Abstract:
The performance of a leaf spring disc clamp can be enhanced by increasing the flatness of an annular contact surface while the clamp is in its fully deflected, installed position. Such a disc clamp exhibits substantially improved flatness at a minor expense in applied axial force. It has been determined that a lapping process can be used to achieve a desired level of performance without fully deflecting the clamp prior to installation in a disc pack. The performance of the disc clamp may be further enhanced by providing one or more slots within a central portion of the disc clamp and joining the slots with a central aperture in the central portion in order to more uniformly distribute the clamping force around a peripheral portion of the disc clamp and thereby improve disc flatness during disc drive operation.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/499,228, entitled “DISC DRIVE DEFLECTED DISC CLAMP LAPPING PROCESS”, filed on Feb. 7, 2000, now U.S. Pat. No. 6,339,516, which claimed the benefit of priority of U.S. Provisional Patent Application Serial No. 60/130,306, entitled “DEFLECTED DISC CLAMP LAPPING PROCESS”, filed Apr. 21, 1999. This continuation in part application also claims the benefit of U.S. Provisional Patent Application Serial No. 60/158,834, entitled “DISC FLATNESS FROM ID LOBES,” filed on Oct. 12, 1999. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to disc drives and more particularly to a lapped disc clamp used to secure a disc platter assembly to a spin motor, as well as, a process for manufacturing the lapped disc clamp and a disc clamp designed to more uniformly equalize the clamping force exerted on the disc. 
     BACKGROUND 
     Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium on a disc. Modern disc drives comprise one or more rigid discs that are typically coated with a magnetizable medium and mounted on the hub of a spin motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks typically by transducers (“heads”) mounted to an actuator assembly for movement of the heads relative to the discs. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment. Critical to both of these operations is the accurate locating of the head over the center of the desired track. 
     The heads are each mounted via flexures at the ends of actuator arms that project radially outward from the actuator body or “E” block. The actuator body typically pivots about a shaft mounted to the disc drive housing adjacent the outer extreme of the discs. The pivot shaft is parallel to the axis of rotation of the spin motor and the discs, so that the heads move in a plane parallel to the surfaces of the discs. 
     Typically, such actuator assemblies employ a voice coil motor to position the heads with respect to the disc surfaces. The voice coil motor typically includes a flat coil mounted horizontally on the side of the actuator body opposite the actuator arms. The coil is immersed in a vertical magnetic field of a magnetic circuit comprising one or more permanent magnets and vertically spaced apart magnetically permeable pole pieces. When controlled direct current (DC) is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces. The actuator thus allows the head to move back and forth in an arcuate fashion between an inner radius and an outer radius of the discs. 
     Modern disc drives typically include one or more discs mounted to the spin motor. Spacers are used to provide the separation between discs necessary for the actuators arms to movably locate the heads in relation with the disc surfaces. The discs and spacers collectively form a disc stack assembly, or disc pack, that is mounted on the spin motor hub and held together with a leaf spring disc clamp. 
     Disc clamps can be either stamped or milled. While milled clamps are more rigid and less prone to deflecting the abutting disc surface, they are relatively expensive to produce. Consequently, stamped leaf spring disc clamps, which are substantially less expensive, have become popular. The clamp is typically a circular spring-steel, sheet metal structure having a central portion and a rib portion at or near the OD of the clamp, with an annular rib formed in the rim portion of the clamp. The central portion of the leaf spring disc clamp has a partial aperture that is bent or deflected toward the center of the clamp, forming a leaf spring above the level of the annular rib, and includes a plurality of screw holes spaced symmetrically about the central portion of the clamp. The screws used to mount the disc clamp springingly bend and deflect the central portion of the clamp toward the upper surface of the motor spindle as the screws are tightened, thereby forcing the annular rib into firm contact with the uppermost disc surface and applying a clamping force to the disc stack. 
     This type of disc clamp is not without problems. The disc clamp is secured with a plurality of screws, typically  3 , circumferentially spaced around the center of the clamp. The majority of the clamping force is exerted by the rib portion adjacent the screw locations, with a somewhat reduced level of clamping force exerted by the rib portion between the screw locations. This variation in clamping force can mechanically distort the discs in a phenomenon sometimes referred to as “potato chipping,” meaning that the portions of the disc nearest the clamp screws are displaced further than the portions of the disc between the screws. 
