18 In
welding long seams, there may be a tendency for the welding current or voltage
to change slowly. This happens
because the shape and electrical characteristics of the circuit change as the
weld progresses. A more
uniform weld frequently can be obtained by connecting the “ground” to
both ends of the object being welded.
If only one “ground” connection is used, welding should proceed in a
direction away from it. When
welding the longitudinal seams of lightweight cylinders in a jig against copper
backing, it is frequently of
advantage to connect the “ground” to the starting end of the backing
bar. Another method is to use a number
of “ground” connectors equally spaced along the copper bar. This procedure
is particularly appropriate
when direct current is used but may also be necessary with alternating current.
The cables carrying current to the welding
head and to the “ground” connection on the workpiece should be
kept as close together as possible.
Both cables should be kept free of coils and, particularly when
alternating current is used, should
not be hung on or wrapped around metal objects. Irregularly
shaped and peaked welds can result from magnetic arc blow. Weld porosity can also
occur if arc blow
becomes severe. The precautions about grounding mentioned above will minimize
this problem. However,
when currents exceed 1000 amperes or at lower currents when welding inside of
pipes, multipass welds
in heavy plate, around large steel fixtures arc blow may still occur. For these
applications, AC welding power
should be employed and arc blow related defects should be eliminated.
The Welding Current Circuit
