Rotary stitch for joining sheet metal stock
A method for securing two pieces of sheet material is provided using a stitch preferably made by sequentially passing the grid member through two die sets. The first die set lances a strap out of the two sheets of metal such that, at the approximate center or midsection of the strap, the first layer of material is displaced completely out of the plane defined by the web, while the second layer is only partially displaced out of the plane of the web so that a portion of the second layer remains in the space vacated by the first layer of the strap. The lanced strap is then passed through a second die set, in which the first and second layers of the strap are deformed or “swaged” such that the first layer is flattened and expanded in width beyond the edges of the hole in the web created when the strap was lanced. This provides interference such that the first layer of the strap cannot pass back through the hole. Additionally, the strap in the second web is deformed and displaced into the first web to bond the two together.
1. A method for securing two adjacent layers of a sheet material to each other comprising:
providing a first layer of metal having first and second faces;
providing a second layer of metal having third and fourth faces;
bringing the second face of the first layer into face-to-face relationship with the third face of the second layer;
lancing a generally elongated strap out of both the first and second layers such that the straps have ends that remain attached to their respective layers and sides that are separated from their respective layers, the straps having midsections that are displaced such that both the first and second faces of the midsection of the strap in the first layer are displaced past the first face of the first layer, while the third face of the midsection of the strap in the second layer is displaced past the first face of the first layer and the fourth face of the midsection of the strap in the second layer is displaced between the first and second faces of the first layer;
swaging the lanced strap such that the midsection of the strap in the first layer is enlarged in width and the midsection of the strap in the second layer is deformed into the first layer.
2. The method of claim 1 wherein swaging the lanced strap enlarges in width a portion of the strap in the second layer.
3. The method of claim 1 wherein the lancing and swaging steps are performed in a continuous operation by a series of rotary die pairs.
4. The method claim 3 wherein one die in each pair of rotary dies comprises a plurality of male elements having side faces, the side faces in the die for performing the lancing step being generally parallel and the side faces in the die for performing the swaging step forming an acute angle.
5. A grid tee for a suspended ceiling comprising sheet metal folded to form a longitudinally extending body with a cross-section having a vertically extending double web and a pair of opposed flanges integral with the web, the web having first and second elements each formed by a layer of sheet metal, the layers being side by side and together forming the plane of the web, the web elements being locked together in abutting contact by stitches integrally formed therein and spaced from one another along substantially the length of the tee, the stitches being formed by first and second straps lanced out of each of the first and second web elements, respectively, the first strap being displaced out of the plane to one side of the web and the second strap being displaced partially out of the plane of the web to the one side of the web, the material of the first strap being deformed so that the first strap is larger than a hole left in the web upon lancing the straps and the second strap is deformed into the first strap.
6. The grid tee of claim 5 wherein the second strap has a portion enlarged in width.