Patent Publication Number: US-2003221959-A1

Title: Method and apparatus for forming grooved journals

Description:
CROSS-REFERENCE TO A RELATED APPLICATION  
     [0001] This invention is based on U.S. Provisional Patent Application Serial No. 60/383,949 filed May 28, 2002, entitled “Dynamic Machining Gap For Cylindrical ECM Applications” filed in the name of Dustin Alan Cochran. The priority of this provisional application is hereby claimed. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The invention relates generally to the field of disc drives, and more particularly to an apparatus and method for forming hydrodynamic grooves in a disc drive.  
       [0004] 2. Description of the Related Art  
       [0005] Disc drives are capable of storing large amounts of digital data in a relatively small area. Disc drives store information on one or more recording media. The recording media conventionally takes the form of a circular storage disc, e.g., media, having a plurality of concentric circular recording tracks. A typical disc drive has one or more discs for storing information. This information is written to and read from the discs using read/write heads mounted on actuator arms that are moved from track to track across surfaces of the discs by an actuator mechanism.  
       [0006] Generally, the discs are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. The spindle motor generally includes a shaft fixed to 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.  
       [0007] 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 discs. For example, to achieve increased storage densities the read/write heads must be placed increasingly close to the surface of the storage disc. This proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the surface of the disc to contact the read/write heads. This is known as a “crash” and can damage the read/write heads and surface of the storage disc resulting in loss of data.  
       [0008] From the foregoing discussion, it can be seen that the bearing assembly which supports the storage disc is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair of races which allow a hub of a storage disc 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.  
       [0009] 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 disc hub. In addition to air, typical lubricants include oil or other fluids. Hydrodynamic bearings spread the bearing interface over a large 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, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeat run out.  
       [0010] Dynamic pressure-generating grooves (i.e., hydrodynamic grooves) disposed on journals, thrust, and conical hydrodynamic bearings generate localized area of high fluid pressure and provide a transport mechanism for fluid or air to more evenly distribute fluid pressure within the bearing, and between the rotating surfaces. The shape of the hydrodynamic grooves is dependant on the pressure uniformity desired. The quality of the fluid displacement and therefore the pressure uniformity is generally dependant 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.  
       [0011] As the result of the above problems, electrochemical machining (ECM) of grooves in a hydrodynamic bearing has been 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 to form hydrodynamic grooves thereon. 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 accurate and simultaneously place grooves on a surface across a gap which must be very accurately measured, as the setting of the gap will determine the rate and volume at which the metal ions are carried away. Even in simple structures, this problem can be difficult to solve. When the structure is the interior surface of a conical bearing, the setting of the gap width can be extremely difficult. Manufacturability issues associated with conical parts often make it difficult to control the diameter of the cones. Due to mechanical tolerances, the work piece may be misaligned with the electrode causing an uneven gap and a correspondingly uneven depth hydrodynamic groove. Therefore, it is almost impossible to make a tool with fixed electrodes that will guarantee a continued consistent work piece to electrode gap to form dimensionally consistent hydrodynamic grooves.  
       [0012] Therefore, a need exists for a method and apparatus to provide a reliable method and apparatus for forming hydrodynamic grooves that is accurate and cost effective.  
       SUMMARY OF THE INVENTION  
       [0013] Embodiments of the present invention relate to a method and apparatus for electromechanically etching grooves in a surface of a conical bearing. In one embodiment, the invention provides a method for aligning an electrode having one or more hydrodynamic bearing groove patterns thereon within a hydrodynamic bearing. The method includes positioning the electrode within a hydrodynamic bearing, and providing a fluid pressure between the electrode and the hydrodynamic bearing to align the electrode and the hydrodynamic bearing.  
       [0014] In another embodiment, the invention provides an apparatus for forming grooves within a hydrodynamic bearing. The apparatus includes a fluidstatic bearing configured to support at least a portion of an electrode having at least one surface carrying a groove pattern to electrochemically etch on an inner surface of the hydrodynamic bearing. The fluid static bearing utilizes a pressurable medium which may comprise liquid or air. The apparatus includes a fluid input configured to couple a fluid flow in a gap between at least some of the electrode and an inner surface of the hydrodynamic bearing to adjust the width of the gap, and a source of electrolyte to be pumped within the gap.  
       [0015] In another embodiment, the invention provides an apparatus for electrochemically forming grooves on a hydrodynamic bearing, including means for fluidly supporting an electrode having a groove pattern thereon, and means for fluidly aligning the electrode within a hydrodynamic bearing. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016] 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.  
