Abstract:
An apparatus and method are provided for electrochemically etching grooves in a working surface. In an aspect, a frame holds a working surface about an axis and facing an electrode movable along the axis. The electrode, axially movable, has surface carrying a groove pattern to fix on the working surface. A source of electrolyte is pumped at a fixed static pressure rate between the surface of the movable electrode and the working surface. In an aspect, a support fixture supports the electrode for movement toward and away from the working surface with minimal frictional restriction. A force biases the electrode surface toward the working surface so that a gap through which the electrolyte flows between the surface of the movable electrode and the working surface is determined primarily by the static flow rate of the electrolyte and the force bias of the electrode toward the working surface.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority from and benefit under 35 U.S.C. sec. 120 as a Divisional patent application of co-pending U.S. non-provisional patent application Ser. No. 10/609,895, filed Jun. 30, 2003, entitled “Critical Orifice Gap Setting For ECM Grooving Of Flat Plates,” assigned to the assignee of the present application and incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to the field of fluid dynamic bearings, and more particularly to gap setting for forming grooves in flat plates and cones used in a disc drive.  
       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 that 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. To provide a stable rotating system and avoid non-repeatable run-out, the latest generation of disc drives utilize a spindle motor having fluid dynamic bearings on the shaft and the thrustplate 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 that helps to reduce non-repeat runout. However, to be effective, the pressure-generating grooves must be very accurately defined, both as to shape and depth, on a high-speed basis.  
         [0006]     Accordingly, there is a need for an apparatus and method for forming grooves in a work piece 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 allow the grooves to be 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 work piece leading to the failure of the bearing and destruction of the disc drive.  
         [0007]     As the result of the above problems, electrochemical machining (ECM) of grooves in a fluid dynamic bearing has been developed. 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 which must be very accurately defined, 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. Therefore, it is very difficult to make a tool with fixed electrodes that will guarantee a continued consistent work piece to electrode gap. As noted above, the distance is paramount to the accuracy of grooved depth.  
         [0008]     In known designs, the gap is varied to yield a predetermined mass flow, and the position of the electrode relative to the work piece is adjusted mechanically to establish the gap. This takes up to thirty seconds in time, which translates directly into manufacturing costs.  
         [0009]     The present invention provides a solution to these and other problems, and offers other advantages over the prior art.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention relates to a critical orifice gap setting for forming grooves in flat plates and conical designs. These accurately placed grooves may be utilized for spindle motors having fluid dynamic bearings.  
         [0011]     In one embodiment according to the present invention, a method is provided for electrochemically etching grooves in a working surface. In another embodiment, an apparatus and method are provided for electrochemically etching grooves in a working surface. A frame holds a working surface about an axis and facing a movable electrode movable along the axis. The electrode is axially movable and has a surface carrying a groove pattern to fix on the working surface. A source of electrolyte is pumped at a fixed static pressure rate between the surface of the movable electrode and the working surface. A support fixture is provided for supporting the electrode for movement toward and away from the working surface with minimal frictional restriction. A force biases the electrode surface toward the working surface so that a gap through which the electrolyte flows between the surface of the movable electrode and the working surface is determined primarily by the static flow rate of the electrolyte and the force bias of the electrode toward the working surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     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.  
         [0013]      FIG. 1  is an exploded perspective view of the basic elements of a disc drive in which a motor incorporating a counter plate, thrust plate or cone formed by embodiments according to the present invention is especially useful;  
         [0014]      FIG. 2  is a sectional side view of a motor incorporating a counter plate formed by embodiments according to the present invention;  
         [0015]      FIG. 3  is a cross-sectional side view of a system used to etch grooves in a counter plate, thrust plate, other flat surface or cone, according to an embodiment of the present invention;  
         [0016]      FIG. 4  is a perspective view with a partial carve-out of a hydrostatic bearing cartridge assembly, according to an embodiment of the present invention; and  
         [0017]      FIG. 5  is a bottom view of an exemplary counter plate having grooves etched therein by an embodiment of an apparatus and method according to the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0018]      FIG. 1  is an exploded perspective view of a magnetic disc drive for which a spindle motor having a fluid dynamic bearing manufactured by the method and apparatus for 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  that are suspended above surfaces  140  of the discs by a suspension arm assembly  150 . In operation, the 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 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.  
         [0019]      FIG. 2  is a sectional side view of a spindle motor  155  of a type which is especially 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 thrustplate  180  on one end. The thrustplate  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 thrustplate  180  fit into a sleeve  185  and a thrustplate cavity  190  in the hub  160 . A counter plate  195  is provided above the thrustplate  180  resting on an annular ring  205  that extends from the hub  160 . An  0 -ring  210  seals the counter plate  195  to the hub  160 .  
         [0020]     A fluid, such as 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  have pressure generating grooves (not shown in this figure) formed to create fluid dynamic bearings. In one embodiment, the grooves are formed in inner surfaces  215  of the hub  160 . In another embodiment, the grooves are formed in the sleeve  185  and in the thrustplate cavity  190 . The grooves in the thrustplate cavity  190  form a fluid dynamic thrust bearing  220  by generating a localized region of dynamic high pressure to form a dynamic cushion that rotatably supports the hub  160  in the direction of thrust. Grooves in the inner surface  215 a of the sleeve  185  form one or more fluid dynamic journal bearings  225  having dynamic cushions that rotatably support the hub  160  in a radial direction.  
         [0021]     Fluid dynamic bearings, as previously implied, are generally formed between rotatable and non-rotatable members 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 is provided with grooves that induce fluid-flow in the interfacial region and generate the localized region of dynamic high pressure referred to previously.  
