Operating instructions

14
BOC Smootharc Advance II MIG 250C Operating manual
3.0 MMA Process
3.1 Introduction
Arc welding, although in the past principally the tool of tradesmen
and fabricators, has in recent years found increasing usage with small
workshops, farmers, handyman-hobbyists amongst others. This has
been brought about by the introduction of low-cost portable arc welding
machines and the ready availability of small diameter electrodes and
thinner section construction materials. Provided the operator isfamiliar
with the basic principles and techniques, arc welding can be a fast,
e󹟾cient and safe method of joining metals.
The main purpose of this manual is to help the welder with limited
experience to obtain a better understanding of the process, and to
acquire a reasonable degree of prociency in the least possible time.
Even welders with some experience will benet from the information in
this manual.
3.2 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 ux-coated ller 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 di󹟽erence between these two power sources is that, in the
case of DC, the current remains constant in magnitude and ows 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 ows rst in one direction and
then the other. Similarly, the voltage in the circuit changes from positive
to negative with changes in direction of current ow. This complete
reversal is called a ‘half cycle’ and repeats as long as the current
ows. The rate of change of direction of current ow 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).
3.3 Welding Machine
The most important consideration when contemplating the use of arc
welding for the rst 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 innitely 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.
Schematic of MMA process in operation
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