Abstract:
Barrel polishing is effected while causing a mass (M) consisting of work and media (polishing material) to rotation-flow by rotating a rotary disk installed in the bottom of a polishing tank by a drive motor. A load on the drive motor for the rotary disk is preset as by a load current value, and the flow of the mass (M) in the polishing tank is controlled, thereby effecting polishing while maintaining the load on the drive motor within the preset range.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of International Application No. PCT/JP2003/011952, filed Sep. 19, 2003, which designated the United States, and also claims the benefit of Japanese Application No. 2002-346870, filed Nov. 29, 2002, the entireties of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a flow barrel polishing method and a flow barrel polishing apparatus for polishing a workpiece while causing a mass configured by the workpiece and a media to centrifugally flow in a polishing bath.  
       BACKGROUND OF THE INVENTION  
       [0003]     Flow barrel polishing is a method for inputting a mass configured by a workpiece that is a polishing target and a media that is a polishing material into a polishing bath, and for polishing the workpiece while causing the mass to centrifugally flow using a rotary disc provided on a bottom of the polishing bath. One example of the method is disclosed in Japanese Patent Application Laid-Open No. 8-11057 (Japanese Patent Number 3343701). As shown in  FIG. 1 , according to this flow barrel polishing method, the workpiece and the media are rubbed against each other to polish the workpiece surface while the mass is caused to flow in a toroidal fashion by a combination of a horizontally rotatable flow running along a rotation direction of the rotary disc and a vertically rotatable flow rising along an inner wall surface of the polishing bath and running downward in a central direction when reaching an uppermost portion by a centrifugal force.  
         [0004]     The conventional flow barrel polishing has, however, disadvantages in that the media is gradually worn as the polishing progresses, a mass amount is reduced, a frictional force between the workpiece and the media is reduced, and a deterioration in a polishing capability is thereby unavoidable. These disadvantages are conspicuous particularly in dry flow barrel polishing.  
       SUMMARY OF THE INVENVTION  
       [0005]     It is an object of the present invention to provide a flow barrel polishing method and a barrel polishing apparatus that can solve the above-stated conventional disadvantages, that can avoid a reduction in a polishing capability following progress of polishing, and that can greatly improve the polishing capability as compared with that of the conventional polishing method or apparatus.  
         [0006]     The inventors of the present invention exerted their utmost efforts for solving the conventional disadvantages. As a result, the inventors discovered that the polishing capability can be considerably enhanced as compared with the conventional polishing capability by controlling a flow of a mass rising along an inner wall of a polishing bath using an appropriate means against a conventional common knowledge that the polishing capability of the flow barrel polishing is deteriorated when a natural flow of the mass is disturbed. Further, a change in the polishing capability of the flow barrel polishing apparatus appears as a change in a work amount transmitted from the rotary disc to the mass, that is, appears as a change in a rotation resistance of the rotary disc. It is, therefore, possible to grasp the change in the polishing capability of the apparatus as a load of a driving motor for the rotary disc from outside. Accordingly, by controlling the flow of the mass to keep the load of the driving motor for the rotary disc constant, the polishing capability deteriorated as the polishing progresses can be kept constant.  
         [0007]     A barrel polishing method according to the present invention achieved based on the above-stated knowledge is a barrel polishing method for polishing a mass while causing the mass to rotatably flow by a rotary disc provided on a bottom of a polishing bath, characterized in that a load range of a driving motor for the rotary disc is set in advance, a flow area of the mass within the polishing bath is controlled with a load of the driving motor used as a parameter if the load of the driving motor is out of a set range, and the mass is polished while keeping the load of the driving motor within the set range. In this case, it is preferable to use, for example, a load current of the driving motor as the load of the driving motor.  
