Patent Publication Number: US-6902469-B2

Title: Work chamfering apparatus and work chamfering method

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a work chamfering apparatus and a work chamfering method, and more specifically to a work chamfering apparatus and a work chamfering method for chamfering a thin work. 
   2. Description of the Related Art 
   As a related art of this kind, a chamfering apparatus is disclosed in the Japanese Patent Laid-Open No. 5-337716. This chamfering apparatus has a bearing tube having two ends each provided with a tool. The tools respectively grind an upper edge and a lower edge of the work, thereby chamfering both upper and lower edges of the work simultaneously. 
   However, since the bearing tube cannot have a thickness smaller than 3 mm, the related art chamfering apparatus cannot chamfer a thin work having a thickness smaller than 3 mm. 
   Further, a thin work may be bonded by adhesive so that the work can be held firmly during the chamfering. However, this method is time consuming, posing a problem of poor productivity. 
   SUMMARY OF THE INVENTION 
   It is therefore a primary object of the present invention to provide a work chamfering apparatus and a work chamfering method capable of chamfering efficiently even if the work is thin. 
   According to an aspect of the present invention, there is provided a work chamfering apparatus for chamfering a work, comprising: a work holding portion including a first surface and a second surface respectively contacting a main surface and another main surface of the work, for holding the work; wherein the first surface includes a portion having a static friction coefficient greater than 0.1. 
   According to another aspect of the present invention, there is provided a work chamfering method using a work holding portion including a first surface and a second surface, in which the first surface includes a portion having a static friction coefficient greater than 0.1. The method comprises a first step of holding the work with the work holding portion by contacting each of the first surface and the second surface with a main surface and another main surface of the work; and a second step of chamfering the work by using a tool. 
   According to the present invention, since the first surface of the work holding portion has a portion having a static friction coefficient greater than 0.1, it becomes possible to increase holding force to the work. Therefore, even if the work is a thin piece which is difficult to hold, it becomes possible to reduce unwanted movement during the chamfering, and to perform the chamfering while holding the work stably. Further, since there is no need for bonding the work, working time can be shortened, making possible to chamfer efficiently. 
   Preferably, the portion having the static friction coefficient greater than 0.1 is formed at two end portions of the first surface, and the two end portions contact the work. In this case, since the work is held by the two end portions having a large static friction coefficient, it becomes possible to provide a plurality of regions having large holding force to the work between the first surface and the work, making possible to further reduce the unwanted movement of the work during the chamfering. 
   Further, preferably, the portion having the static friction coefficient greater than 0.1 has a holding grain projecting out of the first surface. In this case, when the work is held by the work holding portion, it becomes possible to make the portion having the static friction coefficient greater than 0.1 stick into the work. Therefore, even if the pressing force of the work holding portion applied to the work is smaller than convention, the unwanted movement of the work can be reduced due to anchor effect. 
   According to still another aspect of the present invention, there is provided a work chamfering apparatus for chamfering a work, comprising: a work holding portion including a first surface and a second surface respectively contacting a main surface and another main surface of the work, for holding the work; wherein the first surface includes a center portion and two end portions, each of the two end portions having a static friction coefficient greater than that of the center portion, the two end portions contacting the work. 
   According to still another aspect of the present invention, there is provided a work chamfering method using a work holding portion including a first surface and a second surface, in which the first surface includes a center portion and two end portions and each of the two end portions having a static friction coefficient greater than that of the center portion. The method comprises: a first step of holding the work with the work holding portion by contacting each of the two end portions of the first surface with a main surface of the work and contacting the second surface with another main surface of the work; and a second step of chamfering the work by using a tool. 
   According to the present invention, the first surface has two end portions having a static friction coefficient greater than that of the center portion, and the work is held by these end portions. Therefore, it becomes possible to provide a plurality of regions having large holding force to the work. Thus, even if the work is thin and difficult to hold, the unwanted movement of the work can be reduced by the stable holding force during the chamfering. Further, since there is no need for bonding the work, the working time can be shortened and the chamfering can be made efficiently. 
   Preferably, the second surface contacts the work at a plurality of locations, with a center of rotation of the work in between. In this case, since the work can be held evenly on a good balance, the unwanted movement of the work during the chamfering can be reduced. 
   Further, preferably, the work chamfering apparatus comprises a first grinding stone and a second grinding stone for chamfering one edge and another edge of the work respectively as the tool, and a driving portion for moving the first grinding stone and the second grinding stone thickness-wise of the work. 
