THEORY OF OPERATION
Spot Welding Manual for Medar 3-8 MOTOMAN
Shunting can occur when spot welds are made too close to each other. Electricity
follows parallel path and the path of least resistance. See Figure 3-7.
Figure 3-7 Shunting
Pressure and heat generated during the course of welding causes the electrodes to
wear. This wear negatively impacts the welding process and manifests itself in a
“mushrooming” fashion. Heat causes the electrode to become softer than normal,
and pressure provides the mechanical energy required to force the electrode
surface out of shape. This deformity results in an increased area at the tip-to-part
contact surface. See Figure 3-8.
Figure 3-8 Normal vs. Mushroomed Electrode Surface
The consequences of tip deformity can be severe. Because the worn cap has a
larger surface area, it has a lower current density. If both caps were to conduct the
same amount of current, the current would be spread out over a larger area on the
mushroomed cap. The result is that current (heat) is no longer concentrated
precisely where it is needed (at the nugget point); it is distributed and no longer
efficient. A possible counter-measure to this inefficiency is to conduct more heat
into the mushroomed electrode in order to maintain the same current density.
Also affected by mushrooming is pressure density, which affects the resistance of
the tip-to-part contact point. This happens because the force at the contact point is
distributed over a larger area. Resistance depends in part on the pressure of an
electrode against the part. Mushroomed caps result in less pressure at the weld
point.
Desired Current Path
Shunted Current Path
Existing Weld Nugget
NORMAL
MUSHROOMED
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