     [0017]FIG. 1 depicts a plan view of one embodiment of a disc drive for use with aspects of the invention.  
     [0018]FIG. 2 is a vertical sectional depicting one embodiment of a dual conical bearing utilized in the disc drive of FIG. 1 for use with aspects of the invention.  
     [0019]FIG. 3 depicts a simplified sectional view of an electrochemical machining system for use with aspects of the invention.  
     [0020]FIG. 4 depicts a partial sectional view of an electrochemical machining system for use with aspects of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0021]FIG. 1 depicts a plan view of one embodiment of a disc drive  10  for use with embodiments of the invention. Referring to FIG. 1, the disc 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 from 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 been frequently used, and aspects of the invention are not limited to the configuration of the disc drive housing. For example, disc 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 to or adjacent the top of the spindle motor, is functionally the same as the top cover  14 . Disc drive to further includes a disc pack  16  which is mounted on a hub  202  (See FIG. 2) for rotation on a spindle motor (not shown) by a disc clamp  18 . Disc pack  16  includes a plurality of individual discs that are mounted for co-rotation about a central axis. Each disc surface has an associated read/write head  20  which is mounted to disc drive  10  for communicating with the disc surface. In the example shown in FIG. 1, read/write heads  20  are supported by flexures  22  which are in turn attached to head mounting arms  24  of an actuator body  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 body  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 .  
     [0022]FIG. 2 is a vertical sectional view of a hub  202  supported by dual conical and journal bearing  200  for rotation about a shaft not shown. The hub  202  is integrated with the sleeve  204 . The sleeve  204  includes internal surfaces  206  having grooved regions  214 ,  216  forming the hydrodynamic bearing to support the hub during rotation. As is well-known in this technology, a shaft (not shown) is inserted within the sleeve  204  and has dual conical surfaces which face the conical regions  210 ,  212  at the upper and lower ends of the journal bearing  200 . The shaft would further include a smooth center section which would cooperate with a portion of the journal bearing  200  defined by the grooved regions  214 ,  216 . As is well-known in this field of fluid dynamic bearings, fluid will fill the gap between the stationary shaft and the inner grooved surfaces of the sleeve  204 .  
     [0023] As the sleeve  204  rotates, under the impetus of interaction between magnets mounted on an inner surface of the hub  202  which cooperate with windings supported from the base of the hub  202 , pressure is built up in each of the grooved regions  214 ,  216 . In this way, the shaft easily supports the hub  202  for constant high speed rotation. Hydrodynamic grooves  222  on the inner surface of the sleeve  204  can easily be seen FIG. 2. They include, in one example, two sets of grooves  230 ,  232  for the upper cone and a corresponding set  234 ,  236  for the lower cone. This particular design also utilizes two journal bearings  240 ,  242  to further stabilize the shaft.  
     [0024]FIG. 3 is a simplified illustration of a groove forming apparatus  300  and method for making hydrodynamic grooves  222 . FIG. 2 may be referenced as needed in the discussion of FIG. 3. For purposes of clarity, the illustrative apparatus and method are described in terms of hydrodynamic grooves  222 . However, the present invention is not limited to making this particular combination of hydrodynamic grooves  222 . For example, the apparatus and method described could be used to make the hydrodynamic grooves (e.g., grooves)  222  inside a single cone or a single cone cooperating with a single journal bearing or dual cones cooperating with one or more journal bearings  200 . Further, each of the conical bearings could have one or more sets of hydrodynamic grooves  222 . The principles of the present invention are applicable in forming any design of conical or journal bearing. The solution provided by this invention is especially important in defining conical bearings because manufacturability issues associated with conical parts often make it difficult to control the diameter of the cones. Given this, it is extremely hard to make a tool with fixed electrodes that will guarantee a consistent work piece to electrode gap. As described above, this gap distance is paramount to the accuracy of hydrodynamic groove dimensions. Considering fluid dynamic bearings, the importance of the accuracy of hydrodynamic grooves is that a fluid dynamic bearing generally comprises two relatively rotating members having juxtaposed surfaces between which a layer or film or fluid is maintained to form a dynamic cushion with an antifriction medium. To form the dynamic cushion, at least one of the surfaces, in this case the interior surfaces of sleeve  204 , are provided with the hydrodynamic grooves  222  which induce fluid flow in the interfacial region and generate a localized region of dynamic high pressure.  