     One solution to “potato chipping” is to increase the number of mounting screws used to secure the disc clamp to the spin motor hub. As more screws are used and are spaced closer together, the discrepancy in clamping force is reduced but not eliminated. A disadvantage of this approach is that the use of additional screws complicates the manufacturing and assembly process. 
     Mechanical distortion of the disc surface can, in turn, lead to undesirable variations in the read/write signals detected and written by the heads of the disc drive. Since the heads will fly at varying heights around the circumference of the disc while attempting to follow a distorted disc, the signals used to read and write data on the discs may be inadequate to ensure reliable data storage and recovery. 
     SUMMARY OF THE INVENTION 
     Against this backdrop the present invention has been developed. The performance of a leaf spring disc clamp can be enhanced by increasing the flatness of an annular contact surface in its fully deflected, installed position. Such a disc clamp exhibits a substantially improved flatness in the installed state at a minor expense in applied axial force and thus has a more uniform force distribution applied around the annular contact surface. It has further been determined that a lapping process can be used to achieve a desired level of performance without fully deflecting the clamp prior to installation in a disc pack. 
     Accordingly, an aspect of the invention is found in a method of manufacturing a leaf spring disc clamp for use in a disc drive to clamp a data storage disc to a spindle hub of a spin motor. The method includes the steps of forming a piece of spring sheet metal into a generally circular leaf spring disc clamp having an annular rim portion and a central bowed leaf spring portion. The clamp is then placed on a lapping surface, and a force is applied to the central portion of the clamp to partially deflect the central portion of the clamp from an undeflected position toward the lapping surface. The clamp is then moved relative to the lapping surface to abrade and remove a portion of the rim portion to form a flattened annular contact surface on the rim portion. 
     Another aspect of the invention is found in a leaf spring disc clamp for fastening a data disc to a disc spin motor hub in which an annular rim portion forms an annular rib that has a flat annular contact surface thereon for uniformly distributing clamping force onto the data disc. 
     Yet another aspect of the present invention involves providing one or more slots within the central portion of the disc clamp and joining the slots with a central aperture in the central portion in order to more uniformly distribute the clamping force around the annular rib of the disc clamp and thereby improve disc flatness during disc drive operation. 
     These and other features as well as advantages that characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a plan view of a disc drive incorporating a spring disc clamp in accordance with a preferred embodiment of the invention with the disc drive cover partially removed. 
     FIG. 2 is an exploded view of an exemplary disc pack assembly utilizing three leaf spring disc clamp in accordance with a preferred embodiment of the invention. 
     FIG. 3 is a cross sectional view taken along the line  3 — 3  of FIG. 2 of the disc clamp prior to lapping in accordance with a preferred embodiment of the present invention. 
     FIG. 4 is a view as in FIG. 3 showing the application of clamping force on the central portion of the clamp in the force application step in accordance with a preferred embodiment of the present invention. 
     FIG. 5 is a view as in FIG. 4 showing the flattening of the annular rib during the lapping step in accordance with a preferred embodiment of the present invention. 
     FIG. 6 is a cross sectional view of the disc clamp as in FIG. 3 after the lapping step and after reversal of the clamping force. 
     FIG. 7 is a top plan view of the disc clamp shown in FIG.  3 . 
     FIG. 8 is a top plan view of a disc clamp in accordance with another preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A disc drive  100  constructed in accordance with a preferred embodiment of the present invention is shown in FIG.  1 . The disc drive  100  includes a base  102  to which various components of the disc drive  100  are mounted. A top cover  104 , shown partially cut away, cooperates with the base  102  to form an internal, sealed environment for the disc drive in a conventional manner. This assembly is called a head disc assembly (HDA). The components include a spin motor  106  which rotates one or more discs  108  at a constant high speed. Information is written to and read from tracks on the discs  108  through the use of an actuator assembly  110 , which rotates about a bearing shaft assembly  112  positioned adjacent the discs  108 . 
     The actuator assembly  110  includes a plurality of actuator arms  114  which extend over the surfaces of the discs  108 , with one or more flexures  116  extending from each of the actuator arms  114 . Mounted at the distal end of each of the flexures  116  is a head  118  which includes an air bearing slider enabling the head  118  to fly in close proximity above the corresponding surface of the associated disc  108 . 
     The spin motor  106  is typically de-energized when the disc drive  100  is not in use for extended periods of time. The heads  118  are moved over park zones  120  near the inner diameter of the discs  108  when the drive motor is de-energized. The heads  118  are secured over the park zones through the use of an actuator latch arrangement  122 , which prevents inadvertent rotation of the actuator arms  114  when the heads are parked. 
