Miller Auto-Continuum 350, Owner'S Manual

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OM-273473 Page 33
6-7. Arc Blow
Arc blow is the deflection of the welding arc from its normal path due to magnetic forces. This condition is usually encountered in direct current welding of
magnetic materials, such as iron and nickel. Arc blow can happen in alternating current welding under certain conditions, but these cases are rare and
the intensity of the arc blow is always less severe. Direct current flowing through the electrode and base metal will set up a magnetic field around the
electrode. This magnetic field tends to deflect the arc to the side at times, but usually the arc deflects either forward or backwards along the joint.
Back blow is encountered when welding toward the work cable connection on a workpiece near the end of a joint or into a corner. Forward blow is
encountered when welding away from the work cable connection on a workpiece at the start of a joint. In general, arc blow is the result of two basic
conditions:
1. The change of current flow direction as it enters the work and is conducted toward the work cable.
2. The asymmetric arrangement of magnetic material around the arc, a condition that normally exists when welding is performed near the end of
ferromagnetic materials.
Although arc blow cannot always be completely eliminated, it can be controlled or reduced to an acceptable level through knowledge of the two condi-
tions listed above.
Except in cases where arc blow is unusually severe, certain steps can be taken to eliminate or reduces its severity. Some or all of the following steps
may be necessary:
* Place work cable connection as far as possible from joints to be welded.
* If back blow is the problem, place the work cable connection at the start of the joint to be welded and weld toward a heavy tack weld.
* If forward blow is the problem, place the work cable connection at the end of the joint to be welded.
* Position electrode angle so that arc force counteracts arc blow.
* Use the shortest possible arc that maintains good welding practices (this helps arc force to counteract arc blow).
* Reduce welding current if possible.
* Weld toward a heavy tack weld or runoff tab.
* Use the back step sequence of welding.
* Change to an AC welding operation which may require a change in electrode classification.
* Wrap the work cable around the workpiece in the direction that sets up a magnetic field to counteract the magnetic field causing the arc blow.
Notes