Operating instructions

20
BOC Smootharc Advance II MIG 250C Operating manual
The following terms and denitions are important in specifying and
describing llet welds.
Leg Length
A fusion face of a llet weld, as shown above. All specications for llet
weld sizes are based on leg length.
Throat Thickness
A measurement taken through the centre of a weld from the root to the
face, along the line that bisects the angle formed by the members to
bejoined.
E󹟽ective throat thickness is a measurement on which the strength of a
weld is calculated. The e󹟽ective throat thickness is based on a mitre llet
(concave Fillet Weld), which has a throat thickness equal to 70% of the
leg length. For example, in the case of a 20 mm llet, the e󹟽ective throat
thickness will be 14 mm.
Convex Fillet Weld
A llet weld in which the contour of the weld metal lies outside a
straight line joining the toes of the weld. A convex llet weld of specied
leg length has a throat thickness in excess of the e󹟽ective measurement.
Concave Fillet Weld
A llet in which the contour of the weld is below a straight line joining
the toes of the weld. It should be noted that a concave llet weld of
a specied leg length has a throat thickness less than the e󹟽ective
throat thickness for that size llet. This means that when a concave llet
weld is used, the throat thickness must not be less than the e󹟽ective
measurement. This entails an increase in leg length beyond the specied
measurement.
The size of a llet weld is a󹟽ected by the electrode size, welding speed
or run length, welding current and electrode angle. Welding speed and
run length have an important e󹟽ect on the size and shape of the llet,
and on the tendency to undercut.
Insu󹟾cient speed causes the molten metal to pile up behind the arc
and eventually to collapse. Conversely, excessive speed will produce
a narrow irregular run having poor penetration, and where larger
electrodes and high currents are used, undercut is likely to occur.
Fillet Weld Data
Nominal Fillet
Size (mm)
Minimum Throat
Thickness (mm)
Plate Thickness
(mm)
Electrode Size
(mm)
5.0 3.5 5.0–6.3 3.2
6.3 4.5 6.3–12 4.0
8.0 5.5 8.0–12 & over 4.0
10.0 7.0 10 & over 4.0
Selection of welding current is important. If it is too high the weld
surface will be attened, and undercut accompanied by excessive spatter
is likely to occur. Alternatively, a current which is too low will produce a
rounded narrow bead with poor penetration at the root. The rst run in
the corner of a joint requires a suitably high current to achieve maximum
penetration at the root. A short arc length is recommended for llet
welding. The maximum size llet which should be attempted with one
pass of a large electrode is 8.0 mm. E󹟽orts to obtain larger leg lengths
usually result in collapse of the metal at the vertical plate and serious
undercutting. For large leg lengths multiple run llets are necessary.
These are built up as shown. The angle of the electrode for various runs
in a downhand llet weld is shown on the next page.
Concave Fillet WeldConvex Fillet Weld
Actual Throat
Effective Throat
Convexity
Leg
Lengh
Theoretical Throat
Concavity
Actual Throat
& Effective Throat
Leg
Size
LegSize
Theoretical Throat
Actual Throat
Effective Throat
Convexity
Leg
Lengh
Theoretical Throat
Concavity
Actual Throat
& Effective Throat
Leg
Size
LegSize
Theoretical Throat