Abstract:
An apparatus, method and means is provided for electrochemical machining of hydrodynamic bearing assemblies in spindle motors. In an aspect, a cartridge is provided that receives and accurately positions an electrode in three dimensions in a near frictionless manner. The electrode reaches and maintains an equilibrium position in response to a first and second predetermined force, the equilibrium position being a predetermined three dimensional orientation relative to the work piece and defining a critical orifice with the work piece. In an aspect, a hydrostatic bearing is employed within the electrode for radially adjusting the electrode.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on provisional application Ser. No. 60/441,681, filed Jan. 21, 2003, entitled Hydrostatic Bearing Cartridge For ECM Grooving Applications, and assigned to the assignee of this application and incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to spindle motors, and more particularly to electrode positioning for forming grooves on hydrodynamic bearing assembly spindle components in disc drive data storage systems. 
     BACKGROUND OF THE INVENTION 
     Disc drive memory systems are used by computers and currently also widely used by other devices including digital cameras, digital video recorders (DVR), laser printers, photo copiers and personal music players. Disc drive memory systems store digital information that can be recorded on concentric tracks of a magnetic disc medium. Several discs are rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. The read/write heads are located on a pivoting arm that moves radially over the surface of the disc. The discs are rotated at high speeds during operation using an electric motor located inside a hub or below the discs. Magnets on the hub interact with a stator to cause rotation of the hub relative to the shaft. One type of motor is known as an in-hub or in-spindle motor, which typically has a spindle mounted by means of a bearing system to a motor shaft disposed in the center of the hub. The bearings permit rotational movement between the shaft and the hub, while maintaining alignment of the spindle to the shaft. The read/write heads must be accurately aligned with the storage tracks on the disc to ensure the proper reading and writing of information. 
     Spindle motors had in the past used conventional ball bearings between the hub and the shaft and a thrustplate. However, 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, the close proximity requires that the disc rotate substantially in a single plane. 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, disc drives currently utilize a spindle motor having fluid dynamic bearings on the shaft and the thrustplate to support a hub and the disc for rotation. 
     In a fluid dynamic bearing, a lubricating fluid such as gas or liquid or air provides a bearing surface between a fixed member and a rotating member of the disc drive. Dynamic pressure-generating grooves (i.e., hydrodynamic grooves) formed on a surface of the fixed member or the rotating member generate a localized area of high pressure or a dynamic cushion and provide a transport mechanism for fluid or air to more evenly distribute fluid pressure within the bearing and between the rotating surfaces, enabling the spindle to rotate with a high degree of accuracy. Typical lubricants include oil and ferromagnetic fluids. 
     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. As an example, a hydrodynamic groove having a non-uniform depth may lead to pressure differentials and subsequent premature hydrodynamic bearing or journal failure. 
     One known method for producing dynamic pressure-generating grooves presses and rolls a ball over the surface of a work piece. A problem with this method is the displacement of material in the work piece, resulting in ridges or spikes along the edges of the grooves. Removing these ridges is time consuming costly. A further problem is that the demand for higher disk drive rotational speeds requires the shaft and hub work pieces to be made of material that is as hard or harder than the material of the ball. 
     Another known method for producing the grooves of a fluid dynamic bearing uses 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. This approach also is not typically suitable for use with harder metals. Moreover, because each groove or portion of a groove must be individually formed and the workpiece then moved, the process is time consuming. Further, the equipment necessary for this approach is expensive and the metal-removing tool is subject to wear and requires frequent replacement. 
     Another known method for producing grooves involves an etching 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. One problem is the time consumed in applying and patterning the etch resistant coat. 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 can leave resist material on the workpiece surface resulting in the failure of the bearing and destruction of the disc drive. Yet another problem is that the process requires the extensive use of environmentally hazardous and toxic chemicals including photo resists, developers, solvents and strong acids. 
