Patent Publication Number: US-2011070050-A1

Title: Screw and a method for manufacturing the same

Description:
TECHNICAL FIELD 
     The present disclosure relates to a screw and a method of manufacturing the same, and more particularly, to a screw for connecting a hard disk drive cover to a steel plate or the like and a method for manufacturing the same. 
     BACKGROUND ART 
     A hard disk drive (HDD) is a data storage device that is mounted in a main body of a computer or the like. HDDs are electromechanical devices that store digital signals in the form of semi-permanent magnetic charges. Since the HDD is capable of inputting and outputting a great quantity of data at high speed, it has been widely used as a storage device of a computer system or the like. 
     As shown in  FIG. 1 , such an HDD includes a main body  10  and a cover  30  coupled to the main body  10  with a gasket  20  interposed therebetween, wherein the main body  10  is provided with a plurality of female threaded portions  11 , and the gasket  20  and the cover  30  are provided at positions corresponding to the threaded portions  11  with through-holes  21 ,  31  through which screws  40  are fastened to the main body  10 . 
     Accordingly, when assembling the components of the HDD, such as a spindle motor, actuator, head, disk, etc., mounted in the main body  10  using the screws  40 , the cover  30  is placed on the main body  10  with the gasket  20  interposed therebetween, and the cover  30 , gasket  20  and main body  10  are coupled to one another by fastening the screws  40  through the through-holes  21 ,  31  and the female threaded portions  11 , as shown in  FIG. 2 . 
     However, due to the vulnerability of precision devices to foreign substances, the female threaded portions  11 , which are formed in the HDD or at edges of the main body  10 , are not cut using a cutting tap but are roll-formed using a rolling tap, thereby inevitably leaving a groove  11   a  in a screw thread of the female threaded portion  11  during machining. 
     The conventional screw  40  has a complete external threaded portion  41  and an incomplete external threaded portion  42  having a smaller diameter and formed on a leading end of the complete external threaded portion, so that when the screw  40  is fastened into the female threaded portion  11 , the incomplete threaded portion  42  is coupled into the groove  11   a  in the screw thread of the female threaded portion  11 , causing a fastening defect, which leads to low productivity, cost increase and lower joint force. 
     Further, since the leading end of the conventional screw extends straight forward, the screw is not sufficiently fastened into the female threaded portion, thereby causing a fastening defect. 
     The conventional screw is manufactured by roll-forming because it is difficult to machine the screw and productivity is considerably reduced. 
     Roll-forming of a smaller diameter and length screw is carried out by machining a target object into a certain shape, placing the machined target object between threaded dies and subjecting the machined target object to force to move any one of the dies. In contrast to the cutting process, roll-forming provides advantages such as reduced process time, denser metal structure providing improved strength, and the like. 
     In particular, when screws are formed by roll-forming, a reciprocation type method is usually adopted in which a planar metal die is moved. When high precision screw or special screws are formed by shaping or machining, cylindrical roll type dies are used. 
     In manufacturing a screw, a rod-shaped material is roll-formed between an upper die and a lower die. Here, the screw is formed at a leading cut thereof with a thin, sharp fin, which entails a risk of worker injury and also makes it difficult or even impossible to fasten the screw to a female threaded portion. 
     DISCLOSURE 
     Technical Problem 
     One aspect of the present disclosure provides a screw which can be easily fastened to a female threaded portion of a target object with improved joint force even when the female threaded portion has a low depth, and a method of manufacturing the same. 
     Another aspect of the present disclosure provides a screw which permits easy manufacture, improved productivity and reduced manufacturing costs, and a method of manufacturing the same. 
     A further aspect of the present disclosure provides a screw which eliminates a fin, which is generally formed at an edge cut of the screw upon roll-forming, thereby providing a clean appearance and preventing a worker from being injured by the fin, and a method of manufacturing the same. 
     Technical Solution 
     In accordance with one aspect, a screw includes: a head at one end thereof; and a threaded portion on an outer circumferential surface of the other side of the screw, wherein the threaded portion has a uniform outer diameter and is formed at a leading end thereof with an edge cut intersecting an axial direction of the screw. 
     In accordance with another aspect, a method of manufacturing a screw includes: forming a head by forging and cutting a wire-shape material; roll-forming a threaded portion with stationary and movable dies on an outer circumferential surface of a body having the head formed thereon; and cutting the roll-formed threaded portion to a predetermined length using cutting edges formed on the stationary and movable dies. 
