En Iso 13920-bf < 2024 >

1. Overview of EN ISO 13920

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:

  • Steel, aluminum, and other weldable materials
  • Fabrications made by fusion welding
  • Weldments of all sizes (with some restrictions for very small or large parts)

3. Tolerance Classes in EN ISO 13920

| Class | Typical use | |-------|--------------| | C | Fine – machined parts, precise jigs | | D | Medium – general structural steelwork | | E | Coarse – large fabrications, shipbuilding | | F | Very coarse – flame-cut plates, heavy beams | en iso 13920-bf

Default for unmarked drawings is usually Class D.

Why Class B (Medium) is the Industry Standard

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: and parallelism for welded structures

| 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. en iso 13920-bf

Development Review: EN ISO 13920-BF

Standard: EN ISO 13920 (Welding – General tolerances for welded constructions) Class: BF Status: Active (Harmonized across EU)

3. Meaning of "BF" in context

  • B → Backing weld / backing run
  • F → Usually indicates finish or face side (depends on company drawing standard)
  • Most common usage: a backing run is required before filling the weld groove, with specific tolerance class applied to assembly after backing weld.

2. The Second Letter: Geometrical Tolerances

The second letter defines tolerances for straightness, flatness, and parallelism.

  • Class C: Fine tolerance. For components requiring high geometric accuracy.
  • Class D: Medium tolerance. Suitable for standard structural elements where slight warping or bending is acceptable without compromising structural integrity.

4. Symmetry and Eccentricity

For parts where centering is important (e.g., a boss on a plate).

| Nominal Size (mm) | Permitted Deviation (Class B) | | :--- | :--- | | Up to 120 | 1 mm | | 120 to 400 | 2 mm | | Over 400 | 3 mm |

Typical geometry for "BF" (per ISO 9692-1):

  • Root face (f): 0–4 mm (often 2–3 mm)
  • Root gap (b): 0–4 mm (often 0–2 mm)
  • Bevel angle (α): ~45°–50° per side for double V (total groove angle ~90°–100°)
  • Backing: Either a permanent steel backing strip or a removable backing (ceramic/copper)