be a deciding factor. GMAW is often chosen for welding
Like GTA welding, gas metal-arc welding also uses
a shielding gas to protect the weld zone during welding.
The inert gas is fed through the gun into the weld zone
and prevents atmospheric contamination.
GMAW offers many of the advantages of GTAW.
Since there is no flux, GMA welds are clean and there
is no slag to remove. GMAW enables you to produce
sound welds in all positions quickly. Now lets take a
look at the equipment you will use for GMA welding.
GMA WELDING EQUIPMENT
Gas metal-arc welding equipment basically consists
of four units: the power supply, the wire feeding mecha-
nism, the welding gun (also referred to as the torch), and
the gas supply. Figure 8-24 shows atypical GMA weld-
Figure 8-25.Constant voltage (CV) power unit.
When you use a conventional type of welding ma-
chine for GMA welding, the voltage varies depending
on the length of the arc. Whenever the nozzle-to-work
distance changes, the arc length and the voltage changes.
The only way to produce uniform welds with this type
of power source is to maintain the arc length and voltage
at a constant value. Besides producing nonuniform
welds, this inconsistent voltage can cause the wire to
burn back to the nozzle.
A constant voltage (CV) power source was devel-
oped to overcome the inconsistent voltage charac-
teristics of a conventional welding machine, (See fig.
8-25). It can be either a dc rectifier or motor generator
that supplies current with normal limits of 200 to 250
The CV type power source has a nearly flat voltam-
pere characteristic. This means that the machine main-
tains the same voltage regardless of the amount of
current used. With this type of power source, you can
change the wire-feed speed over a considerable range
without causing the wire to burn back to the nozzle.
When the wire-feed speed is set at a specific rate,
a proportionate amount of current is automatically
drawn. In other words, the current selection is based on
the wire-feed speed. When the wire is fed faster, the
current increases; when it is fed slower, the current
decreases. With this type of power supply, variations in
the nozzle-to-work distance will not change the arc
length and burn back is virtually eliminated.
In gas metal-arc welding, direct-current reverse po-
larity (DCRP) is recommended. You should recall from
the previous section that DCRP produces excellent
cleaning action and allows for deeper penetration.
Wire Feed Drive Motor
The wire feed drive motor is used to automatically
drive the electrode wire from the wire spool through the
gun up to the arc point. You can vary the speed of the
wire feed by adjusting the controls on the wire-feed
control panel. The wire feeder can be mounted on the
power unit or it can be separate from the welding ma-
The function of the welding gun is to carry the
electrode wire, the welding current, and the shielding
gas to the arc area. The gun has a trigger switch that
controls the wire feed and arc as well as the shielding
gas. The welding operator directs the arc and controls
the weld with the welding gun. GMA welding guns are
available in many different styles, some of which are