When adjusting the slope of the volt-ampere curve
on the power supply, always adjust it so that the parting
of the molten drop is smooth with a minimum of spatter.
Remember, the main function of the power supply is
to provide amperage to maintain the arc gap or voltage
selected for welding. For this reason the machine is termed
a constant voltage (CV) or constant potential power supply.
• Inductance (pinch effect) adjustment. When
using the short-arc mode in GMAW, the
separation of the molten drops of metal from the
electrode is controlled by the squeezing forces
exerted on the electrode due to the current
flowing through it. Figure 3-4 shows how the
pinch effect operates. This is called inductance
by some machine manufacturers and the pinch
effect by others.
By adjusting the level of pinch effect, the machine
controls the rate of current rise when the electrode
contacts the workpiece and the short circuit starts. If the
current flows rapidly through the electrode, the drop of
metal is squeezed off quickly and causes spatter. If
inductance is added to the circuit and the current is not
applied as rapidly, the number of short circuits per second
will decrease, and the “arc on” time will increase. This
will result in a more fluid weld pool, a smoother weld
crown, and less spatter.
In GMAW spray-arc mode, some inductance is
beneficial in starting the arc process. This limits
explosive starts by slowing down the rate of current rise
at the start of the cycle.
As a general rule, both the amount of short circuit
current and the amount of inductance needed for the ideal
pinch effect are increased as the electrode diameter is
increased.
When setting up the machine for and during welding
with the short-arc mode, the following guide may be
useful:
Maximum inductance (minimum pinch).
• More penetration.
• More fluid weld pool.
• Flatter weld.
• Smoother bead.
Minimum inductance (maximum pinch).
• More convex bead.
• Increased spatter.
• Colder arc.
Preprogrammed Welding Machine Controls
When using a power supply that has built-in
programs for various types and thicknesses of materials,
select the program desired and insert the required
procedure data. This will then automatically set the
correct machine values for the operation and monitor the
machine output during the welding operation. The
machine shown in Figure 3-5 is of this type. The face of
the machine has only push buttons for program selection
and modification of each value desired. Each readout of
the program is displayed on the digital meter. The factory
program may also be modified by manual override, if
desired. Note: Each machine will also have areas where
programs may be installed and modified by the operator
for individual welding procedures. Machines of this type
are usually factory programmed for the welding of steel,
stainless steel, and silicon bronze or aluminum.
Pulsers and Auxiliary Pulsers
Some machines have a pulser system in the main
power supply. Where the power supply does not contain
a pulser, an auxiliary machine may be added for pulsing
the welding operation. Units like the one shown in
Chapter 3 Equipment Set-up and Control 29
Steep curve
Amperage
Volts
Flat curve
Figure 3-3. The steep curve (slope) setting is normally used for
short-arc welding. The flat curve (slope) setting is normally
used for spray-arc welding.
Figure 3-4. As electrical current flows through the electrode,
heating takes place until the wire melts off the end. Each cycle
is completed many times per second during the welding opera-
tion. This is referred to as inductance or the “pinch effect.”