Patent Publication Number: US-6656416-B2

Title: Powder feeding apparatus, pressing apparatus using the same, powder feeding method and sintered magnet manufacturing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a powder feeding apparatus, a pressing apparatus using the same, a powder feeding method and a sintered magnet manufacturing method. More specifically, the present invention relates to a powder feeding apparatus for feeding a powder into a cavity formed in a die, a pressing apparatus using the same, a powder feeding method and a sintered magnet manufacturing method. 
     2. Description of the Related Art 
     Currently, as sintered rear-earth alloy magnets, two kinds, i.e. a samarium-cobalt magnet and a rare-earth-iron-boron magnet, are used extensively in many fields. Of the two, the rare-earth-iron-boron magnet is appreciated in application to variety of electronic devices and apparatuses. (Hereinafter, the rare-earth-iron-boron magnet will be called “R-T-(M)-B magnet”, where R represents a rare-earth element including yttrium, T represents iron or iron partially substituted by a transition metal element, M represents a doped element, and B represents boron.) A reason for this is that the R-T-(M)-B magnet is the most superior of many kinds of magnets in terms of magnetic energy product and relatively inexpensive in terms of price. The transition metal included as T may be cobalt for example. Boron can be partially substituted by carbon. 
     In manufacture of such a rare-earth magnet, first, a magnetic alloy powder made by milling a rare-earth alloy is pressed into a compact (green compact) by a pressing apparatus. When making the compact, the magnetic alloy powder is fed into a cavity formed by a die hole (through hole) provided in a die and a lower punch inserted into the die. The magnetic alloy powder fed in the cavity is pressed by an upper punch. The compact thus obtained is then sintered at a temperature of 1000° C. -1100° C. approx., and then finished as the sintered rare-earth magnet. 
     Conventionally, a variety of methods are proposed for feeding the magnetic alloy powder into the cavity in the pressing apparatus. 
     For example, Japanese Utility Model Publication (of examined Application for opposition) No. 59-32568 and Japanese Patent Laid-Open No. 61-147802 each discloses a technique of vibrating a container which holds the powder and thereby supplying the power into the cavity in sieving action through a metal net. 
     According to Japanese Patent Laid-Open No. 61-147802, there is described an apparatus comprising a feeder cup (the powder container) having a bottom portion provided with a metal net. The feeder cup is vibrated relatively rigorously by using a solenoid coil, thereby feeding the granular magnetic powder through the metal net into the cavity in a short time. 
     However, according to the conventional apparatus disclosed in Japanese Patent Laid-Open No. 61-147802, the vibration is generated by means of attracting force between the solenoid coil and an iron core, and of restoring force provided by a spring, and the vibration is given to the feeder cup itself which holds the powder. The iron core (moving part) is fastened to the feeder cup by a connecting hardware. With this arrangement, the vibrating force transmitted to the powder in the feeder cup is only a reciprocating force, and the transmitted force is still not sufficient to break down a lump of powder. In such an apparatus, in order to supply the granular powder into the cavity while preventing bridge formation, one possibility is to use the metal net having a fine grid (mesh). However, use of such a fine-mesh metal net poses another problem that the powder is not quickly sieved and there is a significant increase in the time for feeding the powder. 
     Another problem with the above conventional apparatus is that it is difficult to increase the stroke (amplitude) of vibration given to the feeder cup. If the feeder cup is moved only in a short stroke, it is difficult to feed the powder uniformly in the cavity. 
     There is still another problem. Specifically, corner and/or edge regions of the cavity is more difficult to feed with the powder than a center region of the cavity. According to the conventional apparatus therefore, when the rare-earth alloy powder is supplied through the metal net which is provided at a position relatively high above the die surface, the powder tends to form a high portion in the center region. If the powder is fed in such a non-uniform density in the cavity, the compact formed by the pressing operation has an unacceptably large difference in its pressing density, between the corner and/or edge regions and the center region. This density difference can cause a crack in the compact. 
     This problem is presumable also in an apparatus disclosed in Japanese Utility Model Publication (of examined Application for opposition) No. 59-32568. 
     Other techniques for feeding the powder into the cavity are proposed in Japanese Patent Laid-Open No. 11-49101 and Japanese Patent Laid-Open No. 2000-248301. 
     According to the technique disclosed in Japanese Patent Laid-Open No. 11-49101, a feed is fed into a container by means of pneumatic tapping and via a supplying hopper. An arrangement is made so that the feed is present in both of the supplying hopper and the container after the pneumatic tapping. Then, of this mass of the feed present in both of the supplying hopper and the container, a portion of uniform density formed in the container is separated from the feed remaining in the supplying hopper. 
     Japanese Patent Laid-Open No. 2000-248301 discloses a supplying apparatus, in which a feeder box having an opening in a bottom is moved to above a cavity formed in a die tooling, allowing a rare-earth alloy powder to be supplied into the cavity from the opening. The supplying apparatus comprises rod members which are moved at the bottom portion horizontally within the feeder box. The rod members are reciprocated when the rare-earth alloy powder in the feeder box is supplied to the cavity. 
     However, according to the technique disclosed in Japanese Patent Laid-Open No. 11-49101, since the feeding into the container is performed by the pneumatic tapping, the feeding density of the feed in the container becomes higher than by means of natural gravitational fall. For example, a rare-earth alloy powder fed by means of natural gravitational fall has the feeding density of 1.8 g/cm 3  approx., versus the feeding density of 3.4 g/cm 3  approx. by means of pneumatic tapping. The feed packed to such a high density does not allow particles of the powder to move easily, requiring a stronger magnetic field in order to orient the powder, leading to increase in manufacturing cost. 
     According to the technique disclosed in Japanese Patent Laid-Open No. 2000-248301 on the other hand, as shown in FIG. 21A, a feeder box  2  is moved toward a cavity  1 . Then, as shown in FIG. 21B, when the feeder box  2  is positioned above the cavity  1 , a powder  3  is supplied into the cavity  1  by the weight of the powder  3  itself. The feeding thus performed is not even, and therefore the powder  3  is not distributed uniformly. Thereafter, as shown in FIG.  21 C and FIG. 21D, a shaker  4  is activated to fill the cavity  1  with the powder  3 . The shaker  4  forces the powder  3  in, to the density of 2.3 g/cm 3  approx., thereby uniformalizing the feeding density. As a result, a stronger magnetic field is necessary in order to obtain a desired level of orientation. FIG. 22 shows state changes in the feeding operation performed by this conventional apparatus. 
     Further, if the cavity is shallow in a direction of the pressing operation provided by the punches, the feeding density inconsistency in the cavity is not easily corrected by the pressing operation, leading to occasional crack development in the compact. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary object of the present invention to provide a powder feeding apparatus, a pressing apparatus using the same and a sintered magnet manufacturing method, capable of feeding the powder uniformly and in a short time into the cavity of the pressing apparatus. 
     Another object of the present invention is to provide a powder feeding apparatus, a pressing apparatus using the same, a powder feeding method and a sintered magnet manufacturing method, capable of providing a desired orientation and a high magnetic characteristic at a low cost. 
     According to an aspect of the present invention, there is provided a powder feeding apparatus for feeding a powder into a cavity formed in a die, comprising: a container including a bottom portion provided with a powder holding portion formed with a plurality of openings capable of allowing the powder to pass through; and an impactor capable of hitting against the container; wherein the impactor is hit against the container to give an impulsive force to the container, thereby feeding the powder contained in the container into the cavity via the openings. 
     According to this invention, by having the impactor hit against the container, a lump of the powder contained in the container can be broken down and the powder in the broken state can be supplied into the cavity. 
     According to another aspect of the present invention, there is provided a pressing apparatus comprising: the above described powder feeding apparatus; and pressing means which presses the powder fed in the cavity by the powder feeding apparatus. 
     According to still another aspect of the present invention, there is provided a sintered magnet manufacturing method comprising: a first step of applying an impulsive force to a container which includes a bottom portion provided with a powder holding portion formed with a plurality of openings capable of allowing the powder to pass through, thereby feeding the powder contained in the container via the openings into a cavity formed in a die; a second step of forming a compact by pressing the powder fed in the cavity; and a third step of manufacturing a sintered magnet by sintering the compact. 
     By pressing the powder which is fed uniformly in the cavity, a compact which has a uniform density, and a small inconsistency in size and weight can be manufactured. 
     Further, by sintering the compact, a magnet which has a small inconsistency in size and weight can be obtained. 