     Accordingly, there is a need for a method for forming accurate grooves in a work piece that does not require the use of a metal-removing tool that must be frequently replaced and does not use etch resistant material that could contaminate the work piece. As the result of the above-mentioned groove forming concerns, electrochemical machining (ECM) of grooves in a fluid dynamic bearing has been developed. The ECM process is generally known. However, the ECM process raises the need to accurately 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 metal ions are carried away from the surface. Deficiencies in mechanical tolerances may cause misalignment of the electrode with the work piece, causing an uneven gap and correspondingly uneven depth hydrodynamic groove. It is extremely difficult to make a tool with fixed electrodes that will guarantee a consistent work piece to electrode gap to form dimensionally consistent hydrodynamic grooves. Known methods to adjust electrodes (axially) include a worm and gear arrangement, which generates significant friction and is not reliably accurate. Some groove forming methods require the use of a coordinate measuring machine (CMM) to change the electrode. The centerline of the electrode has to be determined, and the work holder is positioned to match the centerline of the electrode, which has proven to be unreliable. Therefore, a need exists to reliably and repeatedly be able to set an accurate gap between an electrode and an interior surface of a work piece, in order to establish accurate grooves on the work piece. 
     SUMMARY OF THE INVENTION 
     An apparatus, method and means for ECM grooving of hydrodynamic bearing assemblies in spindle motors is provided. In an embodiment, the invention provides a reliable and repeatable process for setting a machining gap between an electrode and a work piece in order to create accurate grooves on the work piece. In an embodiment, a self-contained cartridge is provided that receives and positions an electrode in a near frictionless manner, in three dimensions relative to a work piece to be grooved. In an application, the cartridge maintains the electrode at a constant vertical (axial) position and pivots the electrode in a horizontal (radial) motion. In another application, the cartridge fixes the electrode in a radial position and adjusts the axial position of the electrode. In an embodiment, the cartridge provides radial adjustment of an electrode by way of an upper and a lower hydrostatic bearing that can be turned on and off, or adjusted to a desired pressure. 
     Features of the invention are achieved, in an embodiment, by utilizing a critical orifice to position and align the electrode relative to the work piece. A fluid (i.e., electrolyte) sets the machining gap between the electrode and the work piece. The machining gap area is varied by a predefined pressure and mass flow. The force of the fluid on a work surface displaces the electrode upward until equilibrium is reached with a downward force on the electrode provided by a vertical control displacement device. The machining gap is established without the need to make external adjustments. 
     Further, an adaptable cartridge is provided that allows quick changes of electrodes without the need to realign or disassemble the cartridge. The cartridge can receive and employ various electrodes including those used for grooving flat plates (i.e., thrustplates, counterplates), cylinders and cones. 
     Other features and advantages of this invention will be apparent to a person of skill in the art who studies the invention disclosure. Therefore, the scope of the invention will be better understood by reference to an example of an embodiment, given with respect to the following figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a top plain view of a disc drive data storage system in which the present invention is useful; 
         FIG. 2  is a sectional side view of a hydrodynamic bearing spindle motor, in which the present invention is useful; 
         FIG. 3  is a simplified sectional view of a hydrostatic bearing cartridge assembly, in accordance with an embodiment of the present invention; 
         FIG. 4  is a partial and simplified sectional view of an electrode within in a hydrostatic bearing cartridge assembly, in accordance with an embodiment of the present invention; 
         FIG. 5  is a partial and simplified sectional view illustrating the electrode positioning process with a flat surface work piece, in accordance with an embodiment of the present invention; 
         FIG. 6  is a perspective view of electrode active region illustrating injection ports and grooves, in accordance with an embodiment of the present invention; 
         FIG. 7  is a partial and simplified sectional view illustrating the electrode positioning process with a cylinder or cone, in accordance with an embodiment of the present invention; and 
         FIG. 8  is a perspective view of a hydrostatic bearing cartridge joined to a superstructure, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Exemplary embodiments are described with reference to specific configurations. Those of ordinary skill in the art will appreciate that various changes and modifications can be made while remaining within the scope of the appended claims. Additionally, well-known elements, devices, components, methods, process steps and the like may not be set forth in detail in order to avoid obscuring the invention. 
     An apparatus, method and means for electrochemically forming grooves on a work piece is described herein. A reliable and repeatable process for setting a machining gap between an electrode and a work piece in order to create accurate grooves on the work piece is provided. In an embodiment, features of the discussion and claims may be applied to and utilized for forming grooves on hydrodynamic bearing assembly spindle components in disc drive data storage systems. The spindle components include flat surfaces (i.e., thrustplates and counterplates), cylinders and cones. 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates a typical disc drive data storage device  110  in which the present invention is useful. Disc drive  110  includes housing base  112  that is combined with top cover  114  to form a sealed environment. 
