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180 MULTI-PROCESS MIG-ARC-TIG WELDER
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ARC LENGTH
To strike the arc, the electrode should be gently scraped on the work until the arc is established.
There is a simple rule for the proper arc length; it should be the shortest arc that gives a good
surface to the weld. An arc too long reduces penetration, produces spatter and gives a rough
surface finish to the weld.
An excessively short arc will cause sticking of the electrode and result in poor quality welds.
General rule of thumb for down hand welding is to have an arc length no greater than the
diameter of the core wire.
ELECTRODE ANGLE
The angle that the electrode makes with the work is important to ensure a smooth, even transfer of
metal. When welding in down hand, fillet, horizontal or overhead the angle of the electrode is
generally between 5 and 15 degrees towards the direction of travel. When vertical up welding the
angle of the electrode should be between 80 and 90 degrees to the workpiece.
TRAVEL SPEED
The electrode should be moved along in the direction of the joint being welded at a speed that will
give the size of run required. At the same time, the electrode is fed downwards to keep the correct arc
length at all times. Excessive travel speeds lead to poor fusion, lack of penetration etc., while too slow a
rate of travel will frequently lead to arc instability, slag inclusions and poor mechanical properties.
MATERIAL AND JOINT PREPARATION
The material to be welded should be clean and free of any moisture, paint, oil, grease, mill scale,
rust or any other material that will hinder the arc and contaminate the weld material. Joint
preparation will depend on the method used include sawing, punching, shearing, machining,
flame cutting and others. In all cases edges should be clean and free of any contaminates. The
type of joint will be determined by the chosen application.
MIG (METAL INERT GAS) WELDING
MIG (metal inert gas) welding, also known as GMAW (gas metal arc welding) or MAG (metal
active gas welding), is a semi-automatic or automatic arc welding process in which a
continuous and consumable wire electrode and a shielding gas are fed through a welding gun.
A constant voltage, direct current power source is most commonly used with MIG welding.
SHORT-CIRCUIT TRANSFER
Short-circuit transfer is the most common used method. The wire electrode is fed continuously
down the welding torch until it exits from the contact tip. The wire touches the workpiece and
causes a short-circuit. The wire heats up and begins to form a molten bead, the bead separates
from the end of the wire and forms a droplet that is transferred into the weld pool. This process
is repeated about 100 times per second, making the arc appear constant to the human eye.
1. The wire approaches the workpiece and touches the work creating a short-circuit between
the wire and the base metal, because there is no space between the wire and the base
metal there is no arc and current flows through the wire (Fig. 27)
2. The wire cannot support
all the current flow,
resistance builds up and
the wire becomes hot and
weak and begins to melt
(Fig. 28)
3. The current flow creates a
magnetic field that begins
to pinch the melting wire
forming it into droplet
(Fig. 29).
Fig. 27
Fig. 28
Fig. 29
