Tweco 250 Weldskill MIG - Manual (Page 37)

7- Manual No 0-5072
basic welding technique weldskill 250, 350
December , 2008
SECTION 7:
BASIC WELDING TECHNIQUE
7.01 Setting of the Power Source &
Wirefeeder
Power source setting requires some practice by the
operator, as the welding plant has two control settings
that have to balance. These are the Wirespeed control
and the welding Voltage Control. The welding current
is determined by the Wirespeed control, the current
will increase with increased Wirespeed, resulting in
a shorter arc. Less wire speed will reduce the current
and lengthen the arc. Increasing the welding voltage
hardly alters the current level, but lengthens the arc.
By decreasing the voltage, a shorter arc is obtained
with a little change in current level.
When changing to a different electrode wire diameter,
different control settings are required. A thinner
electrode wire needs more Wirespeed to achieve the
same current level.
A satisfactory weld cannot be obtained if the Wire-
speed and Voltage settings are not adjusted to suit
the electrode wire diameter and the dimensions of
the work piece.
If the Wirespeed is too high for the welding voltage,
“stubbing” will occur as the wire dips into the molten
pool and does not melt. Welding in these conditions
normally produces a poor weld due to lack of fusion.
If, however, the welding voltage is too high, large
drops will form on the end of the wire, causing spatter.
The correct setting of voltage and Wirespeed can be
seen in the shape of the weld deposit and heard by a
smooth regular arc sound.
7.02 Position of MIG Torch
The angle of MIG torch to the weld has an effect on
the width of the weld.
Forehand Vertical Backhand
Art # A-07185
7.03 Distance from the MIG Torch
Nozzle to the Work Piece
The electrode wire stick out from the MIG Torch nozzle
should be between 0mm to 20.0mm. This distance
may vary depending on the type of joint that is being
welded.
7.04 Travel Speed
The speed at which the molten pool travels influences
the width of the weld and penetration of the welding
run.
7.05 Electrode Wire Size Selection
The choice of Electrode wire size and shielding gas
used depends on the following:
• Thickness of the metal to be welded
• Type of joint
• Capacity of the wire feed unit and Power
Source
• The amount of penetration required
• The deposition rate required
• The bead profile desired
• The position of welding
• Cost of the wire
Weld metal deposition rate is proportional to current
density. Current density is defined as the current
per cross sectional area of the electrode wire and is
normally expressed as amps per mm²
.
An example
is shown below:
Electrode
Wire Size
mm (inch)
Current
Current
Density
(A/mm 2 )
Deposition
Rate (kg/
hour)
0.9 (0.035) 200A 34 3.2
.2 (0.045) 200A 77 2.8
Table 7- Electrode Wire Deposition Rate