     The radial position of the heads  118  is controlled through the use of a voice coil motor (VCM)  124 , which typically includes a coil  126  attached to the actuator assembly  110 , as well as one or more permanent magnets and return plates  128  which are spaced apart and establish a vertical magnetic field between them in which the coil  126  is immersed. The controlled application of current to the coil  126  causes magnetic interaction between the permanent magnets  128  and the coil  126  so that the coil  126  moves in accordance with the well known Lorentz relationship. As the coil  126  moves, the actuator assembly  110  pivots about the bearing shaft assembly  112  and the heads  118  are caused to move across the surfaces of the discs  108 . 
     A flex assembly  130  provides the requisite electrical connection paths for the actuator assembly  110  while allowing pivotal movement of the actuator assembly  110  during operation. The flex assembly includes a printed circuit board  132  to which head wires (not shown) are connected; the head wires being routed along the actuator arms  114  and the flexures  116  to the heads  118 . The printed circuit board  132  typically includes circuitry for controlling the write currents applied to the heads  118  during a write operation and for amplifying read signals generated by the heads  118  during a read operation. The flex assembly terminates at a flex bracket  134  for communication through the base deck  102  to a disc drive printed circuit board (not shown) mounted to the bottom side of the disc drive  100 . 
     The discs  108  are secured to the hub  230  of a spin motor  106  in spaced-apart fashion. As illustrated in FIG. 2, three discs  108  are alternatively stacked together with spacers  220  that provide the vertical spacing necessary for actuator assembly function (described hereinafter). Any combination of discs  108  and spacers  220  can be assembled together to form a disc pack. The stacked set of discs  108  and spacers  220  are mounted to the spin motor  106  via the disc clamp  210 . This combination of discs  108  and spacers  220 , along with disc clamp  210  can be referred to as a disc assembly or disc pack. Preferably, three mounting screws (not shown) are used to secure disc clamp the  210  to the spin motor hub  230  using threaded bores  222  in the hub  230 . 
     FIG. 2 shows the leaf spring disc clamp  210  according to a preferred embodiment of the present invention. The leaf spring disc clamp  210  has a bowed central leaf spring portion  218  that has a central aperture  216 . The clamp  210  also has an annular rim portion  240  that forms an annular rib  330  (better seen in the sectional views of FIGS.  3 - 6 ). As described hereinafter, annular rib  330  preferably provides the contact surface between the disc clamp  210  and the upper surface of the uppermost disc  108 . In the embodiment illustrated, the leaf spring disc clamp  210  has a total of three screw mounting holes  212  and three spanner slots  214 . The spanner slots  214  are used for alignment during the assembly process. Without these spanner slots  214 , the disc clamp  210  would simply rotate as one attempts to tighten the first mounting screw (not shown). If desired, the spanner slots  214  may be replaced with holes for receiving guide pins. It should be noted that the deflection and lapping process in accordance with a preferred embodiment of the present invention can be used just as easily with a spring clamp having any number of screw holes and can also be applied to a solid or milled clamp. The invention is described herein with particular reference to a stamped leaf spring disc clamp utilizing three screws, as exemplary, for description purposes only. The deflection and lapping process used to manufacture these clamps is illustrated in FIGS. 3-6. 
     A downward deflective force is applied around each screw hole location during the lapping process of the present invention. FIG. 4 illustrates the positioning of the downward or deflective force applied near each screw hole location  340 . The effect of the deflective force can be seen in this figure by the position of central leaf spring portion  320  relative to lapping plane  420 . Lapping plane  420  represents the working surface of a lapping station and represents a flat surface from which relative positions can be described. 
     Lapping can begin once the leaf spring disc clamp  210  has been partially deflected as seen in FIG.  4 . FIG. 5 illustrates disc clamp  210  after lapping has been completed. The annular rib  330  now has an annular flat contact surface  335  that approximately corresponds to the portion of the disc clamp  210  that contacted the lapping surface. Because stamped materials can present variations in configuration, it is possible, indeed probable, that dissimilar amounts of material may be removed from the annular rib  330  in creating the annular flat contact surface  335 . This is illustrated in FIG. 5, in which annular flat contact surface  335  is represented as having an uneven width. The annular flat contact surface  335  is wider, for example, at location  530  in comparison with location  535 . 
