Abstract:
The present invention relates to an anti-vibration arrangement ( 10 ) for a power sander ( 1 ) which comprises a housing ( 2 ), a motor ( 4 ) arranged in the housing ( 2 ), a rotary drive shaft ( 11 ), a first outer or ring-shaped pad surface ( 16 ) for attaching a first sanding paper ( 8 ) and a second inner or circular pad surface ( 22 ) for attaching a second sanding paper ( 9 ). The anti-vibration arrangement ( 10 ) serves to transfer energy from the motor ( 4 ) to the pads ( 16, 22 ) with out-of-phase motions to dynamically compensate for inertial and friction forces. For this purpose, twin cams ( 18   a   , 18   b ) are fixed on the rotary drive shaft ( 11 ). The cams ( 18   a   , 18   b ) rotate the central axes ( 15, 21 ) of the pads ( 16, 22 ) about the rotary drive shaft axis ( 12 ) with a phase differential of typically 180°. Vibration which would otherwise be transmitted to the rotary drive shaft ( 11 ) and from there to the operator of the machine ( 1 ) are drastically reduced irrespective of whether or not the operator increases the applied force ( 1 ) in order to increase the sanding depth or to speed up the sanding operation.

Description:
REFERENCES TO RELATED APPLICATIONS 
   This is a non-provisional application claiming priority to European Application Number 05252417.0, entitled Anti-Vibration Device, filed 18 Apr. 2005, which is hereby incorporated by reference in its entirety. 
   FIELD OF THE INVENTION 
   The present invention relates to an anti-vibration arrangement for an eccentrically rotary and oscillatory tool (eg an abrasive power tool) such as an orbital sander or polisher, to a power tool incorporating the anti-vibration arrangement and to a method for abrading a work piece. 
   BACKGROUND OF THE INVENTION 
   Orbital power tools such as sanders and polishers generally include a pad that is normally adapted to support an abrasive element such as sanding paper. The pad is coupled by a transmission means to a motor arranged in a housing. The transmission means can incorporate a cam rotationally driven by the rotary drive shaft. The cam is housed in a circular aperture that is positioned in the centre of the pad. The rotation of the cam drives every point of the pad in a circular orbit whose radius equals the eccentricity of the cam ie the distance between the rotary axis of the rotary drive shaft and the centre of the circular aperture which is substantially coincident with the centre of the pad. By allowing the pad to rotate around the centre of the circular orbit, it describes a combined rotary/orbital motion referred to as a “random orbit”. 
   The orbital motion can be envisaged as a linear motion (or stroke) in which the pad mass is accelerated in a certain direction. The acceleration produces a reaction force directed in the opposite direction. This reaction force manifests itself as an unwanted vibration which is transmitted to the housing and ultimately to the operator&#39;s hand and arm. The amplitude of this unwanted vibration depends on the diameter of the orbit and on the ratio between the mass of the pad and the mass of the tool. 
   In order to keep vibrations beneath an acceptable level, conventional tools are designed in such a way that the working surface of the pad and the orbital diameter are relatively small. However, these limitations reduce the efficiency of the machine. In order to compensate for these limitations in efficiency, operators frequently apply a certain pressure or load to the tool in order to increase the friction on the work piece with the result that vibrations are amplified. In order to counteract the resulting increase in vibrations, the operator tends to grasp and apply the tool with even more force to the work piece. By doing this, the effective mass of the machine is increased and the vibrations are absorbed by the operator&#39;s hand and arm with often severe consequences for the operator&#39;s health. For example, even low usage operators may experience numbness and tingling in their fingers, hand and arm within a few minutes of operation and this may be lead to an unpleasant loss of feeling and control in the fingers that can last for hours after use has ceased. If use is prolonged for hours, a full recovery can take several days. The consequences for professional workers can be even more severe and long term may lead to retirement and high social costs. On the other hand, adopting strict guidelines relating to vibration threshold values would have a severe impact on productivity and costs. 
