Abstract:
A dental crown polishing apparatus capable of improving the efficiency of polishing dental crowns and contributing to reducing the cost of the polishing process is disclosed. The dental crown polishing apparatus has a motor, a ball bearing assembly arranged around a motor shaft of the motor and equipped with a plurality of balls which rotate on its axis and circularly move around the motor shaft by receiving torque from the motor shaft, a brush assembly mounted on the outer periphery of the ball bearing assembly, equipped with brushes that contact the plurality of balls to rotate and that have bristles in the same direction as an axial direction of such rotation and a tray disposed opposite and open to the brush assembly so as to dispose a crown toward the brushes. The motor shaft is fixedly mounted at a position offset from the center of the brush assembly. The brush assembly is able to move in orbital fashion by the rotation of the motor. The tray is able to rotate separately from the brush assembly. The rotation of the tray is not synchronous with the rotation of the brush assembly. Dental crown polish is achieved by an orbital movement of the brushes.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims the benefit of Japanese Patent Application No. 2005-306564 filed on Oct. 21, 2005, the entire contents of which are hereby incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a dental crown polishing apparatus for polishing dental crowns and the like.  
         [0004]     2. Description of the Related Art  
         [0005]     Dental crowns, bridges, dentures and the like (hereinafter referred to collectively as “crowns”) are produced in a dental crown laboratory. Typically, crowns are polished in the finishing stage. The crowns are machined to a complicated, uneven shape so as to give them the same capabilities as a natural tooth.  
         [0006]     In the conventional polishing step, as shown in  FIG. 11 , a dental lathe or a hand piece  100  (hereinafter “hand piece or the like”) is used and each crown  110  is polished individually by hand. In general, the hand piece or the like  100  is mounted with a disc-like polishing brush  102  disposed on a rotary shaft  101 . The disc-like polishing brush  102  is so large compared to the size of the deeply uneven surface of the crown  110  that the deep grooves can not be polished precisely without breaking the crown  110 .  
         [0007]     The inventor of the present invention previously invented an orbital dental crown polishing device equipped with small brushes adequate to the task of polishing crowns one at a time, as disclosed in Japanese Laid-Open Patent Application Publication No. 2003-048145 (“Pat. Pub. No. 2003-048145”).  FIG. 12  shows a side view of the device disclosed in Pat. Pub. No. 2003-048145. As shown in  FIG. 12 , a brush  151  is contact with the uneven surface of a crown  160  and the crown  160  is moved manually while a device  150  supplies fluid containing a polishing agent (polishing slurry) to the brush  151  so as to polish the surface of the crown  160 . The apparatus  150  makes precise polishing of the crown  160  possible without damaging it.  
         [0008]     However, the apparatus disclosed in Pat. Pub. No. 2003-048145 has room for further improvement. In particular, since it is necessary to hold each crown in hand and polish it, a great deal of time is required to polish many crowns, and consequently the cost of processing the crowns increases.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, the present invention is conceived in light of the above-described circumstances, and has the purpose of providing a dental crown polishing apparatus capable of improving the efficiency of polishing the crowns and contributing to reducing the cost of the polishing process.  
         [0010]     To achieve the above-described purpose, the present invention provides a dental crown polishing apparatus comprising:  
         [0011]     a motor;  
         [0012]     a ball bearing assembly arranged around a motor shaft of the motor and equipped with a plurality of balls which rotate on its axis and circularly move around the motor shaft by receiving torque from the motor shaft;  
         [0013]     a brush assembly mounted on the outer periphery of the ball bearing assembly, equipped with brushes that contact the plurality of balls to rotate and that have bristles in the same direction as an axial direction of such rotation; and  
         [0014]     a tray disposed opposite and open to the brush assembly so as to direct a crown toward the brushes,  
         [0015]     wherein the brush assembly is attached to the motor shaft in a position offset from the center of the motor shaft,  
         [0016]     the brush assembly is able to move in orbital fashion by the rotation of the motor,  
         [0017]     the tray is able to rotate separately from the brush assembly, the rotation of the tray is not synchronous with the rotation of the brush assembly, and,  
         [0018]     the brushes polish the dental crown in an orbital movement.  
         [0019]     Therefore, by setting multiple crowns in the tray and supplying polishing agent thereto, multiple crowns can be polished in a single polishing operation. Furthermore, since the brush rotates independently of the tray and moves orbitally, the crowns are polished while contacting the brush at multiple locations. As a result, even the most uneven parts of the crowns can be thoroughly polished.  