     Preferably, the apparatus further comprises a vibrating mechanism connected to an upper portion of the container. The impactor is provided so as to hit against a lower portion of the container, and the vibrating mechanism vibrates an upper portion of the container, thereby allowing the impactor to hit against the lower portion of the container. In this way, by connecting the vibration mechanism with the container and by separating the impactor from the vibration mechanism, it becomes possible to reduce whirling up of the powder, thereby reducing binding of the powder in the vibrating mechanism. Further, by hitting the impactor on the lower portion of the container, the impact can be transmitted more directly to the opening of the container, making possible to transmit the impact to the entire mass of the powder present at the opening, thereby feeding the cavity with the powder uniformly. 
     Further, preferably, the powder holding portion is formed of a net having a mesh size of 2-14. More preferably, the powder holding portion is formed of a net having a mesh size of 2-8. By using a relatively coarse net as the above, the powder can be fed uniformly into the cavity while remarkably reducing the time necessary for the powder feeding. 
     Preferably, the powder holding portion is provided at a height smaller than 2.0 mm from a surface of the die. More preferably, the powder holding portion is provided at a height smaller than 1.0 mm from the surface of the die. This arrangement makes possible to allow only a small amount of the powder to project from within the cavity above the surface of the die. Therefore, an amount of the extra powder to be wiped is small, and a lump produced in the wiping operation by the container is not unwontedly fed into the cavity at the next cycle of powder feeding. 
     Further, preferably, the container can move when the impulsive force is given to the container by the hitting of the impactor against the container. With this arrangement, it becomes possible to have the moving container be hit by the impactor, and to give a reverse impact to the container, and therefore to feed the cavity with the powder more uniformly. 
     Preferably, the apparatus comprises a plurality of the impactors disposed outside of the container in an opposing relationship, with the container in between. With this arrangement, the impulsive force can be given continuously to the container. 
     Further, preferably, the apparatus further comprises a partition plate provided inside the container. With this arrangement, when the impactor hits a side wall of the container, the impulsive force can be transmitted dispersively to the powder inside the partitioned container, making possible to feed the powder more efficiently. This arrangement can remarkably reduce feeding time of the powder into the cavity. 
     Further, preferably, a size of the openings provided in the powder holding portion is in accordance with a location of the opening. By changing the coarseness according to the location of the opening in this way, the amount of powder to be fed into the cavities can be controlled according to region. 
     If the powder is a rare-earth alloy powder, the powder particles are angular, and with addition of a lubricant, the powder decreases its flowability and forms a lump, into a state not to easily drop from the opening of the powder holding portion. However, according to the present invention, even if the powder is a rare-earth alloy powder mixed with a lubricant and poor in flowability, the powder can be fed in the cavity uniformly and efficiently in a short time. 
     According to another aspect of the present invention, there is provided a powder feeding apparatus for feeding a powder into a cavity formed in a die, comprising: a feeder box movable to above the cavity, including a bottom portion formed with an opening, and containing the powder; a rod member provided inside the feeder box and pushing the powder downwardly; a linear member provided at the opening of the feeder box; and orienting means which aligns the powder fed from the feeder box in the cavity. 
     According to still another aspect of the present invention, there is provided a powder feeding method for feeding a powder into a cavity formed in a die, the method comprising: a step of moving a feeder box to above the cavity of the die, with the feeder box containing the powder, being provided inside thereof with a rod member movable in a horizontal direction, and having an opening provided with a linear member; a step of feeding the powder into the cavity while moving the rod member in the horizontal direction within the feeder box, when the feeder box is above the cavity; and a step of orienting the powder by applying a magnetic field to the powder in the cavity. 
     According to this invention, by providing the linear member at the opening of the feeder box, the powder does not fall into the cavity even when the feeder box has moved to above the cavity. The powder can be fed into the cavity thereafter, by activating the rod member in the feeder box. In this feeding, the powder can be fed into the cavity uniformly at a natural feeding density (1.7 g/cm 3 -2.0 g/cm 3  for example). Since the powder is not fed at a high density, the powder particles can move easily, and a desired orientation can be achieved by an orienting magnetic field of a relatively low strength. This makes possible to prevent manufacturing cost from increasing. Further, since the density distribution in the feeding can be made uniformly, a product having a superb magnetic characteristic can be obtained by orienting the powder in the cavity. 
     Preferably, the rod member is spaced from the linear member by a distance not smaller than 0.5 mm and not greater than 10 mm. With this arrangement, flow of the powder near the linear member is assisted, making possible to smoothly feed the powder into the cavity at a density suitable for the orientation. 
     According to still anther aspect of the present invention, there is provided a pressing apparatus comprising: the powder feeding apparatus described above; and pressing means which presses the powder fed in the cavity by the powder feeding apparatus. 
     According to this invention, by pressing the powder which is fed in the cavity by the above powder feeding apparatus, a compact high in density uniformity can be obtained, and thus crack and fracture development due to inconsistent density can be prevented. 
     If the powder is produced by using a rapid quenching process, and a particle distribution pattern of the powder is made narrow, the powder has an extremely poor flow ability. However, according to the present invention, since the powder flowablity can be improved by the natural gravitational feeding, density consistency of the powder in the cavity can be improved even if the powder is produced by using the rapid quenching process and the particle distribution pattern of the powder is made sharp. Further, each powder particle can be easily moved, and therefore it becomes possible to form a magnet having a high magnetic anisotropy for example. 
     Preferably, the interval between the linear members is not smaller than 2 mm and not greater than 12 mm. 
     According to still anther aspect of the present invention, there is provided a sintered magnet manufacturing method comprising: a step of obtaining a compact by pressing a powder in a cavity, the powder being fed by the above described powder feeding method; and a step of manufacturing a sintered magnet by sintering the compact. 
     According to this invention, by pressing the powder fed into the cavity by means of the above described method, a compact high in density uniformity can be obtained, and thus crack and fracture development in the compact can be reduced. As a result, sintered magnet obtained by sintering the compact has a decreased rate of defects due to cracking and/or fracturing, and a decreased rate of deformation. Therefore, it becomes possible to improve yield in manufacturing process, to improve productivity of the sintered magnet, and to manufacture a sintered magnet having a favorable magnetic characteristic. 
     The above objects, 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 a principal portion of a pressing apparatus as an embodiment of the present invention; 
     FIG.  2 A and FIG. 2B are views showing a principal portion of a powder feeding apparatus used in the embodiment in FIG.  1 ; FIG. 2A is a plan view with a lid removed, whereas FIG. 2B is a sectional view with a powder present; 
     FIG.  3 A and FIG. 3B are sectional views showing a fall of the powder from a net member caused by an impact force; FIG. 3A illustrates a state before applying the impact force, whereas FIG. 3B illustrates a state right after the application of the impact force; 
     FIG. 4 is an enlarged sectional view of a part of a powder container for illustrating a gap between a die surface and the net member; 
     FIG. 5 is a graph showing a relationship of the gap between the die surface and the net member with a thickness inconsistency; 
     FIG. 6 is a schematic diagram showing the pressing apparatus in FIG. 1 and a surrounding setting; 
     FIG. 7 is a sectional view of a powder container in a powder feeding apparatus according to another embodiment; 
     FIG.  8 A and FIG. 8B are plan views each showing a variation of the net member; 
     FIG.  9 A and FIG. 9B are views each showing a principal portion of a powder feeding apparatus used in still another embodiment; FIG. 9A is a plan view with a lid removed, whereas FIG. 9B is a sectional view with a powder present; 
     FIG. 10 is a perspective view showing a principal portion of the pressing apparatus according to another embodiment of the present invention; 
     FIG. 11 is a side view showing a section of a principal portion of the embodiment in FIG. 10; 
     FIG. 12 is an end view taken in line C—C (shown in FIG.  11 ), showing a principal portion of the embodiment in FIG. 10; 
     FIG. 13 is a side view showing a principal portion of a powder feeding apparatus used in the embodiment in FIG. 10; 
     FIG. 14 is a perspective view showing a feeder box provided with a shaker and linear members; 
     FIG.  15 A through FIG. 15D are views illustrating a powder feeding operation according to the embodiment in FIG. 10; 
     FIG. 16 is a diagram illustrating state changes in the powder feeding according to the embodiment in FIG. 10; 
     FIG. 17A is a view showing a compact formed in an experiment, whereas FIG. 17B is a table showing a result of the experiment; 
     FIG. 18 is a schematic diagram showing another embodiment of the present invention; 
     FIG. 19 is a schematic diagram showing still another embodiment of the present invention; 
     FIG.  20 A and FIG. 20B are graphs showing a result of another experiment; 
     FIG.  21 A through FIG. 21D are diagrams illustrating a powder feeding operation performed by a conventional apparatus; and 
     FIG. 22 is a diagram illustrating state changes in the powder feeding according to the conventional apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
     Referring to FIG.  1  and FIG. 2, a powder pressing apparatus  10  as an embodiment of the present invention comprises a pressing portion  12  and a powder feeding apparatus  14 . 
