Abstract:
A system and method for characterizing and measuring hydrodynamic grooves made by ECM processes is disclosed. The method includes a procedure for alignment of the work piece to the measurement apparatus as well as a technique for accurately reliably measuring the erosion pattern quickly. Additionally, the invention provides a system for characterizing and measuring the erosion of these grooves.

Description:
This application claims priority from U.S. provisional application Ser. No. 60/401,796, filed on Aug. 6, 2002 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of fluid dynamic bearings and more particularly to etching grooves in a hub used in a spindle motor of a disk drive to form such bearings. 
     2. Description of the Related Art 
     Conventional disc drives use magnetic properties of materials to store and retrieve data. Typically, disc drives are incorporated into electronic equipment, such as computer systems and home entertainment equipment, to store large amounts of data in a form that can be quickly and reliably retrieved. The major components of a disc drive include magnetic media, read-write heads, motors and software. Motors, which are used to spin media at several thousand revolutions per minute, are constructed to spin with minimal vibration and to be reliable and efficient. One of the ways this is done is by insuring proper lubrication of critical moving components in the motor with oil. Proper lubrication of a motor is typically achieved by incorporating grooves in the bore of the hub and shaft through cutting processes such as electrochemical machining (ECM) processes. The bore is defined as the inner surface of the hub. Since these grooves are important for maintaining proper oil circulation, erosion of the groves can cause improper oil circulation leading to motor failure because of lockup. Therefore, measuring and understanding the erosion of the grooves in the bore and shaft is important to building a motor robust enough for hard drives. 
     The cutting process may be performed in any of various electro-erosive machining modes. In electrical discharge machining (EDM), the cutting liquid is dielectric liquid, e.g. deionized water, and the machining electric current is supplied in the form of a succession of electrical pulses. In electrochemical machining (ECM), the cutting medium is a liquid electrolyte, e.g. an aqueous electrolytic solution, and the machining current is a high-amperage continuous or pulsed current. In electrochemical-discharge machining (ECDM), the liquid medium has both electrolytic and dielectric natures and the machining current is preferably applied in the form of pulses, which facilitate the production of electric discharges through the conductive liquid medium. 
     The work piece may be disposed in a bath of the cutting liquid medium to immerse the cutting region therein. More typically, however, the cutting zone is disposed in the air or ambient environment. Advantageously, one or two nozzles of a conventional design are disposed at one or both sides of the work piece to deliver the cutting liquid medium to the cutting region disposed in the air or immersed in the liquid medium. The cutting liquid medium is conveniently water as mentioned, which is deionized or ionized to a varying extent to serve as a desired electro-erosive cutting medium. 
     Since modern hard drives require smaller and faster motors having finer critical features, there is a real challenge in both making and measuring the finer features made using these ECM processes and the like. For example, smaller motors have correspondingly smaller and finer groves built into their bores and shafts than ever before. The ECM process is generally known in the art. However, the ECM process raises the need to accurately and simultaneously place grooves on a surface across a gap that must be accurately measured. Deficiencies in mechanical tolerances may cause misalignment of the electrode with the work piece, causing an uneven gap and correspondingly uneven depth hydrodynamic groove. Accurate measurement of these grooves is needed to understand their wear patterns and ultimately design and build better motors. Conventional methods used to measure component wear in motors are inadequate for measuring the small dimensions found in modern bore and shaft grooves because they were developed for measurements of larger features. 
     Therefore, what is needed is a system and method which overcomes these deficiencies and enables measuring fine features, such as groves, on the bore and shaft of motors. 
     SUMMARY OF THE INVENTION 
     The invention provides a system and method for characterizing and measuring hydrodynamic grooves made by ECM processes. Additionally, the invention provides a system and method for characterizing and measuring the erosion of these grooves. 
     The method for measuring bore erosion includes aligning a stylus with a gauge pin, covering the length of a journal, moving a stylus to an apex region, locating a grove minimum by rotating a hub, rotating said hub to a fixed position, scanning between a first endpoint and a second endpoint collecting data during said scan, analyzing said data by fitting said data to a line, locating a lowest peak in said groove, and calculating the erosion. This process is then repeated after rotating the work piece to a new position. Typically three such measurements are taken with each measurement being taken after the work piece has been rotated by 120 degrees. 
     The system for measuring bore erosion includes a gauge pin for alignment, a theta chuck for supporting said work piece, a theta stage capable of rotating said theta chuck and said work piece about an axis of rotation, a stylus tip for probing a topography on said work piece while said work piece is rotated, a stylus for supporting said stylus tip, and a surface scanner for measuring the response of said stylus to said topography of said work piece and for controlling and moving said stylus along a direction of stylus motion. The gauge pin can have size substantially the same as that of a bore diameter to be measured and the gauge pin has a region on it along a line that has a maximum variation in height of 30 microns over a length of about 20 mm. 
    
