Patent Publication Number: US-6213851-B1

Title: Abrading apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION 
     The present invention relates to an apparatus for abrading material from a workpiece, and more particularly relates to an abrading apparatus for woodworking and metal working applications. The present invention even more particularly relates to an improved eccentric orbit sanding and grinding apparatus. The present invention finds application in woodworking, metalworking, and in other fields wherein material is abraded by sanding or grinding a workpiece. Examples of woodworking and metal working applications in which the present invention may be used include smoothing or shaping wooden workpieces and deburring metal workpieces. 
     BACKGROUND OF THE INVENTION 
     Description of the Invention Background 
     As is known in the art, powered eccentric orbit sanding and grinding devices provide a means for sanding and grinding the surfaces of workpieces of various types of material. Sanding and grinding, as well as other similar operations wherein material is removed from a surface of a workpiece are collectively referred to herein as “abrading”. Also, as used herein, an “eccentric orbit” sanding or grinding device is one having an abrading surface for removing material from a workpiece and the abrading surface both rotates and orbits simultaneously. More specifically, the abrading surface rotates about a rotational axis, and the rotational axis simultaneously orbits about a point offset from the rotational axis. As a point of reference only, the combined rotational and orbital motions of the abrading surface of an eccentric orbit abrading device may be compared generally with the motion of the Earth, which rotates about its axis while also orbiting about the Sun. 
     The combination of rotational and orbital motions of the abrading surface of an eccentric orbit abrading device is advantageous for at least the reason that during use a point on the abrading surface is less likely to describe a repeat pattern relative to the abraded surface of the workpiece than, for example, a point on the belt of a belt sander or the abrading disk of a disk sander in which the disk rotates about an axis which does not orbit about an offset point. If the abrading surface of an abrading device contacts the workpiece and a point on the abrading surface describes a repeat pattern on the workpiece, sand grains or other particles of abrasive material resident on the abrading surface may leave grooves, indentations, or other unsightly lines on the workpiece and mar its surface finish. Familiar examples of abrading devices having abrading surfaces that have a high tendency to generate repeat patterns include rotational disk sanders (that is, rotating sanders that do not provide for orbital motion of the rotational axis of the abrading surface) and powered belt sanders. The combined rotational and orbital motions of the abrading surface of an eccentric orbit abrading device may reduce the occurrence of such unsightly lines because the more complex motion described by a point on the abrading surface lessens the likelihood that repeated patterns will occur. 
     A subset of eccentric orbit abrading devices are the random orbit abrading devices. The abrading surface of random orbit abrading devices also moves in a combined rotational/orbital motion as described above, but the abrading surface freely rotates about the above-described rotational axis and is not positively driven to rotate. If left unchecked, the impulse imparted to the abrading surface as it orbits around the point offset from the surface&#39;s rotational axis causes the abrading surface to rotate about the rotational axis, and the rate of rotation of the abrading surface about the rotational axis may match the rate at which the abrading surface orbits about the axis point offset from the rotational axis. Reaching such a rotational rate is facilitated by, for example, mounting the abrading pad on a shaft that is received by low friction bearings. The shaft then defines the rotational axis and a means is provided for imparting the orbital motion to the surface. As the abrading surface contacts the workpiece with varying pressures, the frictional forces generated against the surface&#39;s rotation will vary the rotational speed of the pad and will prevent it from approaching the surface&#39;s orbital speed. The abrading surface&#39;s combined varying rotational speed and relatively constant orbital speed results in the random movement of points on the abrading surface, and this feature of random orbit abrading devices further reduces the possibility of sanding lines or other indentations being generated during the abrading operation. When sufficiently random, the action of a powered abrading device may simulate hand sanding, but will remove material from the workpiece at a substantially greater rate and will significantly speed the abrading operation. 
