Tweco 95 S Thermal Arc(Jan2013) - Manual (Page 32)

THERMAL ARC 95 S OPERATION
Manual 0-5086 4-6 Operation
4.13 Arc Welding Technique
A Word to Beginners
For those who have not yet done any welding, the simplest
way to commence is to run beads on a piece of scrap
plate. Use mild steel plate about 6.0mm thick and a 3.2mm
electrode. Clean any paint, loose scale or grease off the
plate and set it firmly on the work bench so that welding
can be carried out in the downhand position. Make sure
that the work clamp is making good electrical contact with
the work, either directly or through the work table. For light
gauge material, always clamp the work lead directly to the
job, otherwise a poor circuit will probably result.
4.14 The Welder
Place yourself in a comfortable position before beginning
to weld. Get a seat of suitable height and do as much work
as possible sitting down. Don’t hold your body tense. A
taut attitude of mind and a tensed body will soon make
you feel tired. Relax and you will find that the job becomes
much easier. You can add much to your peace of mind
by wearing a leather apron and gauntlets. You won’t be
worrying then about being burnt or sparks setting alight
to your clothes.
Place the work so that the direction of welding is across,
rather than to or from, your body. The electrode holder
lead should be clear of any obstruction so that you can
move your arm freely along as the electrode burns down.
If the lead is slung over your shoulder, it allows greater
freedom of movement and takes a lot of weight off your
hand. Be sure the insulation on your cable and electrode
holder is not faulty, otherwise you are risking an electric
shock.
4.15 Striking the Arc
Practice this on a piece of scrap plate before going on to
more exacting work. You may at first experience difficulty
due to the tip of the electrode “sticking” to the work piece.
This is caused by making too heavy a contact with the
work and failing to withdraw the electrode quickly enough.
A low amperage will accentuate it. This freezing-on of the
tip may be overcome by scratching the electrode along
the plate surface in the same way as a match is struck. As
soon as the arc is established, maintain a 1/16" (1.6mm)
to 1/8" (3.2mm) gap between the burning electrode end
and the parent metal. Draw the electrode slowly along as
it melts down.
Another difficulty you may meet is the tendency, after the
arc is struck, to withdraw the electrode so far that the arc
is broken again. A little practice will soon remedy both
of these faults.
Art # A-07696_AB
20°
1.6 mm (1/16”)
Figure 4-11: Striking an arc
4.16 Arc Length
The securing of an arc length necessary to produce a
neat weld soon becomes almost automatic. You will find
that arc produces a crackling or spluttering noise and
the weld metal comes across in large, irregular blobs.
The weld bead is flattened and spatter increases. A short
arc is essential if a high quality weld is to be obtained
although if it is too short there is the danger of it being
blanketed by slag and the electrode tip being solidified
in. If this should happen, give the electrode a quick twist
back over the weld to detach it. Contact or “touch-weld”
electrodes such as E7014 electrode do not stick in this
way, and make welding much easier.
4.17 Rate of Travel
After the arc is struck, your next concern is to maintain
it, and this requires moving the electrode tip towards the
molten pool at the same rate as it is melting away. At the
same time, the electrode has to move along the plate to
form a bead. The electrode is directed at the weld pool at
about 20° from the vertical. The rate of travel has to be
adjusted so that a well-formed bead is produced.
If the travel is too fast, the bead will be narrow and strung
out and may even be broken up into individual globules.
If the travel is too slow, the weld metal piles up and the
bead will be too large.
4.18 Making Welded Joints
Having attained some skill in the handling of an electrode,
you will be ready to go on to make up welded joints.
A. Butt Welds
Set up two plates with their edges parallel, as shown in
Figure 4-12, allowing 1/15" (1.6mm) to 3/32" (2.4mm)
gap between them and tack weld at both ends. This is to
prevent contraction stresses from the cooling weld metal
pulling the plates out of alignment. Plates thicker than 1/4"
(6.0mm) should have their mating edges beveled to form