Abstract:
The present invention relates to a method and apparatus for properly and consistently spacing an electrode from a workpiece while electrochemically etching (ECM) grooves to a precise depth in a surface of the workpiece to form a fluid dynamic bearing. The electrode is especially designed for imparting a grooved pattern to a flat surface, the electrode comprising a surface carrying the pattern to be formed on the flat surface, and a central rod extending a short distance above the electrode surface. The central rod precisely sets the gap between the electrode and the flat surface. The electrode is adapted to be electrically connected to a power supply so that the electrode serves as the cathode, and the flat work piece serves as the anode in an ECM system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims benefit of United States Provisional Application No. 60/274,387, entitled COUNTER PLATE ELECTRODE WITH SELF ADJUSTING Z-AXIS, filed Mar. 9, 2001 by Mark G. Steele, which is hereby incorporated by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention pertains generally to the field of fluid dynamic bearings, and more particularly to etching grooves in a counter plate of a fluid dynamic bearing.  
         BACKGROUND OF THE INVENTION  
         [0003]    Disc drives, including magnetic disc drives, optical disc drives and magneto-optical disc drives, are widely used for storing information. A typical disc drive has one or more discs or platters which are affixed to a spindle and rotated at high speed past a read/write head suspended above the discs on an actuator arm. The spindle is turned by a spindle drive motor. The motor generally includes a shaft having a thrust plate on one end, and a rotating hub having a sleeve and a recess into which the shaft with the thrust plate is inserted. Magnets on the hub interact with a stator to cause rotation of the hub relative to the shaft.  
           [0004]    In the past, conventional spindle motors frequently used conventional ball bearings between the hub and the shaft and the thrust plate. However, over the years the demand for increased storage capacity and smaller disc drives has led to the read/write head being placed increasingly close to the disc. Currently, read/write heads are often suspended no more than a few millionths of an inch above the disc. This proximity requires that the disc rotate substantially in a single plane. Even a slight wobble or run-out in disc rotation can cause the disc to strike the read/write head, damaging the disc drive and resulting in loss of data. Because this rotational accuracy cannot be achieved using ball bearings, the latest generation of disc drives utilize a spindle motor having fluid dynamic bearings on the shaft and the thrust plate to support a hub and the disc for rotation.  
           [0005]    In a fluid dynamic bearing, a lubricating fluid such as gas or a liquid or air provides a bearing surface between a fixed member and a rotating member of the disc drive. Dynamic pressure-generating grooves formed on a surface of the fixed member or the rotating member generate a localized area of high pressure or a dynamic cushion that enables the spindle to rotate with a high degree of accuracy. Typical lubricants include oil and ferromagnetic fluids. Fluid dynamic bearings spread the bearing interface over a large continuous surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, improved shock resistance and ruggedness is achieved with a fluid dynamic bearing. Also, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeat runout.  
           [0006]    One generally known method for producing the dynamic pressure-generating grooves is described in U.S. Pat. No. 5,758,421, to Asada, (ASADA), hereby incorporated by reference. ASADA teaches a method of forming grooves by pressing and rolling a ball over the surface of a workpiece to form a groove therein. The diameter of the ball is typically about 1 mm, and it is made of a material such as carbide which is harder than that of the workpiece. This approach and the resulting fluid dynamic bearing, while a major improvement over spindle motors using a ball bearing, is not completely satisfactory. One problem with the above method is the displacement of material in the workpiece, resulting in ridges or spikes along the edges of the grooves. Removing these ridges, for example by polishing or deburring, is often a time consuming and therefore a costly process. Moreover, to avoid lowering yields, great care must be taken not to damage the surface of the workpiece.  
           [0007]    A further problem with the above method is due to a recent trend in disc drives toward higher rotational speeds to reduce access time, that is the time it takes to read or write data to a particular point on the disc. Disc drives now commonly rotate at speeds in excess of 7,000 revolutions per minute. These higher speeds require the shaft and the hub to be made of harder material. Whereas, in the past one or more of the shaft, the sleeve or the hub, could be made of a softer material, for example brass or aluminum, now all of these components must frequently be made out of a harder metal such as, for example, steel, stainless steel or an alloy thereof. These metals are as hard or harder than the material of the ball. Thus, the above method simply will not work to manufacture fluid dynamic bearings for the latest generation of disc drives.  