   In this case, after the work is held by the work holding portion, one edge of the work is chamfered by the first grinding stone. Then, the first grinding stone and the second grinding stone are moved, and the second grinding stone chamfers the other edge of the work. Therefore, a variety of works having a different thickness can be chamfered easily. 
   Conventionally if the work is a R—Fe—B alloy containing cobalt at a rate not smaller than 0.3 wt % and not greater than 10 wt %, chipping is increased and uniform chamfering is difficult, for the work is fragile. However, according to the present invention, since holding force to the work can be increased, even if the work is such a fragile piece, the work can be held stably, chamfered easily, and chipping can be reduced. 
   If the rotating speed of the grinding stone is slower than 2000 rpm, machining load of the grinding stone is large, resulting in an increased number of chippings of the work. If the rotating speed of the grinding stone is faster than 5000 rpm, a coolant is not supplied to a grinding edge enough, the number of chippings increases, too. Therefore, the rotating speed of the grinding stone is preferably not slower than 2000 rpm and not faster than 5000 rpm. 
   Further, if the circumferential speed of the grinding stone is slower than 125.6 m/min, the machining load of the grinding stone is large, resulting in an increased number of chippings of the work. If the circumferential speed of the grinding stone is faster than 314 m/min, the number of chippings increases, too. Therefore, the circumferential speed of the grinding stone is preferably not slower than 125.6 m/min and not faster than 314 m/min. 
   According to another aspect of the present invention, there is provided a chamfering method for chamfering a rare-earth sintered magnet by using a rotating grinding stone, wherein the grinding stone is rotated at a speed not slower than 2000 rpm and not faster than 5000 rpm and relative speed of the grinding stone with respect to an outer circumferential portion of the rare-earth sintered magnet is not slower than 0.5 mm/sec and not faster than 7.0 mm/sec, for chamfering the rare-earth sintered magnet. 
   According to still another aspect of the present invention, there is provided a chamfering method for chamfering a rare-earth sintered magnet by using a rotating grinding stone, wherein the grinding stone is rotated at a circumferential speed not slower than 125.6 m/min and not faster than 314 m/min and relative speed of the grinding stone with respect to an outer circumferential portion of the rare-earth sintered magnet is not slower than 0.5 mm/sec and not faster than 7.0 mm/sec, for chamfering the rare-earth sintered magnet. 
   In the case where the grinding stone is rotated at a speed not slower than 2000 rpm and not faster than 5000 rpm, and in the case where the grinding stone is rotated at the circumferential speed not slower than 125.6 m/min and not faster than 314 m/min, if the relative speed of the grinding stone with respect to the outer circumferential portion of the rare-earth sintered magnet is slower than 0.5 mm/sec, the grinding efficiency goes down, on the other hand, if the relative speed is faster than 7.0 mm/sec, the grinding stone exerts large machining load, resulting in an increased number of chippings in the rare-earth sintered magnet. According to the present invention, by setting the relative speed of the grinding stone within a range not slower than 0.5 mm/sec and not faster than 7.0 mm/sec, the number of chippings can be reduced and the chamfering can be performed efficiently. 
   If an average diameter of an abrasive grain is smaller than 100 μm, the grinding stone is clogged easily by sludge produced during chamfering. Furthermore, the abrasive grain wears prematurely, thereby reducing the productivity. On the other hand, if the average diameter of the abrasive grain is greater than 270 μm, the number of chippings increases when a fragile work such as a rare-earth sintered magnet is chamfered, since the diameter of the abrasive grain is too large. Especially, Such a problem is apt to occur in the case of a thin work. Therefore, preferably, the grinding stone includes the abrasive grain having the average diameter not smaller than 100 μm and not greater than 270 μm. 
   Preferably, a coolant having a surface tension not smaller than 25 mN/m and not greater than 60 mN/m is supplied to a grinding region. In this case, the coolant has a good permeability to a grinding edge of the grinding stone, improving grinding efficiency. 
   The present invention is especially effective if the rare-earth sintered magnet contains cobalt at a rate not smaller than 0.3 wt % and not greater than 10 wt %. 
   It should be noted here that in this specification, the term “chamfering” means to chamfer sequentially along an outer circumferential portion of a work, and includes copy chamfering and profile chamfering for example. 
   The above object, other objects, characteristics, aspects and advantages of the present invention will become clearer from the following description of embodiments to be presented with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view showing an outline of an embodiment of the present invention; 
       FIG. 2  is a diagram showing an example of a tool; 
       FIG. 3  is a perspective view showing a primary portion of the embodiment in  FIG. 1 ; 
       FIG. 4A  is a perspective view of a work table,  FIG. 4B  is a sectional view showing a primary portion thereof; 
     FIGS.  5 A˜ 5 C are diagrams for describing a single face chamfering according to the embodiment; 
       FIG. 6  is a diagram for describing a simultaneous two-face chamfering according to the embodiment; 
       FIG. 7A  is a perspective view showing an example of work,  FIG. 7B  is a diagram showing a chamfering amount X; 
       FIG. 8A  is a table showing results of an experiment example, and  FIG. 8B  is a graphical representation thereof. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. 