     The pressing portion  12  includes a die set  16  and a die tooling  18 . The die tooling  18  includes a die  20 , lower punches  22  and upper punches  24  (See FIG.  6 ). The die  20  has a saturated magnetism not smaller than 0.05 T and not greater than 1.2 T for example. The die  20  is fitted into the die set  16 . Each of the lower punches  22  is disposed so as to be inserted into a die hole  26  from below. The die hole  26  is a through hole running vertically through the die  20 . An upper end surface of the lower punch  22  and an inner circumferential surface of the die hole  26  provide a cavity  28  (See FIG. 2B) of a variable volume. With this arrangement, the upper punch  24  is inserted into the cavity  28  to press a powder m (to be described later) fed in the cavity  28  into a compact. Further,a magnetic field generating coil  29  is provided near the die  20 . By using the coil  29  for generation of magnetic field, an orienting magnetic field, having a strength of 1.2 T for example, is applied to the powder m in parallel with the pressing direction. 
     The powder feeding apparatus  14  includes a base plate  30  disposed in abutment on the die set  16 . On the base plate  30 , a feeder box  32  is disposed. The feeder box  32  is moved by a cylinder rod  36  of a cylinder  34  which is driven hydraulically or pneumatically for example (or by an electric servo motor), in a reciprocating pattern between a predetermined position on the die  20  and a stand-by position. Near the stand-by position of the feeder box  32 , there is provided a replenishing apparatus  38  for replenishing the feeder box  32  with the powder m. 
     The replenishing apparatus  38  includes a weighing scale  40 , a feeder cup  42  disposed thereon, and a vibrating trough  44  which drops the powder m by a small amount into the feeder cup  42 . The weighing operation is performed while the feeder box  32  is moved onto the die  20 . When the weight of the powder m in the feeder cup  42  reaches a predetermined level, a robot  46  grasps the feeder cup  42 , and when the feeder box  32  returns the stand-by position, the robot  46  replenishes the feeder box  32  with the powder m in the feeder cup  42 . The amount of the powder m in the feeder cup  42  replenishes an amount of the powder in the feeder box  32  used in a cycle of pressing operation. Therefore, the feeder box  32  holds a constant amount of the powder m. Because of the constancy in the amount of the powder m held in the feeder box  32 , pressure in gravitational fall of the powder m into the cavity  28  is constant, and an amount of the powder m fed into the cavity  28  is constant. The powder m may be a rare-earth alloy powder for example. 
     Reference is now made to FIG.  2 A and FIG. 2B, and description will be made for a principal portion of the powder feeding apparatus  14 . 
     The feeder box  32  of the powder feeding apparatus  14  includes an enclosing member  48  and a lid  50  which is disposed on an upper surface of the enclosing member  48  and can be opened and closed. Inside the enclosing member  48 , a powder container  52  is disposed. The powder container  52  is disposed between a pair of opposed impactors  54 . The feeder box  32 , with the powder container  52  containing the powder m, is moved to above the cavity  28  formed in the die  20  of the pressing apparatus  10 , allowing the powder m to be supplied into the cavity  28 . 
     The lid  50  provided on the upper surface of the enclosing member  48  can seal the inside of the enclosing member  48 . Preferably, inside the enclosing member  48  an inert gas such as nitrogen gas is supplied, preventing the powder m contained in the powder container  52  from oxidization by the atmosphere. The lid  50  can be opened and closed automatically by an air cylinder for example. 
     The powder container  52  has a bottom portion provided with a net member  56  which is capable of holding the powder m and of allowing the powder m to pass through upon impact from the impactor  54 . Preferably, the net member  56  is made of a stainless steel such as SUS  304 , and has a mesh size of 2-14 (sieve aperture not smaller than 1.8 mm and not greater than 12.7 mm). More preferably, the mesh size is 2-8 (sieve aperture not smaller than 3.2 mm and not greater than 12.7 mm). For example, the net member of a mesh size of  8  can be made of a metal wire having 0.6 mm diameter weaved into a net having 3.0 mm grids. The net member  56  preferably is plated with nickel for example. This decreases surface coarseness of the net member  56 , making possible to improve flowability of the rare-earth alloy powder at the time of feeding. 
     Each of the impactors  54  is provided with and driven by an air cylinder  58 , independently of the other. The impactor  54  can be moved quickly by the air cylinder  58  toward the powder container  52 , to hit on a side wall of the powder container  52  thereby applying an impulsive force (an impacting force). By this impact, the powder m contained in the powder container  52  is supplied into the cavity  28  through the net member  56 . Preferably, the impactors  54  are driven by the air cylinders  58  to hit the powder container  52  at a rate of 50-120 times per minute. Each of the impactors  54  has a reciprocating stroke of 10 mm-20 mm for example. 
     Preferably, upon impact from one of the impactors  54 , the powder container  52  can move toward the other impactor  54 . In order to allow this, the enclosing member  48  is provided with a pair of guide members  60  extending in parallel with each other in the direction in which the impactors  54  are moved. The powder container  52  can move linearly in the enclosing member  48  along the guide members  60 . With this arrangement, the other impactor  54  can be hit against the approaching powder container  52 , and it becomes possible to give the powder container  52  an impact in the reverse direction of the direction of the container movement. This makes possible to feed the powder m in the cavity  28  uniformly. 
     The powder container  52  has a bottom edge provided with a sliding member  62  (thickness: 5 mm approx. for example) made of such material as a thin plate of fluororesin or felt. The sliding member  62  reduces chance for the powder m to be caught between the powder container  52  and the die  20 , making possible for the powder container  52  to slide smoothly on the die  20 . A similar sliding member  64  is provided at a bottom edge of the enclosing member  48 . The sliding member  64  reduces chance for the powder m to be caught between the enclosing member  48  and the die  20 , making possible for the enclosing member  48  to slide smoothly on the die  20 . With these arrangements, the feeder box  32  can slide smoothly on the die  20  of the pressing apparatus  10 . 
     Next, reference is made to FIG.  3 A and FIG.  3 B. FIG. 3A shows a state before the impactor  54  gives an impact. If the powder m is a rare-earth alloy powder produced by using a strip cast process, each powder particle is angular. Further, if a lubricant is added to the powder m, the powder m decreases in its flowability and forms a lump. In this case, the powder m, i.e. the rare-earth alloy powder, is in a state not to easily drop from the opening  56   a  (grid) of the net member  56 . For this reason, the net member has a relatively coarse grid of 2-14 mesh approx., with the opening  56   a  having a relatively large width (gap) d 1 , which is a few millimeters through ten plus a few millimeters. 
     Thereafter, as shown in FIG. 3B, the impact is given by the impactor  54 , to break up the lump, allowing the powder m or particles smaller than the mesh to fall through the opening  56   a  of the net member  56 . A note should be made here that in FIG.  3 A and FIG. 3B, the illustrated particles of the powder m are relatively oversized. In reality however, the particle of the powder m provided by a rare-earth alloy powder typically has a diameter not greater than 10 μm, which is by far smaller than the width d 1  (a few millimeter through a ten plus a few millimeter) of the opening  56   a.    
     As has been described, according to the present embodiment, unlike the prior art in which the container itself is vibrated, the impactors  54  are hit against the powder container  52  as shown in FIG.  2 A and FIG.  2 B. This makes possible to break down the powder m, which is poor in flowablity and subject to lump formation in the powder container  52 , and to supply the cavity  28  with the powder m under a broken state. Use of the impactors  54  makes possible to apply the powder container  52  with a very large force which acts in a significantly short period of time (instantaneous force), which transmits to the powder m and effectively breaks the lump of powder m into finer state. According to the present embodiment, by using a relatively coarse net of 2-14 mesh size approx., it becomes possible to uniformly feed the powder m in the cavity  28  in a remarkably reduced time. 
     Next, reference is made to FIG.  4 . According to the powder feeding apparatus  14 , after supplying the cavity  28  with the powder m, and when the feeder box  32  is moving away from above the cavity  28 , a bottom edge of the powder container  52  wipes a top portion of the fed powder. This makes possible to accurately feed a predetermined amount of powder m which is to be pressed into compact, into the cavity  28 . In order to properly adjust the amount of the powder by the wiping operation, the net member  56  is attached closely to the surface of the die  20 , at the bottom portion of the powder container  52 . The net member  56  is spaced from the surface of the die  20  by a distance d 2 , which is preferably smaller than 2 mm, and more preferably smaller than 1 mm. 
     If the gap d 2  between the net member  56  and the surface of the die  20  is small as described, only a small amount of the powder m is allowed to project from within the cavity  28  above the upper surface of the die  20 . Therefore, an amount of the extra powder m to be wiped is small, and a lump of the powder resulting from the wiping operation by the powder container  52  is not fed into the cavity  28  in the next cycle of powder feeding. Further, it becomes possible to reduce an amount of powder m dropped between the surface of the die  20  and the net member  56  in a region other than the cavity  28 , making possible to prevent this extra amount of powder m from being fed (pushed) into the cavity  28  at the time of wiping. Further, even if the cavity  28  has corner and/or edge regions which are difficult to supply with the powder m as compared with a cavity center region, it is possible to prevent the powder m from projecting in the center region (i.e. to prevent extra amount of powder from being fed), and to uniformly feed the powder m in the corner and/or edge regions of the cavity  28  up to the surface of the die  20 . 