         [0020]     In addition, according to one aspect of the present invention, the brush assembly comprises a power transmission means for transmitting torque to the tray, and the tray comprises a passive means for receiving torque from the power transmission means with a rotation permitting means for permitting the tray to rotate. The tray intermittently receives force in the direction of rotation by receiving the eccentric drive from the brush assembly through the passive means and rotates separately from the brush assembly.  
         [0021]     Accordingly, even without providing a separate drive source for rotating the tray, by using only the motor for driving the brush assembly the brush assembly and the tray can be rotated separately from each other and the brush assembly is made to move orbitally.  
         [0022]     In addition, according to another aspect of the present invention, the power transmission means comprises a first projecting member projecting from the periphery of the brush assembly and the passive means comprises a second projecting member projecting from the periphery of the tray.  
         [0023]     Accordingly, caused by the orbital movement of the brush assembly; the drive force of the brush assembly is easily transmitted to the tray in the circumferential direction of the tray with a simple structure.  
         [0024]     In addition, according to another and further aspect of the present invention, the rotation permitting means is a ball bearing assembly disposed between the tray and a pedestal positioned beneath the tray.  
         [0025]     Accordingly, the tray receiving the drive force from the brush assembly rotates smoothly in the circumferential direction.  
         [0026]     In addition, according to yet another and further aspect of the present invention, the dental crown polishing apparatus further comprises a means for adjusting the rotation of the tray.  
         [0027]     Accordingly, when the tray receives the drive force from the brush assembly and rotates, resistance against the rotation can be adjusted, and thus a rotation of the tray that is suited to each polishing process can be achieved.  
         [0028]     In addition, according to still another and further aspect of the present invention, the dental crown polishing apparatus further comprises a means for adjusting the distance of the tray from the brush assembly.  
         [0029]     Therefore, without setting the crowns in the tray at the optimum height, good polishing can still be achieved. Moreover, by adjusting the height of the tray according to the extent of polishing, it is possible to accommodate a variety of polishing conditions.  
         [0030]     In addition, according to still another and further aspect of the present invention, the dental crown polishing apparatus further comprises a means for adjusting the horizontal angle of the brush with respect to an open surface of the tray.  
         [0031]     Accordingly, by setting the brush at a slight angle, it is possible to thoroughly polish even the deepest grooves, even if there is wide variation in the depth of the grooves in the uneven parts of the crowns.  
         [0032]     The present invention improves the efficiency of polishing the crowns and contributes to reducing the cost of the polishing process.  
         [0033]     Other features, objects and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]      FIG. 1  shows a side view of the dental crown polishing apparatus according to an embodiment of the present invention;  
         [0035]      FIG. 2  shows a front view of the dental crown polishing apparatus according to an embodiment of the present invention;  
         [0036]      FIG. 3  shows an enlarged view of the area of the roller mounting plate shown in  FIG. 1 ;  
         [0037]      FIG. 4  shows a state in which the height of a pedestal is adjusted by lifting in the direction indicated by an arrow X to a position at which the brush contacts a crown in the tray in the dental crown polishing apparatus shown in  FIG. 1 ;  
         [0038]      FIG. 5  shows an open surface of the tray as seen from above with a columnar post cut off between a fixed shaft that fixedly mounts an arm so as to allow the arm to rotate and the uppermost part of a multi-stage stopper in the dental crown polishing apparatus shown in  FIG. 1 ;  
         [0039]      FIG. 6  shows an enlarged view of a partial cross-section of the tray periphery in the dental crown polishing apparatus shown in  FIG. 1 ;  
         [0040]      FIG. 7  shows a head in the dental crown polishing apparatus shown in  FIG. 1  as seen from the brush side (that is, from below);  
         [0041]      FIG. 8  shows the first brush unit removed so as to allow the interior of the ball bearing assembly to be seen in the state shown in  FIG. 7 ;  
         [0042]      FIG. 9  is a diagram illustrating the ideal movement and the actual movement of the brush assembly in the state shown in  FIG. 7 ;  
         [0043]      FIG. 10  shows an enlarged view of the area of the roller mounting plate different from that shown in  FIG. 3 ;  
         [0044]      FIG. 11  is a diagram illustrating the conventional method of polishing a crown or the like; and  
         [0045]      FIG. 12  is a diagram illustrating a method of polishing a crown or the like using the polishing apparatus previously invented by the inventor of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]     Preferred embodiments of the present invention will now be described, with reference to the accompanying drawings.  
         [0047]      FIG. 1  shows a side view of a dental crown polishing apparatus according to an embodiment of the present invention.  FIG. 2  shows a front view of the dental crown polishing apparatus according to an embodiment of the present invention. It should be noted that, in the following description, up, down, left and right in  FIG. 1  are called “top”, “bottom”, “front” and “rear”, respectively.  
