Abstract:
A method of forming a hollow, bullet-shaped article having a strap extending across the normally closed end. The method includes the steps of drawing a metal blank into a cylindrical cup, piercing the bottom of the cup in at least two locations to form a strap, and reducing the diameter of the cup while leaving the length of the strap substantially unchanged so that the strap arcs away from the cup.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the stamping of metal products, and more particularly to deep-drawn stamping. 
     There is a demand for metallic items or parts that are bullet-shaped and hollow, and that include one or more holes in the normally closed end. For example, one such item is a spark plug ground shield  10  (FIGS. 31-32) incorporated into a spark plug  20  (FIG.  3 ). The ground shield is essentially bullet-shaped and hollow, having a cylindrical side wall  12  and a strap  14 . The strap is integral with the side wall and extends diametrically across the normally closed end of the ground shield  10 . 
     Typically, the ground shield  10  is fabricated by, first, stamping a metal blank into a bullet shape and, second, machining the closed end to create the strap  14  arcing away from the remainder of the piece. This process is undesirably labor-intensive and therefore expensive and prone to quality and consistency issues. 
     While at first blush, the ground shield  10  appears to be capable of manufacture using deep-drawn stamping exclusively (i.e. no machining), such manufacture is not possible. Specifically if the strap  14  were formed by piercing the piece, the strap could not thereafter be pulled away from, or arced away from, the remainder of the piece because insufficient metal is left in the strap to withstand such forces. 
     SUMMARY OF THE INVENTION 
     The aforementioned problems are overcome in the present invention wherein an elongated, hollow piece having one or more straps over the normally closed end is produced solely using deep-drawn stamping. The process includes the steps of drawing a metal blank into a cup, piercing the bottom of the cup to create a strap in place of the bottom, and reducing the width of the cup without reducing the length of the strap so that the strap arcs away from the remainder of the piece. In the preferred embodiment, the initial forming step is a series of draws and/or redraws to create the cup. And, after the bottom is pierced, the item is reduced in width through a series of push reductions. 
     Each of the deep-drawn steps is individually known to those skilled in the deep-drawn stamping art. However, the process of the present invention is novel sequence of the steps, enabling the deep-drawn manufacture of a wide variety of parts nor previously possible. 
     The present invention eliminates the need to machine the pieces following stamping. Consequently, the pieces are produced with less labor and therefore are less expensive. Further, the quality and consistency of pieces is improved. 
     These and other objects, advantages, and features of the invention will be more readily understood and appreciated by reference to the detailed description of the preferred embodiment and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a spark plug ground shield manufactured in accordance with the process of the present invention; 
     FIG. 2 is a perspective view of the ground shield; 
     FIG. 3 is a perspective view of a spark plug incorporating the ground shield; 
     FIGS. 4-15 are sectional views (even-numbered Figs.) and perspective views (odd-numbered Figs.) of the piece at various stages of the manufacturing process through piercing; 
     FIGS. 16-18 illustrate alternative piercing patterns; and 
     FIGS. 19-32 are sectional views (odd-numbered Figs.) and perspective views (even-numbered Figs.) of the piece at various stages of the manufacturing process following piercing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A spark plug ground shield fabricated in accordance with the process of the present invention is illustrated in FIGS. 1 and 2 and generally designated  10 . The ground shield  10  is an elongated, hollow, bullet-shaped object. The shield  10  includes a cylindrical side wall  12  of uniform diameter throughout its height and a strap  14  extending diametrically across one end of the side wall  12 . The upper or open end  16  of the side wall  12  is flared outwardly. The strap  14  arcs away from the side wall  12 . 
     As illustrated in FIG. 3, the ground shield  10  is ultimately incorporated into a spark plug  20 . The construction of the spark plug  20  is generally well known to those having ordinary skill in the spark plug art and therefore will not be described in this specification. 
     The ground shield  10  of the present invention is fabricated using only deep-drawn stamping techniques. FIGS. 4-32 show the configuration of the shield  10  (and several alternative embodiments) at each stage of the stamping process. The implementation of the dies and fixtures to implement each stage are known to those skilled in the deep-drawn stamping art and therefore are not illustrated or described. 
     The first step in forming the shield  10  is a blanking step to create a circular planar disk  10   a  of material (FIGS.  4 - 5 ). The material is a non-resiliently deformable material such as 0.042 Inconel 600. Other materials now or later known to be suitable for stamping may be substituted. For the disclosed spark plug ground shield, the material should also be electrically conductive. However, the invention has applications well beyond the spark plug environment, and electrical conductivity may or may not be desirable in any particular application. 
     The second step is to draw the blank  10   a  into a cup  10   b  (FIGS.  6 - 7 ). This step initially forms a side wall  12  and a solid bottom  13 . A top or shoulder  15  extends radially outwardly from the open end of the cup  10   b.    