     ADVANTAGEOUS EFFECTS 
     According to embodiments of the present disclosure, an incomplete threaded portion is eliminated from the screw, and the threaded portion is formed at a leading end thereof with a guide portion and has a uniform outer diameter, thereby facilitating a fastening action of the screw and minimizing defect rate when the screw is fastened to a low-height female threaded portion of a target object. Consequently, the screw according to the embodiments provides improved productivity, lower costs, and higher joint force. 
     Further, the screw having the threaded portion on the circumferential surface thereof and cut to a predetermined length may be subjected to re-roll forming to form a smooth circumferential surface of the threaded portion, thereby providing a clean appearance and preventing worker injury. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1 to 3  illustrate a fastening state of a conventional screw, wherein 
         FIG. 1  is an exploded perspective view of the conventional screw fastened to a computer hard disk, 
         FIG. 2  is an enlarged cross-sectional view of the conventional screw fastened to the hard disk, and 
         FIG. 3  is a cross-sectional view of the conventional screw with a fastening defect; 
         FIG. 4  is a front view of a screw according to an exemplary embodiment of the present disclosure; 
         FIG. 5  is a block diagram illustrating a procedure of a method of manufacturing a screw according to an exemplary embodiment of the present disclosure; 
         FIGS. 6 to 8  illustrate dies for manufacturing the screw according to one exemplary embodiment of the present disclosure, wherein 
         FIG. 6  is a perspective view of the dies, 
         FIG. 7  is a perspective view of the dies combined to each other, and 
         FIG. 8  is a partially enlarged cross-sectional view of the screw manufactured by the method according to the exemplary embodiment; 
         FIG. 9  is a partially enlarged cross-sectional view of the screw manufactured by the method according to the exemplary embodiment in a fastened state; 
         FIGS. 10 to 14  illustrate a screw according to another exemplary embodiment of the present disclosure, wherein 
         FIG. 10  is a front view of an example of a screw and a target object to which the screw is fastened, 
         FIG. 11  is a view of the screw fastened to the target object in  FIG. 10 , 
         FIG. 12  is a front view of a modified example of the screw and the target object to which the screw is fastened, 
         FIG. 13  is a view of the screw fastened to the target object in  FIG. 12 , and 
         FIG. 14  is a front view of another modified example of the screw; 
         FIGS. 15 and 16  illustrate dies for manufacturing a screw according to another exemplary embodiment of the present disclosure, wherein 
         FIG. 15  is a perspective view of the dies, and 
         FIG. 16  is a partial cross-sectional view of the screw manufactured by the dies and cut at a lower portion thereof; and 
         FIG. 17  is a partially enlarged cross-sectional view of the lower portion of the screw subjected to re-roll forming 
     
    
    
     BEST MODE 
     Exemplary embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. 
       FIG. 4  shows a screw according to an exemplary embodiment of the present disclosure, wherein the screw  400  includes a head  410  formed at one end thereof and a complete threaded portion  420  on the other side thereof The threaded portion  420  has a uniform outer diameter and is formed at a leading end thereof with an edge cut  430  that is formed in a linear fashion perpendicular to an axial direction of the screw. 
     Next, a method of manufacturing the screw will be described with reference to  FIG. 5 . 
     The method includes supplying a wire-shaped material such as a steel wire in S 10 , forging the material to form a screw preform A having a head  410  in S 20 , roll-forming an outer circumferential surface of the screw preform A having the head  410  to form a threaded portion  420  thereon in S 30 , cutting the threaded screw perform A to a predetermined length in S 40 , and finishing the cut screw preform A to form a final screw  400  in S 50 . 
     Specifically, when the wire-shaped material is supplied in S 10 , the material is forged and cut to a predetermined length by pre-machined dies of a forging machine to form the screw preform A having the head  410  at one end thereof, in S 20 . 
     Then, in S 30 , the screw preform is roll-formed by a pair of dies, i.e. stationary and movable dies  500 ,  600 , which are located to face each other, as shown in  FIGS. 6 and 7 , to form the threaded portion  420  on the outer circumferential surface of the screw preform A. 
     The stationary die  500  is composed of a lower die section  510  and an upper die section  520  coupled to the lower die section  510 . The movable die  600  is also composed of a lower die section  610  and an upper die section  620  coupled to the lower die section  610 . The stationary and movable dies  500 ,  600  are held in a holder, which is not shown in the drawings, so as to machine the threaded portion  420 . 
