Chapter 29 Shielded Metal Arc Welding 791
Copyright Goodheart-Willcox Co., Inc.
Welding Current and Polarity
Welding current is expressed in amperes (amps,
or A). The amperage required to weld is determined
by the electrode diameter, the thickness of the pieces
to be welded, and the position of the welding.
Amperage is the most important setting on a SMAW
welding machine. The amount of amperage controls
the heat input at the weld joint, which affects the
weld penetration, the deposition of fi ller material, and
the conditions of the weld pool. Figure 29-21. Most
welding machines designed for SMAW automatically
control the voltage. The voltage in a welding circuit
determines the ability of the arc to form between the
electrode and the base metal.
Refer to the welding machine manufacturer’s
recommendations when setting the amperage.
Manufacturers of welding equipment supply these
charts in several different ways. Most often, amperage
charts are attached to the power source for easy
reference. They are also available in printed form and
on company websites. Free smartphone apps can be
used to determine the correct amperage setting. Search
for welding apps using the key word welding.
Amperage charts provide a range of suggested
amperages for a given set of conditions. Set the
amperage on the machine within the recommended
range. You can weld on scrap pieces to fi ne-tune the
amperage setting. Each machine is a little different, so
making minor adjustments when switching machines
is common. Generally, a smaller electrode and lower
amperage is needed to weld a small piece than would
be needed for a larger piece of the same thickness. Thin
metals require less current than thick metals, and a
small electrode requires less amperage than a large one.
Many SMAW machines can be switched to
different welding polarity settings. On some machines,
polarity is switched by simply turning a dial or
pushing a button. See Figure 29-22. On other machines,
the welding leads must be moved to different terminals
to change polarity.
Polarity is the direction of fl ow of electric current.
Electric circuits have negative and positive poles. Direct
current (DC) fl ows in only one direction, resulting in
static polarity. Alternating current (AC) fl ows in one
direction half the time and in the opposite direction
the other half the time. AC current changes polarity
120 times per second with the 60-hertz current supplied
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Figure 29-21. Amperage is the most important
setting on a SMAW welding machine. Adjustments
vary greatly on different machines.
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Figure 29-22. The smoothest welding characteristics
are obtained by using the correct polarity for the
chosen electrode.