         [0048]     The dental crown polishing apparatus  1  according to the present embodiment comprises a rectangular planar fixed base  10 , a support post  11  disposed so as to protrude in a direction perpendicular to the fixed base  10  from a position offset from a center top point on the fixed base  10 , an annular ring member  12  into which the support post  11  is inserted and which is mounted so as to be movable vertically in the direction of the length of the support post  11 , a planar pedestal  13  connected to the ring member  12  and having a substantially square broad surface extended parallel to the top surface of the fixed base  10  toward a perpendicular line that penetrates the top of the fixed base  10 , an arm  14  provided on top of the support post  11  and extended toward a perpendicular line that penetrates the top of the fixed base  10 , and a head  18  connected to the arm  14  and disposed above the pedestal  13 .  
         [0049]     A cut of predetermined length is made in the top of the support post  11  from the top toward the bottom. One end of the arm  14  is inserted into this cut. The end of the arm  14  inserted into the cut is penetrated by a fixed shaft  15  inserted from a side of the support post  11  at substantially a right angle to the point at which the end of the arm  14  is inserted into the cut in the support post  11 . One end of the arm  14  has two planar projecting parts  14   a ,  14   b  extended from the hole that is penetrated by the fixed shaft  15  to the tip of the end. The tips of the projecting parts  14   a ,  14   b  project beyond the cut in the support post  11 . A shaft  16  penetrates the tip of the projecting part  14   a . The shaft  16  has a handle at one end and a slot for a screw at the other end. The handle  17 , the shaft  16 , the fixed shaft  15  and the arm  14  form one example of a brush angle adjustment means. A flange part  16   a  having a diameter larger than the diameter of the screw hole is provided on the upstream end of the screw slot of the shaft  16  so as to allow the projecting part  14   a  to be pressed by being screwed toward the projecting part  14   b  of the shaft  16 . A screw hole is formed on the surface opposite the projecting part  14   a  that is the tip of the projecting part  14   b  into which the tip of the side having the screw slot of the shaft  16  is screwed.  
         [0050]     Accordingly, as the handle  17  is turned and the shaft  16  is screwed into the projecting part  14   b , the gap between the projecting part  14   a  and the projecting part  14   b  narrows and the circumference of the fixed shaft  15  is tightened. Conversely, as the handle  17  is rotated so as to withdraw the shaft  16  from the projecting part  14   b , the gap between the projecting part  14   a  and the projecting part  14   b  widens and the circumference of the fixed shaft  15  is loosened. As a result, after the handle  17  is turned so as to loosen the hold of the one end of the arm  14  on the fixed shaft  15  and the arm  14  is set to a desired angle, the handle  17  is then turned so as to tighten the hold of the one end of the arm  14  on the fixed shaft  15 , enabling the head  18  to be held at a set angle. As shown in  FIG. 1  by the solid lines and the dot-and-dash lines, the angle of the head  18  about the fixed shaft  15  can be changed.  
         [0051]     The head  18  is substantially bell-shaped, the bottom of which is substantially flat. A switch  18   a  is provided on the outside of the head  18 . A motor  19  (indicated by the dotted line in  FIG. 1 ) is contained in the interior of the head  18 . A power cord  25  is connected to one side of the fixed base  10 . The power cord  25  is electrically connected to the motor  19 . By plugging the power cord into a household outlet or the like and turning the switch  18   a  ON and OFF, the rotation of the motor  19  can be started and stopped. Although the motor  19  described in the present embodiment can run at several thousand to several tens of thousands of revolutions per minute, it is possible to use motors that can rotate at more or fewer revolutions per minute. Moreover, a variety of motors may be used for the motor  19 , including a DC motor, an AC motor and a stepping motor.  
         [0052]     A brush assembly  21  mounted on a projecting rotary shaft  20  that is coupled to the motor shaft of the motor  19  is provided on the bottom of the head  18  facing the pedestal  13 . The brush assembly  21  is disc-shaped, and has brushes  22  on the surface facing the pedestal  13 . A total of four first projecting members  23  spaced 90 degrees apart are connected to the periphery of the brush assembly  21 . The first projecting members  23  have a L shape, extending beyond the edge of the brush assembly  21  in the direction of the diameter of the brush assembly  21  and then downward toward the pedestal  13  (below) at (substantially) a right angle. The first projecting members  23  are examples of a power transmission means (to be described later) that transmits the power of rotation to a tray on the pedestal  13 .  
