Abstract:
A method for manufacturing a tool to form a dynamic pressure bearing in which at least one of the bearing surfaces has a plurality of pressure generating grooves defined by projections and in which lubricating fluid is contained in the grooves to provide lubrication between the bearing surfaces when they rotate relative to one another. A flat bevel is created at one or more of the corners of each of the projections that define the grooves thereby generating a wedge-shaped space that communicates between the grooves and the gap between the bearing surfaces. This wedge-shaped space facilitates passage of lubricating fluid from the groove to the gap when movement, normally rotation, occurs between the two bearing surfaces thereby minimizing wear between the surfaces.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a dynamic pressure bearing apparatus and a method for manufacturing thereof and to a tool for forming a dynamic pressure generating groove in the apparatus and a method for manufacturing the groove, in which a dynamic pressure is generated in a lubricating fluid wherein a fixed member and a rotatable member hold each other relatively rotatable due to the dynamic pressure. 
     Various suggestions have been made regarding a dynamic pressure bearing apparatus holding various rotating bodies, such as a polygon mirror, a magnetic disc and an optical disc, while rotating at a high speed. In such a dynamic pressure bearing apparatus, a dynamic pressure bearing surface on the side of a fixed member is placed to face a dynamic pressure bearing surface on the side of a rotatable member with a narrow space or gap in which a dynamic pressure bearing portion is formed. Also, dynamic pressure generating grooves are formed on at least one of the dynamic pressure bearing surfaces wherein a lubricating fluid inserted in the dynamic pressure bearing portion, such as air and oil, is pressurized by a pumping effect generated by the dynamic pressure generating grooves during rotation such that the fixed member and the rotatable member are rotatably held thereat without contacting each other due to the dynamic pressure of the lubricating fluid. 
     The dynamic pressure bearing grooves are formed as a concavity extending on the dynamic pressure bearing surfaces in a shape of a spiral or a herringbone. A projecting portion, as the rest of the space on the dynamic pressure bearing surfaces, comprises a projecting surface. When the rotatable member is not in rotation, the fixed member and the rotatable member are in contact with each other wherein the lubricating fluid is held inside the dynamic pressure generating grooves. Once relative rotation of the rotatable member and the fixed member starts, the lubricating fluid in each of the dynamic pressure generating grooves flows towards a specific pressure point and generates a dynamic pressure by confluence thereof to provide a given floating power. 
     As described above, in an ordinary dynamic pressure bearing apparatus, it takes a period of time to obtain a given floating power by a dynamic pressure after rotation starts; in other words there is a period of time at the beginning of the rotation when there is no dynamic pressure in the lubricating fluid. Therefore, the fixed member and the rotatable member rub each other while being in contact during the above period of time. As a result, abrasion of the members progresses in the early stage of their life, resulting in life shorter than expected. 
     Such an issue becomes noticeable when original surface  2 ′ (indicated as a dotted line), as shown as a thrust dynamic pressure bearing surface in FIG. 17, is flattened to increase flatness of projecting portion  2 , comprising a projecting surface as the remains area thereat other than dynamic pressure generating groove  1 . In other words, projecting portion  2  after the flattening treatment has its surface slightly lower than original surface  2 ′; then, the opening edge of dynamic pressure generating groove  1  is pushed inward by the difference in the height. As a result, reverse tapering portion  1   b  starts from the bottom of the side wall of the groove. Such reverse tapering portion  1   b  tends to keep back the lubricating fluid which tries to flow out from dynamic pressure generating groove  1  towards projecting portion  2 ; especially, when rotation has just started, the time period without the lubricating fluid on the side of projecting portion  2  is further extended such that abrasion of the fixed member and the rotatable member is significantly increased. 
     Hence, a purpose of the present invention is to provide a dynamic pressure bearing apparatus, which has a simple configuration wherein abrasion of a fixed member and a rotatable member is decreased, and a method for manufacturing thereof and a tool for forming a dynamic pressure generating groove used therewith and a method for manufacturing thereof. 
     BRIEF DESCRIPTION 
     In brief, the dynamic pressure bearing has first and second facing bearing surfaces having a gap between them. Typically, one of these bearing surfaces might be the surface of a shaft of a motor and the other bearing surface might be the cylindrical bearing portion of the stator of the motor. As is known in the art, a plurality of pressure generating grooves are formed on one of the two bearing surfaces. These grooves contain lubricating fluid. When the rotor rotates, these grooves tend to pump fluid into the gap between the two bearing surfaces. Thus the fluid flows from the grooves to the space between the top of the projections that define the grooves and the opposed surface. It is the top of the projections which bear most of the load. In order to enhance the flow of the lubricating fluid into the zone adjacent to the top of the projections, a beveled surface is provided at each corner of the projections. This bevel provides a wedge-shaped space that enhances lubricating fluid flow from the grooves to the gap. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a cross section of a dynamic pressure bearing motor with a rotatable shaft according to the present invention. 
     FIG. 2 is a plan view showing the entire configuration of a dynamic pressure generating groove formed at a thrust plate. 
     FIG. 3 is a cross section of the dynamic pressure generating groove formed at the thrust plate in the direction perpendicular to the length of the groove. 