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
         FIG. 1  is a flowchart showing the preferred steps used to measure the land erosion of the hydrodynamic motor with groves in the bore and the shaft in accordance with one embodiment of the invention; 
         FIG. 2  is a block diagram representing a groove measurement system, in accordance with one embodiment of the invention; 
         FIG. 3  is a diagram illustrating scan lines of a typical measurement done on a hydrodynamic motor bore with groves; 
         FIG. 4  is a diagram illustrating a typical profile of the groves in the bore including the radial erosion, apex region, original diameter and lowest peak 
         FIG. 5  is a plot showing a typical scan of a hydrodynamic motor bore with groves. 
         FIG. 6  is a plot showing a typical scan showing analysis and results of an outer bore erosion pattern measurement. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention provides a system and method for characterizing and measuring hydrodynamic grooves made by ECM processes. Additionally, the invention provides a system and method for characterizing and measuring the erosion of these grooves. 
       FIG. 1  is a flowchart showing the preferred steps used to measure the land erosion of the hydrodynamic motor with groves in the bore and the shaft in accordance with one embodiment of the invention. First in step  105 , the rotary fixture is aligned so that its axis of rotation coincides with the direction of motion of the stylus. Next in step  110 , the end points for the movement of the stylus during the measurements are defined by moving the stylus from a first journal to a second journal. The first journal and second journal are typically set at 11.8 mm. 
     Next in step  115 , the stylus is moved to the apex region which is determined by the motor design. In step  120 , a groove minimum (MIN) is located by holding the stylus fixed and the rotating the hub. In step  125 , after the MIN is located, the hub is rotated by another θ degrees so that the position of the stylus is at MIN+θ degrees. Although the value of θ is typically set at 12, it is determined based on the number of groves. 
     Next in step  130 , the stylus is moved across the fixture from the first journal point to the second journal point scanning the surface and creating a profile of the surface as shown in the attached figures. A line of best fit for the apex region and the original diameter is then calculated using a least square fit algorithm in step  135 . Next in step  140 , the lowest peak is located and the distance from the least square fit line to the lowest peak is calculated. In step  145  the radial erosion (R i ) for this hub angle is calculated. Typically, three radial erosions (R i ) will be calculated, one for each hub angle, which is, offset from the previous angle by 120 degrees, resulting in three values R 1 , R 2  and R 3 . 
     Next in step  150 , the hub is rotated by 120 degrees to a position of MIN+θ+120 degrees. In step  155 , a decision is made as to whether the hub has been rotated to a position greater than MIN+θ+360 degrees. If it is determined in step  155  that the position of the hub is not greater then MIN+θ+360 degrees, then steps  130  through  155  are repeated. Typically, step  155  results in performing three scans and calculating three erosion values R 1 , R 2  and R 3 , at three different angles, as was discussed with reference to step  145  above. Although the hub is rotated by 120 degrees in step  150  it can be rotated by any amount such as 30 degrees or 60 degrees, for example. There is no restriction on the amount of rotation. If it is determined in step  155  that the position of the hub is greater then MIN+θ+360 then step  160  is performed. 
     In step  160 , the total erosion is calculated by averaging the three measured erosions R 1 , R 2  and R 3  and multiplying the average by 2. If N scans are performed instead of only three, as described with reference to steps  145  to  155  above, then the total erosion is determined by calculating the average of all the erosions measured and multiplying that by 2 (ie. Total erosion=Σ(R i ) I=1 TO N /N). Finally, in step  165 , the fixture is removed. 