         [0053]     The ring member  12  coupled to the pedestal  13  is mounted on the support post  11  in such away as to engage a multi-stage stopper  26  provided on the rear of the support post  11  up to a optimum height from the bottom of the support post  11 . A knob  27  for adjusting the tightness of the ring member  12  against the support post  11  is mounted on the ring member  12 . At least one of the knob  27  and the multi-stage stopper  26  constitutes a means for adjusting the height of the tray. When moving the ring member  12  along the length of the support post  11 , the knob  27  is turned to loosen the force with which the ring member  12  is held against the support post  11 . When fixing the ring member  12  in place at an intermediate position along the support post  11 , the knob  27  is turned to tighten the force with which the ring member  12  is held against the support post  11 . The stopper  26  is a multi-stage one, and thus the ring member  12  can be moved up and down in stages.  
         [0054]     A tray mounting plate  30  is disposed on the surface of the pedestal  13  facing the head  18 . The tray mounting plate  30  is connected to the pedestal  13  through a ball bearing assembly  31 . The ball bearing assembly  31  constitutes a means to permit the tray mounting plate  30  to rotate. A container (what is herein called a “tray”)  32  is fixedly mounted on the tray mounting plate  30  with the open face towards the brushes  22 . The side of the tray  32  is composed of a transparent cylindrical panel  34 . It should be noted that the cylindrical panel  34  may be opaque or semi-transparent. A total of four second projecting members  33  spaced 90 degrees apart are connected to the periphery of the cylindrical panel  34 . The second projecting members  33  are flat panels extending in the direction of the diameter of the tray  32 . As will be described later, the second projecting members  33  constitute passive means for receiving torque from the first projecting members  23  of the brush assembly  21 .  
         [0055]     In the tray  32 , for example, multiple dental crowns A fixed in a resin agent B can be placed (In  FIG. 1  and in the remaining diagrams, although only some of the crowns A are given the reference designation “A”, it is to be understood that reference designation A applies to all crowns depicted therein.). In  FIG. 1  and  FIG. 2 , the cylindrical panel  34  is transparent, and thus the crowns are drawn as if they are visible from the outside. The exact method of polishing the crowns is described in detail later.  
         [0056]     On the pedestal  13 , a L-shaped roller mounting shaft  35  is fixedly mounted on the rear side of the tray mounting plate  30 . A roller  36  is mounted on the roller mounting shaft  35  so as to contact the periphery of the tray mounting plate  30 . The roller  36  thus contacts the side of the tray mounting plate  30  so as to enable the tray mounting plate  30  to rotate smoothly, without rolling. It should be noted that a plurality of rollers  36  may be provided around the periphery of the tray mounting plate  30 . Moreover, a roller mounting plate  37  is fixedly mounted on the front of the tray mounting plate  30  at a position 180 degrees from the roller  36 .  
         [0057]      FIG. 3  shows an enlarged view of the area of the roller mounting plate  37 .  
         [0058]     The roller mounting plate  37  comprises a fixed vertical panel  37   a  that extends vertically from the pedestal  13  and two horizontal panels  37   b  and  37   c  extending substantially parallel to the top of the pedestal  13  in the direction of the tray mounting plate  30  from the fixed vertical panel  37   a . The two horizontal panels  37   b  and  37   c  are fixedly mounted on the vertical panel  37   a  at positions separated in the vertical direction by a predetermined interval. A roller  38  whose center is penetrated by a rotary shaft  38   a  mounted vertically with respect to the horizontal panels  37   b  and  37   c  is provided between the horizontal panel  37   b  and the horizontal panel  37   c . The roller  38  is provided between the horizontal panel  37   b  and horizontal panel  37   c  in such a state as to be rotatable while contacting the periphery of the tray mounting plate  30 .  
         [0059]     A shaft  39  is disposed on a side 180 degrees opposite the point of contact between the roller  38  and the tray mounting plate  30 . The tip of the shaft  39  contacts the roller  38 . A knob  40  is mounted on the other end of the shaft  39  on a side 180 degrees opposite the portion of the shaft  39  that contacts the roller  38 . Screw threads  39   a  are cut into the shaft  39  over a predetermined length from the end of the shaft  39 , and the threaded shaft  39   a  is screwed into a threaded through-hole  37   d  similarly provided on the vertical panel  37   a  of the roller mounting plate  37 . Accordingly, as the knob  40  is turned and the shaft  39  is screwed toward the back of the apparatus, the tip of the shaft  39  strongly contacts the roller  38 , which not only restrains the rotation of the roller  38  but also increases the resistance to the rotation of the tray mounting plate  30 . In the present embodiment, the knob  40  and the shaft  39  constitute an example of rotation adjustment means.  