   Referring to  FIG. 1 , a work chamfering apparatus  10  as an embodiment of the present invention comprises a driving portion  11  for driving a tool  34  (to be described later). The driving portion  11  includes a base  12 . The base  12  has an upper surface provided with a bed  14 . The bed  14  has an upper surface provided with a pair of rails  16   a ,  16   b  parallel to each other. On the rails  16   a ,  16   b , a generally L-shaped column  18  is disposed movably in horizontal directions. The column  18  is driven by a profile-following cylinder  20 . The tool  34  is adjusted so as to chamfer at a predetermined constant profile-following pressure not smaller than 20 N and not greater than 30 N for example, as far as moving horizontally within a stroke range of the profile-following cylinder  20 . 
   The column  18  has a front surface provided with a slide  22  for vertically moving a grinding stone shaft  32  (to be described later). The slide  22  is mounted with an electric motor attaching portion  24  slidably in vertical directions. The motor attaching portion  24  is provided with a grinding stone shaft motor  26 . The grinding stone shaft motor  26  is provided with a bearing  28  extending downwardly from a lower end of the grinding stone shaft motor  26  and held by a bearing holding portion  30 . The grinding stone shaft  32  is held by the bearing  28  and has a tip mounted with the chamfering tool  34 . With this constitution, the grinding stone shaft motor  26  rotates the grinding stone shaft  32  and the tool  34  in a direction indicated by Arrow A (See  FIG. 3. ) at 3600 rpm for example. 
   As shown in  FIG. 2 , the tool  34  includes grinding stones  36   a ,  36   b . The grinding stones  36   a ,  36   b  respectively includes base members  38   a ,  38   b  made of iron for example. The base members  38   a ,  38   b  have respective surfaces formed with abrasive grains  40   a ,  40   b  such as diamond abrasive grains rendered by electrocasting of Ni layers  39   a ,  39   b . Preferably, average diameters of the abrasive grains  40   a ,  40   b  are not smaller than 100 μm and not greater than 270  82  m respectively. If the average diameters of the abrasive grains  40   a ,  40   b  are smaller than 100 μm, the grinding stones  36   a ,  36   b  are clogged easily by sludge produced during chamfering. Furthermore, the abrasive grains  40   a ,  40   b  wear prematurely, thereby reducing the productivity. On the other hand, if the average diameters of the abrasive grains  40   a ,  40   b  are greater than 270 μm, the number of chippings increases when a fragile work  85  (to be described later) such as a rare-earth sintered magnet is chamfered, since the diameters of the abrasive grains  40   a ,  40   b  are too large. Especially, Such a problem is apt to occur in the case where the work  85  is thin. 
   The grinding stones  36   a ,  36   b  described above are disposed in such a way that respective tapered portions  42   a ,  42   b  for chamfering are opposed to each other, and connected with each other by a screw  44 . A bearing  46  is placed between the grinding stones  36   a ,  36   b . When chamfering, the bearing  46  is contacted onto a side surface of the work  85 , and the grinding stones  36   a ,  36   b  chamfer an upper edge  86   a  and a lower edge  86   b  of the work  85  respectively (See FIG.  5  and FIG.  6 .). Depending on a size of the work  85 , selection is made between a simultaneous two-surface chamfering and a single surface chamfering. If the work  85  has a thickness not greater than a thickness of the bearing  46  for example, then the grinding stones  36   a ,  36   b  are moved vertically to perform the single surface chamfering to the work  85 . 
   Returning to  FIG. 1 , the column  18  has an upper surface provided with a cylinder  48  for vertically moving the grinding stone shaft  32 . The cylinder  48  has two ends respectively formed with holes  50   a ,  50   b . The holes  50   a ,  50   b  respectively receives threaded portions  52   a ,  52   b  projecting out of the upper surface of the column  18 . The threaded portions  52   a ,  52   b  are provided with blocks  54   a ,  54   b  at positions corresponding to the thickness of the work  85  respectively. The cylinder  48  is connected to the motor attaching portion  24  by an arm  56 . With this constitution, the blocks  54   a ,  54   b  limit upward movement of the cylinder  48 , and determine a vertical stroke of the cylinder  48 , and vertical strokes of the grinding stone shaft  32  and the tool  34  moved vertically by the cylinder  48 . 
   A container  58  is disposed near the base  12 . Inside the container  58 , a work holding portion  59  is provided for holding the work  85 . The work holding portion  59  includes a turntable  60  disposed inside the container  58 . The turntable  60  is rotated by a table rotating motor  62  disposed right beneath the container  58  at a speed not slower than 1 rpm and not faster than 10 rpm for example, and in a direction indicated by Arrow B. As shown also in  FIG. 3 , the turntable  60  has an upper surface provided with a work table  64  on which the work  85  is to be placed. 
   Referring to FIG.  4 A and  FIG. 4B , the work table  64  includes a base  66 . The base  66  has an upper surface  68  including a center portion  70  and two end portions  72   a ,  72   b  each having a static friction coefficient greater than that of the center portion  70 . The static friction coefficient of the end portions  72   a ,  72   b  is greater than 0.1 and smaller than 1.0. The end portions  72   a ,  72   b  are respectively formed with holding grains  76  made of diamond and so on, rendered by electrocasting of Ni layers  74 . For example, the Ni layer  74  is formed to a thickness of 50 μm, and the holding grains  76  has a grain diameter D of 100 μm approximately, and the holding grains  76  project out of the upper surface  68 . With this constitution, when the work is held, the holding grains  76  stick into the work  85 , reducing unwanted movement of the work  85  during the chamfering due to an anchoring effect. Further, by constituting the work holding portion  59  as described above, even if the work  85  is made of a highly abrasive material such as a rare-earth alloy magnet, it becomes possible to apply stable and firm holding force, without causing wear in the work holding portion  59 . 