   Operators of power tools tend to apply a certain load to the tool so that the speed of the work is increased. The increase in the working efficiency that is achieved by the increased load is due exclusively to the increase in friction between the pad and the work piece. On the other hand, the increased load unbalances the tool and increases the unwanted vibrations. The diameter of the unwanted vibrations is subtracted from the orbital diameter of the pad. In practice, the effective working orbital diameter is the result of the theoretical orbit diameter less that of the unwanted vibrations. 
   An arrangement for overcoming the above-mentioned drawbacks adopts one or more counterbalances (eg eccentric masses or counterweights) that move in a direction opposite to that of the pad to counterbalance the vibrations. Examples of this kind of arrangement are illustrated in U.S. Pat. No. 4,660,329, U.S. Pat. No. 4,729,194, U.S. Pat. No. 5,888,128, U.S. Pat. No. 6,244,943, US-A-6206771, U.S. 2001/0003087, DE-A-3922522, EP-A-303955, EP-A-0455618, WO-A-98/01733 and WO-A-02/068151. In general, this type of arrangement works satisfactorily when the pad is not touching the work piece but displays major limitations in normal use. As soon as the pad is placed on the work piece, the load effectively modifies the mass of the pad and the ratio between the mass of the pad and the mass of the counterbalance is altered. As a result, the counterbalance fails to eliminate the vibrations induced by the heightened effective mass of the pad. The higher the load, the greater the system imbalance and the higher the level of unwanted vibrations. With a load tending to infinity, the pad will be at a standstill and the tool will vibrate with an amplitude equal to the radius of the orbit of the pad. 
   Another arrangement for overcoming the above-mentioned drawbacks uses elastic materials as an interface between the tool and the operator&#39;s hands for dampening vibrations. The kinetic energy of the vibrations is converted into thermal energy. Examples of this type of arrangement are illustrated in U.S. Pat. No. 4,905,772, U.S. Pat. No. 5,453,577, U.S. Pat. No. 5,347,764, U.S. 2001/0011856 A1, WO-A-03/049902. However, by interposing an elastic element between the housing and the operator&#39;s hand, the tool is free to vibrate with greater amplitude than if it was firmly held by the operator. In practice, the operator instinctively feels the decreased efficiency of the machine and tends to grasp it with increased force in an attempt to restore efficiency. By doing this, the effeciency of the elastic element is minimized so that vibrations are transmitted to the operator&#39;s hand and arm. Moreover, the increased muscular force reduces the human body&#39;s natural capability to dampen vibrations. 
   OBJECT OF THE INVENTION 
   An object of the present invention is to overcome certain of the above-described drawbacks by exploiting two or more pads exbiting out-of-phase orbital motion. 
   SUMMARY OF THE INVENTION 
   Thus viewed from one aspect the present invention provides an anti-vibration arrangement for an eccentrically rotatable and oscillatory tool (eg a motor-driven abrasive tool), the arrangement comprising:
         a first pad having a first external pad surface for fitting a first abrasive element;   a second pad having a second external pad surface for fitting a second abrasive element, wherein the first external pad surface and the second external pad surface are substantially coplanar; and   transmission means driveable by a rotary drive shaft of the motor, wherein the transmission means is adapted to transmit drive to the first pad and to the second pad to cause the first external pad surface and the second external pad surface to orbit out-of-phase about a first orbital axis and a second orbital axis respectively.       

   The anti-vibration arrangement dynamically compensates for inertial and frictional forces and reduces or eliminates vibrations that are otherwise transmitted to the rotary shaft. Thus at relatively low cost, the anti-vibration arrangement significantly reduces the risks to the operator&#39;s health. The arrangement is easy to use and convenient to maintain and even when the load is unequally shared by the abrasive elements, the residual vibrations are lower than in a conventional machine provided (for example) with a counter-balance mechanism. 
   The motor can be electric or pneumatic. 
   Preferably the first pad has essentially the same mass as the second pad. 
   Preferably the first external pad surface has essentially the same area as the second external pad surface. 
   Preferably the centre of gravity of the first pad and the centre of gravity of the second pad are aligned along a straight line intersecting the rotary axis. 
   Preferably the second external pad surface is arranged substantially peripherally and eccentrically with regard to the first external pad surface. 
