← The Bench · Sheet C-08
Welding symbol decoder
A welding symbol is a complete fabrication instruction compressed into about two centimetres of line work. Here is how to unpack it, and every symbol you are likely to meet.
Anatomy — per AWS A2.4
The one rule that matters: a symbol below the reference line is the arrow side — the side the arrow physically touches. A symbol above the line is the other side. This is the opposite of what most people guess, it is the same in every revision of A2.4, and getting it backwards is the single most common welding-symbol error on a drawing.
Arrow side — one weld
Symbol below the line. A 6 mm fillet on the joint face the arrow physically touches. Nothing is deposited on the far side.
Both sides — two welds
Symbol above and below. Two welds — twice the deposited metal, twice the heat input, and shrinkage that now pulls evenly instead of bowing the assembly.
Every symbol you will actually meet.
Drawn here on the arrow side. Move the same glyph above the line and it means the other side.
| Symbol | Weld | What it asks the shop to do |
|---|---|---|
| Fillet | A triangular deposit in the corner of a lap or tee joint. No edge preparation. The cheapest weld there is and by far the most common — size is the leg length, not the throat. | |
| Square groove | Butt the two edges with a root gap and fill it. Only good for thin material — beyond about 6 mm you cannot get penetration to the root. | |
| V-groove | Both edges bevelled to form a V. The default full-penetration butt weld. Cheap to prepare, more filler than a U. | |
| Bevel groove | One edge prepared, one left square. Used where only one member can be machined, or on a tee joint that needs full penetration. | |
| U-groove | A machined radius at the root. Far less filler than a V on thick sections, so less shrinkage and less distortion — but it needs machining, so it costs more up front. | |
| J-groove | The single-sided U. Same trade: preparation cost against filler volume and distortion. | |
| Flare-V groove | The natural groove formed between two rounded members — two tubes, or two formed edges. No preparation at all. | |
| Flare-bevel groove | A rounded member against a flat one. Common on tube-to-plate. | |
| Plug or slot | Fill a hole or slot in the near member to fuse it to the far one. Used where you cannot reach an edge. Size is the hole diameter, not the weld. | |
| Spot | A discrete fused nugget, usually resistance welded. Size is the nugget diameter, and strength is per spot. | |
| Seam | A continuous line of fusion, effectively overlapping spots. Used where the joint has to seal as well as hold. | |
| Back or backing | Deposited on the far side of an already-welded groove. As a back weld it comes second; as a backing weld it comes first. The symbol is identical, so the tail has to say which. | |
| Melt-through | Complete joint penetration required from one side, with visible reinforcement on the far face. Says what the far side must look like, not what to deposit. | |
| Surfacing | Build-up or hardfacing on a face, not a joint at all. The dimension is deposit thickness. | |
| Edge | Joins the flanged edges of two members lying against each other. Thin sheet work. |
Contour and supplementary symbols
| Flush contour | Finish the weld face level with the parent metal. Add a finishing letter in the tail — G ground, M machined, C chipped, R rolled, H hammered. | |
| Convex contour | A crowned face. Adds nothing to strength — the throat is what carries load — and adds a stress raiser at the toe. Rarely what you want in fatigue service. | |
| Concave contour | A dished face. Reduces the toe angle and helps fatigue life, at the cost of throat. Specify the throat if it matters. | |
| Field weld | Not welded in the shop. A flag on this joint changes the sequence, the access, the position, the inspection plan and the cost. | |
| Weld all around | Continue the weld the whole way round the joint. On a tube through a plate this is one symbol and a great deal of arc time. |
Read the numbers in the right order
Left of the symbol is the size. Right of it is length and pitch, written
L–P. So 6 ◢ 50–150 is a 6 mm
fillet, 50 mm long, repeating every 150 mm centre to centre — an
intermittent weld, not a continuous one. Miss the dash and you have specified
roughly three times the arc time.
The tail carries the real content
Process, procedure specification, filler classification, inspection level.
GTAW, WPS-104, 100% RT. A drawing that
leaves the tail empty has quietly delegated all of that to whoever is holding
the torch.
Symmetry beats correction
Every weld shrinks as it cools, and shrinkage pulls the structure toward the weld. Two fillets either side of a web cancel; one fillet on one side bows the assembly. Most distortion problems are decided at the symbol stage, long before anyone strikes an arc.
Draw the torch, not just the joint
A symbol can specify a weld that no human hand can reach and no machine can see. Before releasing the drawing, ask where the torch goes, where the welder’s eyes go, and how the inspector gets a probe onto it. I have never regretted asking. I have regretted not asking.
GD&T reference
All fourteen characteristics, with a frame builder.
Open C-01ASME VIII-1 pressure vessel
Joint efficiency is where welding meets the code.
Open C-06Thermal expansion & restraint
The other reason things move when they get hot.
Open C-10The circularity bench
Draw a circle. Get a real Y14.5 number back.
Open