between functional accuracy and manufacturing cost. It requires skilled welders and decent fixturing but does not mandate expensive post-weld heat treatment or machining. Detailed Tolerance Values for EN ISO 13920-B To truly understand "EN ISO 13920-BF," you must know the numbers. Below are the key tolerance ranges for Class B (Medium). 1. Linear Dimensions (Lengths and Heights) These tolerances apply to external and internal lengths, step heights, and center distances.
Example: A 2,500 mm long beam should not bow more than 3 mm over its entire length. For parts where centering is important (e.g., a boss on a plate). en iso 13920-bf
"USE STANDARD WELDING TOLERANCES" Correct usage (per ISO guidelines): "GENERAL TOLERANCES: EN ISO 13920 – B (MEDIUM). ALL OTHER DIMENSIONS ± 0.5 mm" Advanced usage (with modifiers): If your "BF" actually means "B + Filter," you might add: "EN ISO 13920-B, BUT FLATNESS OF MOUNTING FACE F1 (reference surface) TO BE HELD TO CLASS A (±0.5 mm)" This notation tells the welder: "Use Class B for most features, but tighten up this specific surface." Common Mistakes and Pitfalls with EN ISO 13920-BF Even experienced fabricators sometimes misinterpret this standard. Avoid these errors: Mistake #1: Using Class B for Precision Interfaces If part A slides into part B with a clearance of 0.2 mm, do not use ISO 13920-B . Class B allows ±3 mm over 400 mm. Your sliding fit will jam or wobble. Use Class A or machine after welding. Mistake #2: Ignoring the "Free State" Condition ISO 13920 tolerances apply to the weldment in its free state (no external clamping force). If you force a distorted frame onto a flat table before measuring, you are cheating the inspection. Release clamps and let the part relax. Mistake #3: Forgetting Weld Shrinkage Class B does not account for accumulated shrinkage. If you weld a 10-meter truss, expect 5–10 mm of overall shrinkage. The tolerance of ±8 mm may still be violated if you do not pre-set angles or use back-step welding techniques. Mistake #4: Confusing Symmetry with Coaxiality 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. Inspection Methods for EN ISO 13920-BF Compliance How do you prove your weldment meets the standard? You need the right tools. between functional accuracy and manufacturing cost
Note: For small parts, the tolerance is looser because relative distortion is higher. This is critical for base plates and mounting surfaces. Below are the key tolerance ranges for Class B (Medium)
| Nominal Size Range (mm) | Permitted Deviation (Class B) | | :--- | :--- | | 30 to 120 | ± 2 mm | | 120 to 400 | ± 3 mm | | 400 to 1,000 | ± 4 mm | | 1,000 to 2,000 | ± 5 mm | | 2,000 to 4,000 | ± 6 mm | | 4,000 to 8,000 | ± 7 mm | | > 8,000 | ± 8 mm + 0.5 mm per 1,000 mm |