- 8 -
risks to persons (e.g. electric shock) and objects (e.g. fire).
5.4 CONNECTION OF THE WELDING CABLES
WARNING! BEFORE MAKING THE FOLLOWING CONNECTIONS MAKE
SURE THE WELDING MACHINE IS SWITCHED OFF AND DISCONNECTED FROM
THE POWER SUPPLY OUTLET.
Table
(TAB. 1)
gives the recommended values for the welding cables (in mm
2
)
depending on the maximum current supplied by the welding machine.
5.4.1 TIG welding
Connecting the torch
- Insert the torch current cable into the appropriate quick terminal (-)/~. Connect the
three-pin connector (torch button) to the appropriate socket. Connect the torch gas
pipe to the appropriate connector.
Connecting the welding current return cable
- This is connected to the piece to be welded or to the metal bench on which it rests,
as close as possible to the joint being made.
This cable is connected to the terminal with the (+) symbol (~ for TIG machines
designed for AC welding).
Connecting the gas bottle
- Screw the pressure reducing valve to the gas bottle valve, first inserting the special
reduction accessory supplied when argon gas is used.
- Connect the gas inflow hose to the pressure reducing valve and tighten the hose
clamp supplied.
- Loosen the ringnut for adjusting the pressure reducing valve before opening the
valve on the bottle.
- Open the valve on the bottle and adjust the quantity of gas (l/min) according to the
suggestions for use given in the table
(TAB. 4)
; if it is necessary to adjust the gas
flow during welding this should always be done by adjusting the ring nut on the
pressure reduction valve. Make sure there are no leaks in the piping and connectors.
WARNING! Always close the gas bottle valve at the end of the job.
5.4.2 MMA WELDING
Almost all coated electrodes are connected to the positive pole (+) of the power
source; as an exception to the negative pole (-) for acid coated electrodes.
Connecting the electrode-holder clamp welding cable
On the end take a special terminal that is used to close the uncovered part of the
electrode.
This cable is connected to the terminal with the symbol (+)
Connecting the welding current return cable
This is connected to the piece being welded or to the metal bench supporting it, as
close as possible to the join being made.
This cable is connected to the terminal with the symbol (-)
Warnings:
- Turn the welding cable connectors right down into the quick connections (if present),
to ensure a perfect electrical contact; otherwise the connectors themselves will
overheat, resulting in their rapid deterioration and loss of efficiency.
- The welding cables should be as short as possible.
- Do not use metal structures which are not part of the workpiece to substitute the
return cable of the welding current: this could jeopardise safety and result in poor
welding.
6. WELDING: DESCRIPTION OF THE PROCEDURE
6.1 TIG WELDING
TIG welding is a welding procedure that exploits the heat produced by the electric arc
that is struck, and maintained, between a non-consumable electrode (tungsten) and
the piece to be welded. The tungsten electrode is supported by a torch suitable for
transmitting the welding current to it and protecting the electrode itself and the weld
pool from atmospheric oxidation, by the flow of an inert gas (usually argon: Ar 99.5)
which flows out of the ceramic nozzle
(FIG. G)
.
To achieve a good weld it is absolutely necessary to use the exact electrode diameter
with the exact current, see the table
(TAB. 3).
The electrode usually protrudes from the ceramic nozzle by 2-3mm, but this may reach
8mm for corner welding.
Welding is achieved by fusion of the edges of the joint. For properly prepared thin
pieces (up to about 1mm) weld material is not needed
(FIG. H)
.
For thicker pieces it is necessary to use filler rods of the same composition as the base
material and with an appropriate diameter, preparing the edges correctly
(FIG. I)
. To
achieve a good weld the pieces should be carefully cleaned and free of oxidation, oil,
grease, solvents etc.
6.1.1 HF and LIFT strike
HF strike:
The electric arc is struck without contact between the tungsten electrode and the piece
being welded, by means of a spark generated by a high frequency device. This strike
mode does not entail either tungsten inclusions in the weld pool or electrode wear and
gives an easy start in all welding positions.
Procedure:
Press the torch button, bringing the tip of the electrode close to the piece (2 -3mm),
wait for the arc strike transferred by the HF pulses and, when the arch has struck, form
the weld pool on the piece and proceed along the joint.
If there are difficulties in striking the arc even though the presence of gas is confirmed
and the HF discharges are visible, do not insist for long in subjecting the electrode to
HF action, but check the integrity of the surface and the shape of the tip, dressing it
on the grinding wheel if necessary. At the end of the cycle the current will fall at the
slope down setting.
LIFT strike:
The electric arc is struck by moving the tungsten electrode away from the piece to
be welded. This strike mode causes less electrical-radiation disturbance and reduces
tungsten inclusions and electrode wear to a minimum.
Procedure:
Place the tip of the electrode on the piece, using gentle pressure. Press the torch
button right down and lift the electrode 2-3mm with a few moments’ delay, thus striking
the arc. Initially the welding machine supplies a current I
LIFT
, after a few moments the
welding current setting will be supplied. At the end of the cycle the current will fall to
zero at the slope down setting.
6.1.2 TIG DC welding
TIG DC welding is suitable for all low- and high-carbon steels and the heavy metals,
copper, nickel, titanium and their alloys.
For TIG DC welding with the electrode to the (-) terminal the electrode with 2% thorium
(red band) is usually used or else the electrode with 2% cerium (grey band).
It is necessary to sharpen the tungsten electrode axially on the grinding wheel,
as shown in
FIG. L
,
making sure that the tip is perfectly concentric to prevent arc
deviation. It is important to carry out the grinding along the length of the electrode.