     Disc drive  110  further includes disc pack  116 , which is mounted for rotation on a spindle motor (not shown) by disc clamp  118 . Disc pack  116  includes a plurality of individual discs, which are mounted for co-rotation about a central axis. Each disc surface has an associated head  120  (read head and write head), which is mounted to disc drive  110  for communicating with the disc surface. In the example shown in  FIG. 1 , heads  120  are supported by flexures  122 , which are in turn attached to head mounting arms  124  of actuator body  126 . 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  128 . Voice coil motor  128  rotates actuator body  126  with its attached heads  120  about pivot shaft  130  to position heads  120  over a desired data track along arcuate path  132 . This allows heads  120  to read and write magnetically encoded information on the surfaces of discs  116  at selected locations. 
       FIG. 2  is a sectional side view of a hydrodynamic bearing spindle motor  255  used in disc drives  110  in which the present invention is useful. Typically, spindle motor  255  includes a stationary component and a rotatable component. The stationary component includes shaft  275  that is fixed and attached to base  210 . The rotatable component includes hub  260  having one or more magnets  265  attached to a periphery thereof. The magnets  265  interact with a stator winding  270  attached to the base  210  to cause the hub  260  to rotate. Core  216  is formed of a magnetic material and acts as a back-iron for magnets  265 . Magnet  265  can be formed as a unitary, annular ring or can be formed of a plurality of individual magnets that are spaced about the periphery of hub  260 . Magnet  265  is magnetized to form one or more magnetic poles. It is to be appreciated that spindle motor  255  can employ a fixed shaft as shown in  FIG. 2 , or a rotating shaft. 
     The hub  260  is supported on a shaft  275  having a thrustplate  280  on one end. The thrustplate  280  can be an integral part of the shaft  275 , or it can be a separate piece which is attached to the shaft, for example, by a press fit. The shaft  275  and the thrustplate  280  fit into a sleeve  285  and a thrustplate cavity  290  in the hub  260 . A counter plate  295  is provided above thrustplate  280  resting on an annular ring  205  that extends from the hub  260 . Counterplate  295  provides axial stability for the hydrodynamic bearing and positions hub  260  within spindle motor  255 . An O-ring  212  is provided between counterplate  295  and hub  260  to seal the hydrodynamic bearing and to prevent hydrodynamic fluid from escaping. Hub  260  includes a central core  216  and a disc carrier member  214 , which supports disc pack  116  (shown in  FIG. 1 ) for rotation about shaft  275 . Disc pack  116  is held on disc carrier member  214  by disc clamp  118  (also shown in  FIG. 1 ). Hub  260  is interconnected with shaft  275  through hydrodynamic bearing  217  for rotation about shaft  275 . Bearing  217  includes radial surfaces  215  and  225  and axial surfaces  220  and  222 . 
     A fluid, such as lubricating oil or a ferromagnetic fluid fills interfacial regions between the shaft  275  and the sleeve  285 , and between the thrustplate  280  and the thrustplate cavity  290  and the counter plate  295 . Although the present figure is described herein with a lubricating fluid, those skilled in the art will appreciate that a lubricating gas can be used. In order to promote the flow of fluid over the bearing surfaces which are defined between the thrust plate  280  and the counterplate  295 ; between the thrust plate  280  and the sleeve  285 ; and between the shaft  275  and the sleeve  285 , typically one of the two opposing surfaces of each such assembly carries sections of pressure generating grooves (not shown). The grooves induce fluid flow in the interfacial region and generate a localized region of dynamic high pressure. As sleeve  285  rotates, pressure is built up in each of its grooved regions. In this way, shaft  275  easily supports hub  260  for constant high speed rotation. The grooves are separated by raised lands or ribs and have a small depth. It can be extremely difficult to form grooves having small dimensions that are relatively closely packed on a surface. The effective operation of the pressure generating grooves depends in part on the pressure generating grooves being within a specified depth tolerance. 