           [0008]    Another method for producing the grooves of a fluid dynamic bearing is described in U.S. Pat. No. 5,878,495, to Martens et al. (MARTENS), hereby incorporated by reference. MARTENS teach a method of forming dynamic pressure-generating grooves using an apparatus, such as a lathe, having a metal-removing tool and a fixture that moves the workpiece incrementally in the direction in which a pattern of grooves is to be formed. The metal-removing tool forms the grooves by carrying out a short chiseling movement each time the workpiece is moved. This approach, while an improvement over the earlier one in that it does not produce ridges that must be removed, is also not completely satisfactory. For one thing, this approach like that taught by ASADA is typically not suitable for use with harder metals, which in addition to being more difficult to machine are often brittle and can be damaged by the chiseling action. Moreover, because each groove or portion of a groove must be individually formed and the workpiece then moved, the process tends to be very time consuming and therefore costly. Furthermore, the equipment necessary for this approach is itself expensive and the metal-removing tool is subject to wear and requires frequent replacement.  
           [0009]    A final method for producing the grooves involves a conventional etching process as described in U.S. Pat. No. 5,914,832, to Teshima (TESHIMA), hereby incorporated by reference. TESHIMA teaches a process in which the workpiece is covered with a patterned etch resistant coating prior to etching so that only the exposed portions of the workpiece are etched. While this approach avoids many of the problems of the previously described methods, namely the formation of ridges around the grooves and the inability to form grooves in hard metal, it creates other problems and therefore is also not wholly satisfactory. One problem is the time consumed in applying and patterning the etch resistant coat. This is particularly a problem where, as in TESHIMA, the resist coat must be baked to prior to patterning or etching. Another problem is that the coating must be removed after etching. This is frequently a difficult task, and one that if not done correctly can leave resist material on the workpiece surface resulting in the failure of the bearing and destruction of the disc drive. Yet another problem with this approach is that each of the steps of the process requires the extensive use of environmentally hazardous and often toxic chemicals including photo resists, developers, solvents and strong acids.  
           [0010]    Accordingly, there is a need for an apparatus and method for forming grooves in a workpiece made of a hard metal to manufacture fluid dynamic bearings suitable for use in a disc drive. It is desirable that the apparatus and method that allows the grooves to formed quickly and cheaply. It is also desirable that the apparatus and method not require expensive equipment or the use of a metal-removing tool that must be frequently replaced. It is further desirable that the apparatus and method not use an etch resistant material during manufacture that could contaminate the workpiece leading to the failure of the bearing and destruction of the disc drive.  
           [0011]    As the result of the above problems, electrochemical machining of grooves in a fluid dynamic bearing has been developed as described in the above-incorporated patent application. A broad description of ECM is as follows. ECM is a process of removing material metal without the use of mechanical or thermal energy. Basically, electrical energy is combined with a chemical to form a reaction of reverse electroplating. To carry out the method, direct current is passed between the work piece which serves as an anode and the electrode, which typically carries the pattern to be formed and serves as the cathode, the current being passed through a conductive electrolyte which is between the two surfaces. At the anode surface, electrons are removed by current flow, and the metallic bonds of the molecular structure at the surface are broken. These atoms go into solution, with the electrolyte as metal ions and form metallic hydroxides. These metallic hydroxide (MOH) molecules are carried away to be filtered out. However, this process raises the need to accurately and simultaneously place grooves on a surface across a gap between the electrode and the workpiece, which gaps must be very accurately set. This requires the use of a work holder which can accurately locate and constrain a workpiece within an electrochemical machining process environment (ECM). ECM is used to place grooves on the moving parts of a fluid dynamic bearing. The depth of these grooves has a typical tolerance of ±0.003 mm. Therefore the electrode/workpiece position error must be no greater than this.  