     FIG. 4 is an enlarged partial cross section of FIG.  3 . 
     FIG. 5 is a longitudinal section showing an example of a coining processing apparatus to manufacture the thrust plate. 
     FIG. 6 is a flow chart showing a method of manufacturing a coining punch and a thrust plate using the same. 
     FIG. 7 is a schematic view of an apparatus for manufacturing a coining punch according to the present invention. 
     FIG. 8 is an enlarged longitudinal section of a blank for a coining punch manufactured by the apparatus of FIG.  7 . 
     FIG. 9 is a longitudinal section showing an example of a spindle motor having a dynamic pressure bearing apparatus according to the present invention. 
     FIG.  10 ( a ) is a longitudinal section and FIG.  10 ( b ) is a view from the bottom of the shaft of the spindle motor of FIG.  9 . 
     FIG. 11 is a longitudinal section showing an example of a mold for integrally forming a shaft and a thrust plate according to the present invention. 
     FIG. 12 is a longitudinal section showing another example showing a spindle motor having a dynamic pressure bearing apparatus according to the present invention. 
     FIG.  13 ( a ) is a longitudinal section and FIG. ( b ) is a view from the bottom of the shaft and thrust plate of the spindle motor of FIG.  12 . 
     FIG. 14 is an enlarged and cross-sectioned partial front view of another example of a shaft and a thrust plate applicable to the present invention. 
     FIG. 15 is a longitudinal section showing yet another example of a spindle motor having a dynamic pressure bearing apparatus according to the present invention. 
     FIG. 16 is a longitudinal section showing yet another example of a shaft and a thrust plate applicable to the present invention. 
     FIG. 17 is an enlarged partial cross section of a known dynamic pressure generating groove. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following describes an embodiment of a spindle motor having a thrust dynamic pressure bearing according to the present invention in reference to drawings. 
     FIG. 1 shows a cross section of dynamic pressure bearing motor  50  with a rotatable shaft. Motor  50  is configured mainly of: fixed member  52 , which is assembled on frame  51  formed of an aluminum material; and rotatable member  54  which rotates around rotatable shaft  53  in relation to fixed member  52 . Hub  55  is mounted at the upper end of rotatable member  54  to hold a disc. Also, the inner surface of outer cylindrical portion  55   a  of hub  55  has drive magnet  57  facing stator core  56 . Additionally, fixed member  52  comprises: frame  51 ; cylindrical holder  58  which is formed integrally with frame  51 ; stator core  56  around which coil  59  is wound and which is fixed to the outer surface of holder  58 ; cylindrical bearing  60  which is fixed to the inner surface of holder  58  and which holds rotatable shaft  53  rotatably; and counter plate  62  which is fixed to the opening of holder  58 . 
     A dynamic pressure generating groove portion is formed on at least one of the inner surface of bearing  60  and the outer surface of rotatable shaft  53 ; the dynamic pressure generating groove portion configures radial dynamic pressure bearing portion  67 . Also, disc-shaped thrust plate  61  is engaged to the bottom end of rotatable shaft  53 . Thrust plate  61  is placed between the bottom of bearing  60  and the top surface of counter plate  62 . Additionally, each end surface of thrust plate  61  in the axial direction has a dynamic pressure generating groove portion which configures thrust dynamic pressure bearing portions  61   a ,  61   b . The dynamic pressure generating portion needs to be formed on only one of end surfaces of thrust plate  61  in the axial direction; also, the dynamic pressure generating groove portion can be formed on the top surface of counter plate  62  or the bottom surface of bearing  60 , both of which face an end surface of thrust plate  61  in the axial direction. 
     Lubricating fluid  65  fills radial dynamic pressure bearing portion  67  and thrust dynamic pressure bearing portions  61   a ,  61   b . Therefore, when rotatable member  54  rotates, the outer surface of rotatable shaft  53  and the inner surface of bearing  60  are not in contact with each other having lubricating fluid  65  therebetween, as well as at the end surfaces of thrust plate  61  in the axial direction, the top surface of counter plate  62  and the bottom surface of bearing  60 . 
     In this embodiment, the inner surface of bearing  60 , the bottom surface of bearing  60 , and the top surface of counter plate  62  are a part of fixed member  52  wherein a dynamic pressure bearing portion formed of these surfaces is called the first dynamic pressure bearing surface. Also, the outer surface of rotatable shaft  53  and both end surfaces of thrust plate  61  in the axial direction are a part of rotatable member  54  wherein a dynamic pressure bearing portion formed of these surfaces is called the second dynamic pressure bearing surface. 
     FIG. 2 shows an example of thrust dynamic pressure generating groove  11  according to the present invention. This thrust dynamic pressure generating groove  11  is formed on at least one of fixed member  52  and rotatable member  54 , which are placed to be rotatable in relation to each other; such a groove, for example, can be formed on a dynamic pressure bearing surface of thrust plate  61  and is shaped as a herringbone wherein a plurality of V-shaped groove portions  11   a  are lined in the circumferential direction. In each of groove portions  11   a , the lubricating fluid flows from both ends towards the summit of the V such that given dynamic pressure is generated by the pressure at the summit. Also, the area of the dynamic pressure bearing surface other than thrust dynamic pressure generating groove  11  is projecting portion  12  having a projecting surface. 