   The grain diameter D of the holding grain  76  is preferably not smaller than 50 μm and not greater than 300 μm approximately. Within this range, sticking depth of the holding grain  76  into the work  85  can be within an approximate range not smaller than 5 μm and not greater than 10 μm. Therefore, marking of the work  85  can be made shallow, while holding the work  85  firmly due to an anchoring effect. 
   As shown in FIG.  1  and  FIG. 3 , inside the container  58 , a clamper  80  operated by a clamping cylinder  78  is disposed. The clamper  80  has a tip provided with a generally U-shaped member  82 . The U-shaped member  82  has two ends each having a lower surface  84  (a total of two lower surfaces) contacted onto an upper surface  87   a  of the work  85 . In this state, a rotating center P of the work  85  is between the lower surfaces  84 , the lower surfaces  84  are apart generally equally from the rotating center P, and the work  85  is pressed at two regions. 
   With the above constitution, when chamfering, the work  85  is held by the two end portions  72   a ,  72   b  of the work table  64  included in the work holding portion  59  and the lower surfaces  84  at the end portions of the U-shaped member  82 . 
   The present invention can be effective if the work  85  is a hard and fragile work such as a rare-earth alloy magnet for obtaining a magnet used in a voice coil motor for a HDD. When considering the fact that the related art chamfering apparatus in which both the upper and lower edges of the work is chamfered simultaneously can only chamfer the work having a thickness not thinner than 3.0 mm, the present invention can be especially effective, if the work  85  is thinner than 3.0 mm, difficult to hold and has little margin to grind. Further, the present invention is also effective to the work formed into a shape including a curved line such as a sector-shaped work. 
   Further, in order to supply a coolant to the work  85  when chamfering, a coolant nozzle  88  of a coolant supplying device (not illustrated) is disposed near the work holding portion  59  in the container  58 . 
   The coolant is primarily made of water. The coolant has a surface tension not smaller than 25 mN/m and not greater than 60 mN/m. If the primary ingredient is water, cooling capability is high because of a high specific heat and a high evaporation heat. If the surface tension is not smaller than 25 mN/m and not greater than 60 mN/m, the coolant has a good permeability to grinding edges of the grinding stones  36   a ,  36   b , improving grinding efficiency. 
   It should be noted here that an antifoaming agent may be added by the coolant so that rapid temperature increase caused by foaming can be prevented at a grinding region. The additives for the coolant may include a surfactant or synthetic type lubricant, a rust inhibitor, a non-ferrous metal anticorrosive, an antiseptic and an antifoaming agent. 
   The surfactant added to the coolant including water as a primary ingredient can be an anionic surfactant or a nonionic surfactant. Examples of the anionic surfactant are a fatty acid derivative such as fatty acid soap and naphthenic acid soap; a sulfate ester surfactant such as long-chain alcohol sulfate ester and sulfated oil of animal or vegetable oil; and a sulfonic acid surfactant such as petroleum sulfonate. Examples of the nonionic surfactant are a polyoxyethylene surfactant such as polyoxyethylene alkylphenyl ether and polyoxyethylene monofatty acid ester; a polyhydric alcohol surfactant such as sorbitan monofatty acid ester; and an alkylol amide surfactant such as fatty acid diethanol amide. Specifically, the surface tension and the coefficient of dynamic friction can be adjusted within the preferred ranges by adding to water approximately 2 wt % of a chemical solution type surfactant, JP-0497N (manufactured by Castrol Limited). 
   The synthetic type lubricant can be any of a synthetic solution type lubricant, a synthetic emulsion type lubricant and a synthetic soluble type lubricant, among which the synthetic solution type lubricant is preferred. Specific examples of the synthetic solution type lubricant are Syntairo 9954 (manufactured by Castrol Limited) and #870 (manufactured by Yushiro Chemical Industry Co., Ltd.). When any of these lubricants is added to water in a concentration of approximately 2 wt %, the surface tension and the coefficient of dynamic friction can be adjusted within the preferred ranges. 
   Furthermore, when the coolant includes a rust inhibitor, corrosion of the rare-earth alloy can be prevented. In this embodiment, pH of the coolant is preferably set to 9 through 11. The rust inhibitor can be organic or inorganic. Examples of the organic rust inhibitor are carboxylate such as oleate and benzoate, and amine such as triethanol amine, and examples of the inorganic rust inhibitor are phosphate, borate, molybdate, tungstate and carbonate. 
   An example of the non-ferrous metal anticorrosive is a nitrogen compound such as benzotriazole, and an example of the antiseptic is a formaldehyde donor such as hexahydrotriazine. 
   Silicone emulsion can be used as the antifoaming agent. When the coolant includes an antifoaming agent, the coolant can be prevented from foaming up so as to attain high permeability. As a result, the cooling effect can be enhanced, and the temperature increase at the grinding edges of the grinding stones  36   a ,  36   b  can be avoided. Thus, the abnormal temperature increase and the abnormal abrasion of the grinding edges of the grinding stones  36   a ,  36   b  can be suppressed. 
   Now, primary operations of the work chamfering apparatus  10  with the above constitution will be described. 
   Referring to FIG.  5 A˜ FIG. 5C , description will cover a case in which the upper edge  86   a  and the lower edge  86   b  of the work  85  are chamfered sequentially, one edge at a time. This single surface chamfering is used for example, if the thickness of the work  85  is smaller than the thickness of the bearing  46 . 
