The horizontal line--called the
reference line--is the anchor to which all the other
welding symbols are tied. The instructions for making the weld
are strung along the reference line. An arrow connects the
reference line to the joint that is to be welded. In the example
above, the arrow is shown growing out of the right end of the
reference line and heading down and to the right, but many other
combinations are allowed.
Quite often, there are two sides to the
joint to which the arrow points, and therefore two potential
places for a weld. For example, when two steel plates are joined
together into a T shape, welding may be done on either side of
the stem of the T.
The weld symbol distinguishes between the
two sides of a joint by using the arrow and the spaces above and
below the reference line. The side of the joint to which the
arrow points is known (rather prosaically) as the arrow side,
and its weld is made according to the instructions given below
the reference line. The other side of the joint is known (even
more prosaically) as the other side, and its weld is
made according to the instructions given above the reference
line. The below=arrow and above=other
rules apply regardless of the arrow's direction.
The flag growing out of the junction of
the reference line and the arrow is present if the weld is to be
made in the field during erection of the structure. A weld
symbol without a flag indicates that the weld is to be made in
the shop. In older drawings, a field weld may be denoted by a
filled black circle at the junction between the arrow and the
reference line.
The open circle at the arrow/reference
line junction is present if the weld is to go all around the
joint, as in the example below.
The tail of the weld symbol is the place
for supplementary information on the weld. It may contain a
reference to the welding process, the electrode, a detail
drawing, any information that aids in the making of the weld
that does not have its own special place on the symbol.
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Each type of weld has its own basic
symbol, which is typically placed near the center of the
reference line (and above or below it, depending on which side
of the joint it's on). The symbol is a small drawing that can
usually be interpreted as a simplified cross-section of the
weld. In the descriptions below, the symbol is shown in both its
arrow-side and other-side positions.
The fillet weld (pronounced
"FILL-it," not "fil-LAY") is used to make lap joints, corner
joints, and T joints. As its symbol suggests, the fillet weld is
roughly triangular in cross-section, although its shape is not
always a right triangle or an isosceles triangle. Weld metal is
deposited in a corner formed by the fit-up of the two members
and penetrates and fuses with the base metal to form the joint.
(Note: for the sake of graphical clarity, the drawings below do
not show the penetration of the weld metal. Recognize, however,
that the degree of penetration is important in determining the
quality of the weld.)
The perpendicular leg of the triangle is
always drawn on the left side of the symbol, regardless of the
orientation of the weld itself. The leg size is written to the
left of the weld symbol. If the two legs of the weld are to be
the same size, only one dimension is given; if the weld is to
have unequal legs (much less common than the equal-legged weld),
both dimensions are given and there is an indication on the
drawing as to which leg is longer.
The length of the weld is given to the
right of the symbol.
If no length is given, then the weld is to
be placed between specified dimension lines (if given) or
between those points where an abrupt change in the weld
direction would occur (like at the end of the plates in the
example above).
For intermittent welds, the length of each
portion of the weld and the spacing of the welds are separated
by a dash (length first, spacing second) and placed to the right
of the fillet weld symbol.
Notice that the spacing, or pitch,
is not the clear space between the welds, but the
center-to-center (or end-to-end) distance.
The groove weld is commonly used to
make edge-to-edge joints, although it is also often used in
corner joints, T joints, and joints between curved and flat
pieces. As suggested by the variety of groove weld symbols,
there are many ways to make a groove weld, the differences
depending primarily on the geometry of the parts to be joined
and the preparation of their edges. Weld metal is deposited
within the groove and penetrates and fuses with the base metal
to form the joint. (Note: for the sake of graphical clarity, the
drawings below generally do not show the penetration of the weld
metal. Recognize, however, that the degree of penetration is
important in determining the quality of the weld.)
The various types of groove weld are:
The square groove weld, in which
the "groove" is created by either a tight fit or a slight
separation of the edges. The amount of separation, if any, is
given on the weld symbol.
The V-groove weld, in which the
edges of both pieces are chamfered, either singly or doubly, to
create the groove. The angle of the V is given on the weld
symbol, as is the separation at the root (if any).
If the depth of the V is not the full
thickness--or half the thickness in the case of a double V--the
depth is given to the left of the weld symbol.
If the penetration of the weld is to be
greater than the depth of the groove, the depth of the
effective throat is given in parentheses after the depth of
the V.
The bevel groove weld, in which the
edge of one of the pieces is chamfered and the other is left
square. The bevel symbol's perpendicular line is always drawn on
the left side, regardless of the orientation of the weld itself.
The arrow points toward the piece that is to be chamfered. This
extra significance is emphasized by a break in the arrow line.
(The break is not necessary if the designer has no preference as
to which piece gets the edge treatment or if the piece to
receive the treatment should be obvious to a qualified welder).
The U-groove weld, in which the
edges of both pieces are given a concave treatment.
The J-groove weld, in which the
edge of one of the pieces is given a concave treatment and the
other is left square. It is to the U-groove weld what the bevel
groove weld is to the V-groove weld. As with the bevel, the
perpendicular line is always drawn on the left side and the
arrow (with a break, if necessary) points to the piece that
receives the edge treatment.
The flare-V groove weld, commonly
used to join two round or curved parts. The intended depth of
the weld itself are given to the left of the symbol, with the
weld depth shown in parentheses.
The flare bevel groove weld,
commonly used to join a round or curved piece to a flat piece.
As with the flare-V, the depth of the groove formed by the two
curved surfaces and the intended depth of the weld itself are
given to the left of the symbol, with the weld depth shown in
parentheses. The symbol's perpendicular line is always drawn on
the left side, regardless of the orientation of the weld itself.
Common supplementary symbols used with
groove welds are the melt-thru and backing bar
symbols. Both symbols indicate that complete joint penetration
is to be made with a single-sided groove weld. In the case of
melt-thru, the root is to be reinforced with weld metal on the
back side of the joint. The height of the reinforcement, if
critical, is indicated to the left of the melt-thru symbol,
which is placed across the reference line from the basic weld
symbol.
When a backing bar is used to achieve
complete joint penetration, its symbol is placed across the
reference line from the basic weld symbol. If the bar is to be
removed after the weld is complete, an "R" is placed within the
backing bar symbol. The backing bar symbol has the same shape as
the plug or slot weld symbol, but context should always make the
symbol's intention clear.
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