     Dynamic pressure generating groove  11  and projecting portion  12  are formed to have a cross section as shown in FIGS. 3 and 4. In other words, groove portion  11   a  of dynamic pressure generating groove  11  is shaped as an approximate rectangular in a cross section thereof in a direction perpendicular to the length of the groove. Slanted guide surfaces  13  are formed on facing edges of dynamic pressure generating groove  11 , as opening edges thereof. Slanted guide surface  13  is formed as a connection between side wall  11   b  of dynamic pressure generating groove  11  and projecting surface  12   a  of projecting portion  12 ; also, it extends at a given angle in relation to projecting surface  12   a  of projecting portion  12 . Slanted guide surfaces  13  provides a shape of the opening of dynamic pressure generating groove  11  to expand outwards (upward in the figures). Therefore, groove portion  11   a  of dynamic pressure generating groove  11  is configured of: bottom  11   c ; side walls  11   b  which stand almost perpendicular to bottom  11   c ; and slanted guide surfaces  13 . Slanted guide surface  13  may be formed only on the side of the opening edge of dynamic pressure generating groove  11  from which the lubricating fluid flows during rotation, or on both sides of the opening as shown in FIG.  3 . In reference to FIGS. 2 and 3, the side from which the lubricating fluid flows during rotation is defined as the opening edge of dynamic pressure generating groove  11  on the downstream of the lubricating fluid which moves counterclockwise as indicated by an arrow in FIG. 2 when the fixed member and the rotatable member rotates respectively; in other words, it is the side of V-shaped dynamic pressure generating groove  11  on the downstream. In FIG. 3, when the lubricating fluid moves from the right to the left, for example, it is preferable to form slanted guide surface  13  on at least the right side of the facing edges of groove  11   a . This slanted guide surface  13  is a flat bevel which provides a wedge-shaped space  15  that provides communication between the fluid holding grooves and the gap between the flat bearing surface  12   a  and the opposing bearing surface. 
     Dynamic pressure generating groove  11  and projecting portion  12  in this embodiment are formed by performing a coining treatment to a blank of a thrust plate. The projecting surface of projecting portion  12 ′ right after the coining treatment, as indicated with a dotted line in FIG. 4, is slightly higher than final surface  12  indicated with a solid line. Also, original shape  13 ′ of slanted guide surface  13  right after the coining treatment is a relatively gentle slope. Then, flattening treatment is performed to the projecting surface of projecting portion  12 ′ right after the coining treatment, as indicated with a dotted line, from the top of the figure; as a result, final projecting portion  12 , which is slightly squished down, is excellently flattened. In this case, slanted guide surface  13  has a slope sharper than original shape  13 ′ by the amount of projecting portion  12  squished down by the flattening treatment. 
     When rotatable member  54  stops, opposing member  14  comes into contact with projecting surface  12   a  of projecting portion  12  on a thrust dynamic pressure bearing formed on a thrust plate. Wedge-shaped space  15  is formed between slanted guide surface  13  and opposing member  14  (indicated with a two-dotted line in FIG.  4 : for example, it is bearing  60  or counter plate  62 ). Therefore, the lubricating fluid in dynamic pressure generating groove  11  when rotation is suspended immediately flows out via slanted guide surface  13  on the opening edge of dynamic pressure generating groove  11  toward projecting portion  12 ; then, it swiftly flows between fixed member  52  and rotatable member  54  due to the wedge effect of slanted guide surface  13  to form a film. As a result, a contact between fixed member  52  and rotatable member  54  is prevented right after rotation starts such that abrasion of the members can be minimized. 
     With reference to FIG. 3, the following dimensions are provided in one specific preferred embodiment. The width of the groove  11  and of the projection  12  are essentially the same and are each equal to 0.35 mm. The full height of the projection is 0.009 mm. However, the perpendicular sidewalls  11   b  are 0.0075 mm. The coined flat top portion  12   a  of the projection  12  is 0.15 mm. The bevel  13  is at an acute angle of 6 degrees to the horizontal. The flat bevel  13  is a hypotenuse of a right triangle having a height of 0.0015 mm and a length of 0.1 mm. This means that the two bevels  13  constitute approximately 35% of the width of the projection  12 . It is preferred that the upper flat bearing portion  12   a  of the projection be over fifty percent of the total width of the projection  12 . 
     It should be noted that the size of the grooves  11  and projections  12  vary appreciably from application to application and have a typical range from 0.2 mm to 0.5 mm with consequent proportionate variations in the dimensions of the bevel  13 . 