     A variety of hand -held eccentric and random orbit abrading devices are known in the art. Such known devices typically incorporate an arrangement of two disc members driven by a motor shaft. The motor shaft is coupled to a small electric drive motor, thus providing for hand-held operation of the device. Typically, a first disc member is coupled to the motor shaft and rotates with the motor shaft. The motor shaft thereby defines a rotational axis for the first disc member. A second disc member is rotatably mounted on the first disc member, typically received in low friction bearings, so as to substantially freely rotate relative to the first disc member. The second disc member&#39;s rotational axis is offset from the rotational axis of the first disc member. By this arrangement of elements, the second disc member may both rotate about its own rotational axis and revolve or “orbit” about the rotational axis defined by the motor shaft, thereby providing the second disc member, to which abrasive material is mounted, with the aforementioned combined rotational and orbital motions. As noted, if the second disc member may freely rotate about its rotational axis, the movement of points on the abrading surface will be random in use, and such random movement will significantly reduce the likelihood of scarring or gouging of the workpiece caused by the generation of repeat patterns. 
     The known hand-held eccentric and random orbit abrading devices are subject to several inherent shortcomings. For example, when working with relatively small wooden workpieces, the operator of hand-held eccentric or random orbit sanding devices typically may operate the device with one hand while manipulating and adjusting the orientation of the workpiece with the other hand. In such circumstances, it may be difficult to steady the workpiece, and safety concerns also arise because of the risk that the operator&#39;s hand may contact the driven sanding surface, resulting in possible injury. Additionally, the known handheld eccentric or random orbit sanding devices provide little or no precision in the control of the angle at which the workpiece contacts the device&#39;s sanding surface. Such a drawback is particularly troublesome when sanding small or easily abraded workpieces or when sanding adjacent workpiece surfaces that meet at an angle which the operator wishes to maintain in the finished article. 
     To address the foregoing shortcomings of the known hand-held eccentric and random orbit abrading devices, additional equipment such as a jig or vise have been used to immobilize the workpiece in a desired orientation and allow the operator to use both hands to manipulate the abrading device. Certain jigs or vises also have been employed to ensure that the abrading surface of hand-held abrading devices contact the workpieces at specific angles. The jigs and vises require additional expense, require time for proper mounting of the workpiece, exert pressure on the workpiece that may mar its surface, and significantly complicate the abrading process. 
     A further drawback of known hand-held eccentric and random orbit abrading devices is that as they approach a particular size and/or weight, they become difficult or impossible to use. An operator cannot readily manipulate large and/or heavy hand-held devices, and the vibrations and inertial forces generated by the combined rotational and orbital movements of the abrading surface makes the steady handling and accurate positioning of the device relative to the workpiece increasingly difficult. Certain known hand-held eccentric and random orbit abrading devices incorporate counterbalance weight means that will to some extent offset the vibrational forces generated by the eccentric rotation of the abrading surface about the motor shaft. However, as the size of the abrading surface of such devices becomes greater, the vibrational forces generated as the abrading surface eccentrically rotates about the motor shaft become increasingly significant. Dampening of such forces by counterbalance weight means eventually becomes impractical because the weight means significantly augments the weight of the device. 
     Accordingly, considering the deficiencies of the known hand-held eccentric orbit abrading devices, the need exists for an improved eccentric orbit abrading apparatus. 
     SUMMARY OF THE INVENTION 
     The present invention provides an abrading apparatus that includes a housing having a base for supporting the housing on a surface. The abrading apparatus also includes a workpiece support member that is mounted on the housing and that is for supporting a workpiece on the abrading apparatus. A motor is mounted within the housing and includes a motor shaft that may be selectively rotated about a first rotational axis. The abrading apparatus also includes a platen and a transmission for transmitting rotational motion of the motor shaft to the platen so that as the motor shaft rotates, the platen is urged to rotate about a second rotational axis. The second rotational axis, about which the platen rotates, is offset from and orbits about the first rotational axis. The first and second rotational axes may be generally parallel. 
     The transmission of the abrading apparatus of the present invention may include an intermediate member that is connected to the motor shaft and may rotate about the first rotational axis as the motor shaft rotates, and the intermediate member may also include a mounting structure that defines the second rotational axis. The mounting structure optionally may include a spindle to which the platen is operably mounted and on which the platen is rotatable so that an axis of the spindle defines the second rotational axis, about which the platen may rotate. 