     The lower die sections  510 ,  610  are provided at one side of upper portions thereof with cutting edges  511 ,  611  that protrude to face each other, and the upper die sections  520 ,  620  are provided at one side thereof with thread-forming portions  521 ,  621  that face each other to form the threaded portion  420 . 
     The cutting edges  511 ,  611  are formed on front sides of the upper portions of the lower die sections  510 ,  610  to face each other, have a gentle gradient as seen in a plan view, and also have cutter portions at their leading ends to cut the screw  400  to a predetermined length. 
     The lower die section  510  ( 610 ) and the upper die section  520  ( 620 ) have contact steps  512 ,  522  that are oppositely provided and come into contact with each other when the lower and upper die sections are combined with each other, thereby preventing the upper and lower die sections from slipping out of place during machining. 
     Then, when the screw preform machined to have the head  410  in the previous operation is supplied between the stationary and movable dies  500 ,  600 , the screw preform is machined to have the threaded portion  420  on its outer circumferential surface by the action of the movable die  600  moved in an arrow direction in  FIG. 6  by means of a separate drive unit such as a cylinder relative to the stationary die  600 . 
     Specifically, as shown in  FIG. 8 , when the screw preform having the head  410  formed in the previous operation is supplied between the stationary die  500  and the movable die  600 , the movable die  600  moves to one side and presses the screw preform to form the threaded portion on the outer circumferential surface thereof. At the same time, the threaded screw is cut to a predetermined length by the cutting edges  511 ,  611 , which are placed at a position corresponding to the predetermined length of the screw, finally forming a screw  400  having an edge cut  430  at the leading end thereof. 
     The cut length of the screw  400  is determined by a distance from the upper portions of the upper die sections  520 ,  620  to the cutting edges  511 ,  611 , that is, the upper portions of the lower die sections  510 ,  610 , which is preset by taking the length of the screw  400  into consideration when manufacturing the dies. 
     The threaded portion  420  of the screw  400  has a uniform outer diameter. 
     Referring to as  FIG. 9 , a final screw for hard disk drives includes the threaded portion  420  having a uniform outer diameter and the edge cut  430  at the leading end of the threaded portion, so that when the screw  400  is fastened to a female threaded portion  11  of a main body  10  of a hard disk drive through a gasket  20  and a cover  30 , the threaded portion  420  is not coupled into the a groove  11   a  of the female threaded portion  11  unlike a conventional screw, thereby reducing fastening defects while improving fastening force. 
     Further, through interaction between the stationary and movable dies  500 ,  600 , the threaded portion  420  is formed on the outer circumferential surface of the screw  400  and at the same time the screw is cut to a predetermined length by the cutting edges  511 ,  611 , simplifying the manufacturing process and thereby improving yield and reducing manufacturing costs. 
     The screw according to the embodiments is not limited to the configuration wherein the edge cut  430  is formed in a linear fashion perpendicular to the axial direction of the screw  400 . In an alternative embodiment, the screw may have a guide portion  440  formed on the edge cut at the lower leading end of the threaded portion  420  to facilitate fastening of the screw to a female threaded portion  110  of a target object  100  made of steel, as shown in  FIGS. 10 and 11 . 
     The guide portion  440  may have a trapezoidal step having inclined side faces  441 , a diameter of which decreases from an upper side to a lower side thereof. 
     The upper portion of the trapezoidal step, i.e. the lower leading end of the threaded portion  420 , may have a length that is equal to or smaller than a root diameter D 1  of the threaded portion  420 . 
     An angle between the inclined side face  441  and the lower leading end of the threaded portion  420  may be 30 to 60 degrees. 
     The screw according to the embodiments is not limited to the configuration wherein the guide portion has the trapezoidal step having the inclined side faces  411 . In an alternative embodiment, the guide portion may be formed with a rectangular step having linear side faces  442  which extend in the axial direction of the screw, as shown in  FIGS. 12 and 13 . 
     The diameter of the rectangular step may be equal to or smaller than a root diameter D 1  of the screw. 
     Further, as shown in  FIG. 14 , the guide portion  440  may have a step with rounded corners  443  at the leading end thereof so that the threaded portion  420  of the screw  400  can be easily inserted into the female threaded portion  110 . 
