MESSER SolidWELD MIG 250GS Guide D'utilisation page 14

014 MIG 250GS 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, efficient 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 proficiency in the least possible
time. Even welders with some experience will benefit 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 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).
Schematic of MMA process in operation
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 Canada is 60 Hz (Hertz).
3.3 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.
Messer 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
• output amperage required, e.g. 120 amps or 240 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.