It is a common manufacturing practice to secure nuts directly to sheet metal pieces through a welding process. In the manufacture of motor vehicles today, there are virtually hundreds of applications for such "projection weld nuts" which form attachments for air bag supports, door hinges, motor and transmission mounts, safety belts, and many other industrial applications.
Conventional resistance projection welding techniques employ a method by which metal workpieces are joined together at one or more predetermined points such as projections, embossments, or intersections. Workpieces are held together at the predetermined points under force by one or more electrodes. The contacting points are heated by a pulse of high amperage welding current generated by contact with an electrode. The resistance to the flow of welding current through the workpieces produces a weld at the faying surface of the joint. The projections concentrate the welding current and force at the weld area. The high current generates sufficient heat so that the metal surfaces reach a plastic state. The force applied before, during, and after the current forges the heated parts together so that coalescence occurs at the faying surface.
Projection weld nuts are commonly attached to sheet metal, usually over an opening in the sheet to permit a screw or other fastener to extend through and into the internal threads of the nut. One of the two surfaces of each projection weld nut typically has small projections. These projections are consistently measurable to obtain an exact dimension from projection weld nut to projection weld nut. The other surface of the projection weld nut is flat and smooth. The projection weld nut may have a pilot ring extending out from the same surface having the projections for the purpose of properly locating the projection weld nut relative to the sheet. The pilot ring is also consistently measurable to obtain an exact dimension from projection nut to projection nut.
For proper projection welding, the surface of the projection weld nut having the projections must engage the sheet. When an electrode conducts electric welding current through the projection weld nut and the workpiece, the electric current travelling through the flat surface of the projection weld nut is a low density current. The current at this interface is low density because it is able to travel through a large contact area defined by the entire flat surface of the projection weld nut. The low density current converts into a high density current through the projections. Since the projections are the only paths making electrical contact with the workpiece, the internal resistance to electric current flow is relatively much greater than the flat, smooth surface. The high internal resistance causes intense heat to develop through the projections. The heat melts the sheet causing the projections to penetrate the metal surface when force is applied. After the current terminates, the projection weld nut is welded to the workpieces as a result of coalescence.
A problem arises when the projection weld nut is loaded onto the workpiece in an upside down or an inverted position. This problem can occur when using a vibrational nut feeder for supplying the projection weld nuts. As explained above, the flat surface provides a relatively low resistance path because all of its surface area is in contact with the workpiece. Consequently, the smooth surface of the projection weld nut inadequately penetrates the workpiece due to the lack of heat generated. These weakly welded projection nuts can easily escape detection during the projection welding process.
A primary disadvantage associated with existing resistance projection weld nut welding systems is the fact that it is unknown when a projection weld nut has been welded upside down or laterally out of position with respect to an aperture on the workpiece. This disadvantage exposes itself when a failure occurs during an attempt to screw a bolt into the welded projection nut or subsequently when the nut is loaded. Many times the required torque applied to secure the bolt to the threads of an inverted welded projection nut breaks the weak weld and the projection nut spins off the workpiece. If the projection nut was welded out of position, it is impossible to extend the bolt through the aperture on the workpiece. Therefore, it is extremely desirable to not apply electric power to either an inverted or misaligned projection weld nut or to at least terminate the power before the projection nut has been heated sufficiently to weld. Early detection allows for the projection weld nut to be easily removed while the workpiece has little or no damage and can be reused.
A second problem in resistance welding is in controlling the weld process satisfactorily in order to consistently produce good welds. Many different factors must be controlled, such as voltage, current, pressure, heat loss, shunting, water temperature and electrode wear, as well as the thickness and composition of the workpiece material. Many of these variables are difficult to consistently control because of contaminants at the faying surfaces such as dirt, grease, oil or paper. Improper welding conditions can cause defective welds due to expulsion and low or over penetration of the weld nut projections. Expulsion occurs when an excessive application of power blows out the projections at the faying surfaces. Over penetration results from the weld current being too strong or applied for too much time. Too little power causes low penetration. Defectively welded projection weld nuts have distinguishing features from good welds such as being weaker, being misaligned, weld expulsion bonded to its internal threads rendering the projection nut non-usable, and other defects.
Several attempts have been made to automatically control resistance projection welding processes. For example, some techniques have been designed to regulate the amount of energy used during the weld cycle. To this end, current sensors and voltage regulators have been incorporated into feedback systems to compare the detected levels with certain preset reference values. These feedback systems are disadvantageous because they do not directly detect physical characteristics of the weld itself, but instead rely upon detection of secondary parameters. This can lead to poor weld quality when uncontrolled parameters vary from nominal operating conditions.
A product that has weak welds can fail to perform under extreme stress, fail at some point during the course of normal operation or can break after other parts have been welded to it. A welded product such as a car seat, automobile engine cradle, or an automobile frame can easily have dozens or even hundreds of separate welds. Often the projection weld nuts welded to a workpiece are fabricated as a box section such as an automobile motor/transmission mount. Obviously, it is very expensive and impractical to repair or replace a bad weld after a unit has been welded together and assembled in a vehicle. Usually the entire assembly is removed and scrapped.
Many products such as an air bag support will perform only once under extreme stress conditions during its lifetime. If the air bag is supported by defective welds, it may not perform its intended function. Finally, if the incorrectly welded projection nut has enough strength to allow the required torque at assembly to hold, it can fail later while in use due to structural vibrations or other stresses transferred to this weak joint.
A further obstacle in resistance welding occurs when an electrode becomes fused to a welding surface after completion of a weld. This condition is known as a "stuck gun condition". If the welding system does not detect the stuck gun before attempting to move the electrodes from a closed welding position to an opened position, extensive damage to the electrodes, weld gun, a work cell, and even human weld operators may occur.
Another consideration in resistance welding is to ensure that the electrodes apply welding current to the projection weld nut once and only once. The strength of the weld between a projection weld nut presently welded to a workpiece is substantially weakened with the application of subsequent electric welding power. The subsequent welding power causes the projections to overheat, and thus become brittle.