COMMON VARIABLES IN RESISTANCE SPOT WELDING
Wallace A. Stanley, "Resistance Welding, Designing, Tooling, and Applications" McGraw-Hill 1950
A minimum variation in welding current is of paramount importance in the production of satisfactory welds, inasmuch as it is one of the three basic factors that enable resistance welds to be made, i.e., current, pressure, and time.
Pressure is one of the most poorly regulated of the three variables, current, pressure, and time, but is vitally important in producing consistent spot welds.
Weld time is of critical importance, especially for light gauge steel, also for welding aluminum, copper, and other high-conductivity metals having very narrow or critical plastic temperatures.
Human Element (top)
The human element is the least consistent variable in spot welding because of the variations encountered in speed, attitude, diligence, capability, carefulness, and many other factors. Though skilled operators have been responsible for producing highly successful products, some of them have produced some of the most defective welds imaginable. If parts are carelessly placed in improper position on the machine, if welding is done at the wrong angle, if tips are allowed to mushroom, the product is certain to be rejected. On the other hand, if craftsmanship is used, quality can be controlled.
For this reason it is good for the tooling engineer to attempt to eliminate as many of these human variables as possible.
Though all the remaining variables in a welding setup might be perfect, the human element, because of poor attitude, can destroy perfection in the final result. On the other hand, with the other nine variables at less than maximum efficiency, the human element, with proper skill and attitude, can still produce excellent results.
Machine Characteristics (top)
One of the variables of importance is that group of machine characteristics such as power factor, welder impedance, attachments and fixtures, all of which determine the efficiency of the machine and any one of which could contribute to the possibility of variation in production.
Although little can be done to minimize the variables of the machine characteristics after proper installation, the power supply can be kept within a narrow plus or minus range.
Condition of the Weld Head (top)
The sixth general variable is the condition of the weld head, specifically its physical geometry. The actuation of electrodes, condition of bearings, secondary connections, alignment and rigidity of the machine, the air cylinders, rams, dies, etc., are possible variables which should be minimized from the start by good construction and by vigilant maintenance.
Condition of the Electrodes (top)
This seventh variable, which can be most readily detected and which most readily occurs is the condition of electrodes. Particular attention should be paid at all times to assure that electrodes are dressed to original geometry as possible, and that they are properly aligned and maintained. Replace when worn or damaged so that they are always at the correct height and of the proper size for the particular application.
Condition of Material and Surfaces of Material (top)
Far too commonly overlooked, is the condition of the metals and the surfaces of the parts to be welded. Such surface conditions as accumulated rust, scale, slag, burrs, oxides, etc., are commonly left uncorrected before welding.
Satisfactory production of resistance welds on metal parts can best be achieved only after such factors as surface condition, coatings, etc., have been carefully taken into account in establishing of machine settings.
Throat Depth and Throat Height (top)
The ninth and tenth variables, throat depth and throat height, present two of the most easily correctable conditions. The variables of height and depth directly affect the other factors of current, pressure, time, human element, and machine characteristics. Usually if the throat depth or height is greater than actually needed more current is required and more care must be exercised in pressure application to prevent undue electrode deflection and misalignment.
It is wise at all times to keep both the secondary throat depth and height to an absolute minimum.
Wallace A. Stanley, Resistance Welding, Designing, Tooling, and Applications McGraw-Hill 1950