     [0025] With continuing reference to FIG. 3, groove-forming apparatus  300  includes an fluidstatic bearing  306 . Fluidstatic bearing  306  includes an air inlet  308  to receive fluid  310  such as pressurized air, clean dry air (CDA), liquid and the like. Internal surfaces  307  of fluidstatic bearing  306  define a longitudinal bore  309 . Longitudinal bore  309  inside diameter is sized to hold a floating electrode  302  therein. Floating electrode  302  has an outside diameter sized smaller than longitudinal bore  307  to define a gap  316  there between. Fluid flow through inlet  308  into gap  316  is at sufficient viscosity or pressure provides force FX 1  between internal surfaces  307  and floating electrode  302 . FX 1  is of a magnitude capable of supporting floating electrode  302  to maintain gap  316 . In this embodiment, pressure within gap  316  between internal surfaces  307  and floating electrode  302  center and support such floating electrode  302  within longitudinal bore  309 . Fluidstatic bearing  306  may include one or more end walls not shown to prevent floating electrode  306  from moving outside longitudinal bore  309 .  
     [0026] Floating electrode  302  includes an extension  304  extending from one end thereof. Extension  304  has an outside diameter sized to fit within an inside diameter of journal bearing  200  (i.e., work piece) to form a fluid gap  322  there between. The journal bearing  200  is rigidly held in place by a clamping apparatus not shown. Extension  304  is configured with a hydrodynamic journal pattern  324  juxtaposed to inside surfaces  206 . Hydrodynamic journal pattern  324  may be used to form hydrodynamic grooves  222  on the journal bearing  200 , for example. During a hydrodynamic groove forming operation, electrolyte  320  is pumped through an electrolyte inlet  321  into fluid gap  322 . As electrolyte  320  is generally non-compressible, electrolyte  320  fills fluid gap  322  centering electrode extension  304  within journal bearing  200 . In this embodiment, electrolyte  320  is used to center the extension  304  within journal bearing  200 .  
     [0027] In another aspect of the invention, Floating electrode  302  may further include a fluid delivery bore  315  extending axially there through, and at least partially through extension  304 . Fluid delivery bore  315  includes a positioning fluid inlet  314  on one end and a plurality of fluid jets  328 A-C coupled to an opposite end of fluid bore  315 . Fluid jets  328 A-C are disposed so that positioning fluid  312  received from fluid inlet  314  exits at least partially against an inside surfaces  206  of journal bearing  200 . To maximize centering pressure FX 2  and holding force FY 2 , fluid jets  328 A-C may be angled at an angle α approximately 45 degrees relative the inside surfaces  206  they contact. Positioning fluid  312  may be any fluid configured to work with electrolyte  320 , and may be an electrolyte similar to or the same as electrolyte  320 .  
     [0028] As illustrated in FIG. 4, fluid jets  328 A-C may be radially spaced approximately uniformly about extension  304  so that positioning fluid  312  discharged from fluid jets  328 A-C provide uniform centering forces FX 2  and FY 2  against the journal bearing  200 . Positioning fluid  312  exits from fluid gap  322  via an end of journal bearing  200 . During another alignment operation of extension  304  within journal bearing  200 , positioning fluid  312  is pumped though fluid inlet  314  and forced through fluid jets  328 A-C. Fluid forces FX 2  and FY 2  balance force FX 1  in an equilibrium condition so that extension  304  is horizontally and vertically centered within journal bearing  200 . While three fluid jets  328 A-C are illustrated spaced so the angle Θ is approximately 120 degrees apart to provide an equal fluid force FX 2  to center the extension  304  within the journal bearing  200 , any number or configuration of fluid jets  328  may be used to provide such centering and aligning forces.  
     [0029] The ECM process can then be executed by then applying an electrical potential to the work piece  200  and floating electrode  302 , the work piece receiving the positive potential and the floating electrode  302  serving as the cathode and receiving the negative potential. By timing the current flow, an imprint in the form of the groove patterns  222  shown in FIG. 2 are placed on the work piece  200 . As is well-known, the width and depth of the resulting hydrodynamic grooves  222  is controlled by the duration and level of current applied to the work piece  200  and the floating electrode  302 . The current level being modified primarily by the fluid gap  322  which has now been adjusted by fluidstatic bearing  306 , electrolyte  320 , and positing fluid  312  via fluid jets  328 A-C.  
     [0030] 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.