This is the critical section for quality control.
Before diving into the "BF" subclass, it is essential to understand the parent standard.
EN ISO 13920 (full title: Welding — General tolerances for welded constructions — Linear and angular dimensions — Shape and position) is the European adoption of the international ISO standard. It serves as a universal reference for permissible deviations in welded assemblies. en iso 13920-bf
Unlike machining tolerances (which are very tight), welding tolerances account for the inherent variability of heat-based joining processes. The standard provides five distinct tolerance classes: A (fine), B (medium), C (coarse), D (very coarse), and E (extremely coarse). These classes allow designers to specify how much deviation is acceptable based on the function of the part.
| Code | Meaning | Application | |------|---------|--------------| | BF | Butt weld – Flat root face + backing | Used when full penetration is required and backing is retained | This is the critical section for quality control
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.
Even experienced fabricators sometimes misinterpret this standard. Avoid these errors: Comparison: If the standard were set to Class
EN ISO 13920 is the international standard titled "Welding — General tolerances for welded constructions."
Its primary purpose is to simplify technical drawings. Instead of adding individual tolerance values for every single dimension (length, angle, straightness), a manufacturer and client can agree on a general tolerance class. Unless otherwise specified, the dimensions of the welded structure must fall within the limits set by that class.
Imagine you are building a support frame for a conveyor system.
If the final measurement shows length = 2,008 mm (violation by 3 mm), the part fails. The solution? Re-cut side members 5 mm short next time to compensate for weld shrinkage.