     FIG. 6 shows the leaf spring disc clamp  210  after lapping and after the deflection force applied in the previous lapping step has been removed. Central leaf spring portion  320  can be seen in its undeflected position in which it is relatively further away from an imaginary line drawn across the bottom of the disc clamp  210 . In comparison with FIG. 5, it can be seen that annular flat contact surface  335  appears narrower at location  635  than at location  535 , since portions of annular flat contact surface  335  can twist or deflect from a planar position once the deflective force has been removed. It is intended that the annular flat contact surface  335  be as flat as possible when the disc  210  is installed. It may not be as flat when in an undeflected position. 
     When installed, the disc clamp  210  may not have a flat annular contact surface  335  that is entirely planar. The lapping process is preferably carried out while the disc clamp  210  is only partially deflected. Thus, while the resulting annular contact surface  335  would be even flatter if lapping took place while the clamp  210  was fully deflected, full deflection may structurally damage the clamp  210 . When the clamp  210  is fully deflected (as installed), it undergoes plastic deformation. It is important that plastic deformation of the disc clamp  210  not occur until final installation. As a result, it is preferred that the disc clamp  210  be deflected to a position approximately halfway between an undeflected position and a fully deflected, installed position during lapping. This corresponds to a deflective force during lapping that is about 70 percent that of the force applied to the fully installed clamp  210 . 
     Lapping can be accomplished in a variety of manners. A combination of lathe and mandrel can be used, although a lapping station is preferred. A lapping station includes a flat plane bearing an abrasive material suitable to remove small amounts of material from the spring disc clamp. The clamp can be made from a variety of materials, although stainless steel is preferred. The flat plane of the lapping station moves relative to the disc clamp  210 . The flat plane can be square, circular or some other shape and can either vibrate, oscillate or rotate at a speed sufficient to remove material at a desirable rate. Alternatively, especially for low volume production, the lapping station could be stationary and the disc clamp  210  could be moved about on the abrasive surface. Preferably, only a small amount of material is removed from the annular contact surface. In a preferred embodiment, less than about 0.004 inches is removed. More preferably, the amount of material thickness removed is between about 0.001 inches and about 0.003 inches. 
     The following non-limiting example is intended only to illustrate one preferred embodiment of the invention. 
     WORKING EXAMPLE 
     Standard production stamped clamps were processed according to the following procedure: 
     Deflect clamp to a position approximately halfway between an undeflected position and a fully deflected (installed) position. 
     Remove a small amount of material (approximately 0.004 inches) from annular contact ring of clamp using a mandrel on a lathe. 
     Subsequent inspection under a microscope indicated that more material was removed from the annular contact ring in areas in radial alignment with the screw holes. 
     Five disc packs were assembled using the processed clamps, and five were assembled using unprocessed clamps. The average axial force was reduced by 15 to 20 percent in the disc packs using the processed disc clamps. Average flatness, which was measured at peak acceleration at an ID radius) was improved by about 70 percent. This roughly corresponds to the flatness of an unclamped disc. 
     Another aspect of the present invention is described below with reference to the plan view of the disc clamp  210  in FIG.  7 . The disc clamp  210  is a generally circular disc shaped body having a central portion  218  and a peripheral annular rim portion  240  forming an annular rib  330 . The central portion  218  has a central aperture  216  therethrough and screw mounting holes  212  equidistantly spaced around the central aperture  216  of the disc clamp  210 . Although FIG. 7 shows three screw mounting holes  212 , the disc clamp  210  may have three or more screw holes  212 . The screw mounting holes  212  each receive a screw (not shown) to fasten the disc clamp  210  to the hub  230  of the spin motor  106 . The disc clamp  210  further includes a slot, such as the spanner slots  214 , between every two adjacent screw mounting holes  212  within the central portion  218  of the disc clamp  210 . Each of the slots  214  has a closed end  213 , an open end  215  that joins the central aperture  216 , and has parallel sides  217  which form the generally rectangular shape of slots  214 . The slots  214  serve the additional function of equalizing a clamping force exerted by the annular rib  330  against the disc  108 , which reduces “potato chipping” of the disc  108 . The slots  214  thereby improve the flatness of the disc  108 . The slots  214  perform the equalizing function in two ways. First, the slots  214  reduce the magnitude of the peak to valley pressure fluctuations of the clamping force around the annular rib  330 . Second, the slots  214  reduce the number of peak pressure fluctuations in the clamping force around the annular rib  330  of the disc clamp  210 . 