     As has been described, by attaching the net member  56  closely to the surface of the die  20 , it becomes possible to feed the powder m uniformly in the cavity  28 . It should be noted here that if the net member  56  is provided closely to the surface of the die  20  as described above, in order to prevent the net member  56  from contacting the surface of the die  20 , it is preferable that the net member  56  does not easily sag down. For this reason, the net member  56  is preferably made of a rolled mesh which is not distorted easily. 
     FIG. 5 is a graph showing a relationship of the distance (gap) d 2  between the net member  56  and the surface of die  20  with thickness inconsistency of the sintered compact (sintered body). The thickness inconsistency was measured as follows: First, block-like compacts each having a size of 55 mm width, 45 mm length and 16 mm height were manufactured by the pressing apparatus  10 . The compacts were then sintered, and then thickness measurements were made at a total of five locations, i.e. four locations near respective corners as well as one center location, on an upper surface of the sintered body. The thickness inconsistency (percent) was calculated by dividing a difference between a maximum measurement and a minimum measurement of the five measurements, by an average of the five measurements. For each setting of the gap d 2 , the thickness inconsistency was obtained for thirty sintered bodies, an average of which is then plotted on the graph as the thickness inconsistency (percent) at each particular gap d 2 . 
     As understood from the graph, the thickness inconsistency could be reduced to not greater than 4% when the gap d 2  is smaller than 2 mm, and compacts of a desired shape having a relatively uniform thickness could be manufactured. Also, it was learned from the graph that in order to reliably manufacture a compact having a small thickness inconsistency, the gap d 2  should preferably be smaller than 1 mm, and further, if the gap d 2  is set to not greater than 0.5 mm, it becomes possible to manufacture a highly accurate sintered body having a remarkably reduced thickness inconsistency. 
     As has been described, in the powder feeding apparatus  14  according to the present embodiment, the impactors  54  provide impulsive force to break down the lump of powder m in the powder container  52 , and to allow the powder m to be supplied into the cavity  28  through the relatively coarse net member  56  provided closely to the surface of the die  20 , whereby it became possible to feed the powder m uniformly regardless of the depth or region in the cavity  28 . Further, it became possible to remarkably reduce the time necessary for the powder supply. The powder feeding apparatus  14  according to the present embodiment was applied to the feeding operation of a rare-earth alloy powder which had poor flowability due to addition of a lubricant made of raw material to be described later, and was found to have a significant effect. Further, the effect was particularly remarkable when the depth of the cavity  28  to which the powder m was fed was not greater than 30 mm. 
     Now, description will cover an operation of the pressing apparatus  10 . 
     An inert gas such as nitrogen gas is supplied to the powder container  52  in the feeder box  32 . Under this state, the lid  50  of the feeder box  32  is opened, and the robot  46  supplies the powder container  52  with a predetermined amount of powder m measured in the feeder cup  42 . After supplying the powder m, the lid  50  is closed so as to maintain the inside of the powder container  52  filled with the inert gas. The supply of the inert gas into the powder container  52  is continuous, not only when the feeder box  32  is moving above the cavity  28 , in order to prevent the powder from catching fire. The inert gas may alternatively be argon or helium gas. 
     Under the above condition, the feeder box  32  containing the powder m is moved to above the cavity  28 , and then the powder supply is performed. As shown in FIG.  2 A and FIG. 2B, the powder supply is performed by driving the air cylinders  58  connected with the impactors  54  thereby applying impulsive force to the powder container  52 . By using the impactors  54  and thereby applying the impact multiple times continually, the powder m contained in the powder container  52  is supplied into the cavity  28  through the net member  56 . 
     A hitting pattern of the impactors  54  can be varied in many ways. For example, the pattern may be that the left impactor  54  hits the powder container  52  whereupon the right impactor  54  leaves the powder container  52 , and then the right impactor  54  hits the powder container  52  whereupon the left impactor  54  leaves the powder container  52 . Along with the hitting action, it is preferable that the powder container  52  is allowed to reciprocate on the die  20 , so that the powder container  52  itself is finely vibrated. By providing the impactors  54  to oppose each other, on the left and right sides, it becomes possible to supply the powder m into the cavity  28  in an appropriate hitting pattern that allows the powder m to easily enter the cavity  28  uniformly. 
     Reference is made to FIG.  6 . Now that the powder m is fed, the upper punches  24  begin to lower, and the coil  29  generates a magnetic field for orientation, which is applied to the powder m in the cavities  28 . The upper punches  24  and the lower punches  22  press the powder m in the cavities  28 , thereby forming compacts  66  in the cavities  28 . Thereafter, the upper punches  24  are raised, and the lower punches  22  are raised to push (to take) the compacts  66  out of the die  20 . FIG. 6 shows a state in which the lower punches  22  have held up the compacts  66  entirely above the die  20 . 
     After the pressing operation is complete, the compacts  66  which are elevated by the lower punches  22  are placed onto a sintering plate  68  (thickness: 0.5 mm-3 mm) by an unillustrated transporting robot. The plate  68  is made of a molybdenum material for example. The compacts  66  are transported on the conveyer  70 , together with the plate  68 , into a sintering case  72  which is placed in a space filled with inert gas atmosphere such as nitrogen atmosphere. The sintering case  72  is preferably made of a thin molybdenum plate (thickness: 1 mm-3 mm approx.). 
     The sintering case  72  is provided with a plurality of molybdenum rods (supporting rods)  74  extending horizontally. The rods  74  support the plate  68 , on which the compacts  66  are placed, generally horizontally in the sintering case  72 . 
     Use of the sintering case  72  as described above allows a plurality of compacts  66  to be sintered efficiently in the sintering furnace while preventing the compacts  66  from being exposed within the furnace during the sintering, making possible to prevent such problems as oxidization of the compacts  66 . 
     Hereinafter, description will cover a method of manufacturing an R-T-(M)-B rare-earth magnet by using the powder feeding apparatus  14 . 
     In order to manufacture an R-T-(M)-B magnet, first, an R—Fe—B alloy is made by using a strip cast process, which is a known method of making an alloy by means of rapid quenching process (quenching speed: not slower than 10 2 ° C./s and not faster than 10 4 ° C./s). The strip cast process is disclosed in the U.S. Pat. No. 5,383,978 for example. Specifically, an alloy having a composition comprising  26  weight percent Nd, 5.0 weight percent Dy, 1.0 weight percent B, 0.2 weight percent Al, 0.9 weight percent Co, 0.2 weight percent Cu, with the rest of ingredient being Fe and unavoidable impurities is melted by a high-frequency melting process into a molten. The molten is maintained at 1,350° C., and then quenched on a single roll, yielding a flaky alloy having a thickness of 0.3 mm. Cooling conditions at this time include a roll peripheral speed of about 1 m/s, a cooling rate of 500° C./s, and a sub-cooling of 200° C. for example. 
     The obtained alloy flake is coarsely pulverized by means of a hydrogen occlusion milling, and then further milled in an nitrogen atmosphere by a jet mill, into a fine alloy powder having an average particle diameter of 3.5 μm approx. It is preferable that the amount of oxygen in the nitrogen atmosphere should be maintained at a low level, at around 10000 ppm for example. Such a jet mill as the above is disclosed in Japanese Patent Publication (of examined Application for opposition) No. 6-6728. Preferably, concentration of oxidizing gas (such as oxygen and moisture) contained in the atmosphere during the fine milling should be controlled, whereby oxygen content (weight) in the finely milled alloy powder is controlled not greater than 6000 ppm. If the oxygen content in the rare-earth alloy powder is excessive, beyond 6000 ppm, then the magnet contains non-magnetic oxide at a high rate, which deteriorates magnetic characteristic of the resulting sintered magnet. 
     Next, a lubricant is added to and mixed with the rare-earth alloy powder at a rate of 0.3 weight percent, for example, in a rocking mixer, so that particle surfaces of the alloy powder are coated with the lubricant. Preferably, the lubricant is a fatty acid ester diluted with a petrol solvent. According to the present embodiment, capronic acid methyl can be used as the fatty acid ester, and isoparaffin can be used as the petrol solvent, suitably. Weight ratio of the capronic acidmethyl to isoparaffin is 1:9 for example. 
     The kind of the lubricant is not limited to the above-mentioned. For example, besides capronic acid methyl, usable fatty ester includes capric acid methyl, lauryl acid methyl, and lauric acid methyl. As for the solvent, isoparaffin is representative but many others can be selected from petrol solvents, as well as naphthene and other solvents. The solvent may be added at a discretionary timing, i.e. before, during or after the fine milling. Further, a solid (dry) lubricant such as zinc stearate can be used together with the liquid lubricant. 
     Next, the pressing apparatus  10  is used to form compacts from the alloy powder described above. 