       FIG. 2  is a block diagram representing a groove measurement system, in accordance with one embodiment of the invention, including a work piece  210 , a theta chuck  215 , theta stage  220 , an axis of rotation  225 , stylus tip  230 , a stylus  235 , a gauge head  237 , a surface scanner  240 , and a direction of stylus motion  245 . Work piece  210  is typically a motor shaft with grooves in it, or a motor sleeve with grooves, or a gauge pin used for calibration. Theta Chuck  215  is a conventional chuck used to securely mount and hold work piece  210  during profiling. Theta stage  220  rotates work piece  210  to a specified position for profiling and typically includes a servo motor or a stepper motor that can rotate work piece  210  from zero to 360 degrees with a resolution of 0.1 degrees. Theta stage  220  rotates work piece  210  about axis of rotation  225  that is typically set to coincide with the symmetry axis of work piece  210 . Typically, theta stage  220  will move work piece to three different orientations (0°, 120°, 240°) wherein the scanning is performed as was further discussed with reference to  FIG. 1  above. 
     The stylus tip  230  moves over work piece  210  by moving the stylus  235  along the same direction as the axis of rotation  225 . As the stylus tip  230  moves over the work piece  210 , the stylus  235  moves up and down according to the topography of the work piece  210 . The movement of the stylus  235  is detected by the gauge head  237 , which in turn produces electrical signals in response to the movement of stylus  235 , which mimics the topographical changes in work piece  210 . Gauge head  237  can produce electrical signals by means well known in the art such as by measuring the mechanical movement of the stylus  235  using a piezoelectric, by measuring the capacitance difference between stylus tip  230  and work piece  210  or by measuring the tunneling of electrons between the stylus tip  235  and the work piece  210 . Stylus tip  230  is mounted to stylus  235  that holds and drives the stylus tip  230  as well as provides a coupling to the head gauge  237 . Surface scanner  240  is a conventional contact surface profiler used to move stylus  235  and stylus tip  230  as well as record and analyze data generated by the electronics in the surface scanner. Surface scanner  240  drives stylus  235  and stylus tip  230  in a direction of stylus motion  245  which is usually parallel to the axis of rotation  225 . 
       FIG. 3  is a diagram illustrating scan lines of a typical measurement done on a hydrodynamic motor bore with groves. Although  FIG. 3  shows only three grooves there is no restriction on the number of grooves. Typically the actual number of grooves can be between 10 and 20. The scan direction is from left to right, as indicated by the direction of the scan line  330 . Further details of the scan are discussed with reference to  FIG. 4  below. Scan line  330  is the direction along which stylus  235  moves and corresponds to the stylus motion  245 . 
       FIG. 4  is a diagram illustrating a typical profile of the groves in the bore including the original diameter  410 , lowest peak  415 , apex region  420 , least square line  425 , and ECM radial erosion  430 . Original diameter  410  is data generated by scanner  240  and represents the topography of the bore grove erosion pattern. Original diameter  410 , which is also known as the quiet zone, includes points corresponding to the diameter of the hub that depicted in  FIG. 4  as the highest peaks similar to the labeled point  410  and above above least square line  425 . Lowest Peak  415  represents the lowest part of the of the erosion pattern and is used to determine the ECM radial erosion  430 . Apex region  420  represents the uppermost part of the erosion pattern and is also used to determine the ECM radial erosion  430 . The least square line  425  is calculated using a least square fitting algorithm that is well known in the art. The least square line  425  is calculated using the apex region and the original diameter  410  (Quiet Zone). 
     ECM radial erosion  430  is defined as the distance between the least square line  425 , passing through the apex region and original diameter (Quiet Zone), and the lowest peak of the ECM bore on the given journal. The ECM radial erosion  430  is determined using the following equations: 
      Ri=Radial Erosion obtained from the ith scan
 