         [0022]     As mentioned herein, it is difficult to make a device with fixed electrodes that guarantees a continued consistent work piece to electrode gap. The distance of the gap is paramount to the accuracy of grooved depth.  
         [0023]     Given the above, it is necessary to create or define a tool or method used to form the grooves incorporating moving electrodes. Utilizing moving electrodes gives rise to another problem (i.e., how to set the gap between the electrode and the working surface on which the grooves are to be defined). The electrode/work piece gap itself is in many instances the “critical orifice.” Critical orifice flow measurement is utilized because the setting of the gap will determine the rate and volume at which the metal ions are carried away, all other parameters being unchanged, and thereby determines the shape and depth of the grooves being formed.  
         [0024]     In known designs, as mentioned herein, the gap is varied to yield a predetermined mass flow and the position of the electrode relative to the work piece is adjusted mechanically to establish the gap. This takes up to thirty seconds in time, which translates directly into manufacturing costs. It is desirable to be able to set a gap quickly and accurately with a consistent gap width each time the gap is set.  
         [0025]     Referring to  FIG. 3 , one embodiment according to the present invention provides a method and apparatus for forming the pressure generating grooves in a working surface of the counter plate  195 . A system  310  comprises counter plate  195 , electrode  312 , plenum  314 , insulation  316 , gap  318  (sometimes referred to as “critical orifice gap” or “machining gap”) and injection port  320 .  
         [0026]     In use, in an embodiment, an electrolyte is supplied (as described herein) through the electrode  312  and into the plenum  314 . In  FIG. 3 , the plenum  314  is shown as having a smaller diameter at a proximal end and a larger diameter at a distal end; however, this need not be the case.  
         [0027]     Before or after the electrolyte is supplied, the electrode  312  is moved into contact with or proximate the counter plate  195  via a constant downward force F. In one embodiment, F is due to a constant pressure P ac  applied by a (substantially) frictionless air cylinder. In other embodiments, F is due to the gravitational pull on a mass or the like.  
         [0028]     In an embodiment, electrolyte is supplied through the electrode  312  and into the plenum  314 . It is envisioned that the electrolyte is supplied into the plenum  314  by penetrating the electrode in one embodiment. In another embodiment, the electrolyte is supplied into the plenum  314  without penetrating the electrode. The electrolyte is supplied at a constant pressure P e  and with a constant flow rate Q e .  
         [0029]     The electrolyte exits the plenum  314  via an injection port  320 . The electrolyte comes into contact with the counter plate  195  and disperses in a radial fashion through the gap  318 . The force of the electrolyte displaces the electrode  312  in a distal (upward) direction until an equilibrium is reached with the downward force F on the electrode  312 . The gap  318  then becomes a critical orifice as the width of the gap  318  will directly affect grooves that will be formed in the counter plate  195 .  
         [0030]     If P e , Q e  and F are constant then the cross-sectional flow area of the gap  318  will remain constant. In this case, the electrode  312  will hover over the counter plate  195 . The gap  318  is automatically established without the need to make an external adjustment.  
         [0031]     The insulation  316  prevents unwanted areas of the counter plate  195  from being scathed. The insulation  316  covers all areas of the electrode  312  that are proximate the counter plate  195  for which it is desired that the electrode  312  areas be made ineffectual in forming grooves in the counter plate  195 . An electric potential is applied between the electrode  312  and the counter plate  195 . Desired grooves are thus formed in the counter plate  195  as described herein.  
         [0032]      FIG. 4  is a perspective view with a partial carve-out of a hydrostatic bearing cartridge assembly  410  according to an embodiment of the present invention. The electrode  312  is slidably positioned within the hydrostatic bearing cartridge assembly  410  and protrudes from a proximal end thereof. The hydrostatic bearing cartridge assembly  410  provides a (substantially) frictionless way for the electrode  312  to slide up and down.  
         [0033]     As mentioned herein, a (substantially) frictionless air cylinder  412  imparts a force F to the electrode  312  in a proximal (downward) direction. The electrode  312  is free to slide up and down with substantially no friction due to hydrostatic bearings  414 . Electrolyte is supplied into the plenum  314  via a first inlet  416 . Electrolyte is supplied to the hydrostatic bearings  414  via a second inlet  418 . P ac  and P e  are controlled and maintained constant via a super-precision regulator(s), which is known to those of ordinary skill in the art.  
         [0034]      FIG. 5  is a bottom view of an exemplary counter plate  195  having grooves etched therein by an embodiment of an apparatus and method according to the present invention.  FIG. 5  merely depicts an exemplary embodiment of grooves  510  formed according to methods described herein. The grooves, which are separated by ribs or raised lands, can have a depth of from about 0.009 to 0.015 mm, although they are not limited to this range. Generally, the grooves are shaped and arranged to form a chevron or herringbone pattern. That is, the grooves are made up of two straight segments that meet at an angle to define a “V” shape. Alternatively, the grooves define a pattern that has an arcuate or sinusoidal shape, or may be of any other pattern; the present invention is useful to form any desirable pattern.  
         [0035]     Thus the present invention represents a significant advancement in the field of fluid dynamic bearing motor design. Wear is significantly reduced by providing an accurate and relatively inexpensive method of forming grooves on a counter plate  195 . It is contemplated that embodiments of the apparatus and methods described herein can be used to etch grooves of varying configurations. Moreover, it is envisioned that embodiments of the apparatus and methods described herein can be used to etch grooves in any suitable plate, conical element or the like.  
         [0036]     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.