           [0012]    In a very commonly used fluid dynamic bearing design, a flat circular plate referred to as a counter plate is used, and must have grooves precisely etched thereon. The invention resulted from the need to accurately locate the distance between a thrust surface type ECM electrode (which defines the groove pattern) and a counter plate (the circular plate used in fluid dynamic motors) within an electro-chemical machining process (ECM). ECM is used to plate grooves on the moving or stationary elements of a fluid dynamic motor. The depth of these grooves has a tolerance of ±0.002-0.003 mm. Therefore the electrode/workpiece maching gap error must be no greater than this. In order to keep the counter plate cost to a relative low, the thickness of the plate has a large size tolerance, typically ±0.025 mm. This shift in plate thickness can alter the machining gap to a point where groove depth consistency is practically unattainable within the specification limits. In addition to the accuracy, the gap adjusting mechanism should be without parts movable while the process is being executed (the salt dissolved in the electrolytes will crystallize and hinder its movement) and be easy to manufacture. The salt dissolved in the electrolyte will crystallize and hinder its movement.  
           [0013]    The present invention provides a solution to these and other problems, and offers other advantages over the prior art.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention relates to a method and apparatus for properly and consistently spacing an electrode from a workpiece while electrochemically etching grooves in a surface of the workpiece to form a fluid dynamic bearing.  
           [0015]    Other features and advantages of this invention will be apparent to a person of skill in this field who studies the following detailed description of an embodiment of the invention given in conjunction with the associated drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 illustrates a disc drive in which a motor incorporating the hydrodynamic bearing whose grooves are formed using the present invention is especially useful.  
         [0017]    [0017]FIGS. 2A is a vertical section of the spindle motor of FIG. 1.  
         [0018]    [0018]FIGS. 2B and 2C are vertical and horizontal sectional views of a portion of the motor, especially the shaft and thrust plate, illustrating the grooves which may be formed utilizing the present invention.  
         [0019]    [0019]FIG. 3 is a perspective view of the diaphragm workholding device of the present invention.  
         [0020]    [0020]FIG. 4 is a cross section of the device of FIG. 3 shown with the diaphragm deflected.  
         [0021]    [0021]FIG. 5 is a view along the same section line as FIG. 4 showing the device in its relaxed state with the air pressure removed.  
         [0022]    [0022]FIG. 6A is a schematic view of the function of the present invention.  
         [0023]    [0023]FIG. 6B is a vertical sectional view of a preferred embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    Other features and advantages of this invention will be apparent to a person of skill in this field who studies the following detailed description of an FIG. 1 is an exploded perspective view a magnetic disc drive for which a spindle motor having a fluid dynamic bearing manufactured by the method and apparatus of the present invention is particularly useful. Referring to FIG. 1, a disc drive  100  typically includes a housing  105  having a base  110  sealed to a cover  115  by a seal  120 . The disc drive  100  has a spindle  130  to which are attached a number of discs  135  having surfaces  140  covered with a magnetic media (not shown) for magnetically storing information. A spindle motor (not shown in this figure) rotates the discs  135  past read/write heads  145  which are suspended above surfaces  140  of the discs by a suspension arm assembly  150 . In operation, spindle motor rotates the discs  135  at high speed past the read/write heads  145  while the suspension arm assembly  150  moves and positions the read/write heads over one of a several radially spaced tracks (not shown). This allows the read/write heads  145  to read and write magnetically encoded information to the magnetic media on the surfaces  140  of the discs  135  at selected locations.  
         [0025]    [0025]FIG. 2A is a sectional side view of a spindle motor  155  of a type which is useful in disc drives  100 . Typically the spindle motor  155  includes a rotatable hub  160  having one or more magnets  165  attached to a periphery thereof. The magnets  165  interact with a stator winding  170  attached to the base  110  to cause the hub  160  to rotate. The hub  160  is supported on a shaft  175  having a thrust plate  180  on an end. The thrust plate  180  can be an integral part of the shaft  175 , or it can be a separate piece which is attached to the shaft, for example, by a press fit. The shaft  175  and the thrust plate  180  fit into a sleeve  185  and a thrust plate cavity  190  in the hub  160 . A counter plate  195  is provided above the thrust plate  180  resting on an annular ring  205  that extends from the hub  160 . An O-ring  210  seals the counter plate  195  to the hub  160 .  
