This invention relates generally to the use of preweld resistance measurements to endure weld repeatability of a resistance weld and, more specifically, to a system for checking both welder resistance and part resistance prior to welding.
Resistance welding is a techique of joining metals in which the heat necessary for welding is generated by the electrical resistance to an electrical current at the joint of parts. For a typical resistance weld, an AC voltage is applied to the input of a step-down transformer and a welding current loop is formed at the transformer output terminals. This loop usually includes a current path from one side of a two-terminal transformer output to a moveable first electrode and a current path from the other terminal to a fixed or moveable second electrode. Two electrically conductive workpieces complete the current loop; the first workpiece being in electrical and physical contact on one surface with the first electrode and on an opposing surface with one surface of the second workpiece. Another surface of the second workpiece is in electrical and physical contact with the second electrode. A ram, or other means for applying pressure, presses the first electrode toward the second electrode, squeezing the two workpieces between the electrodes. Ideally, the maximum resistance in this current loop is at the physical connection between the two workpieces. In this event, the maximum heat loss in the loop occurs at the location to be welded. The welding heat is proportional to I.sup.2 RT, where I is the current through the loop, R is the resistance of the workpiece junction, and T is the length of time current is flowing.
Since flux need not be applied during the welding operation, and since the weld does not require a moving electrode to follow a weld line, resistance welding is well suited to mass production welding of workpieces shaped in a manner permitting them to be held in proper alignment by the squeezing force between the electrodes. Squeezing identical sets of parts with the same force, and passing the same current through them, should result in identical welds.
The two principal, controllable, variables in resistance welding are the electrical energy input to the weld and the electrode pressure. The electrical energy input is typically controlled by electronic means to adjust the portion of the input waveform that is applied to the transformer, thereby controlling the power to the transformer. Current regulators may be used to ensure that a constant welding current, voltage or power is applied to the output loop.
Welding pressure is important because of the nature of the contact between two metallic pieces. When two cold surfaces are pressed together, much of the cross-sectional area is in physical oxide-to-oxide contact. A small fraction of this physically contacting area comes into actual metallic contact due to the oxide film. The remainder of the joint consists of thin oxide-lined voids which provide no current path.
Since voids and cold metal oxides do not conduct electricity, only the scattered points of metallic contact carry current, causing high concentrations of current and associated rapid heat generation. The contact resistance of the interface is inversely proportional to pressure; i.e, the higher the pressure on the interface the more contact among metal points between the voids, and the lower the contact resistance.
The generation of heat at the interface, the natural result of high current conduction, usually lowers the joint resistance. The amount of the reduction is also a function of pressure.
Therefore, in order to get repeatable welds in a production environment, the operator must determine what current and pressure gives a desirable weld, and repeat it for each successive weld. Unfortunately, experience shows that changes in the welding machine and differences between sets of workpieces can produce noticeable changes in welds.