En Iso 13920-bf -
When welding metals, the intense localized heat causes thermal expansion, contraction, and residual stress. This makes it impossible to build a weldment to exact, absolute blueprint dimensions. Rather than forcing design engineers to detail individual, expensive tolerances for every single joint, referencing EN ISO 13920 on an engineering drawing simplifies communication and establishes clear workshop guidelines based on standard workshop accuracy. Anatomy of the Designation: BF Explained
When a drawing specifies , it is identifying two specific tolerance classes:
The chosen class for shape and position . It governs characteristics like straightness, flatness, and parallelism across the welded framework. The positional classes range from E (Fine) through H (Very Coarse), leaving Class F as the standard "medium" benchmark. Class B: Dimensional Tolerances (Linear & Angular)
This standard is essential for ensuring consistency and quality in structural steel fabrication when individual tolerances are not specified on a drawing. 1. Breakdown of the "BF" Designation en iso 13920-bf
Class B allows for the natural thermal distortion that occurs during welding without requiring expensive post-weld machining or straightening processes.
To maintain compliance, manufacturing teams should implement several engineering best practices:
: Guarantees that parts fabricated in different shops or by different welders will fit together seamlessly during final assembly. When welding metals, the intense localized heat causes
: If a critical feature requires tighter control than what Class BF provides, that specific dimension must be toleranced individually on the drawing. The individual tolerance always overrides the general standard.
EN ISO 13920-BF provides a range of tolerance classes, each with its own set of specific tolerance values. The tolerance classes are designated by a letter (e.g., f, m, c, or v), which corresponds to a specific level of precision. For example, the "f" class represents a fine tolerance, while the "v" class represents a very coarse tolerance.
replaces individual tolerance callouts on drawings with general workshop-based standards. CertBetter Tolerance Classes Available Class in "BF" Description Linear & Angular A, B, C, D Medium Accuracy Anatomy of the Designation: BF Explained When a
How do you prove your weldment meets the standard? You need the right tools.
If you need the exact for such a preparation (e.g., root face, gap), that would be found in ISO 9692-1, not ISO 13920.
If you are operating under Class B, you must consult the tables within the ISO 13920 standard.