Understanding EN ISO 13920-BF: A Guide to Welding Tolerances
Angular deviations are measured across the of the angle being inspected. For Class B, these values are expressed proportionally to prevent catastrophic lever-arm misalignment over long spans:
: ±20 minutes of a degree (approx. ±6 mm per meter).
Tolerance depends on the length of the shorter leg of the angle. en iso 13920-bf
For a welded construction, the permissible deviation for shape and position (e.g., the flatness of a large plate or the parallelism between two surfaces) is determined using the tolerance values shown below. The value "l" in the table is the nominal length of the feature being measured.
Symmetry tolerance (Class B = up to 3 mm) is not the same as coaxiality of bores. For two bores that must align for a shaft, use a separate geometric tolerance (GD&T), not ISO 13920.
The classes are independent of each other, meaning a drawing can call for a high-precision size (e.g., class B) while still allowing for a wider tolerance on shape (e.g., class F). This is exactly what the combination signifies: class B for dimensions (lengths & angles) and class F for geometry (shape & position) . The specification ISO 2768-mk is used for a similar purpose but for machined parts, while ISO 13920-BF is used explicitly for welded constructions . Understanding EN ISO 13920-BF: A Guide to Welding
| Nominal Angle Leg Length (mm) | Permitted Deviation (Class B) | | :--- | :--- | | Up to 400 | ± 30 minutes (± 0.5°) | | 400 to 1,000 | ± 20 minutes (± 0.33°) | | Over 1,000 | ± 15 minutes (± 0.25°) |
The most straightforward implementation is to add a note to the drawing's title block, such as . This clearly defines the baseline expectation for all dimensions not otherwise specified, removing ambiguity.
The "BF" combination is a popular choice. Class B for dimensions offers a practical middle ground, avoiding the high cost of extremely tight tolerances while providing sufficient accuracy for most applications. It is commonly used in rail vehicles, motor vehicles, crane construction, steel construction, and machine construction. Tolerance depends on the length of the shorter
The core of the standard is its categorization of dimensional and geometrical tolerances into . These classes, which are the same for both ISO and EN versions of the standard, range from the most precise to the least stringent:
For linear measurements, Class B dictates the following permissible deviations: ±1 mm Over 30 mm to 120 mm: ±1 mm Over 120 mm to 400 mm: ±2 mm Over 400 mm to 1000 mm: ±3 mm Over 1000 mm to 2000 mm: ±4 mm Over 2000 mm to 4000 mm: ±6 mm Over 4000 mm to 8000 mm: ±8 mm Over 8000 mm to 12000 mm: ±10 mm Over 12000 mm to 16000 mm: ±12 mm Over 16000 mm to 20000 mm: ±14 mm Over 20000 mm: ±16 mm Angular Dimensions
Source: DIN EN ISO 13920 Standard.
A welded frame measuring 3000mm in length could deviate by up to 8mm in either direction and still be considered acceptable under the general tolerance rule.