         [0008]     According to the present invention, the flow area of the mass within the polishing bath can be controlled by various methods including a method for elevating movable means that covers an upper portion of the polishing bath and increasing or reducing the flow area of the mass that rises along an inner wall of the polishing bath, a method for increasing or reducing a force of pressing down an upper end the mass that rises along the inner wall of the polishing bath, a method for controlling a rotation speed of the rotary disc, and the like. Further, the load current set range is not always limited to one and a plurality of load current set ranges can be set at predetermined time intervals. By intermittently controlling the flow of the mass within the polishing bath, polishing of the mass while controlling the flow of the mass and free polishing without a control over the flow of the mass can be alternately and repeatedly performed.  1   
         [0009]     Further, a barrel polishing apparatus according to the present invention is characterized by comprising: a polishing bath into which a workpiece and a media are input; a rotary disc, provided on a bottom of the polishing bath, for causing the workpiece and the media to rotatably flow to thereby form a mass within the polishing bath; means for setting a load of a driving motor for the rotary disc; and flow area control means for controlling a flow area of the mass within the polishing bath with the load of the driving motor used as a parameter if the load of the driving motor is out of a set range.  
         [0010]     As the mass flow area control means, any of various means including a combination of movable means provided in an upper portion of the polishing bath and an elevation mechanism for the movable means, a combination of movable means provided in an upper portion of the polishing bath and a pressurization chamber that supplies or discharges a pressurized fluid and that thereby moves the movable means downward or stops moving the movable means downward, a combination of movable means that is provided in an upper portion of the polishing bath and that is expandable and compressible within the polishing bath and a pressurization mechanism that expands or compresses the movable means, control means for controlling a rotation speed of the driving motor for the rotary disc, and the like can be used. Each of these control means can be employed together with the other control means.  
         [0011]     According to the present invention, the load range of the driving motor for the rotary disc is set in advance as a load current or the like, and the flow area of the mass within the polishing bath is controlled with the load of the driving motor used as a parameter if the load of the driving motor is out of the set range, whereby the mass is polished within the load set range. A deterioration in polishing capability following progress of the polishing can be detected as a reduction in the load of the driving motor for the rotary disc as a result of a reduction in the frictional force between the workpiece and the media due to a reduction in a volume of the mass after wearing of the media or polishing of the workpiece. Therefore, if the load is reduced, then the flow area of the mass within the polishing bath is narrowed to keep the frictional force between the workpiece and the media constant, and the load of the driving motor is always kept within the set range, whereby the barrel polishing can be performed while keeping the polishing capability constant. Besides, by area control of the flow of the mass within the polishing bath, the frictional force between the workpiece and the media can be considerably intensified as compared with that of the conventional method or apparatus. While these advantages of the present invention are conspicuous particularly in dry barrel polishing, they are similarly exhibited even in wet barrel polishing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a perspective view that depicts a flow of a mass in conventional flow barrel polishing.  
         [0013]      FIG. 2  is a partial cross-sectional view that depicts a first embodiment of the present invention.  
         [0014]      FIG. 3  is a partial cross-sectional view that depicts a state in which a movable means is moved downward according to the first embodiment of the present invention.  
         [0015]      FIG. 4  is a partial cross-sectional view that depicts a second embodiment of the present invention.  
         [0016]      FIG. 5  is a partial cross-sectional view that depicts a third embodiment of the present invention.  
         [0017]      FIG. 6  is a partial cross-sectional view that depicts a modification of the third embodiment of the present invention.  
         [0018]      FIG. 7  is a partial cross-sectional view that depicts a fourth embodiment of the present invention.  
         [0019]      FIG. 8  is a partial cross-sectional view that depicts a fifth embodiment of the present invention.  
         [0020]      FIG. 9  is a graph that depicts a change in a load current according to a first example.  
         [0021]      FIG. 10  is a graph that depicts a workpiece polishing effect according to the first example.  
         [0022]      FIG. 11  is a graph that depicts a correlation between the number of revolutions of a rotary disc and a load current according to a second example.  
         [0023]      FIG. 12  is a graph that depicts a workpiece polishing effect according to the second example.  
         [0024]      FIG. 13  is a graph that depicts a state of controlling a load current according to a third example.  
