EN
Multiprocess 175. Operating manual.
Multiprocess 175. Operating manual.
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4. Manual Metal Arc Welding (MMAW).
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.
4.3 Welding machine
Basic welding machine and cables
The choice of welding machine is based mostly on the following factors:
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primary voltage, e.g. 240 volt or 380 volt
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output amperage required, e.g. 140 amps
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output required, e.g. AC or DC +/-
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duty cycle required, e.g. 35% @ 140 amps
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method of cooling, e.g. air-cooled or oil-cooled method of output
amperage control, e.g. tapped secondary lugs
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infinitely variable control
For example, the Smootharc Multiprocess 175 connects to 240 volt
supply (15 amps Input), has an output of 175 amps DC @ 35% 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.
4.1 Introduction
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 experience may benefit from the information in this
manual.
4.2 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).
1
Weld metal,
2
Slag,
3
Flux covering,
4
Core wire,
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Arc,
6
Weld pool,
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Workpiece
1
2
3
4
5
6
7
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
current in the middle of the range specified on the electrode package is
considered to be the optimum.
In the case of welding machines with separate terminals for different
size electrodes, ensure that the welding lead is connected to the correct
terminal for the size electrode being used. When using machines with
adjustable current, set on the current range specified.
The limits of this range should not normally be exceeded.
Arc length
To start 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 that is 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 rough deposits that are associated with
slag inclusions.
For downhand welding, it will be found that an arc length not greater
than the diameter of the core wire will be most satisfactory. Overhead
welding requires a very short arc, so that a minimum of metal will be
lost. Certain Ryval electrodes have been specially designed for ‘touch’
welding. These electrodes may be dragged along the work and a
perfectly sound weld is produced.
Electrode angle
The angle which the electrode makes with the work is important to
ensure a smooth, even transfer of metal. The recommended angles for
use in the various welding positions are covered later.
Correct 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.
Correct travel speed for normal welding applications varies between
approximately 125–375 mm per minute, depending on electrode size,
size of run required and the amperage used.
Excessive travel speeds lead to poor fusion, lack of penetration, etc.
Whilst too slow a rate of travel will frequently lead to arc instability, slag
inclusions and poor mechanical properties.
4.4 Welding technique
Successful welding depends on the following factors:
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selection of the correct electrode
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selection of the correct size of the electrode for the job
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correct welding current
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correct arc length
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correct angle of electrode to work
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correct travel speed
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correct preparation of work to be welded
4.5 Electrode selection
As a general rule the selection of an electrode is straight forward, in that
it is only a matter of selecting an electrode of similar composition to the
parent metal. It will be found, however, that for some metals there is a
choice of several electrodes, each of which has particular properties to
suit specific classes of work. Often, one electrode in the group will be
more suitable for general applications due to its all round qualities.
Electrode size
The size of the electrode is generally dependent on the thickness of the
section being welded, and the larger the section the larger the electrode
required. In the case of light sheet the electrode size used is generally
slightly larger than the work being welded. This means that if 1.5 mm
sheet is being welded, 2.0 mm diameter electrode is the recommended
size. The following table gives the recommended maximum size of
electrodes that may be used for various thicknesses of section.
Recommended electrode sizes
Average thickness of plate or
section
≤ 1.5 mm
1.5–2.0 mm
2.0–5.0 mm
5.0–8.0 mm
≥ 8.0 mm
Maximum recommended
electrode diameter
2.0 mm
2.5 mm
3.15 mm
4.0 mm
5.0 mm
For further help on choosing the right electrode for your work please
contact your local Ryval supplier.
Welding current
Correct current selection for a particular job is an important factor in arc
welding. With the current set too low, difficulty is experienced in striking
and maintaining a stable arc. The electrode tends to stick to the work,
penetration is poor and beads with a distinct rounded profile will be
deposited.
Excessive current is accompanied by overheating of the electrode. It will
cause undercut, burning through of the material, and give excessive
spatter. Normal current for a particular job may be considered as the
maximum which can be used without burning through the work, over-
heating the electrode or producing a rough spattered surface, i.e. the