     The intermediate member also may include a motor shaft mounting bore for accepting at least a portion of the motor shaft so as to connect the motor shaft to the intermediate member. In that form, an axis of the motor shaft mounting bore is coincident with the first rotational axis, the motor shaft mounting bore extends into the spindle, and the spindle is configured so that it provides a central axis coincident with the second rotational axis. The perimeter of a cross-section of the spindle preferably is generally circular, and the abrading apparatus preferably also includes a platen support for mounting the platen on the spindle so that the platen support is connected to the platen and includes a spindle receiving bore, and at least a portion of the spindle is disposed within the spindle receiving bore so that the platen support may rotate about the spindle. One or more friction reducing members such as, for example, roller bearings, may be included in connection with the platen support and/or the spindle so as to facilitate rotation of the platen support on the spindle. 
     The platen preferably has an abrasive member mounted on a surface of the platen, and the abrasive member includes an abrasive surface for abrading material from a workpiece as the platen rotates about the second rotational axis and orbits about the first rotational axis. The abrasive member may be directly attached to a surface of the platen or one or more intermediate members may be used to facilitate securely connecting the abrasive member to the platen. 
     The workpiece support member of the present abrading apparatus preferably borders on at least a portion of the platen, and the workpiece support member may include a workpiece supporting surface that is generally co-planer with the abrasive surface of the abrasive member. The abrading apparatus also may include a fence member that may be mounted on, for example, the workpiece support member and that is provided with a fence surface for partially supporting a workpiece on the abrading apparatus in a desired orientation relative to the abrasive member. In one form, the fence member may be adjustable so that an angle formed by a surface of the fence member and a surface of the workpiece support member may be adjusted. 
     The abrading apparatus of the present invention also may include a positive drive feature to urge the platen to rotate about the second rotational axis as the transmission causes the second rotational axis to orbit about the first rotational axis. The positive drive feature optionally includes a first drive member in the form of a first ring having a generally circular perimeter and that is mounted on the base so that a central axis of the first ring generally coincides with the first rotational axis. The positive drive feature also may include a second drive member in the form of a second ring with a generally circular perimeter and that is fixedly mounted on the platen so that a central axis of the second ring generally coincides with the second rotational axis. The inner perimeter of the first ring is greater than an outer perimeter of the second ring, and the second ring is positioned within and rolls along the inner circumference of the first ring so that at least a point on the outer circumference of the second ring contacts the inner circumference of the first ring as the second rotational axis orbits about the first rotational axis. By this arrangement, the platen is urged to rotate about the second rotational axis in an angular direction that is opposite to an angular direction of rotation of the motor shaft about the first rotational axis. 
     The abrading apparatus of the present invention also optionally includes an abraded matter collection system for removing at least a portion of the abraded matter from the vicinity of the abrading member disposed on the platen. In one form, the dust collection system includes vanes radially extending from a portion of the above-discussed intermediate member, and the vanes are disposed within an air guide cavity within the housing. The air guide cavity is in communication with the workpiece support surface (i.e., a pathway exists between the air guide cavity and the workpiece support surface), and the air guide cavity also includes an exhaust port for allowing air and matter entrained by the flow of air to exit from the air guide cavity. Rotation of the intermediate member about the first rotational axis forces air through the exhaust port of the air guide cavity and reduces pressure within the cavity, thereby pulling air from the vicinity of the workpiece support surface, along with entrained matter, into the air guide cavity and through the exhaust port. 
     The present invention also is directed to an abrading apparatus that includes a base for supporting the apparatus on a surface, a housing that is connected to the base, and a workpiece support member that is mounted on the housing. A motor is connected to or mounted on the housing and includes a motor shaft that may selectively rotate on a first axis of rotation. A platen is also provided that is rotatable on a second axis of rotation. The second axis of rotation differs from the first axis of rotation. The platen is coupled to the motor shaft by a transmission that causes the second axis of rotation to orbit in a circular path about the first axis of rotation as the motor shaft rotates. The abrading apparatus may include a positive drive for urging the platen to rotate in a second angular direction on the second axis of rotation as the motor shaft rotates on the first axis of rotation in a first angular direction, the first angular direction of rotation being opposite to the second angular direction of rotation. The positive drive system may include a first drive member having an inner void of a generally circular perimeter. The first drive member is connected to the base so that the central axis of the inner void generally coincides with the first axis of rotation of the abrading apparatus. The positive drive system also includes a second drive member that has a generally circular outer perimeter and that is fixedly mounted on the platen. The second drive member include a central axis that generally coincides with the second axis of rotation, and the generally circular perimeter of the inner void of the first drive member is greater than the generally circular outer perimeter of the second drive member. The second drive member is positioned within the inner void and is capable of rolling along the generally circular perimeter of the inner void as the second axis of rotation orbits about the first axis of rotation. The positive drive system may be configured so that at least a point on the generally circular outer perimeter of the second drive member contacts the generally circular perimeter of the inner void of the first drive member as the second axis of rotation orbits about the first axis of rotation so as to urge the platen to rotate on the second axis of rotation in an angular direction that is opposite to an angular direction of rotation of the motor shaft on the first axis of rotation. 