     First, the rare-earth alloy powder is fed in the feeder box  32  of the powder feeding apparatus  14 , and then the alloy powder is supplied from the feeder box  32  into the cavities  28  formed in the die  20  of the pressing apparatus  10 . By using the powder feeding apparatus  14 , the powder can be fed uniformly without forming a bridge for example, in the cavities  28 . Next, the rare-earth alloy powder in the cavities  28  is pressed (press formation) within a magnetic field, into compacts of a predetermined shape. The compacts are made to have a density of 4.3 g/cm 3  for example. According to the present embodiment, the powder feeding apparatus  14  feeds a predetermined amount of the rare-earth alloy powder uniformly in each of the cavities  28 . Therefore, by pressing the rare-earth alloy powder thus fed, compacts having a uniform density can be formed. Further, since the powder feeding apparatus  14  can uniformly feed a plurality of cavities at one time, crack development in the compact during the pressing operation can be prevented and therefore yields can be improved. 
     Particularly, if the depth of the cavity is not greater than 30 mm, inconsistent feeding of the rare-earth alloy powder into the cavity allows bridge formation by the rare-earth alloy powder, and can increase density inconsistency in the resulting compact. The powder teeing apparatus  14  can feed the powder uniformly even if the cavities are of such a shallow depth. 
     Thereafter, as shown in FIG. 6, the compacts placed on the sintering plate  68  are encased in a sintering case  72 , transported to a sintering apparatus, and then placed in a preparation chamber at an entrance of the sintering apparatus. The preparation chamber is then sealed, and atmosphere inside the preparation chamber is partially vacuumed to 2 Pa approx., in order to prevent oxidization. Next, the sintering case  72  is transported into a de-wax chamber, where a de-wax process (Temperature: 250° C.-600° C., Atmospheric pressure: 2 Pa, Time: 3 hours-6 hours) is performed. The de-wax process allows the lubricant (wax) that coats the particle surfaces of the magnetic powder to evaporate before the sintering process. In order to improve orientation of the magnetic powder at the time of pressing operation, the lubricant is mixed with the magnetic powder before the pressing operation, and is present between the particles of the magnetic powder. During the de-wax process, different gases such as organic gases, vapor and so on are released from the compacts. Therefore, it is preferable that a getter which can absorb these gases should be placed in advance in the sintering case  72 . 
     After completion of the de-wax process, the sintering case  72  is transported into a sintering chamber, where the compacts undergo a sintering process in an argon atmosphere at a temperature of 1000° C. -1100° C. for 2 hours-5 hours approx. During the process, the compacts are sintered while shrinking, into sintered bodies. 
     During the above process, since the compacts have a uniform density according to the present embodiment, the shrinkage inconsistency of the compacts in magnetically anisotropic directions is favorably small. Therefore, the sintered bodies can be finished into a predetermined size in a reduced working time, making possible to improve productivity. 
     Thereafter, the sintering case  72  is transported into a cooling chamber, and cooled to a room temperature. The sintered bodies thus cooled are then placed in an aging furnace to undergo a known aging process. The aging process is preformed under such conditions as within an argon atmosphere of 2 Pa approx., at a temperature of 400° C. -600° C. for 3 hours-7 hours. The sintered bodies may be taken out of the sintering case  72  onto a stainless steel mesh container before the aging process. 
     The sintered bodies of the rare-earth magnet thus manufactured to have a desired magnetic characteristic are then cut and polished into a desired shape. Since the sintered bodies have a favorably small size-inconsistency, working time for shaping operation can be reduced. Thereafter,the shaped magnets undergo surface treatment in order to improve weather resistance as necessary, including formation of a protective coating with such material as Ni and Sn, to be rare-earth magnets as a final product. 
     It should be noted that the rare-earth magnet manufactured by the method according to the present invention is not limited to the magnet of the composition described above. For example, the rare-earth element R can be provided by a raw material that includes at least one of the following elements: Y, La, Ca, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu. In order to attain a satisfactory level of magnetization however, it is preferable that at least 50 atomic percent of the rare-earth element R is provided by Pr or Nd, or combination of both. 
     The transition metal element T that can include Fe and Co may only include Fe. However, addition of Co raises Curie temperature and improves heat resistance. Preferably, at least 50 atomic percent of the transition metal element T should be provided by Fe, since the rate of Fe lower than 50 atomic percent decreases saturation magnetism of Nd 2 Fe 14 B type composites. 
     Addition of B is indispensable in order to allow stable crystallization of the tetragonal Nd 2 Fe 14 B type composites. The amount of B smaller than 4 atomic percent allows crystallization of R 2 T 17  phase, which reduces coercive force, resulting in excessive deformation of a desirable square pattern in demagnetizing curve. For this reason, it is preferable that B should be added at a rate not smaller than 4 atomic percent. 
     Other elements may be doped in order to further increase magnetic anisotropy of the powder. At least one selected from the following group of elements, Al, Ti, Cu, V, Cr, Ni, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta, W can be preferably used as the doping element. The doping element M is not necessary for obtaining magnetically isotropic powder, but addition of Al, Cu, Ga and so on can increase intrinsic coercive force. 
     Next, reference is made to FIG. 7, and description will cover a powder container  76  used in a powder feeding apparatus  14   a  according to another embodiment. A plurality of partition plates  78  are provided inside the powder container  76 . With such a provision as the partition plates  78 , when the impactor  54  hits a side wall of the powder container  76 , the impulsive force can be transmitted dispersively to the powder m that is partitioned by the partition plates  78  in the powder container  76 , making possible to feed the powder m more efficiently. With such an arrangement, the time necessary for the powder feeding into the cavity  28  can be remarkably shortened. Vertical positions (along the height of powder container  76 ) of the partition plates  78  are adjustable. By adjusting the position of partition plates  78  in accordance with the volume of the powder m held in the powder container  76 , the force can be distributed appropriately to the entire mass of the powder. 
     The net member provided at the bottom portion of the powder container may be varied. FIG.  8 A and FIG. 8B show such variation as a net member  80  and a net member  82 . As shown in FIG. 8A, the net member  80  includes two kinds of net assemblies  80   a  and  80   b  each having a different grid coarseness from each other. Likewise, as shown in FIG. 8B, the net member  82  includes two kinds of net assemblies  82   a  and  82   b  each having a different grid coarseness from each other. By changing the grid coarseness as the above, in accordance with locations in the net member, it becomes possible to control the amount of powder m to be fed into the cavities  28  according to region. 
     As has been described earlier, sometimes, corner and/or edge regions of the cavity  28  can receive a smaller amount of powder supply than a center region of the cavity  28 . In such a case, in order to supply the entire cavity  28  with the powder uniformly, it is preferable to make an arrangement to supply a greater amount of the powder m in the corner and/or edge regions of the cavity  28 . 
     For this reason, according to the net members  80  and  82  in FIG.  8 A and FIG. 8B, portions corresponding to the edge regions of the cavity  28  are respectively provided with coarser net assemblies  80   b  and  82   b , whereas the portions corresponding to the center region are respectively provided with finer net assemblies  80   a  and  82   a . With such an arrangement, it becomes possible to feed the edge regions of the cavity  28  with a greater amount of powder m than the center region. 
     Further, according to the net member  82  shown in FIG. 8B, the finer net assembly  82   a  is provided at a rear portion with respect to the moving direction (indicated by an arrow A in the figure) of the net member  82  during the wiping operation which is performed after the powder feeding. The region beneath the finer net assembly  82   a  gets less supply of the powder m. This is because the powder m scattered on the die  20  may be wiped into the edge region of the cavity  28  (the region corresponding to the finer net assembly) during the wiping operation, so the amount of the supply to the region is reduced in advance. Such an arrangement allows the entire cavity  28  to have uniformly fed with an appropriate amount of the powder m upon completion of the wiping. 
     Table 1 shows a result of experiment conducted to the embodiments of the present invention and a comparative example. 
     In Embodiment 1, the powder feeding apparatus  14  shown in FIG. 2 was used to feed the cavities  28  with a rare-earth alloy powder, and then a pressing operation was performed to form compacts. In Embodiment 2, the powder feeding apparatus  14   a  shown in FIG. 7 was used to form compacts. In Comparison 1, compacts were formed by using a shaker type powder feeding apparatus disclosed in Japanese Patent Laid-Open No. 2000-248301. 
     Each of the compacts formed as the above was sintered, and measurements were made to see thickness inconsistency and weight inconsistency of the sintered body. The thickness inconsistency was calculated as follows: First, for each of the sintered bodies, the thickness was measured at nine locations. Then, a difference between a maximum measurement and a minimum measurement of nine measurements was obtained, and the difference was divided by an average of the nine thickness measurements to obtain the thickness inconsistency. Note that the thickness inconsistency value given in Table 1 is an average of the thickness inconsistency values (percent) obtained for 200 sintered bodies. The weight inconsistency was calculated by first obtaining a difference between a maximum weight and a minimum weight of the 200 sintered bodies, and then dividing the difference by an average weight of the 200 sintered bodies. The feed time is a length of time needed for feeding the cavities with a certain amount of the powder. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Weight 
                 Thickness 
               
