△ Fillet weld
Fillet weld · Soldadura en plano · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “△ a 5 L 100 (continuous)” states a fillet weld requirement.
What controls
A weld of roughly triangular cross-section deposited in the corner formed by two intersecting plates, typically in a T-joint, lap joint or corner joint. It needs no edge preparation: it is welded on the corner as-is. Its size is stated by the leg (the side of the triangle, letter z on many drawings) or by the throat (the height of the triangle from the root, letter a); the two values are not the same and in a 90° fillet the throat is approximately 0.7 times the leg. The symbol does not require root penetration unless the drawing specifies it separately.
How to verify
A fillet weld gauge (the set of stepped templates) placed in the corner: there are leg gauges and throat gauges, and you use whichever matches the letter on the drawing. The weld must be clean of slag and spatter before measuring, and it is checked at several points along the weld, not just where it looks best. Effective length is measured excluding start and end craters if the acceptance criterion says so. What the gauge cannot see, such as lack of fusion at the root, is only detected with non-destructive testing or with a macro-etch on a test coupon.
Workshop guideline values
- General
- Legs of 3–8 mm in structures, brackets and mild-steel bases.
- Accuracy
- Legs of 3–5 mm with controlled size and profile in equipment, tanks and frames.
- High precision
- Small fillets on thin sheet or stainless steel, with size and profile inspected weld by weld.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Confusing throat (a) with leg (z): with the same number, specifying the throat requires a considerably larger weld than specifying the leg.
- Assuming a fillet weld penetrates the root: on its own it is a corner weld, and penetration only exists if the drawing or the procedure expressly requires it.
- Making up for a small fillet by welding over it without cleaning: slag trapped between passes is an internal defect the gauge does not detect.
How to get it in the workshop / if it comes out of tolerance
Always check whether the drawing calls for “a” (throat) or “z” (leg): calling for a 5 and welding z 5 leaves the weld clearly short of load-bearing section. The fillet is the most common weld in structures and brackets because it needs no edge preparation, but it puts a lot of heat into the corner: on thin sheet it distorts the part if done in a single run.
Fillet short on leg: lower travel speed or add a pass; raising current alone tends to cause undercut on the upper toe. A fillet unequal between the two sides of the corner (one long leg and one short) indicates a badly aimed electrode angle, not a lack of filler. If the part twists, weld in segments alternating sides and let it cool between passes instead of running the whole weld in one go.
Related symbols
Other symbols in Flat welding
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