     The present invention additionally is directed to an abrading apparatus for abrading material from a workpiece and that include a housing and a base connected to the housing and for positioning the housing on a surface so that an operator may abrade material from the workpiece without the need for manipulating the position of the abrading apparatus. A workpiece support is connected to the housing for supporting the workpiece, and the abrading apparatus also includes a rotatable platen and motor mounted on the housing. The motor includes a motor shaft that may rotate about a first axis of rotation. A transmission is provided that connects to the platen and to the motor shaft and that transmits the rotation of the motor shaft to the platen. The platen is rotatable about a second axis of rotation that is defined by the transmission, and the second axis of rotation is offset from the first axis of rotation and also orbits about the first axis of rotation as the motor shaft rotates. 
     The present invention is further directed to an abrading apparatus for abrading material from a workpiece and that includes a housing, means for positioning the housing on a surface, a workpiece support that is connected to the housing for supporting a workpiece on the apparatus, and a platen that is rotatable about an axis of rotation. A motor having a rotatable motor shaft is also provided, and a transmission couples the motor shaft and the platen and urges the axis of rotation of the platen to orbit about a point as the motor shaft rotates. The point may lie on a line that is coincident with an axis of rotation of the motor shaft. 
     The abrading apparatus of the present invention may be positioned on a surface such as, for example, a work bench or a stand or other dedicated support structure. The platen may rotate and also orbit so as to provide the advantages of a combined rotational/orbital motion that will decrease the tendency for a point on the abrading member of the abrading apparatus to move in a repeat pattern that may mar the surface of the workpiece. Because the abrading apparatus of the present invention is self-supporting, it need not be manipulated by hand as is required with the existing eccentric orbit sanding and grinding apparatuses, all of which are hand-held. Thus, the operator&#39;s hands will be free to manipulate the workpiece and, consequently, fine detail work may be performed with greater ease relative to existing devices. Small workpieces also may be manipulated by both of the operator&#39;s hands when using the present abrading apparatus, and the possibility of the operator&#39;s hands contacting the driven abrading member is lessened. Also, angles, curves, and other complicated surface forms may be abraded with greater ease than with the existing, hand-held eccentric orbit sanding and grinding apparatuses, all of which require that the operator either hold the workpiece in one hand or place the workpiece in a vise or other like device while manipulating the abrading apparatus with his or her one or two free hands. 
     Accordingly, the present invention provides an improved abrading apparatus that addresses certain deficiencies associated with existing abrading devices. These and other details, objects, and advantages will become apparent as the following detailed description of embodiments of the present invention proceeds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, preferred embodiments of the present invention are shown, wherein like reference numerals are employed to designate like parts and wherein: 
     FIG. 1 is a front perspective view of an embodiment of the apparatus of the present invention; 
     FIG. 2 is a front elevational view of the embodiment depicted in FIG. 1; 
     FIG. 3 is a rear elevational view of the embodiment depicted in FIG. 1; 
     FIG. 4 is a left side elevational view of the embodiment depicted in FIG. 1; 
     FIG. 5 is a right side elevational view of the embodiment depicted in FIG. 1; 
     FIG. 6 is an assembly view of the embodiment depicted in FIG. 1; 
     FIG. 7 is a top view of the embodiment depicted in FIG. 1; 
     FIG. 8 is a cut-away left side view of the embodiment depicted in FIG. 1 taken along the line VIII—VIII in FIG. 7; 
     FIG. 9 is a top-view of the platen support of the embodiment depicted in FIG. 1; 
     FIG. 10 is a side cut-away view of the platen support depicted in FIG. 9, and taken along the line X—X in FIG. 8; 
     FIG. 11 is a bottom view of the platen of the embodiment depicted in FIG. 1; 
     FIG. 12 is a top view of the mounting member and rigid ring of the embodiment depicted in FIG.  1 . 