   Preferably the second external pad surface is substantially circular. Preferably the first external pad surface is substantially annular. The second external pad surface may be confined within the first external pad surface. 
   Preferably the first pad is substantially bell-shaped and comprises a conical main body terminating at an apical end in an annular lip and terminating at a non-apical end opposite to the apical end in a radial collar, the radial collar defining the first external pad surface. 
   Preferably the second pad comprises a cylindrical main body capped by a circular plate defining the second external pad surface. 
   Preferably the first pad further comprises at least one dust vent. 
   The first orbital axis and the second orbital axis may be coincident or non-coincident. The first orbital axis and/or the second orbital axis may coincide with the rotary axis of the rotary drive shaft. Preferably the first orbital axis and the second orbital axis are common to the rotary axis of the rotary drive shaft. 
   Preferably the central axis of the first external pad surface and the central axis of the second external pad surface are arranged parallel to the rotary axis substantially in a common plane therewith. Particularly preferably the central axis of the first external pad surface and the central axis of the second external pad surface are equidistant from the rotary axis. This advantageously makes construction simple but there may be occasions where a deviation from this condition is desirable. 
   Preferably the transmission means comprises:
     a monolithic drive shaft assembly mountable on the rotary drive shaft and having a first cam and a second cam for transmitting drive to the first external pad surface and the second external pad surface respectively.   

   The cams may be coupled directly or indirectly to the rotary drive shaft. The cams may be any suitable shape (eg cylindrical or elliptical). 
   Particularly preferably the first cam and the second cam are non-coaxial. Partciuarly preferably the monolithic drive shaft assembly is provided with a central aperture for mounting on the rotary drive shaft, wherein the first cam and the second cam are substantially identical and are longitudinally and angularly displaced. Preferably the first cam and the second cam are angularly displaced by approximately 180°. 
   In a particularly preferred embodiment, the first cam and the second cam are each substantially cylindrical and wherein the eccentricity of the first cam and the second cam with respect to the rotary axis equals the orbital diameter. 
   Preferably the outer diameter of the second external pad surface is slightly smaller than the inner diameter of the first external pad surface so that a minimum gap is maintained between the second external pad surface and the first external pad surface. Particularly preferably the gap defines a passage for emitting debris from a work piece during use. The gap can be connected to suction means such as a fan for removing debris and dust from the work piece. This removes the need for apertures that are normally included in conventional machines. 
   Preferably the transmission means comprises: a first bearing mounted on or in the first pad; and a second bearing mounted on or in the second pad. Particularly preferably the first bearing is mounted on the first cam and the second bearing is mounted on the second cam. 
   Preferably the first external pad surface and the second external pad surface are substantially rectangular or square. 
   The anti-vibration arrangement of the invention may further comprise any number of additional pads (eg third and fourth pads). Typically the central axes of the external pad surfaces of the pads are equidistant from the rotary axis. The total number of pads can be driven by a suitable number of drive shaft assemblies with a suitable disposition (eg a suitable number of cams). 
   In a preferred embodiment, the arrangement comprises four pads with external pad surfaces having individual orbital axes, wherein neighboring pads are adapted to orbit in opposite directions. The individual orbital axes may be non-coincident with the rotary axis. Particularly preferably the four pads are disposed in a square configuration. 
   Preferably the first external pad surface has a first predetermined orientation and the second external pad surface has a second predetermined orientation, wherein the transmission means is adapted to transmit drive to the first external pad surface and the second external pad surface in a manner such that the first and second predetermined orientations are maintained. 
   Viewed from a further aspect the present invention provides a method for abrading a work piece comprising:
     causing a first pad with a first external pad surface having a first predetermined orientation and a second pad with a second external pad surface having a second predetermined orientation to be driven such that the first external pad surface orbits about a first orbital axis and the second external pad surface orbits about a second orbital axis with a phase differential to compensate for vibrations.   

   Preferably the first orbital axis and the second orbital axis coincide. 
   Preferably the first predetermined orientation and the second predetermined orientation are maintained during orbit. 
   Preferably the first external pad surface is substantially annular and the second external pad surface is substantially circular and wherein the second external pad surface is arranged within the first external pad surface and the first external pad surface and the second external pad surface are angularly offset by approximately 180°. 