     With the construction of the trapezoidal guide portion  440  formed on the edge cut  430  and having the inclined side faces  441 , the threaded portion  420  can be easily guided into and fastened to the female threaded portion  110  of the target object  100 , as shown in  FIG. 11 , when the screw  400  is fastened to the target object  100  from an upper side. 
     Further, with the construction of the rectangular guide portion  440  formed on the edge cut  430  of the screw  400  and having linear side faces  442 , the threaded portion  420  can be easily and smoothly fastened to the female threaded portion  110  of the target object  100 , as shown in  FIG. 13 . 
     Furthermore, with the construction of the guide portion  440  formed on the edge cut  430  of the screw  400  and having the rounded corners  443 , the threaded portion  420  can also be easily fastened to the female threaded portion  110  of the target object  100 , as shown in  FIG. 14 . 
     Accordingly, upon fastening of the screw  400 , fastening defects resulting from incorrect fitting between the threaded portion  420  and the female threaded portion  110  are prevented. 
       FIGS. 15 to 17  illustrate the stationary and movable dies  500 ,  600  for manufacturing the screw according to the embodiment. 
     Referring to the drawings, the stationary and movable dies  500 ,  600  have the lower die sections  510 ,  610  and the upper die sections  520 ,  620 , which are combined with one another. 
     The lower die sections  510 ,  610  are formed at opposite surfaces thereof with cutting edges  511 ,  611  that protrude to cut the screw  400  to a predetermined length. 
     The cutting edges  511 ,  611  protrude increasingly outwards and have sharp leading ends to cut the screw  400  that rolls between the dies. 
     The upper die sections  520 ,  620  are provided with thread-forming portions  521 ,  621  that face each other to form the threaded portion on the outer circumferential surface of the screw  400 . 
     Each of the thread-forming portions  521 ,  621  includes a pre-forming section (a), a main-forming section (b), and a finishing section (c) formed in series. 
     The pre-forming section (a) pre-forms a thread on the outer circumferential surface of the screw preform A. The main-forming section (b) completes forming of the thread on the circumferential surface of the screw  400 , and also cuts the screw to a predetermined length to form the edge cut  430   a  at the leading end of the screw using the cutting edges  511 ,  611 . The finishing section (c) is re-rolled along the outer circumferential surface of the screw  400  to smooth the surface thereof. 
     The height of the finishing section (c) may be greater than that of the main-finishing section (b). 
     Even when a certain portion protrudes circumferentially from the edge cut  430  after the screw  400  is cut to a predetermined length by the cutting edges  511 ,  611 , or where the screw  400  is inserted between the upper die sections  520 ,  620  and the lower die sections  510 ,  620  to form a fin, the surface of the screw is smoothed via re-rolling. 
     Thus, it is possible to protect a worker from injury or to prevent insufficient fastening of the screw to the female threaded portion due to a fin, which can be formed on the leading end of the screw after roll-forming. 
     According to the embodiment, the screw is machined to form the threaded portion y roll-forming using the stationary and movable dies  500 ,  600  that are located to face each other. 
     That is, by moving the movable die  600  relative to the stationary die  500  after the screw preform A having the head is placed between the stationary and movable dies  500 ,  600 , the screw preform is machined to have a threaded portion on the outer circumferential surface of the screw through interaction between the opposite thread-forming portions  521 ,  621 . 
     Specifically, when the outer circumferential surface of the screw preform A comes into contact with the thread-forming portions  521 ,  621 , the screw preform is machined to first pre-form the thread at the pre-forming section (a) and the screw preform is then machined to complete formation of the thread at the main-forming section (b), as shown in  FIG. 16 , and at the same time is cut to a predetermined length through interaction between the cutting edges  511 ,  611 , thereby forming the linear edge cut  430   a  on the leading end of the threaded portion. 
     Then, as shown in  FIG. 17 , the threaded screw  400  of a predetermined length passes through the finishing section (c) to form a smooth surface of the threaded portion, thereby removing a fin which can be created due to either a gap between the lower die sections  510 ,  610  and the upper die sections  520 ,  620 , or cutting operation. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to screws that couple components of a hard disk drive, such as a spindle motor, disk, actuator, head, etc., to a cover, screws that couple components and a thin metal or plastic product, or screws or bolts that have a threaded portion having a uniform outer diameter, or a manufacturing method thereof The present disclosure is also applicable to any kind of screw that has a complete threaded portion and can be fastened to a female threaded portion of a thin target object made of steel.