Choosing the right tolerance class is a matter of balancing cost with function. Too fine a tolerance (e.g., specifying Class A and E when only Class B and F are needed) will cause fabrication costs to rise sharply without providing additional functional benefit. Conversely, too coarse a tolerance may compromise the structure's safety or create assembly problems on site. The "BF" class is the industry standard for general engineering because it avoids the high costs associated with fine tolerances while ensuring the finished structure will assemble and function as intended.
For a welded assembly with linear dimension tolerances in class B, the permissible deviations (in millimeters) for a given nominal dimension range are as follows:
Class F defines the allowable variation for the shape and position of the structure. Common tolerances for Class F include: +1 mm. Over 120 to 400 mm: +1.5 mm. Over 400 to 1,000 mm: +3 mm. 4. Why Use EN ISO 13920-BF?
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. en iso 13920-bf
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Angular tolerances protect the structural alignment of intersecting parts. According to the standard, the limit is determined by the length of the of the angle being measured. Up to 400 mm : Allowed deviation is ±45′plus or minus 45 prime (minutes of a degree), equating to a calculated offset of Over 400 mm up to 1,000 mm : Allowed deviation is ±30′plus or minus 30 prime , equating to an offset of Over 1,000 mm : Allowed deviation is ±20′plus or minus 20 prime , equating to an offset of
Mastering EN ISO 13920-BF: The Blueprint for Welded Construction Tolerances Choosing the right tolerance class is a matter
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.
| Feature | Tolerance | | :--- | :--- | | | 10 mm per meter (10 mm/m) | | Parallelism | 8 mm per meter (8 mm/m) | | Symmetry | 12 mm per meter (12 mm/m) | Note: Standard tolerances for these features for "class F" are provided in many references, such as industry-standard defaults.
Angular deviations are calculated based on the length of the shorter leg of the angle: ±45′plus or minus 45 prime ±13plus or minus 13 400 mm to 1,000 mm: ±30′plus or minus 30 prime ±9plus or minus 9 Over 1,000 mm: ±20′plus or minus 20 prime ±6plus or minus 6 3. Geometrical Tolerances (Class F) The "BF" class is the industry standard for
If your assembly requires high-precision robotic integration later, BF might be too loose. But for 90% of general structural steel, it’s the gold standard for balancing cost and quality.
: This refers to the tolerance class for shape and position (straightness, flatness, and parallelism) . Key Features of ISO 13920 ISO 13920 An Explained Guide to Welding General 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: