Resistance Welding

The welder arm we are designing is a spot welder. Spot welders use the process of resistance welding to fuse together two pieces of metal. It performs this task by using electrical current, force, calculated amount of time, and a cooling process. This fusing of metal is made possible because metallic objects have electrical and thermal properties.

Mathematically this is expressed by the equation: E = I x R x T
where E = energy, I = current, R = resistance, and T = time. To increase the amount of heat energy you need, you would simply increase the ampacity and time. To decrease the amount of heat energy, you would do the opposite. Resistance welding is accomplished by passing a controlled density of electrical current (I) through the resistance of the metallic workpieces (R) over a specified amount of time (T).

Electrically, metalic objects have a specific level of resistance to the flow of electrical current.This resistance will cause heat energy (electric current) to pass through the work piece. Thermally, they have a melting point, specific heat content, and thermal conductivity. Knowing these properties allows the ability to calculate the amount of heat energy and force needed to create the fuse, as well as time and cooling the fuse needs to create a weld nugget.

The welding current is applied through copper electrodes under controlled force. The diameter of the electrode (the part that makes contact with the workpieces) will determine the density of the electrical current. The force applied when the electrodes make contact, affect the resistance across all interfacing layers including the weld nugget zone and the electrode to work piece interface areas. Force is adjusted to immediately create heat at the interfacing areas. If the force is not applied with the specified amout, excessive heat marks at the electrode-work piece interface can occur which can cause the metal around the fuse to become weak. For this reason, it is very important to have an effective cooling system to take heat away from the surface of the workpieces that make contact with the electrodes in a quick efficient manner.