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12.0 MMA Process
Schematic of MMA process in operation
12.1 Process
Manual Metal Arc welding is the process of joining metals where an electric
arc is struck between the metal to be welded (parent metal) and a flux-
coated filler wire (the electrode). The heat of the arc melts the parent metal
and the electrode which mix together to form, on cooling, a continuous solid
mass.
Before arc welding can be carried out, a suitable power source is required.
Two types of power sources may be used for arc welding, direct current (DC)
or alternating current (AC).
The essential difference between these two power sources is that, in the
case of DC, the current remains constant in magnitude and flows in the same
direction. Similarly, the voltage in the circuit remains constant in magnitude
and polarity (i.e. positive or negative).
In the case of AC however, the current flows first in one direction and then
the other. Similarly, the voltage in the circuit changes from positive to
negative with changes in direction of current flow. This complete reversal
is called a 'half cycle' and repeats as long as the current flows. The rate of
change of direction of current flow is known as the 'frequency' of the supply
and is measured by the number of cycles completed per second. The standard
frequency of the AC supply in Australia is 50 Hz (Hertz).
BOC Smootharc Advance III MIG 425R & 555R Operating manual
12.2 Welding Machine
The most important consideration when contemplating the use of arc welding
for the first time is the purchase of a suitable welding machine.
BOC supplies a popular range of arc welding machines. Machines range from
small portable welders that operate from standard 240 Volt household power
to heavy-duty welders used by the largest steel fabricators.
Basic Welding Machine and Cables
The choice of welding machine is based mostly on the following factors:
→
primary voltage, e.g. 240 Volt or 380 Volt
→
output amperage required, e.g. 140 amps
→
output required, e.g. AC or DC +/-
→
duty cycle required, e.g. 35% @ 140 amps
→
method of cooling, e.g. air-cooled or oil-cooled method of output
amperage control, e.g. tapped secondary lugs
→
or infinitely variable control.
For example, the Smootharc 170 connects to 240 Volt supply (15 amps Input),
has an output of 170 amps DC @ 50% duty cycle.
Having decided on a welding machine, appropriate accessories are required.
These are items such as welding cables, clamps, electrode holder, chipping
hammer, helmet, shaded and clear lenses, scull cap, gloves and other
personal protective equipment.
BOC stocks a huge range of personal protective equipment. This combined
with BOC's extensive network ensures fast reliable service throughout the
South Pacific.
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