     Dynamic pressure generating groove  11 , projecting portion  12  and slanted guide surface  13 , together forming a thrust dynamic pressure bearing, are formed by a coining treatment apparatus comprising coining punch  21  made of a hard metal material as shown in FIG.  5 . Coining punch  21  is mounted on fixed die  22  to be movable in the vertical direction, end portion  21  a on the bottom of coining punch  21  is pressed against blank  23  of a thrust plate, which is set on fixed die  22  as a work piece, such that a pattern is transferred thereto. End portion  21   a  of coining punch  21  has a corrugation which is a reverse pattern of dynamic pressure generating groove  11 , projecting portion  12  and slanted guide surface  13 ; the corrugation is is transferred to the blank of the thrust dynamic pressure bearing. Projecting portion  12  and slanted guide surface  13  are formed precisely with, a corrugation having a reverse shape of shapes  12 ′ and  13 ′ in FIG. 14; especially, the section of coining punch  21  forming slanted guide surface  13  has, as indicated with a dotted line  13 ′, a slope less steep than the angle of the slope of final slanted guide surface  13 . 
     The coining treatment using coining punch  21  is performed to a blank of the thrust plate as shown at the right in FIG.  6 . End portion  21   a  of coining punch  21  forms the above thrust dynamic pressure bearing surface readily and precisely; hence, the coining treatment dramatically improves efficiency in production compared to an etching treatment, cutting treatment, plating treatment and the like. After the coining treatment, a flattening treatment is performed, and the formation of the groove portion is completed after washing and test processes. 
     As described above, coining punch  21  comprises a projecting portion for forming a groove and a concave portion for forming a projecting surface having a reverse shape of dynamic pressure generating groove  11 , projecting portion  12 ′ and slanted guide surface  13 ′. The concave portion for forming a projecting surface of the corrugation can be efficiently formed by using an ion-milling apparatus as described in the following. 
     As shown on the left in FIG. 6, after a metal mask is adhered to the blank for coining punch  21 , a milling treatment is performed by using an ion-milling apparatus; then, the blank is tested. At last, the metal mask is removed and the blanks is washed to obtain coning punch  21 . 
     To perform the milling treatment, as shown in FIG. 7, a plurality of blanks  21 ′ for coining punch  21  made of the hard metal material are placed around rotatable work table  32  connected to rotation drive  31  while an ion irradiation portion of ion-milling apparatus  33  is positioned diagonally above rotatable work table  32 . Each of blanks  21 ′ on rotatable work table  32  is fixed thereto with its surface to form coining punch  21  in the approximately upward direction; ion irradiation axis  33   a  is directed to form a given angle θ in relation to rotational axis  32   a  of rotatable work table  32 , which extends in the approximately vertical direction, within a range of 5 to 85 In other words, a first direction, perpendicular to the end surface of blank  21 ′, and a second direction, as a direction of ion irradiation of ion-milling apparatus  33 , forms a given angle θ. In this case, the angle between center  21 ′ a  of blanks  21 ′ and the direction of ion irradiation axis  33   a  of ion-milling apparatus  33  is established to be θ. 
     As shown enlarged in FIG. 8, metal mask  34  is mounted at a section of the end surface of blank  21 ′ (towards the top of the figure) corresponding to dynamic pressure generating groove  11  such that this section corresponding to dynamic pressure generating groove  11  is protected from ion irradiation from ion-milling apparatus  33 . Metal mask  34  is made of a thin stainless steel and the like wherein a pattern corresponding to dynamic pressure generating groove  11  is formed by etching and the like. Also, the metal mask is firmly adhered to the end surface of blank  21 ′ (towards the top of the figure) by using a heat-resistant adhesive such as an epoxy resin. By adhering metal mask  34  to blank  21 ′ using an adhesive, one can prevent floating and stripping literal translations due to warping of the metal mask caused when it is magnetically fixed, for example. As a result, the pattern on metal mask  34  is accurately transferred to coining blank  21 ′. 
     Metal mask  34  is, together with blank  21 ′, gradually thinned due to the milling treatment by ion irradiation from ion-milling apparatus  33 ; the thickness of metal mask  34  should be thick enough such that metal mask  34  still exists at the end of the process. In other words, the thickness of such a metal mask  34  can be more even than masks formed by pressing a resin and a resist; as a result, precision of the milling treatment, described in the following, can be improved. 
     A concave portion for forming a projecting surface is formed by ion irradiation from ion-milling apparatus  33  onto the end surface of blank  21 ′ (towards the top of the figure) while rotating rotatable work table  32  having the described above angles. In this case, center  21 ′ a  of blank  21 ′ and ion irradiation axis  33   a  of ion-milling apparatus  33  maintains a given angle. Also, blank  21 ′ can be fixed while ion irradiation axis  33   a  of ion-milling apparatus  33  is rotated. 
     As performing ion irradiation to the end surface of blank  21 ′ (towards the top of the figure) while the positional relationship between blank  21 ′ and ion-milling apparatus  33  is maintained constant, milling treatment is not performed on the section covered by metal mask  34  (only metal mask  34  is treated). A concave portion for forming a projecting surface is formed as a groove on the section  35  not protected from ion irradiation by metal mask  34  by the milling treatment. As mentioned above, ion-milling apparatus  33  and blank  21 ′ maintains a constant positional relationship; therefore, the depth of the formed groove may be varied. 
     In other words, the depth of the groove becomes the deepest at section  35   a  to which ion irradiation is constantly applied without metal mask  34 . Section  35   a , with a deep groove, lays for a given range to configure the bottom of the groove. 