         [0060]      FIG. 4  shows a state in which the height of the pedestal  13  is adjusted and the pedestal  13  is lifted in a direction indicated by an arrow X to a position at which the brushes  22  contact the crowns A in the tray  23  in the dental crown polishing apparatus shown in  FIG. 1 .  
         [0061]     The brush assembly  21  has a disc-like shape of smaller diameter than the opening in the tray  32 . In addition, a portion extending in the vertical direction of the first projecting members  23  contacts the second projecting members  33  provided around the periphery of the tray  32 . The first projecting members  23  and the second projecting members  33  are each disposed 90 degrees apart around the periphery of the brush assembly  21  and the tray  32 , respectively. Therefore, as shown in  FIG. 3 , when a given first projecting member  23  contacts one side of a given second projecting member  33 , the remaining first projecting members  23  also contact the same sides of the remaining second projecting members  33 . It should be noted that the number of the first projecting members  23  or the second projecting members  33  is not limited to the four described in the present embodiment, and may thus consist of one, two, three, or five or more.  
         [0062]      FIG. 5  shows the open surface of the tray  32  as seen from above with the support post  11  cut off between the fixed shaft  15  that fixedly mounts the arm  14  so as to allow the arm  14  to rotate and the uppermost part of the multi-stage stopper  26 .  
         [0063]     The dental crowns A are arranged in a ring, that is, are arranged to match the arrangement of the brushes  22  mounted on the bottom of the brush assembly  21 . However, it should be noted that, where the brushes  22  are arranged in a shape different from the shape of a ring, the crowns A may be arranged in a different yet matching layout as well.  
         [0064]      FIG. 6  shows an enlarged view of a partial cross-section of the periphery of the tray  32 .  
         [0065]     A plurality of stoppers  32   a  is provided around the periphery of the bottom of the tray  32  along the circumference of the bottom. A polishing target holding tray  50  for holding objects to be polished that is separate from the tray  32  is placed inside the tray  32 . The bottom edge of the polishing target holding tray  50  is fixedly mounted on the interior of the tray  32  by the stoppers  32   a . The stoppers  32   a  are so disposed as to project slightly inward from the outer diameter of the bottom of the polishing target holding tray  50 . Accordingly, by pressing the polishing target holding tray  50  into the interior of the tray  32 , it spreads the circle formed by the insides of the stoppers  32   a  so as to fixedly mount the polishing target holding tray  50  in the tray  32 .  
         [0066]     When polishing, the crowns A are placed in a circle in the polishing target holding tray  50  and held in place with the resin agent B. After the resin agent B hardens a polishing slurry C including a polishing agent is poured into the polishing target holding tray  50  to approximately the same height as the highest part of any of the crowns A.  
         [0067]      FIG. 7  shows the head  18  as seen from the brushes  22  (that is, from below).  
         [0068]     The brush assembly  21  is composed of a first brush unit  21   a  and a second brush unit  21   b  located behind the first brush unit  21   a , that is, behind the surface of the sheet of paper in which  FIG. 7  is drawn. The brushes  22  are disposed in the shape of a ring in the first brush unit  21   a . The first brush unit  21   a  is affixed to the second brush unit  21   b  by three screws  70  from the side of the brushes  22 . The screws  70  are mounted in an area inside the ring of brushes  22 . When the brushes  22  are worn out, the screws  70  are removed and the first brush unit  21   b  can be detached from the second brush unit  21   b . A new first brush unit  21   a  is then mounted on the second brush unit  21   b.    
         [0069]     Next, the state of connection between the rotary shaft  20  coupled to the motor  19  and the brush assembly  21  is described.  
         [0070]     The rotary shaft  20  is mounted on the brush assembly  21  through a disc-shaped ball bearing assembly  60 . The axis of the rotary shaft  20  is the central point  20   a  (indicated by a black triangle) shown in  FIG. 7 . The center of the ball bearing assembly  60 , which is also the center of the brush assembly  21 , is the central point  21   c  (indicated by a black dot) shown in  FIG. 7 . Thus, the rotary shaft  20  and the ball bearing assembly  60  are not mounted concentrically. As the rotary shaft  20  rotates, the inner cylinder of the ball bearing assembly  60  rotates in orbital fashion. As a result, the brush assembly  21  also moves in orbital fashion in  FIG. 7 . However, it should be noted that the outer periphery of the ball bearing assembly  60  and the brush assembly  21  do not rotate at same rate as the inner periphery of the ball bearing assembly  60 . Next, this point is described in detail.  
