Distance from the MIG Torch Nozzle to the Work Piece
The electrode wire stick out from the MIG Torch nozzle should be between
10mm to 20mm. This distance may vary depending on the type of joint that is
being welded.
Travel Speed
The speed at which the molten pool travels influences the width of the weld
and penetration of the welding run.
MIG Welding (GMAW) Variables
Most of the welding done by all processes is on carbon steel. The items below
describe the welding variables in short-arc welding of 24gauge (0.024”, 0.6mm)
to ¼” (6.4mm) mild sheet or plate. The applied techniques and end results in
the GMAW process are controlled by these variables.
Preselected Variables
Preselected variables depend upon the type of material being welded, the
thickness of the material, the welding position, the deposition rate and the
mechanical properties. These variables are:
• Type of electrode wire
• Size of electrode wire
• Type of gas
• Gas flow rate
Primary Adjustable Variables
These control the process after preselected variables have been found. They
control the penetration, bead width, bead height, arc stability, deposition rate
and weld soundness. They are:
• Arc Voltage
• Welding current (wire feed speed)
• Travel speed
Secondary Adjustable Variables
These variables cause changes in primary adjustable variables which in turn
cause the desired change in the bead formation. They are:
1. Stick-Out
(distance between the end of the contact tube (tip) and the end of
the electrode wire). Maintain at about 10mm stick-out
2. Wire Feed Speed.
Increase in wire feed speed increases weld current.
Decrease in wire feed speed decreases weld current.
Electrode Stick-Out (Fig 1-8)
Average Arc Length
Gas Nozzle
Tip to
Work Distance
Contact Tip (Tube)
Electrode Wire
Actual Stick-Out
3. Nozzle Angle.
This refers to the position of the welding gun in relation to
the joint. The transverse angle is usually one half the included angle between
plates forming the joint. The longitudinal angle is the angle between the centre
line of the welding gun and a line perpendicular to the axis of the weld. The
longitudinal angle is generally called the Nozzle Angle and can be either
trailing (pulling) or leading (pushing). Whether the operator is left handed
or right handed has to be considered to realize the effects of each angle in
relation to the direction of travel.
Transverse & Longitudinal Nozzle Axes (Fig 1-9)
Transverse Angle
Longitudinal Angle
Axis of Weld
Nozzle Angle, Right Handed Operator (Fig 1-10)
Leading or “Pushing”
Angle (Forward Pointing)
Trailing or “Pulling”
Angle (Backward Pointing)
90
o
Direction of Travel
Establishing the Arc and Making Weld Beads
Before attempting to weld on a finished piece of work, it is recommended that
practice welds be made on a sample metal of the same material as that of the
finished piece.
The easiest welding procedure for the beginner to experiment with MIG
welding is the flat position. The equipment is capable of flat, vertical and
overhead positions.
For practicing MIG welding, secure some pieces of 16 or 18 gauge (1.5mm or
2.0mm) mild steel plate (150 x 150mm). Use (0.8mm) flux cored gasless wire or a
solid wire with shielding gas.
Setting of the Power Source
Power source and Current (Wire Speed) setting requires some practice by the
operator, as the welding plant has two control settings that have to balance.
These are the Current (Wire Speed) control and the welding Voltage Control.
The welding current is determined by the Current (Wire Speed) control, the
current will increase with increased Current (Wire Speed), resulting in a
shorter arc. Less Current (Wire Speed) will reduce the current and lengthen
the arc. Increasing the welding voltage hardly alters the current level, but
lengthens the arc. By decreasing the voltage, a shorter arc is obtained with a
little change in current level.
When changing to a different electrode wire diameter, different control
settings are required. A thinner electrode wire needs more Current (Wire
Speed) to achieve the same current level.
A satisfactory weld cannot be obtained if the Current (Wire Speed) and
Voltage settings are not adjusted to suit the electrode wire diameter and the
dimensions of the work piece.
If the Current (Wire Speed) is too high for the welding voltage, “stubbing” will
occur as the wire dips into the molten pool and does not melt. Welding in these
conditions normally produces a poor weld due to lack of fusion. If, however,
the welding voltage is too high, large drops will form on the end of the wire,
causing spatter. The correct setting of voltage and Current (Wire Speed) can
be seen in the shape of the weld deposit and heard by a smooth regular arc
sound.
Electrode Wire Size Selection
The choice of Electrode wire size and shielding gas used depends on the
following:
• Thickness of the metal to be welded
• Type of joint
• Capacity of the wire feed unit and
Power Source
• The amount of penetration required
• The deposition rate required
• The bead profile desired
• The position of welding
• Cost of the wire
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