ISO 2553 and AWS A2.4: the differences that matter
Two systems, the same set of triangles and Vs, and enough divergence that reading one as the other produces a weld on the wrong face at the wrong size.
Difference one: which side
ISO 2553 system A uses two parallel lines — a continuous reference line and a dashed identification line. A symbol drawn on the continuous line means the arrow side; a symbol on the dashed line means the other side. The dashed line may be above or below the continuous one, which does not change the meaning.
AWS A2.4 uses a single reference line. A symbol below the line means the arrow side; above the line means the other side. Compact, and completely unforgiving if you assume ISO.
Mnemonic for AWS: below the line, in front of your eyes — the arrow side is the one nearest you, and its symbol hangs below.
Difference two: how a fillet is dimensioned
ISO practice is to give the design throat: a5. AWS practice is to
give the leg. A drawing marked 5 in AWS therefore asks for a leg of
5 mm — throat 3.5 mm — while a5 in ISO asks for a throat
of 5 mm, which is a leg of 7.
Getting this backwards either wastes 40 % of the filler or leaves the
joint a third undersized. ISO also allows the leg form, written z7,
which is why the letter matters as much as the number. The arithmetic is in the
fillet weld
calculator.
The smaller divergences
| Element | ISO 2553 | AWS A2.4 |
|---|---|---|
| Reference line | Continuous plus dashed | Single |
| Arrow side | Symbol on continuous line | Symbol below the line |
| Fillet size | Throat a, or leg z | Leg |
| Intermittent weld | n × l (e) — pitch between starts | l − e — length and pitch |
| Process reference | EN ISO 4063 number in the tail | AWS abbreviation, e.g. GMAW |
| All round | Circle at the elbow | Circle at the elbow |
| Field weld | Flag | Flag |
The intermittent-weld difference is worth a second look. ISO writes
3×50 (100) where the bracketed figure is the pitch measured between
the starts of consecutive segments. AWS writes the length and the pitch as
50−100, meaning the same thing but read differently.
In practice
On international projects both systems turn up, sometimes in the same set of drawings from different suppliers. Two habits are worth forming: check the title block for the drafting standard before reading any symbol, and treat a fillet dimension without a letter in front of it as a question rather than an instruction.
Compare the two systems side by side in the symbol builder — the same settings drawn in both conventions make the divergence obvious.
Frequently asked questions
How do I tell which system a drawing uses?
Look for the dashed identification line. If the reference line has a dashed twin, it is ISO 2553 system A. If there is a single line with symbols above and below it, it is AWS A2.4 — or ISO system B, which is the same convention. The title block usually names the standard too.
What is ISO 2553 system B?
An alternative within ISO 2553 itself that follows the AWS convention: one reference line, side given by position. It exists so that companies working to both systems do not have to maintain two drafting standards. It is used mainly in Asia; European drawings are almost always system A.
Are the weld sizes the same?
No, and this is the trap. ISO normally dimensions a fillet by its throat, written a5. AWS dimensions it by the leg. The same number in the two systems is a different weld — 5 mm as a throat is a 7 mm leg.
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