   First, as shown in  FIG. 5A , the work  85  is held by the work table  64  and the U-shaped member  82  of the clamper  80 . At this time, two end portions  72   a ,  72   b  in the upper surface  68  of the work table  64  contact a lower surface  87   b  of the work  85 , whereas the lower surfaces  84  of the U-shaped member  82  contact the upper surface  87   a  of the work  85 . Next, the tool  34  is lowered, and the grinding stone  36   a  for chamfering the upper edge is contacted to the upper edge  86   a  as a grinding portion of the rotating work  85 , so that the upper edge  86   a  is chamfered. Then, as shown in  FIG. 5B , the tool  34  is moved off and raised. Then, as shown in  FIG. 5C , the grinding stone  36   b  for chamfering the lower edge is contacted to the lower edge  86   b  as a grinding portion of the work  85 , so that the lower edge  86   b  is chamfered. 
   Next, a case in which both of the upper and lower edges  86   a ,  86   b  of the work  85  are chamfered simultaneously is described with reference to FIG.  6 . The simultaneous two-surface chamfering is used if the work  85  is thick enough to contact both grinding stones  36   a ,  36   b  simultaneously. 
   In this case, chamfering is performed easily, by holding the work  85  with the work table  64  and the U-shaped member  82  of the damper  80  and then by lowering the tool  34  to allow the grinding stones  36   a ,  36   b  to contact the corresponding upper edge  86   a  or the lower edge  86   b  of the rotating work  85 . 
   As has been described above, according to the work chamfering apparatus  10 , chamfering can be performed in a mode appropriate to the thickness of the work  85 . 
   Further, as shown in FIG.  5 A˜ FIG. 5C , by shifting the tool  34  vertically, i.e. thickness-wise of the work  85 , thereby sequentially chamfering the upper and lower edges  86   a ,  86   b  of the work  85 , a variety of works  85  having a variety of thickness can be chamfered easily. It should be noted here that the works  85  of a variety of thickness can be handled without changing the grinding stones  36   a ,  36   b  but by adjusting the stroke of the cylinder  48 . 
   Further, by keeping a constant profile-following pressure, consistency of a chamfering amount X (to be described later) of the upper and lower edges  86   a ,  86   b  of the work  85  can be improved. 
   Further, according to the work chamfering apparatus  10 , the work  85  is held by the end portions  72   a ,  72   b  each having a static friction coefficient greater than 0.1, which is greater than that of the center portion  70  of the work table  64 . Therefore, it becomes possible to increase holding force to the work  85 . Therefore, even if the work  85  is thin and difficult to hold, unwanted movement of the work  85  caused by grinding reaction during the chamfering can be reduced and the work  85  can be held stably, making possible to chamfer and to increase consistency of the chamfering. Further, since there no longer is need for bonding the work  85  for example, working time can be reduced and the chamfering can be performed efficiently. 
   Further, since the work  85  is held by the end portions  72   a ,  72   b , it becomes possible to provide a plurality of regions having a large holding force to the work  85 , making possible to further reduce the unwanted movement of the work  85  during the chamfering. Further, since the holding grains  76  can be stack into the work  85  when holding the work  85 , the unwanted movement of the work  85  can be reduced due to the anchoring effect even if the clamping force to the work  85  is small. 
   Further, by holding the work  85  at a plurality of locations (at two locations by the lower surfaces  84  according to the present embodiment), with the rotating center P of rotation of the work  85  in between and with the locations being apart generally equally from the rotating center P, the work  85  can be held evenly on a good balance. Further, the work  85  can be fastened and the unwanted movement of the work  85  during the chamfering can be reduced by a smaller pressing force. 
   The present invention is effective when the work  85  is a R—Fe—B rare-earth sintered magnet, and is particularly suitable for chamfering a R—Fe—B rare-earth sintered magnet disclosed in the U.S. Pat. Nos. 4,770,723 and 4,792,368. Among others, the present invention is suitable for chamfering and manufacturing a neodymium magnet primarily comprising neodymium (Nd), Iron (Fe) and boron (B), having a hard main phase (iron-rich phase) made of tetragonal intermetallic compound Nd 2 Fe 14 B and a tough Nd-rich grain boundary phase. A typical neodymium magnet is available under a brand name NEOMAX (manufactured by Sumitomo Special Metals Co., Ltd.) 
   Especially, uniform chamfering is difficult if the work  85  is a fragile R—Fe—B magnet containing cobalt at a rate not smaller than 0.3 wt % and not greater than 10 wt %. 
   A reason for this is presumed as follows. Specifically, the R—Fe—B magnet is inferior to a Sm—Co magnet in heat resistance. For this reason, if the R—Fe—B magnet is to be incorporated in a product, such as an electric motor, used under a high temperature, the heat resistance is improved by adding Co, which substitute part of Fe, at a rate not smaller than 0.3 wt % and not greater than 10 wt %. On the other hand, the added Co is not only captured in the primary phase but also present in the grain boundary phase and forms such compounds as R 3 CO or R 2 CO, which reduce strength of the R—Fe—B magnet and makes the magnet fragile. 
   However, according to the present invention, it becomes possible to increase the holding force to the work  85 . Therefore, even if the work  85  is a thin, fragile R—Fe—B magnet containing cobalt at a rate not smaller than 0.3 wt % and not greater than 10 wt %, the work  85  can be held stably, and chamfered easily, while reducing chipping. 
   It should be noted here that as shown in  FIG. 3 , by rotating the tool  34  in the same direction (Arrow B) as the rotating direction (Arrow A) of the work  85 , the load exerted to the work  85  during the chamfering can be reduced and chipping of the work  85  can be reduced. 
   Next, experiment examples of the work chamfering apparatus  10  will be described. 
   Experiment conditions are shown in Table 1. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
           