         [0025]      FIG. 14  is a graph that depicts a workpiece polishing effect according to the third example. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
     FIRST EMBODIMENT  
     Movable Means and Elevation Mechanism  
       [0026]      FIG. 2  depicts a dry flow barrel polishing apparatus according to a first embodiment of the present invention. In  FIG. 2 , reference number  1  depicts a polishing bath into which a mass M configured by a workpiece and a media is input, and  2  denotes a plate-like rotary disc provided on a bottom of the polishing bath. The rotary disc  2  has a peripheral edge curved upward so that the mass M can easily flow upward. An antifriction lining consisting of urethane rubber or the like is applied to a portion in which the polishing bath  1  contacts with the mass M on the rotary disc  2 . Reference  3  denotes a movable means that consists of a flexible material such as rubber for closing an upper opening  13  of the polishing bath  1 . According to this embodiment, the movable means  3  is of a lid shape and has a peripheral portion fixed to an upper end of the polishing bath  1 . As shown in  FIG. 2 , the peripheral edge of this movable means  3  is preferably curved to contact with an inner wall of the polishing bath  1 . A height of the polishing bath  1  is set smaller than a maximum height at which the mass M freely flows centrifugally so that the upper end of the mass M that flows centrifugally can be controlled by the movable means  3 .  
         [0027]     The rotary disc  2 , which is arranged slightly upward of a bottom plate  10  of the polishing bath  1 , is rotated by a driving motor  20  through a reducer  5  while slidably contacting with the inner wall  12  of the polishing bath  1  with a slidable contact clearance  4  left. A rotation speed of the driving motor  20  is controlled by a control means  50 .  
         [0028]     The rotary disc  2  is provided with small bores  6  and a cavity  14  is formed between the rotary disc  2  and the bottom plate  10  of the polishing bath  1 . A dust collector, not shown, is connected to a dust collection tube  11  provided in a lower portion of the cavity  14 . Dusts generated by polishing are passed through the cavity  14  via the small bores  6  and the slidable contact clearance  4 , and collected by this cavity  14  via the dust collection tube  11 .  
         [0029]     The load of the driving motor  20  for the rotary disc  2  is always detected by a load detection means included in the control means  50 . Although it is practical to use a load current for detecting the load of the driving motor  20 , the present invention is not always limited to this and a load power, for example, may be detected. According to the present invention, the load current or the like can be set by a load setting means  70  in advance, and the flow area of the mass M within the polishing bath  1  is controlled by a flow area control means of any one of various types as will be described later in detail. The polishing is thereby always performed within a set range of setting the load of the driving motor  20 .  
         [0030]     An opening  8  for evading a part of the mass M and providing a smooth flow when the mass M is filled into the polishing bath  1  and the flow of the mass M is not smooth is formed at a center of the movable means  3 . In this embodiment, a support member  31  that strides over the opening  8  is fixedly provided. An elevation mechanism  60  for vertically moving this movable means  3  is provided above the movable means  3 . The elevation mechanism  60  is configured by an arm  62  horizontally rotatably attached to an axis of a column  61 , a driving section  64  that is attached to a tip end of the arm  62  and that vertically moves the movable means  3  through an operation rod  63  protruding upward of the support member  31  of the movable means  3 , and a control section  65  that is included in the speed control means  50 , that receives a signal from the load current detection means, and that drives the driving section  64 . As the driving section  64 , an appropriate type of the driving section  64  such as a hydraulic cylinder type or a ball screw type can be employed. In this embodiment, flow area control means for controlling the flow of the mass within the polishing bath  1  is constituted by the movable mean  3  and the elevation mechanism  60 .  
         [0031]     When the workpiece and the media are input into the polishing bath  1  and the rotary disc  2  is rotated by the driving motor  20 , the workpiece and the media form the mass M and the mass M rises along the inner wall  12  of the polishing bath  1  by the centrifugal force as stated above. According to the present invention, the mass M flows in a toroidal fashion while a flow area of the rising mass M is restricted by the movable means  3  and a flow direction thereof is changed to a direction of the center of the polishing bath  1 . It is a conventional common knowledge that when a natural flow of the mass is disturbed, the polishing capability of the flow barrel polishing is deteriorated. According to the present invention, however, the flow area of the mass that rises along the inner wall of the polishing bath is controlled by the appropriate means, thereby considerably increasing the frictional force between the workpiece and the media and greatly enhancing the polishing capability as compared with the conventional apparatus or method.  
         [0032]     As stated, as the polishing progresses, corners (convex portions) of the media are worn and the workpiece is polished. Due to this, the frictional force between them is reduced and the polishing capability is gradually lessened. According to this embodiment, however, the elevation mechanism  60  that is the mass flow area control means moves the movable means  3  downward so that the load of the driving motor  20  is kept within the range set by the setting means  70  in advance.  