   Of independent patentable significance is a portable tool (eg a sander or a polisher) comprising an anti-vibration arrangement as hereinbefore defined which allows the user to accomplish coarse and/or fine surface sanding work on any material with high efficiency and productivity and with a substantial reduction in vibrations irrespective of the the load applied by the user. 
   Viewed from a yet further aspect the present invention provides an eccentrically rotatable and oscillatory tool (eg a portable abrasive tool) comprising:
         a housing;   a handle mounted on or integral with the housing;   an electric motor supported in the housing, wherein the electric motor has a rotary drive shaft with a longitudinal rotary axis; and   transmission means driveable by a rotary drive shaft of the motor for transmitting drive to a first pad and to a second pad to cause a first external pad surface of the first pad and a second external pad surface of the second pad to orbit out-of-phase about a first orbital axis and a second orbital axis respectively.       

   Preferably the tool comprises: an anti-vibration arrangement as defined hereinbefore, wherein the transmission means couples the rotary drive shaft to the first pad and to the second pad. 
   The functionality of this tool advantageously does not depend on the rotation speed, the weight, the type of abrasive surface, the radius of rotation of the pads or the load conditions. 
   Although the absence of a conventional counterweight advantageously increases the useful energy available for abrasion, a counterweight may be added. The counterweight may be any convenient shape. 
   Preferably the tool further comprises:
         a counterweight associated with the rotary drive shaft, wherein the centre of gravity of the counterweight is located outside the rotary axis.       

   Preferably the tool further comprises:
         a cooling fan mounted radially on the rotary drive shaft; and   a counterweight disposed on the cooling fan, wherein the centre of gravity of the counterweight and the cooling fan is located outside the rotary axis.       

   Preferably the tool further comprises:
         an air and dust vent connected to or integral with the housing for connecting a fan.       

   The tool may be a rotary sander, random orbital sander or finishing sander. For a finishing sander, connection pieces made of a resilient material may be deployed to restrain the tool to regular orbital motion. In a finishing sander the pads maintain their predetermined orientations. 
   Preferably the tool further comprises:
         a first resilient connection piece fixed between the first pad and the housing and   a second resilient connection piece fixed between the second pad and the housing.       

   Preferably the tool further comprises:
         at least one brake for reducing the rotational speed of at least one of the first and second pads at least when no load is applied to at least one of the first and second pads.       

   The brake (or brakes) permit high rotational speeds to be avoided especially when no load is applied to the pads. 
   Viewed from a yet still further aspect the present invention provides a kit comprising a substantially annular sanding paper attachable to a first pad defined hereinbefore and a substantially circular sanding paper attachable to a second pad as hereinbefore defined. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial side view of a first rotary sander incorporating a first embodiment of the anti-vibration arrangement of the invention; 
       FIG. 2  is an exploded perspective view of the anti-vibration arrangement of  FIG. 1 ; 
       FIG. 3  is an exploded cross-sectional view of the anti-vibration arrangement of  FIG. 2 ; 
       FIG. 4  is a view of the anti-vibration arrangement of  FIGS. 1–3  in reduced scale from below; 
       FIG. 5  is an assembled cross-sectional view of the anti-vibration arrangement of  FIG. 3 ; 
       FIG. 6  is a perspective view of the drive shaft assembly of the anti-vibration arrangement of  FIGS. 1–5 ; 
       FIG. 7  is an assembled cross-sectional view of a third embodiment of the anti-vibration arrangement of the invention; 
       FIG. 8  is a schematic view of the path of eight small sanding particles during use of the anti-vibration arrangement of  FIG. 1 ; 
       FIGS. 9–12  illustrate schematically the path of another four small sanding particles during use of the anti-vibration arrangement of  FIG. 1 ; 
       FIG. 13  is a bottom view of a second embodiment of the anti-vibration arrangement of the invention; 
       FIG. 14  is an assembled cross-sectional view of a fourth embodiment of the anti-vibration arrangement of the invention; 
       FIG. 15  is a partial side view of a finishing sander incorporating the first embodiment of the anti-vibration arrangement of the invention; and 
       FIG. 16  is a partial side view of a second rotary sander incorporating the first embodiment of the anti-vibration arrangement of the invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a rotary sander  1  incorporating a first embodiment of an anti-vibration arrangement  10  according to the present invention. The rotary sander  1  generally includes a housing  2  that has a handle or grip  3  and an internal volume  4  for housing an electric motor  5  with a variable speed of 2000 to 12000 rpm. The housing  2  is provided with an exhaust tube  45  beneath the handle  3  for exhausting air and dust from the interior  44 . The electric motor  5  has a rotary drive shaft  11  with a longitudinal rotary axis  12  which is supported at the upper end of the housing  2  by ball, cylinder or oil bearings  6 . A power switch  7  is positioned on the handle  3  and the rotary sander  1  is connected to mains power, a rechargeable battery or a compressed air tank which is not represented in  FIG. 1 . The anti-vibration arrangement  10  couples the rotary drive shaft  11  to abrasive elements (such as abrasive layers or sanding papers) for abrading a work piece (not shown) as described below. 
