©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II
technology makes it possible to design
motor speed controls that will produce the same speed,
even though the load on the motor varies or the input voltage to the motor may
fluctuate. 2.4.5.1
A limited amount of gas metal arc welding
is performed with constant current type power
sources. In this case, the motor speed automatically varies to increase
or decrease the wire
feed speed as the arc length varies to maintain a constant voltage.
2.4.5.2 The
wire feeder also controls the main contactor in the power source for safety
reasons. This assures that the
welding wire will only be energized when the switch on the
welding gun is depressed.
2.4.5.3 The
flow of shielding gas is controlled by a solenoid valve (magnetic valve) in the
wire feeder to turn the shielding gas
on and off when the gun switch is actuated. Most feeders
utilize a dynamic breaking circuit
to quickly stop the motor at the end of a weld to prevent a
long length of wire protruding from
the gun when the weld is terminated. Most feeders have a
burn-back circuit that allows the welding
current to stay on for a short period of time after wire
feeding has stopped, to allow the wire
to burn back exactly the right amount for the next arc initiation.
2.4.5.4 The
feed rolls, sometimes called drive rolls, pull the wire off the spool or reel,
and push it through
a feed cable or conduit to the welding gun. These rolls must usually be
changed to accommodate each different
wire diameter, although some rolls are designed to feed
a combination of sizes. 2.4.6
Welding
Gun -
The function of the welding gun, sometimes referred to as a torch, is
to deliver the welding wire, welding
current, and shielding gas to the welding arc. Guns are
available for semi-automatic operation
and for automatic operation, where they are fixed in the
automatic welding head.
2.4.6.1 Guns
for GMAW have several characteristics in common. All have a copper alloy
shielding gas nozzle, that delivers
the gas to the arc area in a nonturbulent, angular pattern to
prevent aspiration of air. The
nozzle may be water cooled for semiautomatic welding at high
amperage and for automatic welding
where the arc time is of long duration. Welding current is
transferred to the welding wire as
the wire travels through the contact tip or contact tube
located inside the gas nozzle (Refer
to Figure 9). The hole in the contact tip through which the
wire passes is only a few thousandths
of an inch larger than the wire diameter. A worn contact
tip will result in an erratic arc due
to poor current transfer. Figure 15 shows a few different
semiautomatic gun configurations that
are commonly used for GMAW.