     On the other hand, as blank  21 ′ is rotated while ion milling apparatus  33  is positioned with an angle, ion irradiation onto section  35   b , which is temporarily shadowed by metal mask  34 , is interrupted for a period of time; as a result, the depth of the groove thereat corresponds to the length of ion irradiation thereon. In other words, section  35   b , which is interrupted from ion irradiation for a period of time by metal mask  34 , is interrupted from ion irradiation for a longer period of time as it comes closer to metal mask  34 . As ion irradiation is interrupted for most of the time at the area right at the bottom of metal mask  34 , the depth of the groove becomes extremely shallow. In other words, at the section away from the bottom of metal mask  34 , the length of interruption of ion irradiation by metal mask  34  becomes shorter while the length of ion irradiation becomes longer, respectively. As a result, the depth of the groove at section  35   b  gradually becomes deeper towards the center starting from the bottom of metal mask  34 . The depth of the groove can be sufficiently adjusted by adjusting the angle between blank  21 ′ and ion milling apparatus  33 ; therefore, the concave portion of coining punch  21  can be readily shaped for a desired shape. 
     Coining punch  21  formed by the above ion milling apparatus  33  results in extraordinarily improved flatness on its surface, compared to the ones formed by etching and the like. Hence, the accuracy of the thrust dynamic pressure bearing surface is improved, resulting in improvement of the dynamic pressure characteristics. 
     FIG. 9 is a cross section of a spindle motor with a rotatable shaft comprising a dynamic pressure bearing apparatus of another embodiment according to the present invention. Members herein sharing the same functions with ones in spindle motor  50  of FIG. 1 are indicated by identical symbols, and their description will be omitted. In FIG. 9, thrust plate  70 , made of a resin, is integrally formed at the bottom end of rotatable shaft  53  by insert formation. Shaft  53  is inserted into a center hole of bearing member  60 . Also, hub  55 , to hold a disc, is press-fitted to the top end of rotatable shaft  53 , which is projecting out from bearing  60 . Hub  55  and shaft  53  can be integrally formed. In this embodiment, a rotatable member includes hub  55 , shaft  53  and thrust plate  70  while a fixed member includes bearing  60 . 
     Radial dynamic pressure bearing  68  is formed between rotatable shaft  53  and bearing  60 ; rotatable shaft  53  is rotatably held by bearing  60  by the dynamic pressure effect of dynamic pressure bearing  68 . The opening at the bottom end of bearing  60  is covered with counter plate  62  to seal the opening of bearing  60 . Also, lower thrust dynamic pressure bearing  74  is formed between the bottom surface of thrust plate  70  and the top surface of counter plate  62  while upper thrust dynamic pressure bearing  73  is formed between the top surface of thrust plate  70  and the surface of bearing  60  opposing the thrust plate . The thrust load of the rotatable member is supported by thrust dynamic pressure bearings  73 ,  74 . 
     Cylinder  58  of frame  51  is fixed on the outer surface at the bottom of bearing  60  by press-fitting and the like. Also, laminated core  59  is mounted on the outer surface of cylinder  58 ; laminated core  59  comprises a given number of salient poles around which driving coil  56  is wound. The top portion of the center hole of bearing  60  is formed with a gently slanted surface such that the area between the outer surface of rotatable shaft  53  and the inner surface at the top of bearing  60  is a meniscus portion wherein the distance therein gradually becomes larger towards the top. A slight gap connects the meniscus portion, radial dynamic pressure bearing  68 , and thrust dynamic pressure bearings  73 ,  74  and is filled with a lubricating fluid, such as oil and the like. Therefore, when rotatable shaft  53  rotates in relation to bearing  60 , dynamic pressure is generated in the lubricating fluid at radial dynamic pressure bearing  68  and thrust dynamic pressure bearings  73 ,  74 ; as a result, rotatable shaft  53  and thrust plate  70  are rotatably held in the lubricating fluid without coming into contact with bearing  60  and counter plate  62 . 
     Hub  55  is shaped as a face-down cup wherein the bottom of its outer wall covers laminated core  59 . Cylindrical rotor magnet  57  is fixed to the inner surface of hub  55  via cylindrical yoke  66 . The inner surface of rotor magnet  57  faces the end surface of the salient poles of laminated core  56  at a given distance. The step portion on the outside of hub  55  is disc mounting surface  55   a ; a center hole of a hard disc is engaged to the cylindrical outer surface of hub  55  as a guide to place one or a plurality of disc/discs on disc mounting surface  55   a . The mounted discs are clamped integrally to hub  55  by a given clamping means. 
     The rotating position of rotor magnet  57  is detected such that the electric current to each of driving coils  59  is controlled according to the detected signals. As a result, rotor magnetic  57  is driven in the circumferential direction by the electromagnetic attraction/repulsion to rotate rotor magnet  57 , and hub  55 , rotatable shaft  53  and thrust plate  70  which are integrated therewith. 