This operation should be repeated periodically, depending on the amount of use and
wear of the electrode, or when the electrode has been accidentally contaminated,
oxidised or used incorrectly. In TIG DC mode 2-stroke (2T) and 4-stroke(4T) operation
are possible.
6.1.3 TIG AC welding
This type of welding can be used to weld metals such as aluminium and magnesium,
which form a protective, insulating oxide on their surface. By reversing the welding
current polarity it is possible to “break” the surface layer of oxide by means of a
mechanism called “ionic sandblasting”. The voltage on the tungsten electrode
alternates between positive (EP) and negative (EN). During the EP period the oxide
is removed from the surface (“cleaning”or “pickling”) allowing formation of the pool.
During the EN period there is maximum heat transfer to the piece, allowing welding.
The possibility of varying the balance parameter in AC means that it is possible to
reduce the EP current period to a minimum, allowing quicker welding.
Higher balance values give quicker welding, greater penetration, a more concentrated
arc, a narrower weld pool and limited heating of the electrode. Lower values give a
cleaner piece. If the balance value is too low this will widen the arc and the de-oxidised
part, overheat the electrode with consequent formation of a sphere on the tip making it
more difficult to strike the arc and control its direction. If the balance value is too high
this will create a “dirty” weld pool with dark inclusions.
The table
(TAB.
4)
summarises the effects of parameter changes in AC welding.
In TIG AC mode 2-stroke (2T) and 4-stroke (4T) operation are possible.
The instructions for this welding procedure are also valid.
The table
(TAB. 3)
shows suggested values for welding on aluminium; the most
suitable electrode is a pure tungsten electrode (green band).
6.1.4 Procedure
-
Use the knob to adjust the welding current to the desired value; if necessary adjust
during welding to the actual required heat transfer.
-
Press the torch button and make sure the gas flow from the torch is correct; if
necessary, adjust pre-gas and postgas times; these times should be adjusted
according to operating conditions, the postgas delay in particular should be long
enough to allow the electrode and weld pool to cool at the end of welding without
coming into contact with the atmosphere (oxidation and contamination).
TIG mode with 2T sequence:
- Press the torch button (P.T.) right down to strike the arc with a current of I
START
.
The current will increase according to the START SLOPE UP setting to the welding
current value.
-
To interrupt welding, release the torch button so that either the current gradually
decreases (if the FINAL SLOPE DOWN parameter has been enabled) or the arc is
extinguished immediately, followed by postgas.
TIG mode with 4T sequence:
-
The first time the button is pressed it will strike the arc with a current equal to I
START
.
When the button is released the current will increase according to the START
SLOPE UP setting to the welding current value; this value is maintained even with
the button is released. When the button is pressed again the current will decrease
according to the FINAL SLOPE DOWN setting, until it reaches I
END
. The I
END
current
will be maintained until the button is released to terminate the welding cycle and
start the postgas phase. If, on the other hand, the button is released while the FINAL
SLOPE DOWN function is proceeding, the welding cycle will terminate immediately
and the postgas phase will start.
TIG mode with 4T and BI-LEVEL sequence:
-
The first time the button is pressed it will strike the arc with a current equal to I
START
.
When the button is released the current will increase according to the START
SLOPE UP setting to the welding current value; this value is maintained even when
the button is released. Now, every time the button is pressed (the time between
pressure and release should be short) the current will change between the setting
for the BI-LEVEL I
1
parameter and the main current value I
2
.
- When the button is kept pressed down for a longer space of time the current will
decrease according to the FINAL SLOPE DOWN setting, until it reaches I
END
. The
I
END
current will be maintained until the button is released to terminate the welding
cycle and start the postgas phase. If, on the other hand, the button is released while
the FINAL SLOPE DOWN function is proceeding, the welding cycle will terminate
immediately and the postgas phase will start (
FIG. M
).
TIG SPOT mode:
- Welding is carried out by keeping the torch push-button pressed until the pre-set
time has been reached (spot time).
6.2 MMA WELDING
- It is most important that the user refers to the maker’s instructions indicated on the
stick electrode packaging. This will indicate the correct polarity of the stick electrode
and the most suitable current.
- The welding current must be regulated according to the diameter of the electrode in
use and the type of the joint to be carried out: see below the currents corresponding
to various electrode diameters:
Ø
Electrode (mm)
Welding current (A)
Min.
Max.
1.6
25
50
2
40
80
2.5
60
110
3.2
80
160
4
120
200
5
150
280
6
200
350
- The user must consider that, according to the electrode diameter, higher current
values must be used for flat welding, whereas for vertical or overhead welds lower
current values are necessary.
- As well as being determined by the chosen current intensity, the mechanical
characteristics of the welded join are also determined by the other welding
parameters i.e. arc length, working rate and position, electrode diameter and
quality (to store the electrodes correctly, keep them in a dry place protected by their
packaging or containers).
- The properties of the weld also depend on the ARC-FORCE value (dynamic
behaviour) of the welding machine. The setting for this parameter can be made
either on the panel or using the remote control with 2 potentiometers.
- It should be noted that high ARC-FORCE values achieve better penetration and
allow welding in any position typically with basic electrodes, low ARC-FORCE
values give a softer, spray-free arc typically with rutile electrodes.
The welding machine is also equipped with HOT START and ANTI STICK devices to
guarantee easy starts and to prevent the electrode from sticking to the piece.
6.2.1 Procedure
- Holding the mask IN FRONT OF THE FACE, strike the electrode tip on the workpiece
as if you were striking a match. This is the correct strike-up method.
