Method of forming expanded metal grids particularly lead grids for storage battery plates
BACKGGROUND OF THE INVENTION 
     It has previously been proposed, for example in U.S. Pat. No. 3,853,626 to Daniels, Jr. et al, that lead battery grids be made by an expanded metal technique wherein a plurality of opposed pairs of battery grids are produced from a narrow sheet or strip of lead utilizing a modified, in-line, dual-expansion process similar to that employed in making expanded metal lathe or fencing. Each grid produced would comprise an unexpanded, current collecting top header portion and an expanded reticulated portion depending from the header. The two header portions would each be formed from the central portion of the original narrow sheet or strip of lead and such central portion would not be subjected to any expansion operations, but must, of course, necessarily be subject to any stresses induced in the stretching or expanding operation performed on the slitted portions which extend from a line adjacent the unexpanded central header portion to each edge of the strip. 
     As stated in the aforementioned patent No. 3,853,626, the expansion of the slitted portions of the lead strip has the tendency to curl the strip about an axis transverse to the path of movement of the continuous strip through the expansion operation. This tendency to curl results in what is known in the art as the &#34;gull-wing&#34; effect and has heretofore required that special rolls or other forms of straightening apparatus be added to the expanding machine to accomplish the reformation of the expanded metal strip into a uniform plane, rendering the strip suitable for entering continuous pasting machines and other subsequent operations. 
     OBJECTS OF THE INVENTION 
     It is an object of this invention to substantially eliminate the &#34;gull-wing&#34; effect which has heretofore been inherent in the lateral expansion of a slitted metal portion relative to an unsitted portion of a continuous metal strip. More particularly, this invention provides a stress absorbing row of punched out or stamped apertures intermediate the slitted and unslitted portions of the metal strip and such stamped or punched portions are effective in greatly minimizing the transmission of curling forces to the unslit central or edge portions of the strip, thereby minimizing the &#34;gull-wing&#34; effect. 
     Further objects and advantages of this invention will become apparent to those skilled in the art through the following detailed description taken in conjunction with the annexed sheets of drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a perspective view of a finished strip of expanded metal, suitable for fabrication into battery grids, which has been constructed in accordance with this invention. 
     FIG. 2 is a schematic perspective view of the metal strip to be expanded, showing it in the condition after the initial forming operation. 
     FIG. 3 is a schematic perspective view similar to FIG. 2 of a modified form of metal strip fabricated in accordance with this invention. 
     FIG. 4 is a schematic perspective view of the metal strip of FIG. 3 after the expansion is completed. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is illustrated a continuous strip 10 of expanded metal suitable for fabrication into individual battery grids by a subsequent shearing operation separating the strip along the dotted lines indicated in FIG. 1. It will be seen that the strip 10 comprises a solid central metal portion 11 from which the header portions 11a of two adjacent battery grids are later fabricated as indicated by the dotted lines. The header portion 11 is integrally connected with a plurality of diamond-shaped, interconnected grid wires 12 which in turn are integrally connected at their outer edges with a substantially straight line boundary strip 13. At each of the interstices of the grid wires 12, there is preferbly provided an upstanding mass of metal 13a, hereinafer called a nodule which, as is well known to those skilled in the art, performs an important function after the coating of the grid with an appropriate composition to convert the grid into an active positive or negative plate for a storage battery. 
     Referring now to FIG. 2, the strip 10 of FIG. 1 is continuously fabricated by two successive operations on a solid endless strip of metal. In the case of battery grids the metal would, of course, be a lead alloy. 
     In the initial fabrication step, which can be accomplished by impact actuated stamping dies, the solid strip 10a is provided with a plurality of axially and laterally spaced slits 14 which effectively divide both laterally outer portions of the strip 10 into the grid arms 12 after a subsequent expansion operation. Concurrently with the formation of slits 14, the nodules 13a are formed. If desired, the grid wires 12 may have their cross-section formed in any desired pattern, for example they may be diamond-shaped in vertical cross-section rather than the rectangular cross-section indicated in the drawings. Additionally, the formation of the nodule 13a at this stage in the manufacture insures that there is a sufficient mass of metal at the juncture of each of the grid wires 12 to guard against inadvertent splitting of the grid wires at such junctures in the subsequent expanding operation. 
     The important feature of this invention is the complete formation in the stamping operation of the first set of wires 15 which are disposed immediately adjacent to the central solid strip 11. Wires 15 will hereinafter be referred to as the stress relieving elements of the finished grid configuration. They may be shaped to generally conform to the pattern of the expanded grid wires, as shown in FIG. 1 or, one shown in the modification of this invention illustrated in FIG. 3, they may comprise relatively longitudinally extending grid wire elements 25 separated from the central solid portion 11 of strip 20 by a horizontally elongated slot 25a. 
     In either event, the next step in the conversion of the initial form of the grid 10 into the final, expanded form illustrated in FIG. 1, is the application of lateral expansion forces to both sides of the slitted strips of the modifications of FIGS. 2 and 3. Such application of lateral expanding forces is a technique well known in the expanded metal art and a variety of apparatus has heretofore been disclosed in such art for effecting such expansion, hence this step of the operation will not be described in any further detail. In any event, the initial slitted strips 10 and 20 of the modifications of FIGS. 2 and 3 are expanded to assume the configurations respectively illustrated in FIGS. 1 and 4. 
     It is well known that the lateral expansion of a slitted strip produces forces on the central strip 11 or 21 and the lateral edge strips 13 which tend to make the continuous strip curl about a transverse axis. This phenomena is referred to in the art as the &#34;gull wing&#34; effect. When the slitted strip is preliminarily formed in the manner illustrated in FIGS. 2 and 3 with stress relieving wires stamped out of the strip, the tendency of the expanded strip to curl is significantly diminished, in fact, to the extent that no subsequent rolling operation is required to restore the continuous strip to a sufficiently flat condition as to permit its further processing. Instead of the longitudinal forces exerted by the grid wires 13 during the expanding operation being exerted on the solid central strip 11 in FIG. 2, or strip 21 in FIG. 3, such forces are primarily exerted on the pre-formed grid wires 15 or 25 and effectively absorbed by such preformed grid wires. 
     If desired, pre-formed grid wires could also be provided adjacent the lateral edge strips 13 and such would function in the same manner to further reduce the tendency of the total strip to curl about a transverse axis. 
     Modifications of this invention will be readily apparent to those skilled in the art and it is intended that the scope of the invention be determined solely by the appended claims.