Weld symbol
Weld symbol reference line basics
Every welding symbol is built on one horizontal reference line with an arrow at one end and an optional tail at the other. Where information sits on that line is what gives it meaning.
Reads asArrow side: fillet weld, 5/16 size, 3 long, 6 pitch.
How to read it
The reference line is the spine of the whole notation. It is always drawn horizontally, regardless of how the joint sits on the part, and everything else hangs off it: the leader and arrow at one end, an optional tail at the other, the weld symbols above and below it, and the supplementary marks at the elbow where the leader meets it.
Position on that line is meaning. Below the line is arrow-side information, above the line is other-side information, left of a weld symbol is size, right of it is length and pitch, inside it is root opening, and beyond its outer face is angle and contour. None of those placements is decorative. Move a number from one side of a symbol to the other and you have changed the instruction.
The line's horizontality is worth dwelling on because it is the source of the notation's greatest strength and its most common confusion. Because the line never rotates, the same symbol reads identically wherever the joint sits on the drawing - but that also means the symbol's shape carries no information about the joint's orientation on the part. The arrow, and the drawing it points into, carry that.
One reference line describes one joint, however many weld symbols it holds. A fillet below and a fillet above is one joint welded on both faces, not two joints. A groove below and a semicircle above is one joint welded from one side and treated from the other. Keeping that in mind stops the most damaging class of misreading, where one instruction gets executed as two.
Where each dimension goes
- Below the reference line
- Arrow-side information: what happens on the face of the joint the arrow tip lands on.
- Above the reference line
- Other-side information: what happens on the opposite face of that same joint.
- Left of a weld symbol
- Size - leg length, groove depth, nugget diameter, or hole diameter depending on the weld type.
- Right of a weld symbol
- Length, and after it the pitch, for welds that do not run continuously.
- The elbow
- Where the leader meets the line. Home of the all-around circle and the field flag, which govern the whole line.
- The tail
- The far end of the line, carrying process, procedure, or notes the symbol cannot express.
Worked examples
Each one is a drawing, the sentence it comes out as, and then the mark-by-mark account of why. Read the sheet first and say the sentence yourself before you look down.
Example 1: A complete symbol read place by place
TitleA complete symbol read place by placeRefReference line basicsEx1 / 2Reads as
Weld a 1/4 fillet, four inches long on eight-inch centres, by the process in the tail.
Which mark says so
- fillet symbolBelow the reference lineA fillet weld on the arrow side of the joint
- 1/4Left of the symbolLeg size of the fillet
- 4Right of the symbolLength of each weld
- -8Right of the length on the symbolPitch - centre to centre spacing of an intermittent weld
- SMAWIn the tailA reference the drawing carries - a process, procedure or specification
Size sits left of the triangle, length and pitch sit right of it, the triangle sits below the line for the arrow side, and the tail names the process. Every element is located by position.
And what it does not say
- Nothing sits above the reference line, so the far face of the joint stays bare.
- There is no circle at the elbow, so the weld does not continue around the joint.
Example 2: One line describing one joint twice welded
TitleOne line describing one joint twice weldedRefReference line basicsEx2 / 2Reads as
Weld the groove from the arrow side and put a back weld on the far side.
Which mark says so
- V-groove symbolBelow the reference lineA V-groove weld on the arrow side of the joint
- 1/2Left of the arrow-side symbolGroove weld size - how deep the weld itself goes
- 60°Beyond the outer face of the arrow-side symbolIncluded angle the joint is prepared to
- 1/8Inside the arrow-side symbolRoot opening - the gap left between the members before welding
- back symbolAbove the reference lineA back weld on the far side of the joint
Two symbols on one reference line still describe a single joint. The groove below and the semicircle above are two operations on the same seam, not two separate joints.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- There is no circle at the elbow, so the weld does not continue around the joint.
Common mistakes
- Reading one reference line carrying two weld symbols as two separate joints.
- Expecting the symbol's shape or the line's direction to show how the joint sits on the part.
- Putting a size on the wrong side of a weld symbol, which turns it into a length.
- Attaching an all-around circle or field flag to a weld symbol instead of the elbow.