          
             
                 
               Work 
               R-Fe-B permanent 
             
             
                 
                 
               magnet (NEOMAX-48BH) 
             
             
                 
                 
               Dimensions (mm): 40 × 20 × thickness h 
             
             
                 
                 
               Thickness h (mm): 1.5, 2.0, 2.5, 3.0 
             
             
                 
                 
               Shape: Sector (for VCM use) 
             
             
                 
               Grinding stone 
               3600 rpm 
             
             
                 
               rotating speed 
             
             
                 
               Coolant 
               10 L/min 
             
             
                 
                 
               (Water mixed with 2 wt % of chemical 
             
             
                 
                 
               solution) 
             
             
                 
               Profile-following 
               24.5 N 
             
             
                 
               load 
             
             
                 
               Grinding stone 
               Abrasive grain: artificial diamond 
             
             
                 
                 
               Mesh: #100 
             
             
                 
                 
               Grain Diameter: not smaller than 170 μm 
             
             
                 
                 
               and not greater than 210 μm, 
             
             
                 
                 
               electrocast 
             
             
                 
                 
               Diameter and angle: 20 mm × 45° 
             
             
                 
               Clamping force 
               588 N 
             
             
                 
               Turntable speed 
               15 rpm 
             
             
                 
               Work table surface 
               Holding grains: artificial diamond 
             
             
                 
                 
               Grain Diameter: not smaller than 90 μm 
             
             
                 
                 
               and not greater than 110 μm, 
             
             
                 
                 
               electrocast 
             
             
                 
                 
               (Surface roughness Rmax = 50 μm, 
             
             
                 
                 
               estimation) 
             
             
                 
               Measuring location 
               One discretionary point for each 
             
             
                 
                 
               straight portion (upper and lower edges 
             
             
                 
                 
               for each surface): A total of four 
             
             
                 
                 
               measurements per work 
             
             
                 
               Measuring 
               Dial gage 
             
             
                 
               instrument 
             
             
                 
                 
             
          
         
       
     
   
   As the work  85 , a R—Fe—B permanent magnet (NEOMAX-48BH: manufactured by Sumitomo Special Metals Co., Ltd.) having a shape described in Table 1 was used. As the coolant, a chemical solution type coolant JP-0497N (manufactured by CASTROL Limited) mixed with water at an approximate rate of 2 wt % was used, and the coolant was discharged at a rate of 10 litters per minute. The grinding stones  36   a ,  36   b  used had respective tapered portions  42   a ,  42   b  having an average outer diameter of 20 mm with an angle of tilt of 45 degrees. As the abrasive grains  40   a ,  40   b , artificial diamond grain of mesh #100 (grain diameter: not smaller than 170 μm and not greater than 210 μm) was used. The abrasive grains  40   a ,  40   b  were fastened to the grinding stone  36   a ,  36   b  by means of electrocast respectively. The clamping force was 588N. The turntable  60  was rotated at a speed of 15 rpm (one rotation per 4 seconds). Artificial diamond having a grain diameter not smaller than 90 μm and not greater than 110 μm was fastened to the upper surface of the work table  64  by means of electrocast as in the grinding stones  36   a ,  36   b , to provide an estimated surface roughness Rmax of 50 μm. The measurements were made by using a dial gage. 
   First, the work chamfering apparatus  10  and a conventional chamfering apparatus were compared in machinability and working time. 
   The experiment gave results shown in Table 2. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 2 
             
           
          
             
                 
                 
             
             
                 
                 
               Comparative 
                 
             
             
                 
                 
               Example 1 
               Comparative 
             
             
                 
                 
               (Simultaneous 
               Example 2 
             
             
                 
               Work 
               Two-surface 
               (Single-surface 
             
             
                 
               chamfering 
               chamfering 
               chamfering 
             
             
                 
               Apparatus 10 
               apparatus) 
               apparatus) 
             
          
         
         
             
             
             
             
             
             
             
          
             
               Thick- 
                 
               Working 
                 
               Working 
                 
               Working 
             
             
               ness 
               Machin- 
               time 
               Machin- 
               time 
               Machin- 
               time 
             
             
               (mm) 
               ability 
               (sec) 
               ability 
               (sec) 
               ability 
               (sec) 
             
             
                 
             
             