     As described previously, thrust plate  70  is formed at one end of rotatable shaft  53  also, it is formed of a resin integral with rotatable shaft  53  by insert formation. FIG. 10 is a longitudinal section showing details of a configuration of rotatable shaft  53  and thrust plate  70 . In FIG. 10, vertical hole  71  is formed at the bottom end of rotatable shaft  53  in the length direction to pour a resin therefrom; it becomes shallow from the bottom surface of rotatable shaft  53  towards the axial direction. Also, a plurality of horizontal hole  72  are formed from the outside surface in the radial direction; these horizontal holes  72  are connected to vertical hole  71 . Circular groove  76  is formed at a section of the outer surface of rotatable shaft  53  corresponding to the position of horizontal hole  72 . By pouring a resin through vertical hole  71  and horizontal holes  72  by insert formation, thrust plate  70  is integrally formed at the bottom end of rotatable shaft  53 . FIG. 11 shows a mold therefor. 
     In FIG. 11, the mold is roughly configured of lower mold  24  and upper mold  25 . Lower mold  24  comprises first lower mold  241  and second lower mold  242  which is integrated with the inside of first lower mold  241 . Gate  243  is formed along the axis thereof to inject the resin. Upper mold  25  comprises first upper mold  251  and second upper mold  252  which is integrated with the inside of first upper mold  251 . Hole  254  is formed at second upper mold  252  along the axis thereof; rotatable shaft  53  is inserted into hole  254 . When the bottom surface of upper mold  25  is pressed against the top surface of lower mold  24 , a formation space appears between the top surface of second lower mold  242  and the bottom surface second upper mold  252 . Also, when rotatable shaft  53  is inserted into hole  254  with its end having vertical hole  71  and horizontal holes  72  downward, vertical hole  71  and horizontal holes  72  are placed in the formation space such that gate  243 , vertical hole  71 , horizontal holes  72  and the formation space are all connected to each other. 
     A resin is injected from gate  243  while rotatable shaft  53  is pressed by inserting log-shaped pressing member  255  into hole  254  of second upper mold  52  on the top of rotatable shaft  53 ; then, gate  43 , vertical hole  71 , horizontal holes  72  and the formation space are filled with the resin such that thrust plate  70  is integrally formed with rotatable shaft  53  of the resin by insert formation. The remains of the gate portion and a burr are found at a section of the bottom surface of rotatable shaft  53  corresponding to gate  243 ; these can be removed to obtain a smooth surface and do not have any impact on the precision of the dynamic pressure bearing. Also, after removing the remains of the gate portion and the burr, it is preferable to form a resin film on thrust plate  70 . Circular groove  76  of rotatable shaft  53  described in reference to FIG. 10 is to prevent fall off of thrust plate  70  from rotatable shaft  53 . 
     Returning to FIG. 10, both top and bottom surfaces  73 ,  74  of thrust plate  70  are thrust dynamic pressure bearing surfaces on which dynamic pressure generating grooves  11  of a given shape are formed. In other words, dynamic pressure generating grooves  11  of a given shape are formed on both thrust dynamic pressure bearing surfaces wherein the area thereof other than dynamic pressure generating grooves  11  is projecting portion  12  having a projecting surface. FIG. 10 ( b ) shows bottom surface  74  of thrust plate  70  wherein symbols  12  and  11   a  indicate a projecting portion and a dynamic pressure generating groove, respectively. 
     Dynamic pressure generating groove  11  is shaped such that dynamic pressure is generated by a lubricating fluid when rotatable shaft  53  and thrust plate  70  are integrally rotated in a given direction. In the example of FIG. 10, dynamic pressure generating grooves  11  are shaped as herringbones wherein a plurality of V-shaped grooves are placed in the circumferential direction. Such a shape provides an effect of increasing dynamic pressure as the lubricating fluid is gathered from both ends of dynamic pressure generating groove  11  to the summit at the center of the “V” when thrust plate  70  rotates clockwise in FIG. 10 ( b ). Projecting portions and dynamic pressure generating grooves similar to projecting portions  12  and dynamic pressure generating grooves  11  of bottom surface  75  are formed on top surface  73  of thrust plate  70  such that thrust dynamic pressure is generated between, top surface  73 , and the opposing surface of bearing  60 . 
     Configurations of dynamic pressure generating grooves  11  and projecting portions  12  of this embodiment have a cross section similar to the one shown in FIG.  4 . In other words, the cross section of groove  11   a  of dynamic pressure generating groove  11  has an approximately rectangular shape wherein the opening edge of groove  11   a  has slanted guide surface  13 . Slanted guide surface  13  is a surface connecting side wall  11   b  of dynamic pressure generating groove  11  and projecting surface  12   a  of projecting portion  12 . Hence, the opening of dynamic pressure generating groove  11  expands outward (towards the top in the figure) by slanted guide surface  13 . The shape of the surface of the mode described in FIG. 11 for forming thrust plate  70 , that is, the shape of the top surface of second lower mold  242  and the bottom surface of second upper mold  252  should be formed to have projections and concavities to correspond to the configuration of dynamic pressure generating groove  11 , projecting portion  12  and slanted guide surface  13 . 