     FIG. 13 a schematic representation of the interaction between the outer cylindrical raised ring and rigid ring of the embodiment depicted in FIG. 1; 
     FIG. 14 is a top view of the embodiment of FIG. 1 showing the device with the workpiece support and platen removed and particularly showing the arrangement of elements within the opening in the mounting member of the housing; 
     FIG. 15 is a top view of the spindle of the embodiment depicted in FIG. 1; and 
     FIG. 16 is a cross-sectional view in isolation of the fan wheel of the embodiment depicted in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Referring now to the accompanying figures for the purpose of illustrating embodiments of the invention only, and not for the purpose of limiting the same, the several figures show various aspects of a stationary bench-top eccentric orbit sanding device  10  within the scope of the present invention. As shown in particular in FIGS. 1-5, stationary sanding device  10  includes a housing  14  and a workpiece support in the form of a work table  15  for supporting a workpiece (not shown) that is to be abraded. A platen (labeled as  17  in the figures in which it is exposed) having an abrasive sheet  19  fastened to the upper surface thereof is rotatably mounted within the device  10 . The abrasive sheet  19  abrades the workpiece as the platen  17  is driven to rotate and orbit, as is further described below. The perimeter of the circular platen  17  may be of any shape, but preferably is circular as shown in the accompanying figures. As further shown in FIG. 1, the platen  17  preferably is positioned so that its upper surface, to which the abrasive sheet  19  is coupled, is generally centrally located on the upper surface of the table  15  and is generally within the plane formed by the upper surface  20  of the work table  15 . A collar  22  of a corresponding shape is preferably provided attached to the tablel 5 adjacent the perimeter of the platen  17  so as to form a border between the surface of the table  15  and the surface of the platen  17 . The collar  22  fills the gap between the table  15  and the platen  17  and should not extend significantly above the plane of the upper surface  20  of the table  15 . The collar  22  thereby prevents the workpiece from striking the exposed edge of the table  15  or from being lodged between the table  15  and the perimeter of the platen  17 . The collar  22  preferably is constructed of a resilient material such as, for example, soft plastic or rubber, that will not mar the workpiece if it should contact the collar  22 . 
     Sanding device  10  also preferably includes a fence  25 . As best shown in FIGS. 1,  4 , and  5 , a surface  27  of the fence  25  preferably is oriented generally perpendicular to the upper surface  20  of the table  15  and lies across the length of the upper surface  20 , and the fence  25  also preferably is oriented such that it overlies a portion of platen  17 . The surface  27  of the fence  25  may be used to retain a workpiece against the abrasive sheet  19  during sanding, and the perpendicular orientation of the surface  27  also aids in the sanding of 90 degree corners and edges. It will be understood that the inclusion of a fence is optional and, alternately, any other known fence arrangement may be utilized with the sanding device  10  in order to facilitate the accuracy and ease of use of the device  10 . Such alternate fence arrangements include, for example, those wherein the angle of the surface  27  relative to the upper surface  20  of the table  15  is adjustable, and also include those fence arrangements wherein the extent by which the platen  17  is overlain by the fence  25  may be adjusted. As shown in FIG. 4, the sanding device  10  also may include a support piece  28  that will reinforce the fence  25  in the proper angle during use. The table  15  and the fence  25  preferably are constructed of materials that will not mar wooden workpieces supported thereby during sanding. 
     Housing  14  is of sufficient size to fully and independently support the remaining elements of the sanding device  10  on a flat surface such as a table top, workbench, or work stand. As such, in operation the device  10  does not need to be manipulated during sanding and the operator&#39;s hands will be entirely free to manipulate the workpiece relative to the driven abrasive sheet  19 . Preferably, the base  14  includes two or more support members  30  and  31  to aid in laterally stabilizing the device  10  during sanding. In one preferred arrangement, shown in FIGS. 1-5, support members  30  and  31  are configured so as to form individual trays  33  and  35  that may be used to store accessories for the device  10 . It will be understood from a consideration of FIG. 1 that the one or more support members  30  and  31  may be suitably designed to provide a wide base for the device  10  that will inhibit the device from vibrating significantly during use and that will also prevent the device  10  from tipping to either side when force is applied to an end of the table  15 . The support members  30  and  31  also each may include one or more foot members  37  on which the weight of the device will be supported. Preferably, the housing  14  and its support members  30  and  31  are positioned so as to support the table  15  and platen  17  in a substantially horizontal orientation when the device  10  is disposed on a horizontal surface. However, additional arrangements (not shown) are possible in which, for example, the table  15  and the platen  17  may be selectively oriented at various angles relative to the surface on which the device is disposed. All such additional arrangements are within the scope of this disclosure. 
