Chapter 29 Shielded Metal Arc Welding 803
Copyright Goodheart-Willcox Co., Inc.
consistent. The main reason for scratching is to allow a
little more time to establish the arc length before the tip
sticks to the steel. However, scratching tends to leave
objectionable marks on the base metal near the weld.
Under some conditions, the marking creates enough of
a heat-affected zone to be considered a weld defect.
Tapping the electrode is a better method for
starting the arc in the correct location, but it is more
challenging to keep the electrode from sticking while
still maintaining an arc. Tapping is poking the exact
starting point of the weld with the tip of the electrode
and then lifting the electrode back to correct arc length.
Once the arc strikes, it is natural to pull the electrode
away to prevent it from sticking. Novice welders
tend to pull away too far and lose the arc. Sticking is
caused by the fi ller metal melting to the steel before the
electrode is pulled away to the correct arc length.
When the electrode does stick, quickly snap
the electrode holder backward from the direction of
travel to free the electrode. If this does not work, it is
necessary to break the circuit by releasing the electrode
from the holder. See Figure 29-46. Do not switch off
the machine with an electrode stuck. Doing so causes
arcing within the switch and results in premature
part failure.
Practice striking an arc on scrap material
before trying to weld beads. The motion becomes
more natural with repetition. Start with clean metal that is free of major
contaminants to make the process easier. Ensure that the workpiece lead is
connected directly to the workpiece to provide a more positive connection
and improve arcing characteristics. Begin with the scratching technique to
get the feel of the arc, but switch to the tapping technique before developing
the habit of using the scratching technique. Practice makes perfect; however,
practicing poor techniques leads to imperfect habits.
Welding a Bead
Once the arc is struck, it must be held at a consistent distance from the
base metal to form the molten pool. The arc is moved along at a pace that
allows the pool to penetrate the base metal as it accepts the addition of fi ller
metal from the melting electrode. Blending, or tie-in, is encouraged by a
slight crescent-shaped weaving or whipping of the electrode. This pattern
of motion should be kept to one and one-half to two times the diameter of
the electrode and should not extend the arc length. Allowing the electrode
movement to exceed these limits invites collapse of the O
2
-free shield,
leaving the molten pool exposed to the formation of oxides and nitrides
in the bead.
Goodheart-Willcox Publisher
Figure 29-46. When an electrode sticks, heat
accumulates quickly. Safely break the electrode
from the surface with pliers or tongs.
Safety Note
When releasing the
electrode from the holder,
do not grab it with your
hand. This can result in a
bad burn, even if you are
wearing welding gloves.
Use pliers to twist off the
stuck electrode. Leave
your face shield down
and break the rod free
while your eyes continue
to be protected—there
will be a bright flash when
it comes loose. Others
should stand away from
the area just behind the
welder’s elbow so as not
to be struck by the motion
as the stuck electrode is
being freed.