         [0033]     That is, if the polishing capability is deteriorated, the load of the driving motor  20 , e.g., the load current is reduced. Therefore, in response to the signal from the load detection means included in the control means  50 , the control section  65  of the elevation mechanism  60  moves the operation rod  63  downward as shown in  FIG. 3 . By thus bending the central portion of the movable means  3  downward, the upper portion of the mass M that rotatably flows is pressed down, the flow area of the mass M is reduced, a rising force of the mass M is converted into a pressurization force, and a pressure applied to the mass M is increased. As a result, the frictional force between the workpiece and the mass is increased, so that the polishing capability deteriorated as the polishing progresses can be recovered. In addition, the load of the driving motor  20  is recovered. When the operation rod  63  is moved downward and the load of the driving motor  20  reaches a preset upper limit, the control means  50  transmits a signal to stop moving down the operation rod  63  to the control section  65 . The load of the driving motor  20  can be, therefore, recovered to an optimum value.  
         [0034]     As can be seen, according to this embodiment, the polishing is performed while controlling the frictional force between the workpiece and the media to always fall within the certain range by optimally adjusting the height of the movable means  3  with the load of the driving motor  20  used as a parameter. It is, therefore, possible to continuously perform barrel polishing without deteriorating the polishing capability even if the polishing progresses.  
         [0035]     After the polishing is finished, the movable means  3  is raised upward of the polishing bath  1  by the elevation mechanism  60  and the arm  62  is then rotated horizontally about the column  61 . Next, the polishing bath  1  is rotated so that the bath  1  stands and the rotary disc  2  is rotated at a right angle or more, whereby the mass M completed with the polishing can be easily taken out from within the polishing bath  1 .  
       SECOND EMBODIMENT  
     Modification of Movable Means and Elevation Mechanism  
       [0036]     In the first embodiment, the peripheral edge of the flexible movable means  3  is fixed to the upper end of the polishing bath  1 . Alternatively, as shown in  FIG. 4 , the movable means  3  may consist of a rigid material such as metal, and may be provided so as to be able to be vertically slid within the polishing bath  1  by the elevation mechanism  60  working with the load of the driving motor  20 . In this case, an outside diameter of the movable means  3  is set slightly smaller than an inside diameter of the polishing bath  1 . In a left half part of  FIG. 4 , a free flow path of the mass M for the conventional apparatus without the movable means  3  is indicated by a broken line. In this second embodiment, similarly to the first embodiment, the upper portion of the mass M that rotatably flows is suppressed by the movable means  3 , the reduced polishing force can be recovered.  
       THIRD EMBODIMENT  
     Movable Means and Pressurization Mechanism for Movable Means  
       [0037]      FIG. 5  depicts a third embodiment of the present invention. In  FIG. 5 , a movable means  3  having an opening cylinder  32  provided at its center is slidably provided within a polishing bath  1 . In addition, an outer cylinder  16  into which this opening cylinder  32  can be slidably fitted is provided on an upper lid  15  of the polishing bath  1 . An annular pressure chamber  17  is formed between the upper lid and the movable means  3 , and a pressurized fluid such as a compressed air is supplied from a pressurized fluid supply port  18  provided in the upper lid  15  to thereby pressurize the movable means  3  downward.  
         [0038]     In this embodiment, a pressure of the pressurized fluid supplied from the pressurized fluid supply port  18  is increased and the movable means  3  is pressed downward in a piston manner when a load of a driving motor  20  is reduced, thereby controlling a flow area of a mass M. It is thereby possible to increase a frictional force between a workpiece and a medium and always perform barrel polishing within a set range of setting the load of the driving motor  20 .  
         [0039]     Alternatively, as shown in  FIG. 6 , an expandable and compressible movable means  3  consisting of an elastic material such as rubber may be provided in an upper portion of the polishing bath  1 , and the pressurized fluid such as the compressed air may be supplied from a pressurization and depressurization mechanism, not shown, via the pressurized fluid supply port  18  provided in the upper lid  15  to the pressure chamber  17  located above the movable means  3 . By doing so, the movable means  3  can be expanded and compressed like a balloon. Thanks to such a structure, the flow area of the mass M can be controlled and the barrel polishing can be always performed within the set range of the load of the driving motor  20 .  