N=Number of Scans
 
Ravg=Σ(Ri) i=1 to N /N
 
Land Erosion=2*Ravg
 
       FIG. 5  is a plot showing a typical scan of a hydrodynamic motor bore with groves. The x-axis of  FIG. 5  shows the scan length as 11.8 mm. The scan length is typically set to be between 5 mm and 20 mm. The scan length is chosen to optimize both speed of measurement and resolution. The longer the scan length the longer the measurement will take and vice versa. The y-axis represents the depth profile of the erosion pattern so that the combination of the scan length and depth profile gives an accurate view of the groves along the direction of motion of the stylus  235 . 
       FIG. 6  is another example of a typical scan showing analysis and results of an outer bore erosion pattern measurement. The scan in  FIG. 6  is obtained in accordance with this invention by first aligning the surface scanner  240  with the axis of rotation  225  Chuck using a work piece  210 , which is gauge pin having a size equal to the bore diameter. Scanning the gauge pin over a length of 20 mm does the alignment. The scanned profile should be a straight line with a maximum height difference between the ends of less than 30 microns. The alignment for the theta axis (run out) is done by positioning the stylus on the top of the gauge pin and rotating the pin through one complete rotation. The stylus reading should be constant through out the rotation. Once the alignment is done, the theta stage  220  is taken to a home position. The work piece to be measured is then loaded the part on to the theta chuck  215  such that the flat portion of the work piece being measured is at the top and parallel to the upper edge of the stylus  235 . The stylus  235  is then moved inside the bore such that there is about 11.9 mm of travel between the edges of the bore and start point of the scan. The stylus is then brought into contact with the work piece  210 . The User Coordinate System (UCS) consisting of of X and Z is then set to Zero and the work piece is scanned. The position of the stylus  235  is then set on the Apex (for example, 2.5 mm position) of the part and the Y stage is manually moved to the lowest point of the bore. The theta chuck  215  is then rotated to an angle where the stylus indicator indicates the lowest value, indicating the presence of a groove. The theta chuck  215  is the turned about 12°. This position is labeled as the 0° Position, for the remainder of the measurement and serves as a reference point. 
     At this time the UCS X is set to 11.8 and Z is set to Zero as illustrated in FIG.  5 . Once this reference is set the scan is done and surface profile is measured. The profile data is illustrated in FIG.  6 . Once the surface scan is completed the data is analyzed by manually finding the highest point on the scan which is defined as the shortest distance from Z=0. The Delta z, which indicated in  FIG. 6 , is the R 1 _Outer_Bore. The theta stage  220  is rotated by 120 degrees and the same measurements are performed to obtain a second measurement at a second position called R 2 _Outer_Bore. The theta stage  220  is again rotated by another 120 degrees and another measurement is taken to obtain a third measurement R 3 _Outer_Bore. Finally, the bore erosion is calculated using the formula:
 
Ravg_Outer_Bore=(R 1 _Outer_Bore+R 2 _Outer_Bore+R 3 _Outer_Bore)/3
 
Outer_Bore_Erosion=Ravg_Outer_Bore*2
 
     It will also be recognized by those skilled in the art that, while the invention has been described above in terms of preferred embodiments, it is not limited thereto. Various features and aspects of the above-described invention may be used individually or jointly. Further, although the invention has been described in the context of its implementation in a particular environment and for particular applications, those skilled in the art will recognize that its usefulness is not limited thereto and that the present invention can be utilized in any number of environments and implementations.