   Referring to  FIGS. 2–6 , the anti-vibration arrangement  10  comprises a bell-shaped first pad  17  having a substantially conical main body  17   a  enclosing a central lower volume  17   c  and terminating at an apical end in an annular lip  17   b  bounding an aperture  17   e . At a non-apical end (opposite the apical end), the conical main body  17   a  terminates in a radial collar  16  bounding an aperture  16   a  and having a first external pad surface  16   b  for fitting to a substantially planar annular abrasive element  8 . The area of the first external pad surface  16   b  is designated F 1 . As can be seen in  FIGS. 1 ,  3  and  5 , air and dust vents  17   f  are provided in the conical main body  17   a . When a fan (not shown) is connected to the exhaust tube  45 , debris from the work piece can be exhausted from the internal volume  4  through the vents  17   f.    
   The anti-vibration arrangement  10  further comprises a second pad  23  having a cylindrical main body  23   a  capped by a circular plate  22  with a second external pad surface  22   a  for fitting to a substantially planar circular abrasive element  9 . The circular plate  22  is accommodated in the aperture  16   a  of the radial collar  16 . The area of the second external pad surface  22   a  is designated F 2 . 
   The anti-vibration arrangement  10  is adapted to reduce the amplitude of vibrations that are generated by the reaction of the first and second external pad surfaces  16   b ,  22   a  on the work piece. For this purpose, the anti-vibration arrangement  10  is arranged so that the first and second external pad surfaces  16   b ,  22   a  are disposed distinctly and separately from each other. The pads  17  and  23  have substantially identical mass. The first and second external pad surfaces  16   b ,  22   a  have substantially identical surface areas F 1  and F 2  and are located substantially in the same plane P (see  FIGS. 1 and 5 ). 
   The anti-vibration arrangement  10  is adapted to provide orbital motion to the first and second external pad surfaces  16   b ,  22   a  in different phases. Through their out-of-phase motion, the first and second external pad surfaces  16   b ,  22   a  dynamically compensate for inertial and frictional forces and thus reduce the vibrations transmitted back to the rotary drive shaft  11 . For this purpose, the anti-vibration arrangement  10  further comprises a monolithic drive shaft assembly  18  having first (upper) and second (lower) substantially cylindrical cams  18   a ,  18   b . The drive shaft assembly  18  is provided with a central aperture  18   c  coincident with the rotary axis  12  for a firm connection to the rotary drive shaft  11  so that the cams  18   a ,  18   b  rotate at the same speed as the rotary drive shaft  11 . The cylindrical cams  18   a ,  18   b  are substantially identical to each other but they are longitudinally displaced (non-coaxial) and angularly offset relative to the plane of the housing by about 180° to drive respectively the first and second pads  17 ,  23  in an out-of-phase eccentric manner. 
