Iso 13920-bf | En

Even experienced fabricators sometimes misinterpret this standard. Avoid these errors:

For parts where centering is important (e.g., a boss on a plate).

| Nominal Size (mm) | Permitted Deviation (Class B) | | :--- | :--- | | Up to 120 | 1 mm | | 120 to 400 | 2 mm | | Over 400 | 3 mm | en iso 13920-bf

Here is the breakdown:

This is critical for base plates and mounting surfaces. | Nominal Size (mm) | Permitted Flatness Deviation

| Nominal Size (mm) | Permitted Flatness Deviation (Class B) | | :--- | :--- | | Up to 120 | 1 mm | | 120 to 400 | 1.5 mm | | 400 to 1,000 | 2 mm | | 1,000 to 2,000 | 2.5 mm | | 2,000 to 4,000 | 3 mm | | 4,000 to 8,000 | 4 mm | | > 8,000 | 5 mm |

Example: A 2,500 mm long beam should not bow more than 3 mm over its entire length. but machining-level precision is not required.

EN ISO 13920-BF defines a specific set of "General Tolerances" for welded structures. It is intended to replace specific tolerance indications on technical drawings where high precision is not critical. The "BF" classification represents a Medium/Fine tolerance grade, making it the most commonly specified default for structural steel and general machinery welding where aesthetics and fit-up are important, but machining-level precision is not required.