     The internal elements of the embodiment of the invention depicted in the accompanying FIGS. 1-5 and the interrelationship of those elements are shown in accompanying FIGS. 6-15. As particularly shown in the assembly view of FIG.  6  and in the cross-sectional view of FIG. 8, the upper surface of the platen  17  has affixed thereon a transition pad  39  that preferably is constructed so to fixedly receive an undersurface of the abrasive sheet  19 . For example, the transition pad  39  may have hooks that detachably yet securely mate with loops on the undersurface of the abrasive sheet  19 , or the transition pad  39  may detachably yet securely mate with an adhesive compound resident on the undersurface of the abrasive sheet  19 . 
     As best shown in FIGS. 8 and 11, the undersurface of the platen  17  includes an inner cylindrical raised ring  41  and an outer cylindrical raised ring  43 , and the inner and outer raised rings  41  and  43  are concentric. The platen  17  further includes three bores  45  defined entirely through the platen  17  within inner raised ring  41 . 
     The abrading platen  17  is connected to a platen support  47 . The platen support  47  is best shown in FIGS. 9 and 10 and includes sloping sides  48  and is generally in the shape of a frustum of a cone. The platen support  47  includes a circular, raised outer rim  49  and a central cylindrical bore  50 , and the walls of the bore  50  are defined by a bearing  52  so that a shaft seated within the bore  50  may rotate therein with a reduced level of friction. Platen support  47  further includes three threaded bores  54  defined in a pattern matching that of bores  45  through the platen  17 . As illustrated in FIG. 8, the space defined within the rim  49  of the platen support  47  is such that a portion of the inner ring  41  projecting from the undersurface of the platen  17  seats within rim  49 . When seated in that fashion, bores  45  through platen  17  align with threaded bores  54  of the platen support  47 , and threaded fasteners disposed through the aligned bores fasten the platen  17  to the platen support in device  10 . Connected in that arrangement, the central axis of bore  50  of the platen support  47  preferably is substantially coincident with the center point of platen  17 . 
     Sanding device  10  further includes a transmission linking movement of a motor shaft  56  of a motor  58 , mounted within housing  14 , to the platen  17 . In order to mount the platen  17  and its attached platen support  47  for rotation about a point and relative to the upper surface  20  of the worktable  15 , the transmission includes an intermediate member in the form of a fan wheel  60 . As best illustrated in FIG.  8  and in isolation in FIG. 16, fan wheel  60  includes a central hub  62  from which extend laterally several fan vanes  64 . The fan vanes  64  are partially supported by a disk-shaped element  66  which also radiates outward from central hub  62 . A generally cylindrical spindle  68  projects from the central hub  62  of the fan wheel  60 , and the hub  62  and spindle  68  includes a stepped central bore  70  having a lower portion  70 A, an upper portion  70 B of reduced cross-sectional diameter relative to the lower portion, and a rim  72  defined by the transition from the lower portion  70 A to the upper portion  70 B of the central bore  70 . The upper and lower portions  70 B and  70 A share an identical central axis. As further described below, bore  70  accepts the elongate cylindrical motor shaft  56  of the motor  58 , and the rotation of the shaft  56  is imparted to the fan wheel  60 . 
     As indicated in particular in FIGS. 6 and 8, sanding device  10  includes motor  58  mounted within housing  14  so that the rotatable motor shaft  56  is oriented upward and toward platen  17 . Motor shaft  56  may have a configuration that corresponds to the central bore  70  of the fan wheel  60 . Thus, motor shaft  56  may have a stepped cylindrical exterior configuration that will closely conform to the separate upper and lower portions  70 A and  70 B, respectively of central bore  70 . 
