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.
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.
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.
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.
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.
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.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.