         [0071]      FIG. 8  shows the first brush unit  21   a  removed so as to allow the interior of the ball bearing assembly  60  to be seen in the state shown in  FIG. 7 .  
         [0072]     The ball bearing assembly  60  has a double cylinder construction comprised of an inner cylinder  60   a  (the part shown by the thick black line outside the rotary shaft  20 ) and an outer cylinder  60   b  (the part shown by the second thick black line counting in from the screw  71 ). A number of balls  61  are crammed into an annular space between the inner cylinder  60   a  and the outer cylinder  60   b  so as to contact each other. Grooves, not shown, are formed on the outer surface of the inner cylinder  60   a  and the inner surface of the outer cylinder  60   b , and the rotary shaft  20  and the balls  61 , and the balls  61  and the brush assembly  21 , respectively, contact each other through these grooves.  
         [0073]     As the rotary shaft  20  rotates, the balls  61  contacting the rotary shaft  20  move in the annular space and revolve while rotating. As the ball bearing assembly  60  moves in orbital fashion due to the rotation of the rotary shaft  20 , the brush assembly  21  also similarly moves in orbital fashion. However, because the brush assembly  21  contacts the ball bearing assembly  60  through the balls  61 , substantially none of the rotational force of the motor  19  is transmitted to the brush assembly  21 . In other words, the inner cylinder of the ball bearing assembly  60  spins freely with respect to the brush assembly  21 . However, since the ball bearing assembly  60  moves in orbital fashion, the brush assembly  21  moves in orbital fashion.  
         [0074]     In reality, there is friction resistance at the portion of contact between the balls  61  and the brush assembly  21 . Therefore, the brush assembly  21  also receives a force exerted in the circumferential direction by the rotation of the rotary shaft  20 . Consequently, the brush assembly  21  moves in a way that combines both orbital movement and rotation. It should be noted that the frictional force between the balls  61  and the brush assembly  21  is very small, and thus the brush assembly  21 , very unlike the high rpm of the motor  19 , rotates slowly, at the rate of approximately 5-10 rotations per minute. The foregoing brush assembly 21 rpm is one example thereof, and can increase or decrease depending on the size of the frictional force with the balls  61 .  
         [0075]     Next, the orbital movement of the brush assembly  21  is described by using some drawings.  
         [0076]      FIG. 9  is a diagram illustrating the ideal movement and the actual movement of the brush assembly  21 . It should be noted that, in  FIG. 9 , the movement of the brush assembly  21  is simplified as much as possible in order to facilitate an understanding of that movement, and thus only one of the four first projecting members  23  is considered and its movement traced.  
         [0077]     In an ideal state in which the friction between the balls  61  inside the ball bearing assembly  60  and the brush assembly  21  is zero, in the case that the rotary shaft  20  (whose axis is the central point  20   a ) rotates clockwise once, when such a rotation is divided into four steps of 90 degrees each, the brush assembly  21 (A) moves from an initial state indicated by the solid line and moves in orbital fashion to the positions indicated as brush assembly  21 (B), brush assembly  21 (C) and brush assembly  21 (D). At this time, the center of the brush assembly  21  (whose center is the central point  21   c ) moves to the positions indicated by central point  21   c (A), central point  21   c (B), central point  21   c (C) and central point  21   c (D). Furthermore, as shown by the dotted line arrow f, the first projecting member  23  similarly moves to the positions indicated by first projecting member  23 (A), first projecting member  23 (B), first projecting member  23 (C) and first projecting member  23 (D).  
         [0078]     However, in reality, unlike the ideal state described above, the friction between the balls  61  inside the ball bearing assembly  60  and the brush assembly  21  is not zero, and consequently, the brush assembly  21  also rotates as it receives the torque of the balls  61  that are themselves revolving due to the rotation of the rotary shaft  20 . Therefore, the first projecting member  23  moves in the direction of “F” indicated by the solid line arrow while moving along the direction of the dotted line arrow f, in  FIG. 9 .  
         [0079]     By the orbital movement of the brush assembly  21  described above, as the four first projecting members  23  move in a way that combines the movements indicated by the dotted line arrow f and the solid line arrow F, the second projecting members  33  that contact the first projecting members  23  receive forces exerted in the direction of the solid line arrow F and the dotted line arrow f by the first projecting members  23 . Although the tray  32  is rotatably mounted via the ball bearing assembly  31 , it is structurally incapable of moving laterally. Accordingly, the second projecting members  33  receive only a force exerted in the circumferential direction indicated by the solid line arrow F by first projecting members  23 . As a result, the tray  32  rotates.  