       FIG. 3  is a simplified sectional view of a hydrostatic bearing cartridge assembly, in an embodiment of the present invention. Hydrostatic bearing cartridge  300  provides ECM grooving of hydrodynamic bearing assemblies in spindle motors. Hydrostatic bearing cartridge  300  is a self-contained cartridge that receives and positions electrode  324  in three dimensions and in a near frictionless manner. Hydrostatic bearing cartridge  300  is an adaptable cartridge that allows quick changes of electrodes without the need to realign the structure/cartridge, and without the need to disassemble the cartridge. Hydrostatic bearing cartridge  300  can receive and employ various electrodes used for grooving flat plates, cylinders and cones. 
     A general discussion of an embodiment of the components of hydrostatic bearing cartridge  300  is provided in this paragraph. Hydrostatic bearing cartridge  300  comprises a corrosion resistant material (i.e., DHS1), which includes electrolyte inlets. An electrolyte is provided to electrolyte inlet  310  for a machining gap and an electrolyte is provided to electrolyte inlet  312  for a hydrostatic bearing. From electrolyte inlet  310 , electrolyte travels to electrolyte delivery hose  334  and into electrode  324  via a channel through electrode  324 , for use in a machining gap. From electrolyte inlet  312  (in an example), electrolyte travels to electrode  324 , through at least one of upper hydrostatic bearing  316  and lower hydrostatic bearing  318 , into longitudinal bore  330 , and then out electrolyte bearing exit  322 . A longitudinal bore  330  is defined between electrode  324  and bearing surface  328 . Electrode  324  attaches, on a first end, to an electrode attachment point  314 , which repositions and essentially floats with electrode  324  within a cavity to remain near frictionless. The second and opposite end of electrode  324  is the electrode active region  326 , which extends from hydrostatic bearing cartridge  300  to a work piece. Electrode attachment point  314  is situated adjacent to plenum  306 . Also adjacent to plenum  306  is an electrical contact  304  for an ECM process, and an electrolyte splash seal  308 . In an embodiment, a frictionless air cylinder  302  is used to apply a predetermined pressure on electrode  324  along z-axis  332 . A cartridge locating surface  320  is a precision ground surface and is used to position hydrostatic bearing cartridge  300  in a superstructure. 
       FIG. 4  is a partial and simplified sectional view of an electrode within in a hydrostatic bearing cartridge assembly, illustrating the function of the hydrostatic bearing. Internal surfaces of the hydrostatic bearing cartridge define a longitudinal bore  430 . Electrode  424  has an outside diameter sized smaller than longitudinal bore  430  to define a gap there between. Radial clearance exists between electrode  424  and bearing sleeve  428 , with consideration to factors including specific gravity and the viscosity of the electrolyte provided to electrolyte inlet  412 . An upper hydrostatic bearing  416  and a lower hydrostatic bearing  418  is located within longitudinal bore  430  to facilitate positioning of electrode  424  in three dimensions and in a near frictionless manner. Electrode  424  includes an active region  426  extending from one end thereof. Active region  426  has an outside diameter sized to fit within an inside diameter of a work piece. such as a sleeve. 
     Electrolyte for hydrostatic bearings  416  and  418  is injected into an electrolyte inlet (shown in  FIG. 3 ) and travels into electrode  424 . Radial movement of electrode  424  is facilitated when one of upper hydrostatic bearing  416  and lower hydrostatic bearing  418  is activated by flow of electrolyte there through into longitudinal bore  430  and against bearing sleeve  428 . Radial movement, as described herein, is movement by electrode  424  perpendicular to z-axis  432  or movement that otherwise intersects z-axis  432 . As an example, when upper hydrostatic bearing  416  is activated (turned on), electrolyte travels through upper hydrostatic bearing  416  into longitudinal bore  430 , between a surface of electrode  424  and bearing sleeve  428 . Concurrently, lower hydrostatic bearing  418  is turned off by preventing electrolyte from traveling through lower hydrostatic bearing  418 . Electrode  424  is thereby allowed to pivot radially about upper hydrostatic bearing  416 . 