     The projecting surface of projecting portion  12  of lower thrust dynamic pressure bearing  74  of thrust plate  70  shaped as above comes into contact with an opposing member (counter plate  62  in FIG. 9) when it is not rotated wherein wedge-shaped space is formed between the contacting surface of the opposing member and slanted guide surface  13 . Therefore, the lubricating fluid, pooled in dynamic pressure generating grooves  11  when rotation is suspended, flows out to projecting portion  12  via slanted guide surface  13  formed at the opening edge of dynamic pressure generating groove  11  as soon as rotatable shaft  53  and thrust plate are integrally rotated. Then, the lubricating fluid swiftly flows between rotatable shaft  53  and bearing  60  by the wedge effect of slanted guide surface  13  to form a film. As a result, the contact between rotatable shaft  53  and bearing  60  is prevented as soon as rotation starts such that abrasion therein can be minimized. 
     The following describes yet another embodiment of a dynamic pressure bearing apparatus according to the present invention, which is adopted to a spindle motor with a fixed shaft, in reference to FIG.  12 . Any parts shared with the spindle motor of FIG. 9 are indicated by identical symbols, and repeating description will be omitted. In FIG. 12, frame  51  of the motor comprises core holding portion  58  at its center hole and its outer top surface wherein the bottom end of shaft  43  is engaged to the center hole. Core  56  is fixed to the outer surface of core holding portion  58 . Also, cylindrical bearing  40  is rotatably engaged to shaft  43 , standing upward from frame  51 . The center hole of hub  41  is engaged to the outer surface at the top of bearing  40  to integrate bearing  40  and hub  41 . Hub  41  holds a hard disc wherein hub  41  and the hard disc are integrally rotatable. 
     Thrust plate  44  is integrally formed of a resin on the outer surface at the top end of shaft  43  by insert formation. Also, the opening at the top end of bearing  40  is covered with counter plate  45 . Opposing surfaces of counter plate  45  and thrust plate  44 , and opposing surfaces of thrust plate  44  and bearing  40  are a pair of thrust dynamic pressure bearing surfaces facing each other in the axial direction. Additionally, thrust dynamic pressure bearings  74 ,  73  are formed at the top surface and the bottom surface of thrust plate  44 , respectively. FIG. 13 ( a ) shows vertically reversed shaft  43  wherein thrust plate  44  is integrally formed on the outer surface of circular groove  76  with shaft  43  to prevent fall off of thrust plate  44 . In this embodiment, the outer surface of shaft  43  and surfaces on both ends of thrust plate  44  in the axial direction are a part of a fixed member; a dynamic pressure bearing portion configured of these surfaces is called the first dynamic pressure bearing surface. Also, the inner surface of bearing  40 , the surface of bearing  40  facing thrust plate  44  and the surface of counter plate  45  facing thrust plate  44  are a part of a rotatable member; a dynamic pressure bearing portion configured of these surfaces is called the second dynamic pressure bearing surface. 
     FIG. 13 ( b ) shows a plan view of thrust plate  44  on which dynamic pressure generating grooves  11  of a mode identical to the ones of FIG. 10 are formed. The cross section of dynamic pressure generating groove  11  is identical to the one described regarding FIG. 4 wherein slanted guide surfaces are formed on at least an edge of the opening of dynamic pressure generating groove  11 , from which a lubricating fluid flows therein, to guide the lubricating fluid in dynamic pressure generating groove  11   a towards projecting portion  12  by the wedge effect. The other thrust dynamic pressure bearing  74  of thrust plate  44  is similarly configured and has a similar dynamic pressure generating groove. 
     As bearing  40  is rotated in relation to fixed shaft  43  and thrust plate  44 , dynamic pressure is generated in upper and lower thrust bearings  73 ,  74  of thrust plate  44  wherein bearing  40  supports the thrust load without contacting any surface by means of the thrust bearing surfaces. Also, radial dynamic pressure bearing portion  68  is formed between the inner surface of the center hole of bearing  40  and the opposing outer surface of shaft  43 . As bearing  40  rotates in relation to thrust plate  44 , dynamic pressure is generated in radial dynamic pressure bearing portion  68  such that bearing  40  rotates without contacting shaft  43 . 
     Thrust plate  44  having the above configuration can be formed by using a mold. In FIG. 13, symbol  47  indicates a gate used for formation, of the thrust plate. At least one gate  47  is placed on the outer surface of thrust plate  44 . The remains of a gate portion or a burr, which are formed at gate  47  during formation, are removed after the formation process. After the removal, it is preferable to form a resin film on thrust plate  44 . 
     FIG. 14 shows another example of thrust plate  48  which is integrally formed with shaft  43 . A configuration of a thrust dynamic pressure bearing surface formed on thrust plate  48  is substantially identical to the one of FIG. 4 wherein slanted guide surfaces  13  are formed on edges of the opening of dynamic pressure generating groove  11 , consisting of an edge, from which a lubricating fluid flows during rotation of thrust plate  48 , and the opposing edge. Slanted guide surface  13  is a connecting surface between side wall  11   b  of dynamic pressure generating groove  11  and projecting surface of projecting portion  12 . The projecting surface of projecting portion  12  on thrust plate  48  comes into contact with an opposing member when rotation is suspended wherein a wedge-shaped space is formed between a contacting surface of the opposing member and slanted guide surface  13 . Therefore, the lubricating fluid, pooled in dynamic pressure generating groove  11  while rotation is suspended, flows out to projecting portion  12  via slanted guide surface  13  formed at the opening edge of dynamic pressure generating groove  11  as soon as shaft  43  and thrust plate  48  are integrally rotated. Then, the lubricating fluid swiftly flows between shaft  43  and the opposing member by the wedge effect of slanted guide surface  13  to form a film. Slanted guide surface  13  can be formed on at least one of the edges of the opening of dynamic pressure generating groove  11 , which is on the downstream of the lubricating fluid during rotation. 