     FIG. 8 shows a modified disc clamp  710  in accordance with yet another preferred embodiment of the present invention. Similar to disc clamp  210 , disc clamp  710  is a generally circular disc shaped body having a central portion  712  and a peripheral annular rim portion  714  having annular rib  716 . The central portion  712  has the central aperture  718  therethrough and three (or more) screw mounting holes  720  equidistantly spaced around the central aperture  718  of disc clamp  710 . The screw mounting holes  720  each receive a screw (not shown) to fasten the disc clamp  710  to the hub  230  of the spin motor  106 . Disc clamp  710  further includes a modified slot  722  between every two adjacent screw mounting holes  720  within the central portion  712  of the disc clamp  710 . Each of the slots  722  has a closed end  713  and an open end  715  that joins the central aperture  718 . However, instead of having parallel sides  217  like the slots  214 , the slots  722  have diverging sides  724 . FIG. 8 shows rounded diverging sides  724  forming the semi-circular slot shapes; however, the diverging sides  724  and the slots  722  may be of any shape. Disc clamp  710  may further include one or more guide pin holes (not shown) to assist in alignment of disc clamp  710  during the assembly process. 
     Both the slots  214  and the slots  722  redistribute the clamping force and thus make the clamping force exerted by the annular ribs  240  and  716  more uniform by reducing hoop stresses caused by the increased pressure located around the screws (not shown) within the screw mounting holes  212 . The more the clamping force around the annular rib  240  is equalized, the more improved the flatness of the disc  108  will be. 
     In summary, the presently claimed invention may be viewed as a disc clamp (such as  210  and  710 ) for fastening a data storage disc (such as  108 ) to a hub (such as  230 ) of a spin motor (such as  106 ) in a disc drive (such as  100 ). The disc clamp (such as  210  and  710 ) has a generally circular disc shaped body having a central portion (such as  218  and  712 ) and a peripheral portion (such as  240  and  714 ). The central portion (such as  218  and  712 ) has a central aperture (such as  216  and  718 ) therethrough. A concentric annular rib (such as  330  and  716 ) is located on the peripheral portion (such as  240  and  714 ) for asserting a clamping force on the disc (such as  108 ). Three screw mounting holes (such as  212  and  720 ) in the central portion (such as  218  and  712 ) are equidistantly spaced around the central aperture (such as  216  and  718 ) for receiving screws to fasten the disc clamp (such as  210  and  710 ) to the hub (such as  230 ) of the spin motor (such as  106 ) thereby exerting the claming force to the disc (such as  108 ). A slot (such as  214  and  722 ) is formed in the central portion (such as  218  and  712 ) between every two adjacent screw mounting holes (such as  212  and  720 ) for equalizing the clamping force around the annular rib (such as  330  and  716 ) of the disc clamp (such as  210  and  710 ). Each slot (such as  214  and  722 ) has a closed end (such as  213  and  713 ) and an open end (such as  215  and  715 ) wherein the open end (such as  215  and  715 ) joins the central aperture (such as  216  and  718 ). Each of the slots (such as  214  and  722 ) may have parallel sides (such as  217 ) or diverging sides (such as  724 ). Each of the slots (such as  214  and  722 ) may have a generally rectangular shape or a generally semi-circular shape. 
     Stated another way, the presently claimed invention may be viewed as a method for equalizing a clamping force exerted by a disc clamp (such as  210  and  710 ) on a data storage disc (such as  108 ) supported on a hub (such as  230 ) of a spin motor (such as  106 ) in a disc drive (such as  100 ). A slot (such as  214  and  722 ) is provided in the central portion (such as  218  and  712 ) between every two adjacent screw mounting holes (such as  212  and  720 ). The disc clamp (such as  210  and  710 ) is loaded onto the hub (such as  230 ) of the spin motor (such as  106 ) such that the annular rib (such as  330  and  716 ) makes contact with the disc (such as  108 ). And the disc clamp (such as  210  and  710 ) is fastened to the hub (such as  230 ) of the spin motor (such as  106 ) with a screw through each of the screw mounting holes (such as  212  and  720 ) such that the annular rib (such as  330  and  716 ) exerts the clamping force onto the disc (such as  108 ). 
     It will be clear that the present invention is well adapted to attain the ends and advantages mentioned as well as those inherent therein. While a presently preferred embodiment has been described for purposes of this disclosure, numerous changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the invention disclosed and as defined in the appended claims.