     Three dimensional motion for an electrode is provided by the present invention. In an example, when positioning electrode  424  to groove a flat surface, such as a thrustplate or a counterplate, electrode  424  moves axially along the z-axis  432 , which is perpendicular to the horizontal axis of the flat surface. Axial movement, as described herein, is movement by electrode  424  along z-axis  432 . In an embodiment, electrode  424  is fixed radially by setting both upper hydrostatic bearing  416  and lower hydrostatic bearing  418  to a high pressure. In another embodiment, upper hydrostatic bearing  416  and lower hydrostatic bearing  418  are set to other pressures for fixing electrode  424  in other radial orientations. The electrolyte flowing from hydrostatic bearings  416  and  418 , and through longitudinal bore  430 , travels or drains through electrolyte bearing exit  422 . 
     In an example, when positioning electrode  424  to groove a cylinder or cone, electrode  424  moves radially perpendicular to z-axis  432  and is fixed axially. Radial movement is achieved by utilizing upper hydrostatic bearing  416  and lower hydrostatic bearing  418 . Upper hydrostatic bearing  416  and a lower hydrostatic bearing  418  can be set to a pressure ranging from a high pressure to a low pressure. Further, upper hydrostatic bearing  416  can be set to a different pressure than lower hydrostatic bearing  418 . In other applications, electrode  424  has three dimensional freedom (moves radially and axially) to move and comply with the dynamic action of the electrolyte flowing through machining gap  420 . 
       FIG. 5  is a partial and simplified sectional view of an electrode utilized in a hydrostatic bearing cartridge assembly, illustrating the electrode positioning process with a flat surface work piece. A frictionless air cylinder (shown in  FIG. 8 ) creates a constant downward force  502  on electrode  524  in the direction of z-axis  532 . Force (F) is caused by an air cylinder pressure (P ac ) and controlled by a super-precision regulator. In an embodiment, electrode  524  is positioned within 25 microns of its desired grooving position, and then machining gap fluid positioning is utilized, as described herein. 
     With continuing reference to  FIG. 3 , a machining gap electrolyte is injected into one or more electrolyte inlets  310  and passes through one or more electrolyte delivery hoses  334  to electrode  324  by way of plenum  306 . Plenum  306  is defined as a central location in which fluids and other compounds pass to equalize and normalize before passing to electrode  324 . In an embodiment, two electrolyte delivery hoses originating from opposite sides of hydrostatic bearing cartridge  300  are utilized for electrolyte injection symmetry to electrode  324 . Electrolyte travels into electrode  324  via a channel through electrode  324 , for use in a machining gap. An electrolyte splash seal  308  is positioned to shield against any leakage of electrolyte to other parts of hydrostatic bearing cartridge  300 , and to protect against any corrosion including anodic and chemical corrosion. In an embodiment, electrolyte splash seal  308  is situated below electrolyte inlet  312  and above upper hydrostatic bearing  316 . 
     The force of the electrolyte flowing from active region  526  displaces electrode  524  axially upward along z-axis  532  (and in some applications, radially) until equilibrium is reached with the opposing downward force  502  on electrode  524 . The machining gap  520  is itself the critical orifice. In an embodiment, the critical orifice is in the range of 10 to 30 microns. The pressure of the electrolyte (P e ), mass flow of the electrolyte (Q e ) and downward force of the air cylinder (F, P ac ) are held constant, and a desired cross sectional flow area within machining gap  520  is achieved. Electrode  524  is thereby positioned in its desired three dimensional orientation above work piece  522 . The desired machining gap is repeatedly established by using the same predetermined forces during the manufacturing process of numerous work pieces, without the need to make an external adjustment. 
     The ECM process is then executed by applying (for a predetermined interval) an electrical potential to work piece  522  and electrode  524 , work piece  522  receiving the positive potential and electrode  524  serving as the cathode and receiving the negative potential. By timing the current flow, an imprint in the form of a groove pattern is placed on work piece  522 . As is well-known, the width and depth of the resulting grooves is controlled by the duration and level of current applied to the work piece  522  and electrode  524 . The current level is modified primarily by machining gap  520 . 
     Electrical contact  304  is a thin copper ribbon (shown in  FIG. 3 ) and supplies a negative charge to electrode  524  without having a position influence on electrode  524 . As shall be appreciated, a thin copper ribbon or other light weight material may be utilized for electrical contact  304 . 
