of the corresponding locating sockets  83  so as to restrainedly couple the first and the second eccentric sleeve  69 ′,  75 ′ with the first and the second eccentric shaft  62 ′,  73 ′ respectively. 
   The coupling member  77 ′ has an operating body  84  rotatably mounted on the principle drive shaft  3 ′ and an extension pin  86  extending downwardly therefrom. Each of the first and the second eccentric sleeve  69 ′,  75 ′ have an axial receiving bore  88  to accommodate the extension pin  86 . The diameter of the receiving bore  88  approximates to the diameter of the extension pin  86 . The length of the receiving bore  88  is longer than the length of the extension pin  86 . 
   Sanding plates  46  and  48  are coupled to the first eccentric sleeve  69 ′ and the second eccentric sleeve  75 ′ via a bearing  90 . A seal ring  92  is interposed between the sanding plate  46  and the first eccentric sleeve  69 ′. 
   The operating body  84  of the coupling member  77 ′ can be manually rotated to allow the eccentric stroke of the sanding plates  46  and  48  to be adjusted. The principle of adjustment is the same as described above for the first and the second embodiment. 
   The present invention may be embodied in other specific forms without departing from its structures, methods, or other essential characteristics as broadly described herein and claimed hereinafter. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.