         [0080]     In addition, since the first projecting members  23  and the second projecting members  33  contacting each other but are not fixedly attached to each other, the tray  32  and the brush assembly  21  rotate to the same direction, but not in the same speed. When the first projecting members  23  and the second projecting members  33  are not in contact with each other, the tray  32  does not receive the force for the circumferential direction from the brush assembly  21 . Then, the first projecting members  23  and the second projecting members  33  are in contact with each other again, and the tray  32  receives the force for the circumferential direction. The tray  32  rotates at approximately the same rpm with the brush assembly  21 , that is, at approximately 5-10 revolutions per minute.  
         [0081]     As described above, by setting multiple crowns A in the tray  32 , by supplying a polishing agent to the tray  32  and by moving the brushes  22  orbitally, multiple crowns A are polished at same time. Since the brushes  22  rotate at a different speed from the tray  32  with orbital movement, the crowns A are polished with contacting the several positions of the brushes  22 . Therefore, multiple crowns A of different unevenness can all be polished cleanly.  
         [0082]     In addition, the first projecting members  23  which project from the brush assembly  21  are provided as the power transmission means for transmitting the drive of the motor  19  from the brush assembly  21  to the tray  32 , and furthermore, the second projecting members  33  which project from the periphery of the tray  32  are provided as a passive means for accepting the torque from the brush assembly  21 . Therefore, as the brush assembly  21  equipped with the brushes  22  moves orbitally, the driving force of the brush assembly  21  exerted in the circumferential direction of the tray  32  is transmitted easily to the tray  32  using a simple structure, and thus there is no need to provide a drive source for rotating the tray  32 .  
         [0083]     In addition, the ball bearing assembly  31  disposed between the tray  32  and the pedestal  13  beneath the tray  32  is provided as the rotation permitting means for permitting the tray  32  to rotate, and thus the tray  32  rotates smoothly by receiving the circumferential direction drive force exerted by the brush assembly  21 .  
         [0084]     In addition, the shaft  39  and the knob  40  are provided as the rotation adjustment means when in contact with the tray  32  for adjusting the rotation of the tray  32 , and thus when the tray  32  rotates by receiving the drive force from the brush assembly  21 , the resistance to the rotation can be adjusted. Therefore, it is possible to adjust the rpm of the tray  32  depending on the polishing conditions.  
         [0085]     In addition, the knob  27  and the multi-stage stopper  26  are provided as a means for the tray height adjustment means for adjusting the distance of the tray  32  from the brush assembly  21 . Thus, it is possible to adjust the distance between the brushes  22  and the crowns A accurately even when the crowns A are not set in the tray  32  at designated height. By adjusting the height of the tray  32  according to the extent of polishing, a variety of polishing conditions can be achieved.  
         [0086]     In addition, the handle  17 , the shaft  16 , the fixed shaft  15  and the arm  14  are provided as means for adjusting the angle of the brushes  22  with respect to the opening surface of the tray  32 . Thus, by setting the brushes  22  at a slight angle, it is possible to thoroughly polish even the deepest grooves of the crowns A, even if there is wide variation in the depth of the grooves in the uneven parts of the crowns A.  
         [0087]     It should be noted that although one embodiment of the dental crown polishing apparatus according to the present invention is described above, the present invention is not limited to the above-described embodiment and can be implemented in a variety of variations and embodiments.  
         [0088]     For example, instead of putting the polishing slurry (including polishing agent) into the tray  32 , alternatively, the polishing slurry may be supplied continuously or intermittently to the brush assembly  21 . Moreover, alternatively, the polishing agent may be fixed to the brushes  22  and only a fluid such as water or the like can be put in the tray  32  or water or other fluids can be supplied continuously or intermittently from the brush assembly  21 .  
         [0089]     In addition, the brushes  22  need not be arranged on the brush assembly  21  in the shape of rings, and alternatively, may be arranged in a circle or in a multi-angular shape. Moreover, the-brush assembly  21  need not be composed of the first brush unit  21   a  and the second brush unit  21   b , and alternately may be a single unit. Moreover, the tray  32  need not be subjected to torque from the brush assembly  21 , and alternatively may be made to rotate differently from the brush assembly  21  by the motor  19  or by a separate power source.  
         [0090]     Alternatively, the power transmission means may be connected to the brush assembly  21  in the form of a projection contained within the tray  32 . In that case, preferably, the passive means is located inside the tray  32  in the form of a projecting part capable of contacting the power transmission means.  
