Introduction In welding fabrication, achieving perfect dimensions is often impractical and cost-prohibitive. ISO 13920 is the international standard that specifies general tolerances for linear and angular dimensions, as well as for straightness and flatness, of welded structures.
Note: Some drawings use "BF" as a shorthand for "Fine overall" – but strictly per ISO 13920, you must verify if they mean linear class F + angular class B. Occasionally, "BF" is used to mean "Class B – Fine" for both, but that is non-standard. Always confirm with the drawing's tolerance block. iso 13920 bf
| Symbol | Meaning | Applies to | |--------|---------|-------------| | | Fine class | Angular dimensions, straightness, flatness | | F | Fine class | Linear dimensions | Occasionally, "BF" is used to mean "Class B
| Nominal size range (mm) | Permitted deviation (± mm) | |------------------------|----------------------------| | up to 30 | 0.5 | | >30 to 120 | 1.0 | | >120 to 400 | 1.5 | | >400 to 1000 | 2.0 | | >1000 to 2000 | 2.5 | | >2000 to 4000 | 3.5 | | >4000 to 8000 | 5.0 | | >8000 to 12000 | 6.5 | or non-right-angle joints.
| Shorter leg length (mm) | Permitted deviation (± minutes or mm/m) | |------------------------|------------------------------------------| | up to 400 | ±15' (0.25°) or 4 mm/m | | >400 to 1000 | ±10' (0.17°) or 3 mm/m | | >1000 to 2000 | ±7.5' (0.125°) or 2 mm/m | | >2000 to 4000 | ±5' (0.083°) or 1.5 mm/m | | >4000 | ±5' (0.083°) or 1 mm/m |
| Nominal length/width (mm) | Permitted deviation (mm) | |--------------------------|--------------------------| | up to 400 | 0.5 | | >400 to 1000 | 1.0 | | >1000 to 2000 | 1.5 | | >2000 to 4000 | 2.0 | | >4000 to 8000 | 3.0 | | >8000 to 12000 | 4.0 |
A welded bracket with a 500 mm width under BF tolerance would allow ±2.0 mm deviation. 2. Angular Dimensions (Class B) Angular tolerances apply to angles between welded parts, bevel angles, or non-right-angle joints.