   A first bearing  13  is firmly received in the aperture  17   e  of the annular lip  17   b  and is mounted on the cam  18   a . A second bearing  19  is firmly received in the cylindrical main body  23   a  and is mounted on the cam  18   b . The first bearing  13  and the second bearing  19  may be ball bearings or cylinder bearings. The centre of the first bearing  13  is denoted as  13   a , its central aperture as  14  and its central axis as  15 . The centre of the second bearing  19  is denoted as  19   a , its central aperture as  20  and its central axis as  21 . The outer surface of the first cam  18   a  is received in the central aperture  14  of the first bearing  13  (and fixed therein) and the second cam  18   b  is received in the central aperture  20  of the second bearing  19  (and fixed therein). The rotation of the rotary drive shaft  11  is transferred to the first and second cams  18   a ,  18   b  and from there slidingly via the bearings  13  and  19  to the first and second pad  17  and  23  respectively (ie to the first and second external pad surfaces  16   b ,  22   a  respectively). It will be noted from  FIGS. 1–7  that the only connection between the first and second pads  17 ,  23  and the housing  2  are the two ball bearings  13  and  19  respectively. 
   The central axes  15 ,  21  are arranged parallel to the rotary axis  12  substantially in a common plane therewith. The central axis  15  coincides with the central axis of the first external pad surface  16   a  and the central axis  21  coincides with the central axis of the second external pad surface  22   b . The eccentricities e 1 , e 2  of the cams  18   a ,  18   b  with respect to the rotary axis  12  (ie the distances between the axes  15 / 12  and  21 / 12  respectively) are identical (ie e 1 =e 2 ) and equate to the diameter of the desired orbit. 
   The anti-vibration arrangement is such that the centre of gravity  25  of the first pad  17  and the centre of gravity  26  of the second pad  23  are aligned along a straight line  27  passing through the rotary axis  12  (see  FIG. 5 ). During use the central axis  15  orbits about the rotary axis  12 . Also during use the central axis  21  orbits about the rotary axis  12  with a phase differential of 180° with respect to the orbit of the central axis  15 . Consequently, pads  17  and  23  describe eccentric orbits with a phase differential of 180° relative to each other. 
   As can been seen in  FIGS. 4 and 5 , the diameter of the circular plate  22  is slightly smaller than the inner diameter of the radial collar  16  so that a gap  24  is maintained between the radial collar  16  and the circular plate  22  during rotation with a predetermined minimum gap  24   a . The gap  24  between the radial collar  16  and the circular plate  22  defines a passage for debris and dust from the work piece. 
   During use, forces K 1 , K 2  are generated and associated with the radial collar  16  and the circular plate  22  respectively (see  FIG. 4 ). These forces K 1 , K 2  act in opposite directions (due to the phase differential of 180°) and therefore substantially eliminate vibrations which would otherwise be transferred back to the housing  2 . 
   As illustrated in  FIG. 7 , during operation of the rotary sander  1  a small torque may be generated by forces f 1 , f 2  around a point  30  which is the centre of gravity of the arrangement (ie of the first and second pads  17 ,  23 , the bearings  13 ,  19  and the drive shaft assembly  18 ). These forces f 1 , f 2  are generated by centrifugal effects and may lead to vibrations. In order to eliminate the torque, a cylindrical counterweight  28  is associated with the rotary drive shaft  11 . The counterweight  28  may be firmly mounted directly on the rotary drive shaft  11  (as in  FIG. 7 ) or connected to the drive shaft assembly  18  or it may be mounted on the lower outer side of a cooling fan  43  (as shown in  FIG. 14 ) connected to the rotary drive shaft  11  for cooling the motor  4 . The centre of gravity  29  of the counterweight  28  is located outside the rotary axis  12  with an eccentricity denoted j in  FIG. 7  and the total centre of gravity is positioned at point  31 . The mass of the first pad  17  equals the mass of the second pad  23  plus the mass of the counterweight  28  because for balancing purposes not only the mass of the counterweight  28  is essential but also its distance q from point  31 . In a similar manner in  FIG. 14 , the centre of gravity  29 A of the counterweight  28  and of the cooling fan  43  is located outside the rotary axis  12 . 
