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: (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.
Tolerances in EN ISO 13920 scale dynamically with the physical size of the assembly. As the nominal size increases, the allowed variation expands to accommodate global thermal expansion during welding. According to official technical metrics: Linear Dimensions (Class B)
For in Class B, the tolerance is determined by the length of the shorter leg of the angle: Up to 400 mm: ± 45 minutes of a degree. Over 400 to 1,000 mm: ± 30 minutes. Over 1,000 mm: ± 20 minutes. 3. Geometric Tolerances for Class F
| Feature | Tolerance | | :--- | :--- | | | 10 mm per meter (10 mm/m) | | Parallelism | 8 mm per meter (8 mm/m) | | Symmetry | 12 mm per meter (12 mm/m) | Note: Standard tolerances for these features for "class F" are provided in many references, such as industry-standard defaults. en iso 13920-bf
The second letter defines tolerances for straightness, flatness, and parallelism.
"EN ISO 13920-B, BUT FLATNESS OF MOUNTING FACE F1 (reference surface) TO BE HELD TO CLASS A (±0.5 mm)"
By providing globally recognized workshop tolerances, this standard simplifies engineering drawings, optimizes production workflows, and reduces fabrication costs by eliminating the need to specify individual tolerances for every single joint. What is EN ISO 13920? Unlike machining tolerances (which are very tight), welding
Understanding EN ISO 13920-BF: General Tolerances for Welded Constructions
| Class | Typical use | |-------|--------------| | | Fine – machined parts, precise jigs | | D | Medium – general structural steelwork | | E | Coarse – large fabrications, shipbuilding | | F | Very coarse – flame-cut plates, heavy beams |
Linear values dictate lengths, widths, heights, and distances between parts: : ±1plus or minus 1 Over 30 to 120 mm : ±2plus or minus 2 Over 400 to 1,000 mm : ±3plus or minus 3 Over 1,000 to 2,000 mm : ±4plus or minus 4 Over 2,000 to 4,000 mm : ±6plus or minus 6 Angular Dimensions (Class B) ISO 13920 An Explained Guide to Welding General Tolerances Tolerances in EN ISO 13920 scale dynamically with
The core of the standard is its categorization of dimensional and geometrical tolerances into . These classes, which are the same for both ISO and EN versions of the standard, range from the most precise to the least stringent:
Angular deviations are calculated based on the of the angle being measured. This metric minimizes layout errors over long spans. Up to 400 mm : Allowed variance of ±plus or minus 45 minutes of a degree ( ±plus or minus 13 mm/m offset). Over 400 mm up to 1,000 mm : Allowed variance of ±plus or minus 30 minutes of a degree ( ±plus or minus 9 mm/m offset). Over 1,000 mm : Allowed variance of ±plus or minus 20 minutes of a degree ( ±plus or minus 6 mm/m offset). Geometrical and Form Tolerances (Class F)
Welding introduces intense heat, causing metal to expand and contract. This inevitably alters the final dimensions of a component. Class B provides acceptable deviations based on the size of the nominal dimension. Larger dimensions are granted larger tolerances because total thermal distortion accumulates over greater lengths. Linear Dimensions (Lengths, Widths, Heights)