The present invention relates to electric resistance welding and, in particular, to methods and apparatus for attaining precision weld depths in electric resistance welding operations.
In electric resistance welding heat is generated in electrically conductive workpieces by conducting a short electric pulse therethrough by means of electrodes situated on opposite sides of the workpieces. One of the electrodes is movable and bears against one of the workpieces under a given pressure to press that workpiece against the other workpiece. In response to being heated, the workpieces are softened to the point where the workpieces are collapsed toward each other (i.e., forged) under the pressure applied by the movable electrode.
It is conventional to apply a welding force to the movable electrode, e.g., by springs, cams or air pistons for example, to push the workpieces together. However, a problem which has long been present in resistance welders involves the occurrence of so-called "spitting" wherein the weld tends to blow apart during the welding operation. Spitting occurs when the material of the workpieces becomes heated above a threshold temperature in the molten state, which overheating is produced by the electric pulse passing through the workpieces. Since the heat build-up generated by the electrical current flow is a function of the magnitude of the electrical resistance of the workpieces, it is desirable to minimize that resistance by imposing a high welding force upon the movable electrode. That is, it is known that the electrical resistance characteristic of the workpieces varies inversely with the force with which the workpieces are pressed together during forging, i.e., the greater the force, the smaller the electrical resistance.
However, even if such measures serve to reduce the electrical resistance to a level which ensures that no spitting occurs due to high electrical resistance of the workpieces, spitting can still occur once the forging action terminates if the electrical pulse continues. Spitting occurs in such a case because once the forging stops, the workpiece is being heated without the heat being dissipated since the collapsing of the workpieces has terminated. The early termination of the forging action results from the rapid rate of forging produced by the high welding force which, as noted above, was employed to minimize the electrical resistance. Therefore, a reduction in welding force to reduce the rate of forging (and hence avoid premature stoppage of the forging action), will only serve to increase the electrical resistance of the workpieces and thereby promote spitting during the forging step. Also, a reduced welding force may not produce a proper collapsing of the workpieces.
One suggestion heretofore made for dealing with the spitting problem involved the use of a rotary driven cam for pressing against the movable electrode. The cam would be arranged to contact and actuate switches for the purpose of initiating the electrical pulse synchronously with the cam rotation, and terminating the electrical pulse immediately prior to the end of cam rotation (i.e., immediately prior to the end of the forging action). However, such a device never achieved commercial success, apparently because the time delays inherent in the behavior of switches made it infeasible to correlate the stoppage of the electrical pulse with the movement of the cam with the requisite high degree of accuracy. Also, such a device did not compensate for dimensional variations of the workpieces. That is, if the cam is positioned to handle workpieces of a given thickness, the presence of a thinner workpiece might result in the cam applying forces to the workpiece only after the electric pulse has been initiated, thereby resulting in spitting. A workpiece thicker than the given thickness might be compressed by the cam prior to the initiation of the pulse, whereupon the workpiece could break.
Consequently, it has been necessary in the use of electrical resistance welders to either endure a limited amount of spitting, or to terminate the electrical pulse sufficiently prematurely to ensure that the pulse will stop before the forging terminates. Such an early cessation of the pulse will result in a rapid hardening of the workpieces, thereby stopping the forging action prematurely; however, such premature termination of the forging action will be acceptable if the range of dimensional tolerances for that particular product is large enough.
On the other hand, there exist certain products for which the range of tolerances is not sufficiently large, and in which no appreciable amount of spitting can be endured. For example, the manufacture of certain slip rings of the type depicted in FIG. 5 herein, involves the welding of iridium balls 1 of 0.010 inch dia. to the ends of a 0.010 inch dia. beryllium copper wire 2, it being required that the spacing d between the balls be precise, e.g., to .+-.0.002 inch tolerance. A further requirement is that the distance d cannot be achieved by bending of the wire after the welding is achieved, because the memory or creep of the wire would not permit that distance to be maintained to the required tolerance range. Efforts made by a number of makers of electrical resistance welders to meet those very narrow tolerances have failed.