               
                   
                   
                 Feed 
                 Inconsistency 
                 Inconsistency 
               
               
                   
                 Method 
                 Time 
                 (R/Av) 
                 (R/Av) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Embodiment 
                 Hitting-type 
                 12 s 
                 2.67% 
                 1.54% 
               
               
                 1 
                 Feeding 
               
               
                   
                 Apparatus 
               
               
                 Embodiment 
                 Hitting-type 
                 10 s 
                 2.35% 
                 1.12% 
               
               
                 2 
                 Feeding 
               
               
                   
                 Apparatus 
               
               
                   
                 plus 
               
               
                   
                 Partition 
               
               
                   
                 Plates 
               
               
                 Comparative 
                 Shaker-type 
                 15 s 
                 5.40% 
                 2.74% 
               
               
                 Example 1 
                 feeding 
               
               
                   
                 Apparatus 
               
               
                   
               
            
           
         
       
     
     From Table 1 given above, it is clear that as compared with the shaker-type powder feeding apparatus (Comparative Example 1) disclosed in Japanese Patent Laid-Open No. 2000-248301, the powder feeding apparatuses  14  and  14   a  (Embodiments 1 and 2) shown in FIG.  2  and FIG. 7 respectively can feed more quickly and can decrease dimensional and weight inconsistency of the sintered body. 
     Next, reference is made to FIG.  9 A and FIG. 9B, which show a principal portion of a powder feeding apparatus  14   b  according to another embodiment. The powder feeding apparatus  14   b  comprises a vibration mechanism  84  connected to an upper portion of a powder container  52 . The vibration mechanism  84  is connected to a cylinder  86  such as an air cylinder. Further a pair of impactors  88  is attached to the enclosing member  48  so as to hit a lower portion of the powder container  52 . Each of the impactors  88  has a tip  90  made, for example, of a hard resin so that the hitting with the powder container  52  does not produce a spark. Other arrangements including mesh size of the net member  56 , the distance from the surface of the die  20  to the net member  56  are the same as in the powder feeding apparatus  14  shown in FIG.  2 A and FIG.  2 B. 
     According to the powder feeding apparatus  14   b , the cylinder  86  drives the vibration mechanism  84 , and the vibration mechanism  84  vibrates the upper portion of the powder container  52 , whereby the impactors  88  are hit against the lower portion of the powder container  52 . The powder container  52  is moved in a stroke of 1 mm-15 mm for example. 
     According to the powder feeding apparatus  14   b , the vibration mechanism  84  is disposed at an upper portion whereas the impactors  88  are disposed at a lower portion. By such separation, the impactors  88  can be disposed closer to the surface of the die  20 , making possible to apply the impact force more uniformly to the opening  56   a  of the powder container  52  which contains the powder m. Therefore, the powder m can be fed more uniformly and stably into the cavity  28 . 
     Further, if the powder m is provided by a very fine powder having, for example, an average particle diameter not greater than 10 μm, it becomes possible to reduce whirling of the powder m in a feeder box  32   b  out of the powder container  52 , making possible to prevent the powder m from being caught by sliding part between the enclosing member  48  and the air cylinder  86  for example. 
     Further, the powder m fed in the cavity  28  by using the powder feeding apparatus  14   b  can be pressed in the same way as in the embodiment shown in FIG. 1, and then sintered into a sintered magnet. In this way, a sintered magnet having a small inconsistency in size and weight can be obtained. 
     The powder feeding apparatus  14   b  offers generally the same effects as offered by the Embodiment 2 shown in the above Table 1. 
     Next, reference is made to FIG.  10  through FIG. 14, and description will cover a pressing apparatus  100  according to another embodiment of the present invention. 
     The powder pressing apparatus  100  comprises a pressing portion  112  and a powder feeding apparatus  114 . 
     The pressing portion  112  includes a die set  116  and a die tooling  118 . The die tooling  118  includes a die  120 , a lower punch  122  and an upper punch  124 . The die  120  has a saturated magnetism not smaller than 0.05 T and not greater than 1.2 T for example. The die  120  is fitted into the die set  116 . The lower punch  122  is disposed so as to be inserted into a die hole  126  from below. The die hole  126  is a through hole running vertically through the die  120 . An upper end surface of the lower punch  122  and an inner circumferential surface of the die hole  126  provide a cavity  128  of a variable volume. With this arrangement, the upper punch  124  is inserted into the cavity  128 , to press a powder m fed in the cavity  128  into a compact. 
     The powder feeding apparatus  114  includes a base plate  130  disposed in abutment on the die set  116 . On the base plate  130 , a feeder box  132  is disposed. The feeder box  132  is moved by a cylinder rod  136  of a cylinder  134  which is driven e.g. hydraulically or pneumatically (or by an electric servo motor), in a reciprocating pattern between a predetermined position on the die  120  and a stand-by position. Near the stand-by position of the feeder box  132 , there is provided a replenishing apparatus  138  for replenishing the feeder box  132  with the powder m. The replenishing apparatus  138  includes a weighing scale  140 , a feeder cup  142 , a vibrating trough  144  and a robot  146 . The operation of the replenishing apparatus  138  is the same as of the replenishing apparatus  38  described earlier, and therefore repetitive description will not be made. 
     As shown in FIG.  11  and FIG. 12, a shaker (may also be called agitator)  148  is provided inside the feeder box  132 . The shaker  148  includes a plurality of rod members  150  disposed in parallel with an upper surface of the die  120  and with an upper surface of the base plate  130 , and a plurality of generally U-shaped supporting members  152 . Each of the rod members  150  is made for example of a bar material having a circular section of a diameter not smaller than 3 mm and not greater than 10 mm. The bar material may be a square bar. The rod members  150  and the supporting members  152  are each made of a stainless steel (SUS  304 ) for example. According to the present embodiment, three rod members  150  and three supporting members  152  are used. Each of the rod members  150  has its two end portions connected with one of the supporting members  152 , so that three sets of generally rectangular frame-like structure are provided. Two supporting rods  158  extend in parallel with each other, penetrating two side walls  154 ,  156 , which are the walls across moving directions of the feeder box  132 . Each of the supporting members  152  has its upper portion connected to the two supporting rods  158 , whereby the supporting members  152  and the rod members  150  are fastened. Each supporting rod  158  has two ends respectively fastened by e.g. screws to connecting members  160 ,  162  provided by e.g. strip-like pieces, and is connected with each other. The side wall  156  has an outer surface provided with a fixing hardware  164 , to which an air cylinder  166  is fixed. The air cylinder  166  has a cylinder shaft  168  fastened to the connecting member  162 . With this structure, the air cylinder  166  has two ends each supplied with air through an air supply tube  170 . This causes the cylinder shaft  168  to reciprocate, thereby reciprocating the shaker  148 . The rate of reciprocation is determined in accordance with the volume of powder to be fed. 
     Further, a gas supply pipe  172  is provided at a center upper portion of the side wall  156  of the feeder box  132 , for supplying an inert gas such as nitrogen gas into the feeder box  132 . The inert gas such as nitrogen gas is supplied into the feeder box  132  at a higher pressure than the normal atmospheric pressure in order to maintain the inside of the feeder box  132  filled with the inert gas. Because of this arrangement,even if friction is generated between the feeder box  132  and the powder by the reciprocating movement of the shaker  148 , this does not cause catching fire. Likewise, the feeder box  132  is moved, with the powder caught between a bottom surface of the feeder box  132  and the base plate  130 , but friction in this movement does not cause ignition either. Further, movement of the feeder box  132  generates friction among powder particles in the feeder box  132 , but this does not lead to ignition of the powder, either. 