               3.0 
               ⊚ 
               18 
               ⊚ 
               18 
               ⊚ 
               300 
             
             
               2.5 
               ⊚ 
               35 
               X 
               — 
               ⊚ 
               300 
             
             
               2.0 
               ⊚ 
               35 
               X 
               — 
               ⊚ 
               300 
             
             
               1.5 
               ⊚ 
               35 
               X 
               — 
               ⊚ 
               300 
             
             
                 
             
          
         
       
     
   
   Here, the term “working time” refers to an amount of time used from the point when chamfering of a work  85  is started to a point when chamfering of the next work  85  is started. Table 2 shows an average working time of eight discretionary works  85  picked from four-hundred samples. Further, in the comparative example 1 and the comparative example 2, the works  85  were held by means of bonding with an adhesive. 
   In Table 2A, Table 3A and Table 3B, symbols ⊚, X and Δ used in the “machinability” column respectively means “possible”, “impossible” and “Chamfering was possible but the holding force was not enough that the work  85  moved during the machining”. 
   As shown in Table 2, according to the work chamfering apparatus  10 , the working time was increased for the works  85  having a thickness of 2.5 mm or smaller. This is because the upper and the lower edges  86   a ,  86   b  of the works  85  were sequentially chamfered in the single-surface chamfering mode. Further, the comparative example 1 cannot chamfer the work  85  having the thickness smaller than 3.0 mm, so no data is given for the works having the thickness of 2.5 mm and smaller. 
   As understood from Table 2, according to the work chamfering apparatus 10, even the works having the thickness smaller than 3.0 mm can be chamfered in a short time. 
   Next, Table 3A and Table 3B show comparison between the work chamfering apparatus 10 and the comparative examples 1˜3 in terms of the machinability and chamfering amount inconsistency. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 3A 
             
           
          
             
                 
                 
             
             
                 
               Work Chamfering 
               Comparative Example 1 
             
             
                 
               Apparatus 
               (Simultaneous Two-surface 
             
             
                 
               10 
               chamfering apparatus) 
             
          
         
         
             
             
             
             
             
          
             
                 
                 
               Chamfering 
                 
               Chamfering 
             
             
               Thick- 
                 
               amount 
                 
               amount 
             
             
               ness 
               Machin- 
               inconsis- 
               Machin- 
               inconsis- 
             
             
               (mm) 
               ability 
               tency (mm) 
               ability 
               tency (mm) 
             
             
                 
             
             
               3.0 
               ⊚ 
               0.07 
               ⊚ 
               0.07 
             
             
               2.5 
               ⊚ 
               0.07 
               X 
               — 
             
             
               2.0 
               ⊚ 
               0.07 
               X 
               — 
             
             
               1.5 
               ⊚ 
               0.07 
               X 
               — 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
           
             
                 
               TABLE 3B 
             
           
          
             
                 
                 
             
             
                 
               Comparative 
               Comparative Example 3 
             
             
                 
               Example 2 
               (Work chamfering apparatus 
             
             
                 
               (Single-surface 
               10 having the work 
             
             
                 
               chamfering apparatus) 
               table without diamond) 
             
          
         
         
             
             
             
             
             
          
             
                 
                 
               Chamfering 
                 
               Chamfering 
             
             
               Thick- 
                 
               amount 
                 
               amount 
             
             
               ness 
               Machin- 
               inconsis- 
               Machin- 
               inconsis- 
             
             
               (mm) 
               ability 
               tency (mm) 
               ability 
               tency (mm) 
             
             
                 
             
             
               3.0 
               ⊚ 
               0.12 
               Δ 
               0.5 
             
             
               2.5 
               ⊚ 
               0.14 
               Δ 
               0.5 
             
             
               2.0 
               ⊚ 
               0.14 
               Δ 
               0.5 
             
             
               1.5 
               ⊚ 
               0.14 
               Δ 
               0.5 
             
             
                 
             
          
         
       
     
   
