Page 55 - MetalForming Magazine June/July 2022 80th Anniversary Issue
P. 55

   Fabrication: Resistance Welding
As RW joints are forged during the process, take care to control the amount of force applied. Insufficient force results in expulsion (sparks), while exces- sive force reduces the interface resistance and can result in a cold weld.
Welding Current
Secondary welding amperage deliv- ered through the copper electrodes generates the heat required to form an RW joint. The typical welding machine delivers several thousand amps to the weld, which may seem high, but it is important to understand that the aver- age machine delivers those amps with only 6 to 10 secondary volts at the weld.
Low secondary voltage means that the machine will not shock the operator if properly insulated. However, it also means that oxidation and accumulated dirt in the copper-to-copper connec- tions from the transformer to the elec- trodes can reduce welding amperage.
To avoid internal arcing and reduced welding amps, regularly check all joints in the secondary circuit for tightness. Operators also should disassemble and clean these joints as part of a regular preventive-maintenance program, and
ensure that copper shunts, weld cables and electrodes remain in good condition.
Sequential Timing Functions
Modern RW controls provide accurate timing measured in cycles or milliseconds. However, we often see confusion regarding the meaning of the sequential timing functions called squeeze, weld, hold and off.
The unfortunately
named control setting
called “squeeze time” has
nothing to do with regulating the weld force applied. Instead, think of squeeze time as electrode-travel time. When properly set, squeeze time allows the
welding electrode to contact the part and achieve the desired force prior to initiation of the welding current.
Insufficient squeeze time results in
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