         [0026]    A fluid, such as lubricating oil or a ferromagnetic fluid fills interfacial regions between the shaft  175  and the sleeve  185 , and between the thrust plate  180  and the thrust plate cavity  190  and the counter plate  195 . One or more of the thrust plate  180 , the thrust plate cavity  190 , the shaft  175 , the sleeve  185 , or the counter plate  195  have pressure generating grooves (not shown in this figure) formed in accordance with the present invention to create fluid dynamic bearings  220 ,  225 . Preferably, grooves are formed in an outer surface  215  of the shaft  175  to facilitate inspection of the grooves. More preferably, the grooves in the outer surface  215  of the shaft  175  form one or more fluid dynamic journal bearings  225  having dynamic cushions that rotatably support the hub  160  in a radial direction.  
         [0027]    [0027]FIGS. 2B and 2C are a vertical sectional view and top plan view, respectively, of a hub and sleeve combination illustrating the grooves which establish the hydrodynamic bearings used to support the sleeve and hub for rotation relative to shaft  175 . In accordance with design principles well known in this field, the sleeve  200  supports on its outer surface a hub  210  which in turn will support one or more discs (not shown) for rotation. The internal surface of the main bore of sleeve  200  includes a pair of sets of grooves  212 ,  214  which in cooperation with the surface of the shaft and the intervening fluid (not shown) will form the journal bearings which are used to support the hub  210  for rotation about the shaft  201 .  
         [0028]    Typically, such a design also includes a thrust plate supported on one end of the shaft (and shown  180  in FIG. 2A). A recess  216  is provided for the thrust plate  180 ; a second recess  218  is provided for the counter plate which overlies the thrust plate in the assembled motor and is used to define the hydrodynamic bearing gap with the upper surface of the thrust plate. The lower surface  219  of the counter plate  218  faces an axially outer surface  221  of the thrust plate  216 . Either the thrust plate  180  surface or the surface of the counter plate  219  also includes a set of grooves  222  (FIG. 2B) which in this case are in the shape of a succession of chevrons similar to the pattern shown in FIG. 2C and which cooperate with the outer surface  221  of the thrust plate  216  to create a pressure gradient which will support the thrust plate  216  and counter plate  218  for smooth relative rotation. This also prevents tilting of the  210  hub and sleeve  200  relative to the thrust plate  216  and the shaft  201  to which it is affixed so the hub  210  rotates with great stability relative to around the shaft  201 .  
         [0029]    It is clear that because of the very small tolerances between the shaft and the thrust plate it supports and the internal surfaces of the sleeve, that the sleeve must be held with great stability in a jig of some sort while the ECM process is carried out; any variation in the gap between the sleeve and the electrode would cause a variation in the depth, spacing and placement of the grooves. As noted above, the fixture must be capable of holding the circular workpiece so that the depth of grooves will have a typical tolerance of ±0.003 millimeters.  
         [0030]    To achieve these goals, the work holder or fixture of FIGS. 3, 4 and  5  was designed, comprising a frame  300  which supports a diaphragm  302  having a plurality of jaw-like workholders  304  facing a common central axis  306 . As shown more clearly in FIG. 4 which should be considered in conjunction with FIG. 3, as the diaphragm is deflected upward to assume a slightly more spheroidal shape, the jaws  304  are uniformly deflected away from the central axis  306  so that a circular or shaft based workpiece such as shown in FIGS. 2A and 2B can be inserted therein. As the air pressure is withdrawn, the deflected jaws  304  return to their original position as the diaphragm  302  flattens out, capturing the shaft or circular workpiece between the jaws. This operation is more readily apparent from the cross section of FIG. 4 which shows the diaphragm  302  relative to the backing plate  320 . As air is injected through the air inlet  322 , it can be seen that the diaphragm will deflect upwardly along the axis  306  with the upper part of each jaw leaning a little further away from the axis  306  than the lower part. This opening between the jaws  304  allows for the insertion of the shaft or circular workpiece. When the void between the diaphragm  302  and backing plate  320  is depressurized, the diaphragm will snap back to its original position, resting firmly against the backplate. The inner diameter of the generally circular work area defined by the jaws will be reduced, capturing the workpiece with a high level of precision accuracy. FIG. 5 shows these jaws returned to their original position.  
         [0031]    So long as the air pressure does not exceed a predefined amount, the maximum bending moment of the diaphragm will not exceed the allowable, allowing substantial repeatability. Further, since the workpiece is consistently held in a repeatably reliable position, with its axial position being defined by the diaphragm, and its radial position accurately fixed by the jaws, an electrode can easily be inserted along the same axis  306 . With the electrode in place, the electrolyte can be applied, and electrical current applied to the system, carrying out the ECM process to form the desired grooves on the workpiece.  