     Further, a powder containing portion  174  of the feeder box  132  is maintained air tight by a lid  176 . When replenishing the powder m, in order to open an upper surface of the powder containing portion  174 , the lid  176  must be moved toward the cylinder  166  (in a rightward direction as in FIG.  13 ). For this purpose, an air cylinder  178  which opens the lid  176  is provided on a side wall  180 . The lid  176  and the air cylinder  178  are connected with each other by a hardware  182  and fastened together by screws. In order to maintain the inside of the feeder box  132  filled with the inert gas, the lid  176  is disposed to cover the powder containing portion  174  of the feeder box  132  at normal times, and is moved toward the cylinder  166  only when the powder is replenished. The side wall  180  of the feeder box  132  is opposed by a side wall  184 , which is provided with guide means (not illustrated) so that the lid  176  can move smoothly during the open/close operation by the air cylinder  178 . With this arrangement, the air cylinder  178  has two ends each supplied with air through an air supply tube  186 . The air drives the cylinder shaft (not illustrated), thereby opening and closing the lid  176 . 
     The feeder box  132  has a bottom surface provided with a plate member  188 . The plate member  188 , made for example of a fluororesin, has a thickness of 5 mm and is fastened by screws. The feeder box  132  slides on the base plate  130  via the plate member  188 , which prevents the powder m from being caught between the feeder box  132  and the base plate  130 . 
     In addition, as shown in FIG. 14, a plurality of linear members  192  are disposed at an opening  190  of the feeder box  132 , in parallel with a direction of movement of the feeder box  132 . The opening  190  is larger than an upper opening of the cavity  128 . The linear members  192  are made of a nonmagnetic metal wire having a diameter of 0.15 mm approx. The linear members  192  are spaced at an interval not smaller than 2 mm and not greater than 4 mm. The rod members  150  are spaced from the linear members  192  by a distance not smaller than 0.5 mm and not greater than 10 mm. The diameter of the linear members  192  and the distance between the rod members  150  and the linear members  192  are adjusted in accordance with the size of the cavity  128 . 
     Further, a pair of magnetic field generating coils  194  is provided to sandwich the die set  116 , as orienting means. At a center of each coil  194 , a core  195  made of permendur for example is provided. By energizing the magnetic field generating coils  194 , an orienting magnetic field having a strength for example of 1.2 T is applied to the powder m in the cavity  128 , in a direction indicated by Arrow B, and the powder m is oriented. 
     Description will now cover an operation of the pressing apparatus  100 . 
     An inert gas such as nitrogen gas is supplied through the gas supply pipe  172  to the inside of the powder containing portion  174  of the feeder box  132 . Under this state, the lid  176  of the feeder box  132  is opened, and the robot  146  supplies the powder containing portion  174  with a predetermined amount of powder m from the feeder cup  142 . After supplying the powder m, the lid  176  is closed so as to maintain the inside atmosphere of the powder containing portion  174  filled with the inert gas. The supply of the inert gas into the powder containing portion  174  is continuous, not only when the feeder box  132  is moving above the cavity  128 , in order to prevent the powder from spontaneous ignition. The inert gas may alternatively be argon or helium gas. 
     Under the above condition, the air cylinder  134  is activated to move the feeder box  132  to above the cavity  128  of the die  120 . Then, the rod members  150  in the feeder box  132  are reciprocated 5 times-15 times for example in horizontal directions to allow the powder in the feeder box  132  to drop through a screen of linear members  192  into the cavity  128 , in the inert gas atmosphere. The above process allows supplying of the powder into the cavity  128  at a remarkably uniform feeding density, without any risk of ignition. During the process, the powder in the feeder box  132  does not drop naturally when the feeder box  132  comes above the cavity  128 . When the shaker  148  begins its pushing action, the powder begins to pass through the screen of the linear members  192 , being placed in the cavity  128  at a density suitable for the orientation. 
     After the powder m is fed in the cavity  128 , the feeder box  132  is receded, and then the upper punch  124  is lowered. Under this state, while the magnetic field generating coils  194  generate the orienting magnetic field, the powder m in the cavity  128  is pressed. During this process, the feeder box  132  which has been receded is replenished with the powder m. By repeating the above described cycle, the pressing operation of the powder m is performed continually. 
     According to the pressing apparatus  100 , even when the feeder box  132  is moved toward the cavity  128  as shown in FIG. 15A, and even after the feeder box  132  has moved above the cavity  128  as shown in FIG. 15B, the powder m does not fall into the cavity  128  since the powder m is in the state of bridging due to the linear members  192  provided at the opening  190  of the feeder box  132 . Thereafter, as shown in FIG.  15 C and FIG. 15D, each reciprocating stroke of the shaker  148  in the feeder box  132  allows a constant amount of the powder m to be placed in the cavity  128  generally uniformly. Specifically, the powder m is fed in the cavity  128  as illustrated in FIG. 16, and the powder m can be fed uniformly in the cavity  128  at a natural feeding density (1.7 g/cm 3 -2.0 g/cm 3  for example). As described, since the powder m is not fed at a high density, the powder particles can easily move, allowing a desired orientation by an orienting magnetic field of a relatively low strength. This makes possible to prevent manufacturing cost from increasing. Further, since the feeding can be made generally uniformly, a product having a superb magnetic characteristic can be obtained by orienting the powder m in the cavity  128 . 
     It should be noted that preferably the reciprocating operation of the shaker  148  should allow at least one of the rod members  150  to move from one side above the cavity  128  to the other side thereof. This setting allows more uniform feeding of the powder m into the cavity  128 . 
     By setting the distance between the rod members  150  and the linear members  192  to be not smaller than 0.5 mm and not greater than 10 mm, flow of the powder m near the linear members  192  is assisted, making possible to smoothly feed the powder m into the cavity  128  at a density suitable for the orientation. If the distance between the rod members  150  and the linear members  192  is smaller than 0.5 mm, the powder m between the rod members  150  and the linear members  192  develops intense friction with the rod members  150  and the linear members  192 , and the friction can cut the fine liner members  192 . On the other hand, if the distance between the two members exceeds 10 mm, it becomes impossible to let the powder pass through the screen of linear members  192  by the pushing action of the rod members  150 , and therefore a feeding suitable for orientation cannot be achieved. 
     Further, feeding by means of natural gravitational fall performed by the pressing apparatus  100  can improve flowability of the powder m at the time of magnetic orientation. Therefore, even if the powder m is made by a rapid quenching process, particles of the powder m can move easily in the cavity  128 . This makes possible to easily orient the powder m in a given magnetic direction, and to form a magnet having a high magnetic anisotropy for example. 
     Further, the interval between the linear members  192  should preferably be 2 mm-12 mm. If the interval is smaller than 2 mm, it becomes impossible to push the powder m in by the moving action of the rod members  150 . If the interval is grater than 12 mm, the feeding density becomes higher than the natural feeding density, since the bridging force above the cavity  128  is weak. 
     Further, by pressing the powder m which is uniformly fed in the cavity  128 , a compact of a highly uniform density can be obtained. Also, crack and fracture development and deformation due to inconsistent density can be prevented. 
     The compact is then transported to a sintering furnace and sintered in an argon atmosphere at a temperature of 1050° C. for two hours, and then aged in an argon atmosphere at a temperature of 600° C. for an hour, to be the sintered magnet. In this stage of sintered magnet, again, rate of defective products due to cracking and/or fracturing is decreased, and rate of after-sintering deformation is also decreased. Therefore, machining margin reserved for dimension correction can be decreased, which makes possible to improve yield in manufacturing process, to improve productivity of the sintered magnet, and to manufacture a sintered magnet having a favorable magnetic characteristic. 
     Further, by performing the pressing operation using the die  120  which has the saturated magnetism of not smaller than 0.05 T and not greater than 1.2 T, a uniform distribution of magnetic flux density is provided in the cavity  128 , and, it becomes possible to manufacture a sintered magnet without deformation. 
     Next, description will cover an experiment. The experiment was conducted to the pressing apparatus  100  and the pressing apparatus disclosed in Japanese Patent Laid-Open No.2000-248301 (conventional apparatus), and results were compared. 
     The experiment was conducted under the following conditions: 
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Experiment conditions 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Compacts 
                 Size: 80 mm × 52.2 mm × 20 mm 
               
