Abstract:
A method of forming a hydrodynamic bearing surface on a workpiece is provided. The method comprises electrochemically etching away all material in a region of the surface, except for portions of the region defined by a land pattern on an etching cathode. The step of removing forms a continuous recessed region comprising grooves formed between the portions of the region defined by the land pattern, and a relief cut region circumscribing the workpiece.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of U.S. Provisional Application No. 60/383,820, filed May 28, 2002 by Heine et al. (entitled “Grooving Technique For Reduced Power Consumption”), which is herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to electrochemical machining (ECM) and, more particularly, to the ECM of hydrodynamic bearing surfaces. 
     BACKGROUND OF THE INVENTION 
     Disk drives are capable of storing large amounts of digital data in a relatively small area. Disk drives store information on one or more recording media, which conventionally take the form of circular storage disks (e.g. media) having a plurality of concentric circular recording tracks. A typical disk drive has one or more disks for storing information. This information is written to and read from the disks using read/write heads mounted on actuator arms that are moved from track to track across the surfaces of the disks by an actuator mechanism. 
     Generally, the disks are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the disks under the read/write heads. The spindle motor generally includes a shaft mounted on a base plate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the base plate to rotate the hub relative to the shaft. In order to facilitate rotation, one or more bearings are usually disposed between the hub and the shaft. 
     Over the years, storage density has tended to increase, and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage disks. For example, to achieve increased storage densities, the read/write heads must be placed increasingly close to the surface of the storage disk. This proximity requires that the disk rotate substantially in a single plane. A slight wobble or run-out in disk rotation can cause the surface of the disk to contact the read/write heads. This is known as a “crash” and can damage the read/write heads and surface of the storage disk, resulting is loss of data. 
     From the foregoing discussion, it can be seen that the bearing assembly that supports the storage disk is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair of races that allow a hub of a storage disk to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems, such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor because of low damping. 
     One alternative bearing design is a hydrodynamic bearing. In a hydrodynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disk hub. In addition to air, typical lubricants include gas, oil, or other fluids. Hydrodynamic bearings spread the bearing surface over a large surface area, as opposed to 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, the use of fluid in the interface area imparts damping effects to the bearing, which helps to reduce non-repeat run-out. 
     Dynamic pressure-generating grooves (i.e. hydrodynamic grooves) disposed on journals, thrust, and conical hydrodynamic bearings generate a localized area of high fluid pressure and provide a transport mechanism for fluid or air so that fluid pressure is more evenly distributed within the bearing and between the rotating surfaces. The shape of the hydrodynamic grooves is dependent on the pressure uniformity desired. The quality of the fluid displacement and therefore the pressure uniformity is generally dependent upon the groove depth and dimensional uniformity. For example, a hydrodynamic groove having a non-uniform depth may lead to pressure differentials and subsequent premature hydrodynamic bearing or journal failure. 
     As the result of the above problems, electrochemical machining (ECM) of grooves in a hydrodynamic bearing has developed. Broadly described, 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 an etching reaction to remove material from the hydrodynamic bearing, forming hydrodynamic grooves thereon. To perform 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 a cathode. The current is passed through a conductive electrolyte that 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 form a solution with the electrolyte, as metal ions, forming metallic hydroxides. These metallic hydroxide (MOH) molecules are carried away and filtered out from the electrolyte. 
     In current motor designs, “relief cuts” are machined into a work piece at one step in the machining process. These relief cuts have the effect of increasing the bearing running gap in certain areas, hence creating less friction loss by unnecessary shearing of oil. Therefore, power consumed by the bearings is reduced. However, this additional step in the machining process renders the overall process longer and therefore more costly. 
     Therefore, a need exists for an electrochemical machining process that reduces bearing power consumption without requiring additional cost or time during manufacturing. 
     SUMMARY OF THE INVENTION 
     The invention provides a method and apparatus for the electrochemical machining of a hydrodynamic bearing. Relief cuts are formed within the work piece simultaneously with the forming of hydrodynamic grooves, reducing the time and cost incurred by the electrochemical machining process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  depicts a plan view of one embodiment of a disk drive for use with aspects of the invention; 
         FIG. 2  is a vertical sectional view depicting one embodiment of a hydrodynamic bearing utilized in the disk drive of  FIG. 1 , for use with aspects of the invention; 
         FIG. 3  depicts one embodiment of a work piece featuring hydrodynamic grooves, according to aspects of the invention; and 
         FIG. 4  depicts one embodiment of a hydrodynamic groove forming apparatus, according to aspects of the invention. 
