1. Field of the Invention
The present invention relates generally to bench and press type resistance welding units and, more particularly, to press type resistance welding units having automated multiple press modes for added safety.
2. Description of the Prior Art
In bench and press type resistance welding a press operator generally supports a production part with his fingers while two electrodes are pressed together with a significant pressure, typically around 4500 psi, during the welding operation. Unfortunately, the operator's fingers sometimes get injured when a finger is accidentally entangled in the welding electrodes. The prior art has proposed a somewhat limited solution to this problem by moving the two electrodes together first with a lower force, then determining if there is an obstruction between the electrodes before higher pressure is applied.
One such system is disclosed in U.S. Pat. No. 4,041,272, to Burton et al. This system determines if there is an obstruction by testing for current across the production part to be welded, then uses a complex analog logic circuit to manipulate a variety of control valves. This system has various drawbacks. First, the production parts being welded must not only be conductive, but must also have similar electrical characteristics to produce consistent results. Second, consistent electrical contact must be made with the part to be welded. Finally, the system of electronic analysis is susceptible to errors. Thus, there is a need in the art to provide a safer apparatus and method in bench and press type resistance welding machines which does not depend upon the specific characteristics of the parts to be welded, which does not require contact with the parts to be welded, and which can be easily adjusted to be used in a plethora of operations.
The prior art has proposed a variety of controls in order to selectively control the pressure applied to bench or press type resistance welding units. However, the prior art teachings with respect to the solution to this specific problem are somewhat limited in that the emphasis for adjustment of the pressure regulator was to obtain increased efficiency of the welding operation and to influence the quality of the welds to be made. For example, Beneteau, U.S. Pat. No. 4,135,076; Neff, U.S. Pat. No. 4,579,042; and McKendrick, U.S. Pat. No. 4,680,441, are all directed to selectively controlling the pressure regulator in order to effect the efficiency as well as effectiveness of the weld. U.S. Pat. No. 2,199,286 issued to Fischer is particularly directed to a safety control mechanism for the operator. Fischer teaches provision of a safety control mechanism on the stitching head of the machine which acts to automatically latch the staple forming and driving elements to prevent them from completing their staple forming and driving strokes when an obstacle in the path of a part of the mechanism is encountered, without discontinuing the operation of the staple forming and driving elements or the operation of the machine with which the stitching is associated. The safety control mechanism includes means for automatically releasing the elements when the operating device of the later acts to restore the elements to their normal position. Vieser et al, U.S. Pat. No. 3,412,451, teach a force limiting tool wherein force exerting tools are used for staking operations to effect an electrical termination. The tool inserts a post within an eyelet with a predetermined amount of force. The insertion is accomplished by driving the post into the eyelet and conversely driving the eyelet around the post. Vieser et al teach a tool which provides and exerts a predetermined amount of force regardless of the amount of tool travel required due to the varying lengths of the post as well as the variable diameters of the eyelets.
Burton et al, U.S. Pat. No. 4,041,272, teach a resistance welding apparatus affording the operator protection in order to avoid the operator's hand or fingers from being crushed between the welding electrodes. Burton is directed to the problems associated with resistance welding operations wherein a switch is placed at a predetermined position to detect whether an electrode meets an obstruction such as a finger during its low force approach. However, wearing of the electrode makes such a system ineffectiv and a condition can arise in which the power actuated electrode is more tha a finger thickness from the workpiece at the moment that it is switched to its high force operating mode. Burton teaches a resistance welding apparatus which determines if there is an obstruction by testing for curren across the production part to be welded, then uses a complex analog logic circuit to manipulate a variety of control valves. The apparatus according to the Burton reference has various drawbacks. First, the production parts being welded must not only be conductive but must also have similar electrical characteristics to produce consistent results or the parts will not be welded even though there is no obstruction. Second, consistent electrical contact must be made. Finally, the system of electronic analysis is susceptible to errors which may result in the operator being injured. Thus, there is a need in the prior art to provide a method of protecting an operator's hand in bench and press type resistance welding machines which is not dependent on any production part characteristics, which does not rely on contact with the physical part and which, of course. can be easily adjusted.