   In  FIG. 8 , the radial collar  16  is illustrated from below to demonstrate the function of the anti-vibration arrangement  10 . The first and second external pad surfaces  16   b ,  22   a  are located in the same plane P and their surface areas F 1 , F 2  are equal. During use, the central axis  15  of the radial collar  16  describes a small circle  40  around the rotary axis  12  of the rotary drive shaft  11  and the central axis  21  of the circular plate  22  with a phase differential of 180° also describes a small circle  41  around the rotary axis  12 . For illustrative purposes, a connection line  42  connects the axes  15 ,  12  and  21 . The distance between the axes  12  and  15  equals the distance between the axes  12  and  21 . 
   For illustrative purposes with reference to  FIG. 8 , an arrow  16   z  may be assumed to be fixed on the first external pad surface  16   b  of the radial collar  16 . It indicates a predetermined direction or orientation of the radial collar  16  with regard to the rotary sander  1 . For example it is directed from the front side of the rotary sander  1  to the back side. For illustrative purposes an arrow  22   z  may be assumed to be fixed on the second external pad surface  22   b  of the circular plate  22 . It similarly indicates a predetermined direction or orientation of this circular plate  22  with regard to the rotary sander  1 . For instance, it may also be directed from the front side of the rotary sander  1  to the back side. In the embodiment of  FIG. 15 , the orientations  16   z ,  22   z  are maintained during the entire operation of the rotary sander  1 . In other words, in all working positions (five of which are indicated in  FIG. 8  by reference signs ( 1 ) to ( 5 )) the arrows  16   z ,  22   z  are each parallel to a predetermined line which is oriented perpendicular to the rotary axis  12 . 
   For illustrative purposes it may also be assumed that eight small sanding particles a to h are in the illustrated position ( 1 ) on the perimeter of the first and second external pad surfaces  16   a  and  22   b . The particles a–d on the first external pad surface  16   a  are assumed to be separated from each other by 90° and similarly the particles e–h on the second external pad surface  22   b  are also assumed to be separated from each other by 90°. The particles a–h travel along small circles t of the same diameter passing through consecutive positions ( 1 )–( 5 ) thereby causing fine sanding of the work piece. 
   This is again shown in  FIGS. 9–12  where the path of three particles k, l, m is shown when the radial collar  16  and circular plate  22  adopt four consecutive positions ( 1 ), ( 2 ), ( 3 ) and ( 4 ). In this case, the particles k,  1 , m are situated on the first and second external pad surfaces  16   a  and  22   b  remote from the perimeter. Again, the particles k, l, m travel along small circles t having an equal diameter. 
   It must be stressed with regard to  FIGS. 8 to 12  that in addition to the orbital motion around circles t, there is rotation of the radial collar  16  and circular plate  22  about their central axes  15  and  21  respectively caused by the relatively small internal friction generated by the bearings  13  and  19 . These pad rotations (denoted by curved rotation arrows  33  and  34  respectively) cause coarse sanding of the work piece. The speed of these pad rotations is dependent on the load applied to the first and second external pad surfaces  16   b ,  22   a  respectively. If the rotary sander  1  operates with no load (eg if it is held in the air so that there is no friction between the first and second external pad surfaces  16   b ,  22   a  and the work piece), the radial collar  16  and the circular plate  22  start to rotate in the same direction as the rotary drive shaft  11  and each of the radial collar  16  and the circular plate  22  is accelerated until it reaches the same speed as the rotary drive shaft  11 . If a load is applied (ie if the first and second external pad surfaces  16   b ,  22   a  are applied to the surface of the work piece), the radial collar  16  and the circular plate  22  decelerate. The pad rotations tend towards stopping and just a very low rotational speed may remain for coarse sanding. However the speed of orbital rotation (leading to elimination of vibration) and thus fine sanding is strongly related to the motor speed and not to the load applied so that orbital rotations will remain. 
   During use, friction between the radial collar  16  and the work piece on the one hand and the circular plate  22  and the work piece on the other hand is not always the same so that the pad rotations of radial collar  16  and circular plate  22  are not the same. This is unimportant for the anti-vibration performance because low pad rotations do not create vibrations. 