         FIG. 5  is a cross sectional view of a groove forming apparatus and a work piece, according to aspects of the present invention. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
       FIG. 1  depicts a plan view of one embodiment of a disk drive  10  for use with embodiments of the invention. Referring to  FIG. 1 , the disk drive  10  includes a housing base  12  and a top cover  14 . The housing base  12  is combined with top cover  14  to form a sealed environment to protect the internal components from contamination by elements outside the sealed environment. The base and top cover arrangement shown in  FIG. 1  is well known in the industry; however, other arrangements of the housing components have frequently been used, and aspects of the invention are not limited by the particular configuration of the disk drive housing. For example, disk drives have been manufactured using a vertical split between two housing members. In such drives, that portion of the housing half which connects to the lower end of the spindle motor is analogous to base  12 , while the opposite side of the same housing member (which is connected or adjacent to the top of the spindle motor) is functionally the same as top cover  14 . Disk drive  10  further includes a disk pack  16  that is mounted on a hub  202  (see  FIG. 2 ) for rotation on a spindle motor (not shown) by a disk clamp  18 . Disk pack  16  includes one or more of individual disks that are mounted for co-rotation about a central axis. Each disk surface has an associated read/write head  20  that is mounted to the disk drive  10  for communicating with the disk surface. In the example shown in  FIG. 1 , read/write heads  20  are supported by flexures  22  that are in turn attached to head mounting arms  24  of an actuator  26 . The actuator shown in  FIG. 1  is of the type known as a rotary moving coil actuator and includes a voice coil motor (VCM), shown generally at  28 . Voice coil motor  28  rotates actuator  26  with its attached read/write heads  20  about a pivot shaft  30  to position read/write heads  20  over a desired data track along a path  32 . 
       FIG. 2  is a sectional side view of a spindle motor  155  of a type which is especially useful in disk drives  10 . 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 thrustplate  180  on one end. The thrustplate  180  can be an integral part of the shaft  175 , or it can be a separate piece that is attached to the shaft, for example, by a press fit. The shaft  175  and the thrustplate  180  fit into a sleeve  185  and a thrustplate cavity  190  in the hub  160 . A counter plate  195  may be provided above the thrustplate  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 . 
     A fluid, such as a lubricating oil or a ferromagnetic fluid fills interfacial regions between the shaft  175  and the sleeve  185 , and between the thrustplate  180  and the thrustplate cavity  190  and the counter plate  195 . One or more of the thrustplate  180 , the thrustplate cavity  190 , the shaft  175 , the sleeve  185 , or the counter plate  195  has pressure-generating grooves (not shown in this figure) formed in accordance with the present invention to create hydrodynamic bearings  225 . Preferably, grooves are formed in an outer surface  215  of the shaft or an inner surface  230  of the sleeve  185 . More preferably, the grooves form one or more hydrodynamic journal bearings  225  having dynamic cushions that rotatably support the hub  160  in a radial direction. 
     The pressure generating grooves  235  formed on the inner surface  230  of the sleeve  185  will now be described with reference to  FIG. 3 .  FIG. 3  depicts a sleeve  185  featuring hydrodynamic grooves  235  formed thereon. Hydrodynamic bearings, as previously mentioned, are generally formed between a rotatable member (i.e. sleeve  185 ) and a non-rotatable member (i.e. a shaft) having juxtaposed surfaces between which a layer or film of fluid is induced to form a dynamic cushion as an anti-friction medium. To form the dynamic cushion, at least one of the surfaces—here, the sleeve  185 —is provided with grooves  235  which induce fluid flow in the interfacial region  260  and generate the localized region of dynamic high pressure referred to previously. 
     The grooves  235 , which are separated by lands or raised regions  240 , can have a depth of from about 0.009 to 0.015 mm. In one embodiment of hydrodynamic grooves, the grooves  235  are shaped and arranged to form a chevron or herringbone pattern. That is, the grooves  235  are made up of two straight segments, which meet at an angle to define a V shape as shown in  FIG. 3 . To form a hydrodynamic journal bearing  225 , the grooves  235  are configured in a ring about the inner surface  230  of the sleeve  185 . In one embodiment, the sleeve  185  has an inner diameter of slightly more than 3 mm, and chevron patterned grooves are formed on the surface thereof. Typically, hydrodynamic bearings  225  formed on a sleeve  185  in such a way also comprise one or more “relief cuts”  245  that circumscribe the inner surface  230  of the sleeve  185  and separate individual sets of grooves  235 . Relief cuts  245  connect to grooves  235  and are formed at substantially the same depth (for example, 5 to 10 microns) on the sleeve surface. Furthermore, relief cuts  245  have a typical width of 1 mm. These relief cuts  245  have the effect of increasing the running gap of the hydrodynamic bearings  225  in certain areas, hence reducing friction loss by caused by unnecessary shearing of fluid. This is a way of reducing motor power consumption as well. 
