En Iso 13920-bf

EN ISO 13920 is the European adoption of the international standard ISO 13920:1996 – Welding – General tolerances for welded constructions.
It specifies general tolerances for linear and angular dimensions, flatness, straightness, and parallelism for welded structures, where no individual tolerances are indicated on the drawing.

The standard applies to:

Prepared for: [Project Name / Client]
Date: [Current Date]
Standard Reference: EN ISO 13920:1996 — Welding — General tolerances for welded constructions
Symbol reference: BF (Backing / Back weld notation)

When a drawing specifies ISO 13920-BF, it is referring to: en iso 13920-bf

Correction: In the ISO 13920 coding system, the second letter regarding geometrical tolerances is either C (Fine) or D (Medium). There is no "F" class for geometry.

However, the combination "BF" does not strictly exist in the standard's classification list (which lists AC, AD, BC, and BD).

Why do people write "BF"? In many industrial contexts, particularly in heavy machinery and automotive supply chains, engineers often confuse or conflate standards. The most common standard classes are: EN ISO 13920 is the European adoption of

If you see ISO 13920-BF on a document, it is highly likely a typo or a non-standard internal code. It almost certainly implies Class B for linear dimensions. The "F" likely refers to a "Fine" requirement for geometry (which would make it ISO 13920-AD), or it is a typo for D (making it ISO 13920-BD).

Note: If the drawing explicitly writes "BF", you must clarify with the client. However, standard-compliant codes are: AC, AD, BC, BD.

(For the sake of this post, we will assume the user implies a Medium Linear / Medium Geometric requirement, which is standard Class BD, or is asking about the specifics of Class B). Prepared for: [Project Name / Client] Date: [Current

Among the five tolerance classes, Class B (Medium) is the most commonly specified. Here is why manufacturers prefer it over finer classes like A or coarser classes like D:

| Tolerance Class | Typical Application | Cost Impact | | :--- | :--- | :--- | | A (Fine) | Precision machinery, jigs, fixtures | High (requires post-weld machining) | | B (Medium) | General mechanical structures, frames, supports | Optimal (achievable with standard welding) | | C (Coarse) | Heavy construction, shipbuilding, basic frames | Low (minimal inspection) | | D/E (Very coarse) | Simple structural steel, agricultural equipment | Very low |

Class B strikes the perfect balance between functional accuracy and manufacturing cost. It requires skilled welders and decent fixturing but does not mandate expensive post-weld heat treatment or machining.

Specifying EN ISO 13920-BF on a contract or purchase order is not just a suggestion—it is a contractual obligation.