Tweco 175i Transtig - Manual (Page 52)

TRANSMIG 175i
BASIC WELDING GUIDE 4-14 Manual 0-5143
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 20º 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.
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 4-21, allowing 1.6mm to 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 6.0mm should have their mating edges bevelled to
form a 70º to 90º included angle. This allows full penetration of the weld metal to the root. Using a 3.2mm
Ferrocraft 21 electrode at 100 amps, deposit a run of weld metal on the bottom of the joint.
Do not weave the electrode, but maintain a steady rate of travel along the joint sufficient to produce a
well-formed bead. At first you may notice a tendency for undercut to form, but keeping the arc length
short, the angle of the electrode at about 20º from vertical, and the rate of travel not too fast, will help
eliminate this. The electrode needs to be moved along fast enough to prevent the slag pool from getting
ahead of the arc. To complete the joint in thin plate, turn the job over, clean the slag out of the back and
deposit a similar weld.
Art # A-07697_AB
Tack Weld
20°-30°
Electrode Tack Weld
Figure 4-21: Butt Weld
Art # A-07698
Figure 4-22: Weld Build up Sequence
Heavy plate will require several runs to complete the joint. After completing the first run, chip the slag
out and clean the weld with a wire brush. It is important to do this to prevent slag being trapped by the
second run. Subsequent runs are then deposited using either a weave technique or single beads laid down
in the sequence shown in Figure 4-22. The width of weave should not be more than three times the core
wire diameter of the electrode. When the joint is completely filled, the back is either machined, ground or
gouged out to remove slag which may be trapped in the root, and to prepare a suitable joint for depositing
the backing run. If a backing bar is used, it is not usually necessary to remove this, since it serves a similar
purpose to the backing run in securing proper fusion at the root of the weld.