     The combined rotational and orbital motions of platen  17  are provided by the configuration hub  62 . As best indicated in FIG. 8, the central axis of the bore  70  in the hub  62  is offset from the central axis of the spindle  68 . The arrangement of the upper portion  70 B of bore  70  in spindle  68  is represented in FIG. 15, which is a view looking downward onto spindle  68  of the hub  62  as the hub  62  is oriented in FIG.  8 . The spindle  68  of the fan wheel is disposed within the central bore  50  of the platen support  47  which in turn is secured to the undersurface of the platen  17 . The platen support  47  is secured to spindle  68  by washer  74  and threaded fastener  76 , which is disposed through washer  74  and is threadedly secured into upper portion  56 A of motor shaft  56 , which is correspondingly threaded to accept fastener  76 . Washer  74  is sized so as to overlap the bearing  52  and, when secured to spindle  68 , prevents platen support  47  from being removed from spindle  68 . Thus, it will be understood that the central axis of the spindle  68 , which defines the axis of rotation of the platen  17 , is offset from the axis of rotation of the spindle  68 , which coincides with the central axis of the motor shaft  56 . Thus, the axis rotation of the platen  17  is caused to orbit about the axis of rotation of the fan wheel  60 , and the platen support  47  and the platen  17  also may rotate about the central axis of spindle  68  on bearings  52 . The combination of the simultaneous rotational and orbital motions of the platen  17  results in eccentric rotation of the platen  17  about the central axis of the motor shaft  56 . Thus, if the platen  17  were allowed to freely rotate (i.e., absent friction) as it orbits, its rotational rate (i.e., revolutions/minute) would eventually reach its orbital rate (orbits/minute), and the angular directions of rotation and revolution would be the same (i.e., clockwise or counterclockwise). 
     To prevent the platen  17  from either stalling or reaching too great a rate of rotation, a positive drive feature may be provided in the apparatus  10  to impart positive rotational motion to the platen  17  during operation. The above-described outer raised ring  43  may be considered a first drive member of the positive drive feature. As shown in particular in FIG. 11, the underside of platen  17  includes a resilient ring  80  mounted on the outer perimeter of outer raised ring  43 . Resilient ring  80  is therefore centered on the axis formed by the central axis of spindle  68 , as is described above. Resilient ring  80  may be formed from an elastic rubber, plastic, or another similarly resilient material, and may be seated in a trough  82  on the outer perimeter of outer raised ring  43 . The circumference of resilient ring  80  is preferably slightly smaller than that of trough  82 , and, as such, resilient ring  80  may be fitted into trough  82  and held there under elastic tension without the need for additional adhesives or other fastening means and so that the ring  80  experiences little or no independent rotation or slippage relative to trough  82  during operation. 
     As shown in FIG. 6, the positive drive feature further includes a second drive member in the form of a rigid ring  84  that is mounted to a mounting member  86  portion of the housing  14 . A top view of the mounting member  86  and the rigid ring  84  attached to the mounting member  86  by fasteners  88  is shown in FIG.  12 . As shown in particular in FIG. 12, the rigid ring  84  is fixedly mounted on a surface of the mounting member  86  so that the rigid ring  84  leads into an opening  90  in the mounting member  86 . The opening  90  is centered about the axis of motor shaft  56 , and the rigid ring  84  is generally concentric about an axis of the motor shaft  56 . As shown in particular in FIG. 13, the inner circumference of rigid ring  84  is greater than the outer circumference of outer raised ring  43 . When the device  10  is assembled, ring  43  is nested within the interior of rigid ring  84 . As such, the two rings  43  and  84  lie substantially in a single plane. However, it will be understood that the central axes of the two rings  43  and  84  are not coincident because their central axes are offset by the distance by which the axis of spindle  68  is offset from the central axis of the motor shaft  56 . Such offset is sized so that at least one point on the exposed surface of resilient ring  80  contacts at least one point along the inner circumference of the rigid ring  84  at all times. As such, when the device  10  is in operation and the motor shaft  56  is rotating, fan wheel  60  rotates along with motor shaft  56  and platen  17  orbits on spindle  68  about the axis of rotation of the motor shaft  56 . When disc  17  orbits, ring  43  and its resilient ring  80  are caused to roll in an orbit about the inner circumference of the rigid ring  84 . The frictional interaction between rigid ring  84  and resilient ring  80  urges the ring  43  (and thus the platen  17  attached thereto) to rotate in a direction of angular movement that is opposite to the direction of angular movement in which the platen  17  would rotate if it were able to freely rotate as it was caused to orbit through its eccentric linkage with the motor shaft  56 . Only if the frictional interaction between the resilient ring  80  on the outer circumference of the ring  43  and the inner circumference of the rigid ring  84  is overcome, and a slippage in the relative rolling motion of the two rings  43  and  84  occurs, will the rotation of the abrading platen  17  set up by the positive drive feature be retarded. 
