BACKGROUND OF THE INVENTION Hydrodynamic bearings have been the subject of considerable research and development in the past few years. In these types of systems, a lubricating fluid-either gas or liquid-functions as the actual bearing surface between a stationary base or shaft and a rotating sleeve or hub of the motor, or between two such relatively rotating parts.

Such bearings have a number of advantages over conventional ball bearings. Such ball bearings, especially when used in motors which are used in disc drives or the like have problems in a number of areas. Specifically, shocks to the disc drive may in turn create a force across the mechanical bearing system which can lead to deformation and damage to the raceway and balls of the ball bearing. Over time, this could result in a failure of the spindle motor to be able to run smoothly and without vibration. Since the spindle motor is directly coupled to the discs, the vibration could easily be directly transferred.

A misalignment between the disc which the spindle supports and the transducer which is used to access the surface of the disc could also occur. In either case, the transducer which flies close to the disc surface is more likely to impact the disc.

However, the very fact that a fluid is being used as the bearing surface in a hydrodynamic bearing demands that the bearing must have very fine tolerances for the

gap between the shaft and the sleeve, in turn, requiring highly accurate machining of all surfaces. Further, the surfaces must be cleaned of any imperfections which could result in scraping or other damage to the near by facing surface which forms the opposite side of the gap, or in turn the generation of particles. All these goals must be achieved with an economically efficient manufacturing process.

Finally, it is important that provision be made for filling the gap between the shaft and the sleeve. It is important that access be provided to the hydrodynamic bearing gap so that fluid or gas can be inserted into the gap ; the same token, token, must must provided that this gap does not diminish the performance of the hydrodynamic bearing, or make it easy for the fluid to leak out of the bearing, or for air to enter the bearing which could diminish the performance of the bearing fluid.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved hydrodynamic bearing design and an improved method for fabricating the hydrodynamic bearing.

It is a further objective of the invention to provide a hydrodynamic bearing design which incorporates a filling groove so that oil or other lubricating fluid can be efficiently inserted into the bearing gap without risking the loss of fluid during operation.

It is yet another objective of the invention to provide an improved method for fabricating the hydrodynamic bearing and its oil filling groove so that the efficiency of the manufacturing process is not compromised.

These and other objectives of the invention are achieved in a hydrodynamic bearing assembly including a shaft and surrounding sleeve which define the

hydrodynamic bearing gap, the shaft and sleeve defining at the end of the sleeve a flat, radial surface. In forming the bore through the sleeve, which is the outer surface of the hydrodynamic bearing, a boring bar is used having a sharp nose which is used for cutting the interior surface of the bore. The same tool nose, when dragged across a portion of the radial surface at the end of the sleeve forms a groove across this radial surface of the sleeve. The operation may be repeated one or more times, depending on the speed at which the oil is being inserted into the hydrodynamic bearing gap during assembly.

By following this process, the entire boring process is simply achieved without changing the tools which are being used in the boring machine, resulting in an efficient, high speed boring process.

Other features and avantages of the invention may be apparent to a person of skill in the art who studies the following description of a preferred embodiments of the method and apparatus for the present invention given in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a typical hydrodynamic bearing and motor with which the present invention is useful ; Figures IB and I C are end and side views of a portion of the sleeve which is ground and bored in Figure 2, showing the filling grooves of the invention ; Figures 2A and 2B are sectional illustration views of the grooving tool which is used in implementing the present invention ; Figure 3 illustrates the step sequence for the method of boring the sleeve of the present invention ;

Figure 4 is a flow chart of the sequence of Figure 3.

DETAILED DESCRIPTION OF A PREFERRED ENIBODIMENT The following description of a preferred embodiment is showing in conjunction with a single type of hydrodynamic bearing incorporated in a specific spindle motor. However, the apparatus and method disclosed herein is equally useful in any hydrodynamic bearing, especially those which present the difficulty of adequately and rapidly filling the bearing.

Hydrodynamic bearings manufactured according to the method of this invention have many uses other than in hard disc drives, and the invention is not intended to be limited to use in bearings and or motors in such an environment. However, this invention is especially useful in this field because of the importance of reliable manufacturing and easy filling of the bearing, as well as the maintenance of close tolerances throughout the manufacturing and assembly.

The basic structure of the motor with the hydrodynamic bearing, or at least as much as is relevant to this invention, includes a stationary shaft 10 and a hub 12 which rotates around the shaft. The shaft supports a thrust plate 14 at one end, and includes at or near the other end a shoulder 16. The shaft and shoulder incorporate a shaft extension 30 having a threaded region 31 which can be used to mount the stationary shaft into the base 44 of a housing for a disc drive or the like. The hub 12 which will support a disc or discs is mounted on a sleeve 15 and cooperates with the shaft 10 and counterplate 14 to define the gaps of the hydrodynamic bearing. The sleeve 15 also houses or supports a counterplate 17 which rests on a shoulder 18 of the sleeve and completes the definition of the hydrodynamic bearing gap. One of the two opposing

surfaces of the shaft thrust plate assembly and the bushingi'counterplate assembly typically carries on at least a portion of the surface cylindrical sections of spiral grooves to create fluid pressures within the gap which maintains the fluid bearing within the gap when rotation of the sleeve around the shaft is occurring.

