Spot Welding

Questions and Answers

Some resistance welding applications call out forces above the limits of tapered electrodes. The RWMA Manual Fig 18.3 shows force vs taper. It shows that above 2400 Lbs force tapers are not viable. Above this force specially designed High Pressure/High Force Electrodes must be used. These electrodes can be straight shanks, threaded or flanged electrodes. They are available from several manufacturers. Contact your electrode supplier for assistance.

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Yes, there is a minimum spacing for spot welds listed in most sample weld schedules. If welds are placed too close together, the weld current can flow through/across the sheet of metal between the desired weld and previous weld locations as shown. This current does not flow through the desired weld joint and therefore does not help create a nugget. This is called shunting current. It is wasted energy. In the figure below the first nugget is acceptable the second and third welds get smaller due to the shunting currents shown. The third weld is undersized in this example. The severity of this condition depends upon the material, parts, spacing and weld schedule.

Shunting Current 3

Too correct for this increase the spacing between welds. Highly conductive materials like aluminum need larger spacing’s than more resistive materials like steel.

Force is used for two main reasons. One is to contain the weld nugget and prevent expulsion and the other is to strengthen the weld by forging the solidifying weld nugget. It is easy to see that if welding force is changed these actions will be affected to some degree. Expulsion could increase or decrease and the resultant weld strength could also increase or decrease.

A weld schedule is an instruction of how to set up a resistance welder for a specific job. It includes all of the specific settings of the machine, control settings, tools, electrodes, forces, times, and rates and all other pertinent data related to a job. This information is stored and used to set the job up the next time you run it. It can be used as a ground zero if parameters have changed and you want to go back to where you started.

Group C material is a designation of the Resistance Welding Manufacturers Alliance (RWMA).  It describes a group of Specialty materials with high strength and high electrical conductivity. Their properties make them excellent materials to use in the resistance welding industry as electrodes especially for galvanized steel.

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