     The third step is to redraw the cup  10   b  into a deeper cup  10   c  (FIGS.  8 - 9 ). The side wall  12  of the cup  10   c  is longer or higher than in  10   b , and the bottom  13  of the cup  10   c  is narrower than the bottom in  10   b.    
     The fourth step is another redraw to produce the intermediate form  10   d  (FIGS.  10 - 11 ). Again, the side wall  12  has become higher; and the bottom  13  has become narrower. 
     The fifth step is a re-strike to produce the intermediate form  10   e  (FIGS.  12 - 13 ). The re-strike flattens both the bottom  13  and the top  15  to assist in meeting dimensional tolerances. 
     In the sixth step, the bottom is pierced to form the strap  14  on the intermediate form  10   f  (FIGS.  14 - 15 ). “Piercing” is a term of art in the deep-drawn stamping field that refers to the removal of material. Piercing should therefore be broadly interpreted to mean any process which results in the removal of material. Material has been removed from either side of the resulting strap  14  of the piece  10   f . The strap  14  extends diametrically across the side wall  12 . 
     Alternative strap shapes, configurations, and attachment locations are possible and will depend in part on the desired application for a manufactured piece. In fact, the variations are limitless. For example, other possible strap configurations  10   x ,  10   y , and  10   z  for spark plug ground shields are illustrated in FIGS. 16-18, respectively. Each of the forms  10   x ,  10   y , and  10   z  includes a unique strap configuration  14   x ,  14   y , and  14   z . All of the straps  14  have the commonality of formation by removing material from the bottom  13  of the form  10   e  (FIGS.  12 - 13 ). 
     The seventh step is a trimming step wherein the top  15  of the form  10   f  (FIGS. 14-15) is removed to produce the form  10   g  (FIGS.  19 - 20 ). The side wall  12  and the strap  14  of form  10   g  are substantially unchanged from form  10   f.    
     In the eighth step, the upper open end  16  is coned inwardly to produce the form  10   h  (FIGS.  21 - 22 ). The coned-in end  16  defines a mouth  17  that is concentric with the side wall  12  and smaller in diameter than the side wall  12 . The coning in step prepares the form  10   h  for the push reduction of the following step. 
     The ninth step is a push reduction resulting in the form  10   i  (FIGS.  23 - 24 ). The diameter of the form  10   i  is less than that of the preceding form  10   h , and the side wall  12  is higher than that of the preceding form. The mouth  17  is approximately the same diameter as the previous step. The length of the strap  14  in the form  10   i  is substantially identical to that in form  10   h . The strap  14  is pushed or bowed outwardly from the form  10 , which results in the strap arcing away from the side wall  12 . 
     The tenth step is a second push reduction resulting in form  10   j  (FIGS.  25 - 26 ). The diameter of the form  10   j  is once again less than the diameter of the preceding form  10   i , and the side wall  12  of the form  10   j  is higher than that of form  10   i . Again, the length of the strap  14  remains substantially identical to its length in the preceding forms, resulting in a more pronounced arc or extension away from the remainder of the form  10   j . The mouth  17  in the form  10   j  is substantially identical in diameter to that of the side wall  12 . 
     In the eleventh step, the upper end  16  is again coned in to create the form  10   k  (FIGS.  27 - 28 ). This step prepares the piece for another push reduction. 
     The twelfth step is a push reduction resulting in the form  10   l  (FIGS.  29 - 30 ). Once again, the diameter of the form  10   l  is less than the diameter of the preceding form  10   k , and the side wall  12  is higher than in form  10   k . Also again, the length of the strap  14  remains substantially unchanged so that it once again arcs further away from the remainder of the form  10   l.    
     The thirteenth step is a push reduction resulting in the form  10   m  (FIGS. 31-32) wherein the diameter of the side wall  12  is reduced and the height of the side wall  12  is increased. Additionally, the upper end  16  of the side wall  12  is flared outwardly in this step. The length of the strap  14  once again remains substantially unchanged, resulting in a more pronounced arc to the strap. 
     The fourteenth and final step is a re-strike resulting in the finished article  10  illustrated in FIGS. 1-2. The re-strike further flares the upper end  16  of the shield  10 . The restrike also gives the strap  14  its final shape or configuration, which is flatter with more pronounced comers than the previous form  10   m.    
     The shield  10  is fabricated using only conventional deep-drawn stamping steps. However, the steps are performed in a unique order resulting in the novel process. The invention can be used to create virtually any elongated hollow object having openings at a normally closed end. The present invention results in improved products at a lower cost. 
     The above description is that of a preferred embodiment of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the Doctrine of Equivalents.