     In current motor designs, relief cuts  245  are machined into the sleeve  185  simultaneously with the final cutting operation, i.e. in a separate process from the machining of grooves  235 . This additional step in the machining process increases the time and cost expended by the complete machining process. In addition, such a process can create problems with the locations and tolerances of the boundaries between the relief cuts  245  and the active grooves  235 . The tolerances of both the relief cut  245  boundaries and the groove  235  apexes  250  are based on the same component datums. Therefore, if the tolerances of both were at their extremes, the functionality of the bearings  225  could be compromised. The present invention not only provides a way to machine a hydrodynamic bearing  225  in a more timely and cost effective manner, but it also results in reduced motor power consumption and increased bearing reliability by improving the process in which the grooves  235  and relief cuts  245  are formed. 
       FIG. 4  is an illustration of one embodiment of a hydrodynamic groove forming apparatus  400  for use in the electrochemical machining of hydrodynamic grooves  235 .  FIG. 5  may be referenced as needed in the discussion of  FIG. 4  to facilitate an understanding of how the groove forming apparatus  400  functions. Groove forming apparatus  400  is used to form hydrodynamic grooves  235  on the inner surface of a work piece, for example, the sleeve  185  (shown in  FIG. 3 ). The surface  420  of the apparatus  400  carries the pattern  435  of the hydrodynamic grooves  235  to be formed on the sleeve  185 . Additionally, the apparatus  400  also carries the pattern  445  of the relief cut  245  to be formed on the sleeve  185  simultaneously with the formation of hydrodynamic grooves  235 . The surfaces of groove pattern  435  and the relief cut pattern  445 —which are the “active” surfaces of the apparatus  400 —are at substantially equal elevations on the apparatus  400 , so that the grooves  235  and relief cuts  245  formed on the sleeve  185  are coplanar. Finally, the apparatus  400  features land patterns  440  that are at a raised elevation relative to that of the groove pattern  435  and the relief cut pattern  445 . These land patterns  440  further comprise an insulative material. The insulative material is retained by holes  450 . Therefore, the land patterns  440  on the apparatus  400  may be considered the “inactive” portions of the apparatus  400  because current will not flow through the electrolyte from these regions. 
     The apparatus  400  is placed concentrically within the sleeve  185  in a substantially spaced-apart relation (shown in  FIG. 5 ). That is, there is substantially no contact between the outer surface  402  of the apparatus  400  and the inner surface  230  of the sleeve  185 . The apparatus  400  acts as a cathode and the sleeve  185  functions as an anode, with direct current being passed between the two surfaces through a conductive electrolyte ( 502  in  FIG. 5 ), such as sodium nitrate. The current passed through the electrolyte  502  typically falls in the range of 8 to 10 amps. At the sleeve  185  surface, electrons are removed by current flow, except for at those portions of the shaft surface that face the insulated (inactive), land portions of the apparatus  400 . As the metallic bonds of the molecular structure at the surface of the sleeve  185  are broken, material is removed from the inner surface  230  of the sleeve  185 , creating the hydrodynamic grooves  235  and the relief cuts  245 . 
     Essentially, the groove forming apparatus  400  allows for the consumption, during electrochemical machining, of the inner surface  230  of the sleeve  185 . That is, material is removed from all portions of the inner sleeve surface facing active (i.e. groove  435  and relief cut  445 ) surfaces of the apparatus  400 , and only the lands  240  remain on the original inner surface  230 . Therefore, instead of actively cutting hydrodynamic grooves  235  and then relief cuts  245  into the sleeve  185  (as prior methods dictate), the grooves  235  and relief cuts  245  are formed simultaneously by etching everything but the lands  240 . This is significant for two reasons. First, the relief cut  245  boundaries and the groove  235  apexes  250  are locked onto the apparatus  400  and so will always be in the same location relative to each other. This will lead to increased bearing  225  reliability as well as decreased component cost. Second, making the inner surface  230  of the sleeve  185  smooth-cut (i.e. no machined-in relief cuts) aids in the metrology of the inner surface  230  size and form tolerances. Thus, electrochemical machining with the groove forming apparatus  400  results not only in time and cost savings in the machining process, but also ultimately will lead to reduced power consumption by the hydrodynamic bearings so formed. For instance, in a current motor sample, the power consumed by the bearings can be reduced by approximately 46 mW by performing the inventive process described herein. 
     It is important to note that this invention is not limited to forming any specific bearing groove pattern, be it sinusoidal, straight line or other. Therefore, the present invention represents a significant time, cost, and power saving advancement in the field of electrochemical machining of hydrodynamic bearings. Furthermore, it produces bearings that function more reliably than those previously machined. 
     While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.