     The ECM process removes material metal without the use of mechanical or thermal energy. The electrical energy (as described above) is combined with a chemical (the electrolyte) to form a reaction of reverse electroplating. Direct current is passed between work piece  522  and electrode  524 , which carries the pattern to be formed, the current being passed through a conductive electrolyte between the two surfaces. At the surface of work piece  522 , electrons are removed by current flow, and the metallic bonds of the molecular structure at work piece  522  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 from machining gap  520  to be filtered out. The contaminated electrolyte may be reprocessing for reuse. 
       FIG. 6  is a perspective view of an electrode active region  626  utilized to groove a cylinder or cone. Electrode active region includes grooves  602  and injection ports  600 . Injection ports  600  are situated to uniformly provide electrolyte to the machining gap. In an embodiment, the injector ports are 200 microns in diameter. 
       FIG. 7  is a partial and simplified sectional view of an electrode utilized in a hydrostatic bearing cartridge assembly, illustrating the electrode positioning process with a cylinder or cone. Electrode  724  includes an active region  726  extending from one end thereof. Active region  726  has an outside diameter sized to fit within an inside diameter of a work piece, such as a sleeve. Electrolyte is supplied to machining gap  720  through an array of injection ports  600  ( FIG. 6 ) in electrode  724 . 
     In an application, the force of the electrolyte flowing from active region  726  (by way of injection ports  600 ,  FIG. 6 ) displaces electrode  724  axially along z-axis  732  and radially until equilibrium is reached with the downward force  702  on electrode  724 . The machining gap  720  is itself the critical orifice. The pressure of the electrolyte (P e ), flow of the electrolyte (Q e )  704  and the downward force of the air cylinder (F, P ac )  702  are held constant, and a desired cross sectional flow area within machining gap  720  is achieved. Electrode  724  is thereby positioned in its desired orientation within a work piece such as sleeve  722 . The desired machining gap is repeatedly established during the manufacturing process of grooving numerous work pieces without the need to make an external adjustment. Thereafter, the ECM process (as described above) is utilized. The electrolyte contaminated by the ECM process flows from machining gap  720  and is expelled through electrolyte exit  728 , which is on the top and bottom (z-axis  732  orientation) of sleeve  722 . Further, it should be appreciated that the electrolyte to the machining gap can optionally originate from the same source as the electrolyte to the hydrostatic bearings. 
       FIG. 8  is an example of perspective view a hydrostatic bearing cartridge  801  joined to a superstructure  800  for implementation of the grooving process. Superstructure  800  includes a frame  814 , top plate  808 , bottom plate  810 , post  804 , ball bearing bushing  806 , air cylinder  802 , die set  816  and a part holder  812 . An expandable ring workholder or a vacuum chuck may be utilized for part holder  812 . Top plate  808  receives cartridge locating surface  820  and electrode extension passes there through to define a hole within top plate  808 . Bottom plate  810  receives a part holder  812  to define a hole and to receive a work piece such as a plate, cylinder or cone. In one embodiment, the concentricity of the hole within top plate  808  and the lower hole within bottom plate  810  is less than 0.0002 mm. 
     Groove depth is directly related and influenced by the machining gap, as discussed above. In an application, groove depths are measured and a population of groove depth data is generated in cases with and without utilizing an embodiment of the present invention. The standard deviation (sigma) from the target groove depth is calculated for both cases utilizing a binomial distribution curve. For one process, sigma shows an improvement from 0.5 microns to 0.1 microns (improvement factor of 5) when utilizing an embodiment of the present invention. Further, the sigma of 0.1 microns is likely the sigma of the measurement process itself, being below the detectable limit of process deviation. 
     Having disclosed exemplary embodiments, modifications and variations may be made to the disclosed embodiments while remaining within the spirit and scope of the invention as defined by the appended claims. For example, the apparatus and method described herein could be employed to form grooves on a flat plate, inside a cylinder, a single cone, a single cone cooperating with a single journal bearing or inside dual cones cooperating with one or more journal bearings. Further, in the examples discussed above, the use of a hydrodynamic bearing is shown in conjunction with a spindle motor. Clearly, the present invention is not limited to use with this particular design of a disc drive, which is shown only for purposes of the example. Further, it is to be appreciated that the present invention is useful for a wide variety of motors, especially those using fluid dynamic bearings having grooves.