This invention generally relates to a method of welding a cage and nut assembly to a metal article surface. More specifically, this invention relates to a method of welding a cage having a retained nut to a sheet metal surface by placing the cage (containing the nut) upside down in a recess in a welding electrode and locating the sheet metal and an opposing electrode over the inverted cage for welding.
Cage and nut assemblies are commonly attached to metal components and used for bolting or otherwise joining the component to another part in an assembly. The cage with its enclosed nut is welded to, e.g., a first sheet metal component over a bolt hole in the member. Then a second component is connected to the first component with a bolt through the hole into the caged nut. Caged nut bearing components are commonly used in manufacturing applications such as in the assembly of automotive body structures.
Projection welding is a common practice for joining a cage nut to a sheet metal or other workpiece. Integral projections from the cage provide fusible metal for the weld. One or more sets of a relatively small cage and nut and the workpiece with its connector hole(s) must be suitably positioned and assembled for welding. In the present method a copper backup electrode carrying a locator pin is placed against the back side of the sheet metal with the locator pin protruding upwardly through the hole. A cage and loosely enclosed nut are placed over the locator pin with the weld projections extending from the cage resting on the sheet around the hole. A second electrode with a flat welding tip presses against the upper surface of the cage. The two electrodes are co-axially aligned and press in opposition to each other. The assembly is now ready for welding.
A controlled electrical power source delivers a pulse of high amperage AC (or rectified AC) current through the facing electrodes, the workpiece, and then to the interposed cage with its enclosed nut. Typically, a 60 Hertz welding current is applied for several cycles of electrical current application, amounting to a fraction of a second of welding time. Although the current passes through the top and sides of the cage and the region of the workpiece around the hole, the higher current density through the projections tends to selectively fuse them. When the current flow is stopped, the fused projection metal re-solidifies to weld the cage nut to the workpiece surface. The nut, while contained within the cage, does not generally participate in the welding operation. The top welding electrode and the backup electrode with its locator pin separate axially from the welded cage nut assembly and are moved to the next assembly or weld location.
In a welded cage nut to sheet metal assembly as described, the nut is held loosely with its threaded hole over the larger connector hole in the sheet metal. The nut can move laterally over the connector hole within the cage to receive a bolt or like connector during attachment of the sheet metal workpiece to another part. Depending upon how the bolted connection is to be made, the nut may be restrained from rotation by the cage structure that encloses it. Thus the nut and cage may take different but complementary shapes depending upon how they are to be used on the workpiece to which they are attached.
The above described method for assembling a cage and nut over a locator pin through the connector hole has been awkward and inefficient resulting in misaligned cages, and has sometimes produced poor welds between improperly aligned cage projections and the intended workpiece. Thus, it is an object of the present invention to provide a more efficient method of locating a cage nut assembly against a workpiece surface for a more reliable weld operation. It is a further object of the present invention to provide a method of increasing weld quality between a cage nut assembly and a workpiece by using an improved orientation of the parts of the assembly to be welded and an improved design of a welding electrode to assist in more reliably locating a cage and nut combination against a workpiece for welding.
In accordance with a preferred embodiment of the present invention, a combination of welding electrode designs is employed that permits assembly of the cage, nut and workpiece in an inverted relationship as compared to the above described prior art method. Thus, the present method contemplates that the assembly and welding operations will be carried out with upper and lower opposing welding electrodes operating along a vertical axis. Typically, the electrodes will be made of copper or suitable copper alloy for welding.
The lower electrode has an upwardly facing tip with a specially designed face. The face is generally flat with a recess for receiving a cage member that is inverted. That is, the open side of the cage, that is intended to face the workpiece, faces upwardly. The nut lies within the cage. The size and shape of the recess in the electrode is such that the cage and nut can be placed in it for suitable location of the upwardly extending cage projections of weld material. The workpiece is carried in a suitable locating fixture and positioned with its connector hole (if it has one) next to the nut. The-upper electrode, which may be of conventional flat tip design, is moved axially against the back side of the workpiece (now an upper surface of the assembly). The electrodes press towards each other to urge the welding projections on the cage against the workpiece. The nut is now located below the connector hole in the workpiece and a welding current of suitable duration is passed between the electrodes through the sides of the cage, its projections and the above adjacent surface of the workpiece.
Placing the cage and nut upside down in the recessed electrode suitably locates the cage on the axis of the welding operation. The shape of the recess is complementary with the upper surface of the cage so that its welding projections are properly positioned for engagement with the workpiece surface. The workpiece will ordinarily be separately supported and, at some point in the assembling of the parts for welding, moved along the axis of the welding electrodes against the cage projections for the welding operation.
After welding, the cage with its retained nut is, of course, attached to the workpiece. The electrodes are separated axially and the workpiece moved from the welding setup by its locating and support means.
Industrial processes that employ cage nuts for the assembly of parts rely on efficient and inexpensive assembling and welding methods and, most importantly, high weld quality. Weld quality is significantly increased by establishing and maintaining alignment of the cage with its retained nut and the workpiece for the welding process. The method of the present invention permits efficient and robust positioning of the cage, nut and workpiece.
These and other objects and advantages of this invention will become apparent from a detailed description of a preferred embodiment that follows.