It should be pointed out at this time that it is the relative rotation of the surfaces of the shaft and sleeve that is important to establishing and maintaining the bearing action. Therefore, the principes of boring and grooving the sleeve surfaces in carrying out this invention can be used as well in creating a bearing or motor where the shaft is rotating and the sleeve is stationary.

Special notice should be taken of two axial features on the sleeve 15. One of these is the shoulder 42 which extends axially above the radial plane of the thrust plate 14. The counterplate 17 is fitted inside this shoulder. Therefore, machining of this feature to accurate dimensions is important. Further, note should be taken of the upright 18 on which the counterplate rests. The upright defines a surface which accurately locates the axial position of the counterplate 17. Further, the opening 44 between the upright 18 and the shoulder 42 is filled with a O-ring or equivalent 44 which is pressed between the counterplate 17 and the sleeve 15 so that fluid cannot easily escape from the bearing groove.

A final feature appears in Figure 1B, which is a top plan view of the upright 18. This plan view shows a plurality of slightly curved grooves 50 which extend entirely across the upright 18. Each of these grooves (which are shown in side view in Figure 1 C) is slightly curved, so that it may be formed by the radius of the nose of the tool which is also used to machine the other precisely located surfaces of the sleeve and hub as will be explained below. As will become apparent from looking at the tool which is

used for the majority of the steps of the process, as well as the sequence of steps, the use of the tool nose to form this groove 50 clearly increases the speed of manufacture of the sleeve, and eliminates the need for changing tools, which is a time consuming process.

Further, burring is minimized since there is no right angle with the surface on which the groove is being formed.

The tool itself appears in Figures 2A and 2B ; its specific design is not a part of the invention, but is a known grooving tool made by P. H. Horn Corporation. Figure 2A shows the nose of the tool 60 and the shank 62 by which it is grasped by the toolholder ; the tool at work in a view taken from a brochure by the supplier of the tool is shown at Figure 2B.

Figure 3 shows on its left side in a schematic view the sleeve 15 and hub 12 which are formed by the sequence of steps to be discussed. The center of the figure shows the platform 70 on which the tools are to be mounted. Although the grooving tool of Figures 2A and 2B appear in a number of steps, obviously only one such tool needs to be provided on the platform which is why the boring bar 62 which is used in performing a number of different steps is indicated by the same reference number 62.

The boring bar 62 is mounted on the same platform with a grooving tool 70, a tool 72 for finishing the exterior of the hub, and a brush tool 74 for cleaning the interior surface of the bore.

Thus referring to the flow chart of Figure 4 in conjunction with the platform supported tools of Figure 3, the platform moves back and forth (i. e. radially relative to sleeve 15) in the direction indicated by the arrow 76 to bring the appropriate tool into alignment with the sleeve. The first step 80 is to bore the pregrooving dimension in the

sleeve 15. The platform also moves axially to bring the selected tool into contact with the sleeve to carry out each step.

After pre-grooving 80, the platform is then stepped radially to align the grooving tool 70 with the bore of the sleeve. After a flusllillg step 82, the grooves for the hydrodynamic bearing discussed above are formed at step 84. The platform is then again stepped radially to realign the tool 62 with the bore and do a skim cut 86 which removes any material displaced by the grooving so that no excess material remains within the bore. This is an important step because of the close tolerances used for the hydrodynamic bearing. At the next step, 88, the cavity in the sleeve for the thrust plate which sits inside the abutment 18 is machined. This is done so that any distortion from the grooving is removed. At the completion of this step, the nose 60 of the tool 62 is scraped slowly across the top surface of this abutment 18 forming the grooves 50.

While this scraping step is done, the sleeve is slowly rotated so that the grooves are somewhat semi-circular in form as shown in Figure IB.

Looking at Figure 3, it is seen that the next step is to machine the counterplate retaining feature 42 using the same tool 62. According to the flow chart, this step would wait until two further steps have been carried out. Either alternative is very useful and the sequence shown in Figure 3 has been found to be slightly more efficient. The next step 92 would be to machine the hub 12 OD, with its flange. This would necessitate a X-axis or radial movement of the platform to align tool 72 with the outer surface of the hub. Another X-axis movement of the platform aligns a brush tool 74 with the main bore of the sleeve so that the brush step 94 can be accomplished.

Finally, a further X-axis movement of the platform realigns the boring bar with the

abutment 18 and the cavity where the thrust plate will be located, so that the thrust plate radius can be accurately finished at step 96.

In summary, by following this sequence of events, each sleeve is efficiently manufactured utilizing a minimum number of tools mounted on the platform ; each tool is brought in alignment with the sleeve and then stepped or moved in the X-direction or axial direction so that the function of the process can be carried out.

Other features and avantages of the present invention will become apparent to a person of skill in the art who studies the present invention disclosure, who may also adopt alternatives to the preferred sequence disclosed herein. Therefore, the scope of the present invention is to be limited only by the following claims.

It can be seen that by providing a platform for mounting the tools which is movable in a direction parallel with a center axis of the sleeve (axial movement) to bring each tool in contact with the target portion of the sleeve, while also making it movable in a direction perpendicular to the primary axis (radial movement) of the sleeve to bring alternative tools in alignment with the sleeve being worked on, that a very efficient process for machining each sleeve and hub combination is carried out.