         [0032]    The present invention is particularly concerned with providing a work piece holder to be used in conjunction forming a groove pattern such as is shown in FIG. 2C on the surface of counter plate  218  which is to face thrust plate  216  in order to support the counter plate and thrust plate for relative rotation. It is apparent that the same device could be used to support the thrust plate  216  if forming the grooves on that surface is desired.  
         [0033]    The invention resulted from the need to accurately locate the distance between a thrust surface type ECM electrode and a counter plate (circular disk used in fluid dynamic motors) within an electrochemical machining process (ECM). ECM is used to define grooves on the moving or stationary elements of a fluid dynamic motor. The dept of these grooves has a tolerance of ±0.002 to 0.003 mm. Therefore the electrode/workpiece maching gap error must be no greater than this. In order to keep the counterplate cost to a relative low, the thickness of the counterplate or other part has a large size tolerance, typically ±0.025 mm. This shift in plate thickness can alter the machining gap to a point where groove depth consistency is unattainable within the specification limits. In addition to the accuracy, the gap adjusting mechanism preferably should have minimum moving parts and be easy to manufacture. The salt dissolved in the electrolyte will crystallize and hinder movement of moving parts.  
         [0034]    Therefore, the present electrode, with a self-controlling machining gap has been designed. The electrode is designed to face the counter plate  218  across a gap  300  as shown schematically in FIG. 6A. The electrode  310  is made primarily of an electrically conductive material so that the pulsed direct current from the source  320  will pass between the anodic work piece, which in this case is the counter plate  218 , and the cathodic electrode  310  through a conductive electrolyte generally shown at  320  which flows through the gap  322  between anode and cathode. At the anode surface of counter plate  218 , electrons are removed by current flow and the metallic bonds of the molecular structure at the surface are broken. These atoms go into solution with the electrolyte as metal ions and form metallic hydroxides. The MOH molecules are carried away to be filtered out. For this reason, ECM may also be known as “anodic dissolution”. A further element to be noted from FIG. 6A is that the surface  340  of electrode  310  carries the pattern to be formed on the surface of counterplate  218 . This pattern is defined by raised lands of electrically conductive material, usually separated and surrounded by insulating material of equal height. Electrically conductive lands in a pattern as shown at FIG. 2C would produce a pattern on the surface of counterplate  218  which comprises the workpiece of that design. While primarily made of a conductive material, the center of the electrode is an electrically inert material such as ceramic  330 .  
         [0035]    The electrode  310  shown schematically in FIG. 6A is shown in greater detail in the cross-section view of FIG. 6B. As shown in this view, the electrode comprises an annular piece  410  of cylindrical cross-section, with a central rod  420 , typically circular in cross-section, which extends (FIG. 6A, 330) a short distance in FIG. 6A above the axially end surface  430  of the metallic electrode thereby defining and establishing the gap spacing  300 . An inlet  440  for electrolyte is provided axially spaced away from the end surface  430  of the electrode, and a gap  445  is defined between the outer surface of the central rod  420  and the inner surface of the conductive cylinder  410 . The electrolyte flows through this gap to reach the axial outer end  430  of the electrode, and then flows radially away between the counter plate  218  which serves as the anode, and the cathodic electrode  430 . This electrolyte as it flows away can then be captured and filtered or simply replaced by fresh electrolyte through the orifice  440 .  
         [0036]    As is apparent from both FIG. 6B and FIG. 6A, the electrically inert center rod  420  is extended a small and very precise set distance above the electrode surface  430 . This sets the gap  300  which as explained above, is a key variable along with time and volume of current flow in establishing groove depth. That is, the center rod  420  establishes and maintains the machining gap in every ECM operation and thus the depth of the grooves formed in counterplate  218 .  
         [0037]    Other features and advantages of this invention may be apparent to a person of skill in this art who studies the present invention disclosure. The electrode of this invention can be used to define grooves of a desired depth on any metal surface; it is especially useful to form grooves on a counterplate or similar metallic price adapted to be held by the diaphragm of FIGS. 4 and 5. Therefore, the scope of the present invention is to be limited only by the following claims.