   Here, the term “chamfering amount inconsistency” is obtained in the following method. 
   First, the chamfering amount X (See  FIG. 7B ) is measured at one discretionary point in each of four straight portions  90  of the upper and the lower edges of the work  85  shown in FIG.  7 A. Next, difference between a maximum value and a minimum value in the four measurements is obtained. This operation is performed to eight works  85  discretionarily picked from a total of four hundred samples. The eight differences obtained from the eight works are averaged to give the chamfering amount inconsistency. 
   According to the work chamfering apparatus  10 , even if the work  85  is thin, the work  85  can be stably held when chamfering, and unwanted movement of the work  85  when chamfering can be reduced. Therefore, as understood from Table 3A and Table 3B, the chamfering amount inconsistency can be smaller than in the comparative example, resulting in more consistent chamfering. 
   It should be noted here that the comparative example 3 was the same as the work chamfering apparatus  10  differing only in that the work table  64  was not formed with the diamond. In this comparative example, the chamfering amount inconsistency increases if the thickness of the work is not greater than 3.0 mm, because the work  85  is moved by grinding reaction during the chamfering. This supports the effectiveness of the formation of the holding grains  76  in the surface  68  of the work table  64 . 
   From the results of experiments described above, efficient and accurate chamfering is possible according to the work chamfering apparatus  10 . 
   Further, relationship between rotating speed of the grinding stone and chipping in the work chamfering apparatus is shown in FIG.  8 A and FIG.  8 B. In this experiment, relative speed of the grounding stones  36   a ,  36   b  to an outer circumference portion of the work  85  was set to 3 mm/sec, and the chamfering amount X was set to 0.14 mm. 
   In this experiment example, the grinding stone rotating speed was varied within a range not smaller than 500 rpm and not greater than 7000 rpm. The work  85  was chamfered at each of the varied rpm and the number of chippings having a diameter not smaller than 1 mm was counted. Each of the works  85  had the thickness h of 3.0 mm, with the other experiment conditions being equal to those shown in Table 1. 
   It should be noted that the grinding stone circumferential speed is a circumferential speed of the grinding stone contacted with the work. In this experiment example, the grinding stone circumferential speed was obtained from a formula; grinding stone outer diameter×3.14×grinding stone rotating speed. The grinding stone outer diameter was provided by an average outer diameter of the tapered portion of the grinding stone, and the actual value used in this experiment was 20 mm. 
   From the results of the experiment, the grinding stones  36   a ,  36   b  should preferably be rotated at a speed not slower than 2000 rpm and not faster than 5000 rpm. In other words, the circumferential speed of the grinding stones  36   a ,  36   b  should preferably be not slower than 125.6 m/min and not faster than 314 m/min. 
   If the grinding stone rotating speed is slower than 2000 rpm (i.e. the circumferential speed of 125.6 m/min), and if the relative speed of the grinding stones  36   a ,  36   b  i not slower than 3 mm/sec, the grinding stones  36   a ,  36   b  exert large grinding load, resulting in an i creased number of chippings in the work  85 . If the relative speed of the grinding stones  36   a ,  36   b  is decreased, the chipping is eliminated but operation efficiency goes down to an extremely low level. On the other hand, if the grinding stone rotating speed is faster than 5000 rpm (i.e. the circumferential speed of 314 m/min), the coolant is not supplied to the grinding edge enough, and the number of chippings increases. Further, if the rotating speed exceeds 6000 rpm (i.e. the circumferential speed of 376.8 m/min), accompanying air flow around the grinding stones becomes too strong and the supply of coolant to the grinding region becomes insufficient, resulting in a seizure. 
   Even if the work  85  is a R—Fe—B magnet containing cobalt not smaller than 0.3 wt % and not greater than 10 wt %, the number of chippings can be reduced and the chamfering can be efficient if the grinding stone rotating speed is not slower than 2000 rpm and not faster than 5000 rpm, i.e. if the grinding stone circumferential speed is not slower than 125.6 m/min and not faster than 314 m/min. At this time, if the relative speed of the grinding stones  36   a ,  36   b  with respect to the outer circumferential portion of the work  85  is slower than 0.5 mm/sec, the grinding efficiency goes down, on the other hand, if the relative speed is faster than 7.0 mm/sec. the grinding stones  36   a ,  36   b  exert large machining load, resulting in an increased number of chippings in the work  85 . Therefore, the relative speed is not slower than 0.5 mm/sec and not faster than 7.0 mm/sec, and more preferably, not slower than 2.0 mm/sec and not faster than 4.0 mm/sec. 
   More preferably, the grinding stone rotating speed is not slower than 3000 rpm and not faster than 4000 rpm, i.e. the grinding stone circumferential speed is not slower than 188.4 m/min and not faster than 251.2 m/min. In this case, the number of chippings can be further reduces. 
   It should be noted here that the portion having the static friction coefficient exceeding 0.1, i.e. the portion having a surface including the holding grains  76  projecting out of the surface, may alternatively be formed in the lower surfaces  84  of the U-shaped member  82 , or may be formed both in the upper surface  68  of the work table  64  and the lower surfaces  84  of the U-shaped member  82 . 
   Further, according to the embodiment, the work  85  is held on the upper surface  87   a . However, the work  85  can alternatively be held by the lower surface  87   b  of the work  85 , or both of the upper and lower surfaces  87   a ,  87   b  of the work  85 , by a plurality of locations, with the rotating center P of the work  85  being between the locations and the locations being apart generally equally from the rotating center P. The number of locations for holding the work  85  may be three or more per surface of the work  85 . 
   The holding grains  76  formed on the base  66  of the work table  64  may be grains of such a substance as Al 2 O 3 , SiC, cBN and so on. 
   It should also be noted here that in the present invention, the term rare-earth alloy refers to a concept including the rare-earth magnet. A rare-earth magnet is obtained by magnetizing a rare-earth alloy. The magnetization can be performed before or after the grinding step. The present invention can be applicable to the work  85  made of any rare-earth alloy. A rare-earth magnet manufactured from a R—Fe—B rare-earth alloy is suitable as a material for a voice coil motor used in positioning a magnetic head. 
   The present invention being thus far described and illustrated in detail, it is obvious that these description and drawings only represent an example of the present invention, and should not be interpreted as limiting the invention. The spirit and scope of the present invention is only limited by words used in the accompanied claims.