       FOURTH EMBODIMENT  
     Modification of Movable Means by Suction  
       [0040]      FIG. 7  depicts a fourth embodiment of the present invention. In  FIG. 7 , a movable means  3  consists of a flexible material such as rubber and is fixed to an upper end surface of a polishing bath  1 . If an opening  8  is formed at a center of the movable means  3 , another sealing lid  81  is provided so as to be able to close the opening  8 . A dust collection tube  11  is connected to a suction means, such as a dust collector, having an adjustable suction force. When a load of a driving motor  20  is reduced, an internal pressure of the polishing bath  1  is reduced to be lower than an atmospheric pressure, thereby bending the flexible movable means  3  toward an interior of the polishing bath  1  and narrowing a flow area of a mass M. A method for deforming the movable means  3  toward the interior of the polishing bath  1  and controlling the flow area of the mass M as stated above can keep the load of the driving motor  20  to fall within a set range, similarly to the preceding embodiments.  
       FIFTH EMBODIMENT  
     Pressurization by Weight of Movable Means  
       [0041]      FIG. 8  depicts a fifth embodiment of the present invention. In  FIG. 8 , a weight  80  is put on an upper surface of a movable means  3  that can be moved up and down within a polishing bath  1 , a weight or the number of weights is increased or decreased according to changes in a load of a driving motor  20 , thereby controlling a flow area of a mass M. A weight adjustment of this weight  80  may be made either automatically using a robot or the like or manually. Alternatively, the movable means  3  may be a flexible member as shown in the first embodiment, and the weight  80  may be put on the upper surface of the movable means  3 , thereby bending the movable means  3  toward an interior of the polishing bath  1  and increasing or reducing a force of pressing down an upper end of the mass M.  
       SIXTH EMBODIMENT  
     Number-of-Revolutions Control  
       [0042]     In the respective embodiments stated so far, if the load of the driving motor  20  is reduced, the position of the movable means  3  is changed to control the flow area of the mass M and to increase or reduce the pressure applied to the mass M. Besides, if the rotation speed of a rotary disc  2  is controlled to increase or decrease a flow speed of the mass M when the load of the driving motor  20  is changed, the pressure applied to the rotatably flowing mass M can be increased or reduced. Namely, according to this sixth embodiment, control means  50  for controlling the rotation speed of the driving motor  20  is allowed to function as a means for controlling a flow of the mass M. It is noted, however, an upper portion of a polishing bath  1  is covered with a lid so as to prevent the mass M from protruding when the number of revolutions of the rotary disc  2  is increased.  
       SEVENTH EMBODIMENT  
     Intermittent Control  
       [0043]     Furthermore, a control means  50  may be configured to be able to set a constraint polishing time for which a flow direction of a mass M is changed by a movable means  3  and a workpiece is polished in a constraint state, and an unconstraint polishing time for which the mass M is polished while the mass M flows freely without a change in the flow direction thereof by the movable means  3 , and to intermittently control a flow of the mass M. By so configuring, it is possible to efficiently perform barrel polishing (see  FIGS. 13 and 14  for a third example to be described later), similarly to the preceding embodiments.  
         [0044]     Namely, a method according to this embodiment is a method for turning the workpiece into an unconstraint state by raising the movable means  3  up to a height at which the mass M is in no contact with the movable means  3  or the number of revolutions of a rotary disc  2  is decreased so that the mass M is in no contact with the movable means  3  without changing the height of the mass M when the workpiece constraint polishing time reaches a predetermined time. During the constraint polishing, a non-uniform mixture state of mixing up a media and the workpiece often causes a deterioration in polishing efficiency. However, by intermittently releasing the constraint and allowing the mass M to rotatably flow in a free state, the workpiece and the media are mixed together uniformly again. It is, therefore, possible to further enhance the polishing efficiency.  