   In  FIG. 13 , a second embodiment of the anti-vibration arrangement of the invention is illustrated. It works on the general principles of the first embodiment described hereinbefore. There are four pads A 1 , A 2 , A 3 , A 4  arranged coplanarly in a symmetrical square configuration equidistant from the rotary axis  12  of the rotary drive shaft  11 . The pads A 1 –A 4  have a planar square shaped external pad surface B 1 –B 4  with identical surface areas for attachment of equal-size sanding or polishing papers. For illustrative purposes, it is assumed that small sanding particles a, b, c, d are present at the outer corners. During operation, these sanding particles a–d adopt consecutive positions ( 1 ), ( 2 ), ( 3 ), ( 4 ) of which only positions ( 1 ) and ( 3 ) are illustrated. Position ( 3 ) results from a shift in the direction of the corner arrows by 45° with respect to position ( 1 ). The centres including central axes of external pad surfaces B 1 –B 4  are denoted S 1 –S 4 . The external pad surfaces B 1 –B 4  and circular areas C 1 –C 4  in their centres S 1 –S 4  are shown in solid lines in position ( 1 ) and in broken lines in position ( 3 ). 
   There are four orbital axes R 1 –R 4  about which the centres S 1 –S 4  and the central areas C 1 –C 4  orbit consecutively between positions ( 1 ), ( 2 ), ( 3 ), ( 4 ). The orbital axes R 1 –R 4  are at the same distance d 1 =d 2 =d 3 =d 4  from the rotary axis  12 . These distances d 1 –d 4  remain unchanged during use. T 1 , T 2 , T 3 , T 4  denote the direction of orbit. It will be appreciated that all neighboring external pad surfaces A 1 –A 4  orbit in opposite directions with respect to each other whereby the individual orientation O 1 , O 2 , O 3 , O 4  of the external pad surfaces B 1 , B 2 , B 3 , B 4  remains unchanged. In this manner, vibrations are cancelled. 
   The second embodiment is driven by a drive shaft assembly and a gear assembly. The drive shaft assembly may be similar to that of  FIG. 6  ie including two cams for neighboring pads A 1 , A 3  and A 2 , A 4 , wherein each of the two drive shaft assemblies is connected to the rotary drive shaft  11 . By such drive shaft assemblies and the gear assembly, the rotation of the rotary axis  12  is transferred to the four axes S 1 , S 2 , S 3 , S 4  so that the external pad surfaces B 1 –B 4  rotate in the directions T 1 –T 4 . The circles C 1 –C 4  shown in solid line indicate the location of the associated cylindrical cam in the first position ( 1 ) whereas the circles shown in broken lines indicate the location of the associated cylindrical cam in the third position ( 3 ). In this embodiment, a significant reduction of vibrations is obtained. In addition to orbiting, the entire configururation will rotate arround the rotary axis  12 , thereby performing pad rotations for coarse sanding. 
     FIG. 15  is a partial side view of a finishing sander  1  incorporating the first embodiment of the anti-vibration arrangement of the invention. The finishing sander  1  is essentially identical to the embodiment shown in  FIG. 1  but additionally comprises a first connection piece  46  and a second connection piece  47 . The first and second connection pieces  46 ,  47  are elongated and made of an elastic material such as rubber. The first connection piece  46  is fixed between the radial collar  16  and the housing  2  and the second connection piece  47  is fixed between the circular plate  22  and the annular lip  17   b  (ie indirectly between the second pad  23  and the housing  2 ). The first and second connection pieces  46 ,  47  ensure that the first and second pads  17  and  23  cannot rotate about their respective central axes  15  and  21 . Since such rotations are prevented, the sanding papers  8  and  9  are restrained to orbit in small circles t as illustrated in  FIGS. 8 to 12 . In other words, the flexible connection pieces  46 ,  47  prevent the pad rotations whilst allowing orbital rotations. 
   In  FIG. 16  there are illustrated two brakes  50 ,  51  used in a second rotary sander  1  otherwise identical to that of  FIG. 1 . The brakes  50 ,  51  slow down the rotation of the first and second pads  17  and  23  when there is no load applied to the rotary sander  1 . The rotation speed is kept low because the brakes  50 ,  51  simulate a load. The brakes  50 ,  51  are illustrated schematically as rubber rings of different diameter.