A 1500 mm long beam must not bow or warp more than 1.5 mm over its total length. How to Specify ISO 13920 BF on a Drawing The correct drawing notation per ISO 13920 is: ISO 13920 – BF Or fully detailed: General tolerances: ISO 13920 – Linear class F, Angular class B Avoid ambiguous notations like "Tolerance BF" without referencing ISO 13920. BF vs Other Common Tolerance Classes | Class | Linear (mm over 1m) | Angular (over 1m) | Typical application | |-------|--------------------|-------------------|----------------------| | A (Very fine) | ±1.0 | ±2 mm/m | Precision machinery, tooling | | BF (Fine) | ±2.0 | ±3 mm/m | General machine parts, moderate fit | | C (Medium) | ±3.5 | ±4.5 mm/m | Structural steel, platforms | | D (Coarse) | ±5.5 | ±6 mm/m | Heavy frames, agricultural equipment |
| 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 | iso 13920 bf
| 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 1500 mm long beam must not bow or warp more than 1
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. ISO 13920 is the international standard that specifies
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.
| Symbol | Meaning | Applies to | |--------|---------|-------------| | | Fine class | Angular dimensions, straightness, flatness | | F | Fine class | Linear dimensions |
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.