     The following describes another embodiment of the present invention in reference to drawings. This embodiment is a spindle motor with a fixed shaft as a modification of the embodiment of FIG.  12 . Common parts will be indicated by identical symbols while description will be focused on different parts thereof. In FIG.  15 :  43  is a fixed shaft;  41  is a hub;  40  is a bearing;  57  is a rotor magnet;  51  is a frame;  66  is a yoke;  56  is a laminated core;  59  is a driving coil; and  58  is a core holding portion. Thrust plate  80  is integrally formed at the top end of shaft  43  by insert formation using a resin. Also, thrust plate  81  is integrally formed at a position towards the bottom of shaft  43  by insert formation using a resin. Upper thrust dynamic pressure bearing  82  is formed between the bottom surface of thrust plate  80  and top surface of bearing  40  while lower thrust dynamic pressure bearing  83  is formed between the bottom surface of bearing  40  and the top surface of thrust plate  81 . Also, radial dynamic pressure bearing  68  is formed between the outer surface of shaft  43  and the inner surface of bearing  40 . A lubricating fluid fills radial dynamic pressure bearing  68  and upper and lower thrust dynamic pressure bearings  82 ,  83 . 
     Thrust dynamic pressure surfaces of thrust plates  80 ,  81  in the embodiment of FIG. 15 have dynamic pressure generating grooves described in relation to FIGS. 4,  10  and  13 . When shaft  43  and bearing  40  are integrally rotated, dynamic pressure is generated in thrust dynamic pressure bearings  82 ,  83  such that bearing  40  rotates without contacting thrust plates  80 ,  81 . Also, thrust plates  80 ,  81  can be formed integrally at shaft  43  by insert formation using a mold as described previously. 
     A thrust plate, to be formed integrally with the shaft using a resin, can be formed such that it covers the entire end surface of the shaft. FIG. 16 shows such an example. In FIG. 16, circular groove  86  is formed on the outer surface at the bottom end of shaft  84  wherein thrust plate  85  is integrally formed at the bottom surface of shaft  84  by insert formation using a resin. As the resin forming thrust plate  85  fills circular groove  86  as well as covering it, fall off of thrust plate  85  therefrom is prevented. Shaft  84  configured above is, for example, applicable as a shaft in a motor of FIG.  1 . 
     A mold used for manufacturing a dynamic pressure bearing apparatus shown in FIG. 11, especially, a portion forming a thrust bearing surface of a thrust plate can be manufactured by an ion-milling apparatus shown in FIG. 7, for example. As a method for manufacturing thereof is identical to the embodiment of FIG. 7, any description is omitted herein. 
     A mold formed by ion-milling apparatus  33  has a surface with dramatically improved smoothness; as a result, the surface precision of thrust dynamic pressure surfaces is also improved, resulting in improvement of the dynamic pressure characteristics. 
     The above described embodiments of the present invention in detail. However, the present invention is not limited to the above embodiments; various modifications are applicable within the scope of the invention. 
     For example, a hard iron alloy and the like can be employed as a material for a coining punch. 
     Also, the above embodiments described examples of the present invention applied to a dynamic pressure bearing motor with a rotatable shaft; however, it can be a dynamic pressure bearing motor with a fixed shaft. In this case, a thrust plate is engaged to a fixed shaft wherein a cylindrical bearing rotates around the fixed shaft. Additionally, a dynamic pressure generating groove portion is formed on at least one of end surfaces of the thrust plate in the axial direction and the end surface of the shaft opposing the end surface of the thrust plate in the axial direction. 
     Moreover, the above embodiments showed examples of the present invention applied to a thrust dynamic pressure bearing; however, it can be a radial dynamic pressure bearing. In this case, one does not perform a flattening process to improve flatness of a projecting surface of a dynamic pressure generating groove portion. 
     As described above, the present invention provides the following configuration. Slanted guide surfaces (bevel surfaces) are formed at edges of the opening of a dynamic pressure generating groove. As a result, a lubricating fluid, which is pooled in the dynamic pressure generating groove while rotation is suspended, flows out into the gap over the projecting portions via the slanted guide surface as soon as rotation starts. Accordingly, a fixed member and a rotatable member are held, without contacting each other, by the floating power due to the wedge effect of the slanted guide surface. Hence, contact between the fixed member and the rotatable member is avoided from the beginning of rotation such that abrasion between the members can be minimized, resulting in extended life of the dynamic pressure bearing apparatus accompanied with improved reliability. 
     Also, in a method for manufacturing a coining punch according to the present invention, ion irradiation is performed to a blank for a coining punch while positioning an ion-milling apparatus at an angle. As a result, an end portion of a coining punch is efficiently formed with high precision.