               
                   
                 Number of compacts made per press: one 
               
               
                   
                 Raw material: Nd-Fe-B alloy powder 
               
               
                   
                 Produced by a strip cast process 
               
               
                   
                 (Average particle diameter: 2 μm-5 μm) 
               
               
                   
                 Capronic acid methyl was added as a lubricant. 
               
               
                   
                 Pressed density: 4.1 g/cm 3   
               
               
                   
                 Feeding density: 
               
               
                   
                 Pressing apparatus 10; 1.8 g/cm 3   
               
               
                   
                 Conventional Apparatus; 2.3 g/cm 3   
               
               
                 Feeder Box 
                 Shaking: 10 reciprocations in parallel with the 
               
               
                   
                 die surface (in both apparatuses) 
               
               
                   
                 Size of rod members: 3 mm diameter 
               
               
                   
                 Material of rod members: stainless steel 
               
               
                   
                 Size of linear members: 0.15 mm diameter 
               
               
                   
                 Material of linear members: copper 
               
               
                   
                 Spacing between liner members: 2 mm 
               
               
                   
                 Spacing between rod and linear members: 2 mm-4 mm 
               
               
                 Pressing 
                 Pressing method: Pressing in a magnetic field 
               
               
                   
                 Pressing was made while applying a magnetic field 
               
               
                   
                 perpendicularly to the pressing direction. 
               
               
                   
                 Die hole size: 80 mm × 52.2 mm 
               
               
                   
                 Depth of powder feeding: 50 mm 
               
               
                 Measurement 
                 Formed compacts were sintered, aged, cut and then 
               
               
                   
                 measured. Measurement was made for only one 
               
               
                   
                 sintered magnet which was sliced out of the center 
               
               
                   
                 portion. Measurement was made on a main surface 
               
               
                   
                 of the sintered magnet. 
               
               
                   
               
            
           
         
       
     
     In this experiment, a compact as shown in FIG. 17A, which can be used in manufacturing a voice coil motor for example, was manufactured. The size of the compact was 80 mm×52.2 mm×20 mm. One compact was made per cycle of the pressing operation. Pressing was performed in a magnetic field, and the pressing was made while applying the magnetic field perpendicularly to the pressing direction (indicated by Arrow S in FIG.  17 A). The feeder box was a single-cavity feeding type. The shaker was reciprocated ten times in horizontal directions. The powder was a rare-earth alloy powder (Nd—Fe—B alloy powder). A strip cast process was used to produce the alloy powder having an average particle diameter of not smaller than 2 μm and not greater than 5 μm. A lubricant (capronic acid methyl) was added to the alloy powder. The compact shown in FIG. 17A was then sintered, aged, and then cut into sintered magnets. Of these sintered magnets, magnetic characteristic was measured for only one sintered magnet obtained from the center portion (corresponding to the shaded piece P in FIG.  17 A). The measurement was made on a main surface of the sintered magnet. 
     It was found that the conventional apparatus fed the cavity at a feeding density of 2.3 g/cm 3  approx. On the other hand, the pressing apparatus  100  according to the present invention fed at a desired feeding density of 1.8 g/cm 3  approx. Therefore, as understood from FIG. 17B, the sintered magnet obtained from the compact manufactured by the pressing apparatus  100  has an improved residual magnetic flux density Br and a maximum energy product (BH)max than the sintered magnet obtained from the compact manufactured by the conventional apparatus. 
     It should be noted that the pressing apparatus  100  may use the die  20  shown in FIG. 1, which is formed with a plurality of cavities  28 . 
     In this case, as shown in FIG. 18, an arrangement may be made so that each of the cavities  28  is fed with the powder m by one of the rod members  150   a.  In such an arrangement, preferably, a mutually adjacent pair of the rod members  150   a  should be spaced from each other by a distance generally equal to a center-to-center distance between the corresponding rows of the cavities  28 . In the above arrangement, in order for each rod member  150   a  to move from one side to the other side above the corresponding row of cavities  28 , the rod member  150   a  should only move in a stroke L 1  which is generally as wide as the cavity. Further, in the shaking action of the rod members  150   a , none of the rod members  150   a  stops above an unrelated row of the cavities  28 , making possible to prevent non-uniform powder feeding. Further, weight inconsistency in the powder feeding can be decreased if a distance between each rod member  150   a  and the die  20  is set equally. 
     Further, as shown in FIG. 19, each of the cavities  28  may be fed with the powder m by all of the rod members  150   b  (three rod members according to this embodiment: The number of the rod members can be one or more). In this case, a stroke L 2  of the rod members  150   b  is set to allow all of the rod members  150   b  to move from one side to the other side above all the rows of cavities. In this case again, weight inconsistency in the powder feeding can be decreased if a distance between each rod member  150   b  and the die  20  is set equally. 
     Next, another experiment will be described. 
     In this experiment, a die formed with two cavities in a row in a direction of the feeder box movement was used to form two compacts (s intered bodies) per pressing cycle. The sintered body was for manufacture of a VCM (voice coil motor). During the pressing operation, a pressing direction of the powder was perpendicular to an orienting direction of the powder. The sintered bodies were manufactured respectively by using the powder feeding apparatus  114  shown in FIG.  10  and the conventional powder feeding apparatus disclosed in Japanese Patent Laid-Open No. 2000-248301, and comparison was made in terms of the weight distribution. Experiment conditions were as follows: The size and weight of the sintered body to be manufactured were set as 58.63 mm×36.9 mm×18.13 mm, and 217.7 g. The linear members used were provided by a wire of a 0.6 mm diameter made into a metal net having a sieve aperture of 6 mesh. A total of 300 blocks of compacts (sintered bodies) were manufactured in 150 continual stroke cycles of feeding and pressing. 
     A result of the experiment is shown in FIG.  20 A and FIG.  20 B. The weight inconsistency was improved by about 30%, from 9.22 gas achieved by the conventional apparatus to 6.04 g, proving improvement in feeding accuracy. As exemplified, use of the shaker  148  and the linear members  192  in the pressing apparatus formed with a plurality of cavities can also improve the weight inconsistency in feeding to the cavities, as compared with the conventional apparatus. 
     It should be noted that the die  120  should preferably be a low-magnetic metal die disclosed in Japanese Patent Laid-Open No. 2000-248301, or a metal die including a nonmagnetic die and highly magnetic yokes disposed on die hole side surfaces which are perpendicular to a direction of magnetic field application. By using such a metal die, it becomes possible to uniformalize magnetic flux density in the cavity  128 , and therefore to prevent the obtained compact from deforming when sintered. 
     The linear members  192  may be provided perpendicularly to the direction of movement of the feeder box  132  or may be made like a net, at the opening  190  of the feeder box  132 . 
     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.