Urrea SOLM1020-BV Manual De Usuario página 22

5. Place the metal portion of the welding rod
inside the jaws of the Electrode Clamp. Welding
rod types vary for welding different metals.
NOTE: If too much current is drawn from the
welder; the Thermal Overload protector will
activate, the Overload indicator will light, and
the welder will turn off until it cools down. It
will automatically reset.
GAS METAL ARC
Gas metal arc welding (GMAW), sometimes re-
ferred to by its subtypes metal inert gas (MIG)
welding or metal active gas (MAG) welding, is
a semi-automatic or automatic arc welding pro-
cess in which a continuous and consumable wire
electrode and a shielding gas are fed through
a welding gun. A constant voltage, direct cur-
rent power source is most commonly used
with GMAW, but constant current systems, as
well as alternating current, can be used. There
are four primary methods of metal transfer in
GMAW, called globular, short-circuiting, spray,
and pulsed-spray, each of which has distinct
properties and corresponding advantages and
limitations.
Welding gun and wire feed unit
The typical GMAW welding gun has a number
of key parts—a control switch, a contact tip, a
power cable, a gas nozzle, an electrode conduit
and liner, and a gas hose. The control switch, or
trigger, when pressed by the operator, initiates
the wire feed, electric power, and the shielding
gas flow, causing an electric arc to be struck.
The contact tip, normally made of copper and
sometimes chemically treated to reduce spat-
ter, is connected to the welding power source
through the power cable and transmits the elec-
trical energy to the electrode while directing it
to the weld area. It must be firmly secured and
properly sized, since it must allow the passage
of the electrode while maintaining an electrical
contact.
Before arriving at the contact tip, the wire is pro-
tected and guided by the electrode conduit and
liner, which help prevent buckling and maintain
an uninterrupted wire feed. The gas nozzle is
used to evenly direct the shielding gas into the
welding zone—if the flow is inconsistent, it may
not provide adequate protection of the weld
area. Larger nozzles provide greater shielding
gas flow, which is useful for high current weld-
ing operations, in which the size of the molten
weld pool is increased. The gas is supplied to the
nozzle through a gas hose, which is connected
22
to the tanks of shielding gas. Sometimes, a water
hose is also built into the welding gun, cooling
the gun in high heat operations. The wire feed
unit supplies the electrode to the work, driving
it through the conduit and on to the contact tip.
Most models provide the wire at a constant feed
rate, but more advanced machines can vary the
feed rate in response to the arc length and volt-
age. Some wire feeders can reach feed rates as
high as 30.5 m/min (1200 in/min), but feed rates
for semiautomatic GMAW typically range from
2 to 10 m/min (75–400 in/min)
POWER SUPPLY
Most applications of gas metal arc welding use a
constant voltage power supply. As a result, any
change in arc length (which is directly related to
voltage) results in a large change in heat input
and current. A shorter arc length will cause a
much greater heat input, which will make the
wire electrode melt more quickly and thereby
restore the original arc length. This helps opera-
tors keep the arc length consistent even when
manually welding with hand-held welding
guns. To achieve a similar effect, sometimes a
constant current power source is used in combi-
nation with an arc voltage-controlled wire feed
unit. In this case, a change in arc length makes
the wire feed rate adjust in order to maintain
a relatively constant arc length. In rare circum-
stances, a constant current power source and a
constant wire feed rate unit might be coupled,
especially for the welding of metals with high
thermal conductivities, such as aluminum. This
grants the operator additional control over the
heat input into the weld, but requires significant
skill to perform successfully. Alternating current
is rarely used with GMAW; instead, direct cur-
rent is employed and the electrode is gener-
ally positively charged. Since the anode tends to
have a greater heat concentration, this result in
faster melting of the feed wire, which increases
weld penetration and welding speed. The polar-
ity can be reversed only when special emissive-
coated electrode wires are used, but since these
are not popular, a negatively charged electrode
is rarely employed
TECHNIQUE
The basic technique for GMAW is quite simple,
since the electrode is fed automatically through
the torch. By contrast, in gas tungsten arc weld-
ing, the welder must handle a welding torch in
one hand and a separate filler wire in the other,
and in shielded metal arc welding, the operator