     Although a resilient ring  80  is included in the present embodiment, it will be understood that if rings  43  and  84  are constructed of appropriate materials such a resilient ring  80  may not be needed to provide adequate frictional interaction between the rings  43  and  84 . For example, one of the rings may be constructed of a hard rubber material that frictionally contacts the remaining ring, or both rings  43  and  84  may be constructed of the hard rubber material. 
     An example of the relative movement of the rings  43  and  84  as the motor shaft  56  rotates is provided in FIG. 13, which is a view looking downward onto the motor, which would be disposed beneath the plane of the paper. For clarity, FIG. 13 excludes resilient ring  80  from the outer perimeter of ring  43 . As the motor shaft rotates in a clockwise direction, the outer circumference  91  of the ring  43  rolls relative to the inner circumference  92  of the ring  84  in the clockwise angular direction of the external arrow. This rolling motion causes the ring  43  to rotate in the opposite angular direction (counterclockwise), indicated by the internal arrow. 
     It will be understood that in certain conditions of use sufficient force may be applied to the surface of the platen  17  to upset the relative rolling motion of the two rings  43  and  84  and cause ring  43  to slip relative to ring  84 . It will further be understood that the threshold force at which such slippage between rings  43  and  84  occurs can be increased, or such slippage possibly may be entirely prevented, by excluding resilient ring  80  and, instead, providing the outer circumference  91  of the ring  43  and the inner circumference  92  of the rigid ring  84  with corresponding sets of geared teeth. As such, the teeth on ring  43  would be caused to interlock with the teeth on the rigid ring  84  and the ring  43  would orbit about the inner circumference of the rigid ring  84 . In such an embodiment (not shown), the mechanical linkage provided by the intermeshing teeth would have to be overcome before any slippage would occur between the two rings  43  and  84 . Other alternate arrangement for providing positive rotation of the platen  17  as the motor shaft  56  rotates will be apparent to those of ordinary skill in the art upon consideration of the present description of the invention. 
     The present embodiment  10  additionally may incorporate a dust collection feature. As indicated in FIG. 8, the fan wheel  60  is nested within a cylindrical air guide cylinder  96  that defines the walls of an air guide cavity  98  that is in communication with the upper surface  20  of the work table  15 . The fan wheel  60  is provided with a plurality of radially emanating vanes  64 . In operation, the fan wheel  64  rotates along with the motor shaft  56 , and the vanes  64  create a partial vacuum within the air guide cavity  98  by forcing air within the air guide cavity  98  out through exhaust port  100 , which communicates the air guide cavity  98  with the atmosphere exterior to the housing  14 . Dust and other abraded matter from the abrading process, pulled to the edge of the abrading platen  17  by centrifugal force, is drawn by the partial vacuum into the housing and into the air guide cavity  98  as indicated generally by the arrow in FIG. 8, and is expelled through the exhaust port  100 . It will be understood that the exhaust port  100  optionally may be connected to a dust collection device or chamber as are known in the art as a means to collect the dust. 
     The oscillation of the abrading platen  17  may create a vibration force. Accordingly, as shown in particular in FIG. 14, one or more counterbalance weights may be mounted as needed on the fan wheel  60 , or on other elements of the device  10 , to counteract the vibrations. In the present embodiment, two weights  102  are shown positioned between vanes  64  on the surface of the fan wheel  60 . In one manner to counteract vibrational forces, the weights may be symmetrically disposed about a line connecting the axes of spindle  68  and motor shaft  56  as is indicated in FIG.  14 . 
     Those of ordinary skill in the art will, of course, appreciate that various changes in the details, materials and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.