       FIRST EXAMPLE  
     Elevation of Movable Means  
       [0045]     Using the barrel polishing apparatus including the polishing bath  1  having an inside diameter of 440 mm as shown in  FIG. 4 , the mass M that is a mixture of a triangular prism media and a test piece {circle around ( 1 )} (SS 400 ; a cylinder having a diameter of 15 mm and a length of 20 mm) serving as a workpiece is input into the polishing bath  1  at 95% relative to an internal volume of the polishing bath  1 , this mass M is constrained while the flow direction thereof is changed by the movable means  3 , and the barrel polishing is performed. In polishing, the number of revolutions of the rotary disc  2  is set at 350 min −1 , an upper limit of the load current of the driving motor  20  is set at 5.2 A, and a lower limit thereof is set at 5.0 A, and the height of the movable means  3  is controlled so that the load current is kept within this set range. A change in the load current with passage of the polishing time as well as a comparison example in which the movable means  3  is kept fixed is shown in  FIG. 9 . Namely, in the first example, the movable mean  3  is moved downward when a polishing resistance is reduced and the current is reduced to 5.0 A, and the current is, therefore, increased to 5.2 A repeatedly. In the first comparison, the polishing is continuously performed while the movable mean  3  is fixed to an initial position, and the current is, therefore, gradually reduced.  
         [0046]     A polishing amount per workpiece is 115 mg/hr as indicated by {circle around ( 1 )} in  FIG. 10 . In the comparison example in which the movable mean  3  is kept fixed, a polishing amount per workpiece is 13 mg/hr. Thus, the polishing amount according to the first example is 8.8 times as large as that of the first comparison example.  FIG. 10  depicts examples of using the other workpieces. Data indicated by blank is data obtained when the polishing is performed with the movable means  3  kept fixed, data indicated by hatching is data obtained when the polishing is performed by the method according to the first example. Materials and sizes of the other workpieces are as follows:  
         [0047]     {circle around ( 2 )}: A column of stainless steel, a diameter of three mm, and a length of 21 mm;  
         [0048]     {circle around ( 3 )}: A ring of steel, an outside diameter of 14 mm, an inside diameter of 13 mm, and a thickness of 12 mm; and  
         [0049]     {circle around ( 4 )}: A plate of spring steel, a length of 54 mm, a width of 27 mm, and a thickness of 4.5 mm.  
         [0050]     Ratios of the method according to the present invention to the conventional method in the polishing amount per workpiece are: 9.9 for the workpiece {circle around ( 2 )}, 14.3 for {circle around ( 3 )}, and 18.6 for {circle around ( 4 )}. It is thus confirmed that the method according to the present invention can enhance the polishing capability of polishing any workpiece.  
       SECOND EXAMPLE  
     Rotation Speed of Rotary Disc  
       [0051]      FIG. 11  depicts a result of searching correlations between the rotation speed of the rotary disc  2  and the load current of the driving motor  20  while changing a filling rate of the mass M at which the mass M is filled into the polishing bath  1  to 95%, 90%, and 85%.  FIG. 12  depicts polishing amounts for the respective cases. At any filling rate of the mass M, the load current suddenly increases and the polishing amount greatly increases as the rotation speed of the rotary disc  2  increases.  
         [0052]     Using these correlations, the load is controlled by changing the rotation speed of the rotary disc  2  between 250 and 400 min −1 . The workpiece used herein is the workpiece {circle around ( 1 )} shown in the first example, and the polishing bath, the media, and the like are the same as those of the first example. The polishing amount per workpiece is over 80 mg/hr according to the present invention relative to 13 mg/hr according to the conventional method. Good results can be thus obtained.  
       THIRD EXAMPLE  
     Intermittent Control  
       [0053]     In this example, the upper limit of the load current is set at 5.2 A, and the lower limit thereof is set at 5.0 A in the same polishing conditions as those of the first example. Further, as shown in  FIG. 13 , one polishing cycle is set to ten minutes, in which cycle the mass M in a constraint state is polished for nine minutes and 45 seconds and the mass M in an unconstraint state is then polished for 15 seconds. By repeating the cycle, the barrel polishing is performed. As a result, as shown in  FIG. 14 , the polishing efficiency can be further enhanced in this third example as compared with the first example. By thus intermittently controlling the polishing, the workpiece and the media are mixed together uniformly again during unconstraint polishing. It is, therefore, possible to uniformly polish the mass M without forming scars on the workpiece surface and causing non-uniform wear.  
         [0054]     In this third example, the mass M is intermittently controlled to freely flow by raising the movable means  3  up to the height at which the movable means  3  is in no contact with the mass M. Needless to say, the workpiece can be polished while intermittently controlling the flow of the workpiece by a method for increasing or decreasing the rotation speed of the rotary disc  2 .