Shot welding
Spot welding is a method used to join two pieces of metal together. This is accomplished by clamping the two pieces together and then passing a large electric current through them for a short period of time. Assuming the right amount of current for the right time, this will weld the two pieces of metal together. Spot welding was invented by Earl J. Ragsdale, a mechanical engineer at the Budd Company, in 1932 for the purposes of welding stainless steel. This welding method was used to construct the Pioneer Zephyr.
The method
The E. G. Budd Company, of Philadelphia recognized the important metallurgical characteristics of stainless steel (18-8) and developed a spot welding machine to take advantage over the oxidized layer of stainless steel. Heat treating the 18-8 stainless steel leaves a metal with non-magnetic and ductile properties. Reheating the metal to 1900 - 2000 degrees Fahrenheit repeatedly impairs the characteristics of the metal. The metal becomes susceptible to corrosion, carbide migration and less fatigue resistance. The important factor to control the metals properties is the time at those temperatures. Using a controlled time element and recorder, a power supply with smooth current and high brief high currents a satisfactory spot weld may be produced.
The corona of the spot weld essentially should not exist on the metal and the equipment used produce satisfactory welds with a smaller than normal (three times the spot diameter) diameter. Sufficient pressure is applied to hold the two sheets of metal together and the peak current rapidly melts the interface and produces a small nugget of weld metal, when cooled, produces a shear resistant metal interface. Good shotwelds have twice the shear strength of a rivet of similar diameter and can be placed 50 percent closer together to each other. Distortion is eliminated, a problem that plagues the gas welding processes.
References
- Ragsdale, Earl J. W., (August 20 1932), Patent number 1,944,106 - Method and product of electric welding (PDF). Retrieved February 27 2005.
- “Shot Welding - A Forward Step in Resistance Welding”, The Welding Engineer, 17 (8), August 1932, p25-26