         [0091]     In addition, the passive means need not be the same panel-like projecting members as the power transmission means. Thus, alternatively, a groove formed along the outer periphery of the tray  32  may be employed as the passive means. Where projecting members like the first projecting members  23  are employed, the tray  32  can be rotated by contacting the tips of the projecting members against the interior of the groove. Transmission of power is facilitated if the projecting members are composed of soft rubber. Furthermore, the bottoms of the brush assembly  21  and the tray  32  are not limited to circular shapes and thus alternatively may have other shapes.  
         [0092]     The rotation permitting means need not be the ball bearing assembly  31 , and alternatively may comprise a rotary shaft inserted into a hole formed in the center of the tray mounting plate  30 . Moreover, where there is no tray mounting plate  30  and only the tray  32  is placed directly on the pedestal  13 , a hole may be provided in the tray  32  and a rotary shaft inserted in the hole as the rotation permitting means.  
         [0093]     The rotation adjustment means need not be the means for controlling the rotation of the tray  32  by a method of adjusting the force exerted on the roller  38 , and alternatively (by a method that adjusts the force exerted on the tray mounting plate  30 ) may be a means for controlling the rotation of the tray  32 . Moreover, an arrangement in which something is pressed against the tray  32  or a portion connected to the tray  32  at a location other than the tray mounting plate  30  as a brake to control the rotation of the tray  32 .  
         [0094]     Adjusting the height of the pedestal  13  need not be the only means for adjusting the height of the tray  32 . Alternatively adjusting the height of just the tray  32  or just the tray mounting plate  30  may be the means for adjusting the height of the tray  32 . Furthermore, where the tray height adjustment means can be implemented not by adjusting the force with which the ring member  12  is tightened against the support post  11  by rotation of the knob  27  so as to raise or lower the pedestal  13  or fix it at a desired position but by setting the frictional force or the engaging force between the ring member  12  and the support post  11  so that the ring member  12  does not descend naturally under its own weight and raising and lowering the ring member  12 , the ring member  12  may be moved up and down with a force greater than the force with which the ring member  12  is kept from descending under its own weight. Moreover, the ring member  12  needs not be moved manually, and may alternatively be movable vertically by electric power.  
         [0095]     The brush angle adjustment means need not be means for changing the angle between the brushes  22  and the crowns A by the inclination of the head  18 , and alternatively may be a means for tilting only the brush assembly  21  or only the brushes  22 . Moreover, even when changing the angle of the head  18 , instead of rotating the arm  14  with respect to the support post  11 , alternatively the brush angle adjustment means may be implemented by tilting the head  18  with respect to the arm  14 . Moreover, instead of comprising the shaft  16 , the handle  17  and the two planar projecting parts  14   a ,  14   b , alternatively the brush angle adjustment means may be implemented by increasing the frictional force between the fixed shaft  15  and the hole in the arm  14  and either manually or electrically rotating the arm  14  around the fixed shaft  15 .  
         [0096]     In addition, the roller  38  shown in  FIG. 3  may be provided in the structure shown in  FIG. 10  so as to restrict the rotation of the tray mounting plate  30 .  FIG. 10  shows an enlarged view of the area of the roller mounting plate in a state different from that shown in  FIG. 3 . The roller mounting plate  37  shown in  FIG. 10  comprises the fixed vertical panel  37   a  which extends perpendicularly upward from the pedestal  13  and two horizontal panels  37   b  and  37   c  extending substantially parallel to the top of the pedestal in the direction of the tray mounting plate  30  from the vertical panel  37   a . The two horizontal panels  37   b ,  37   c  are fixedly mounted on the vertical panel  37   a  separated by a predetermined interval in the vertical direction. The roller  38 , which is penetrated through its central axis by the rotary shaft  38   a  mounted perpendicular to the two horizontal panels  37   b ,  37   c , is provided between the two horizontal panels  37   b  and  37   c . The roller  38  is mounted between the horizontal panels  37   b  and  37   c  so as to be rotatable while contacting the periphery of the tray mounting plate  30 .  
         [0097]     The tip of the shaft  39  contacts the vertical panel  37   a . The knob  40  is mounted on the end of the shaft  39  opposite the tip that contacts the vertical panel  37   a . Screw threads  39   a  are cut into the shaft  39  over a predetermined length from the tip of the shaft  39 , and this threaded portion  39   a  is screwed into a threaded through-hole  41   a  provided on a plate  41  disposed on the outer side of the roller mounting plate  37 . Accordingly, as the knob  40  is turned and the shaft  39  is screwed toward the back of the apparatus, the tip of the shaft  39  pushes the vertical panel  37   a  so as to strongly contact the roller  38  against the tray mounting plate  30 , which causes the resistance to the rotation of the tray mounting plate  30  to increase.  
         [0098]     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific preferred embodiments described above thereof except as defined in the claims.