These dictate how "flat" a sheet or plate must be and how "straight" a long member (like an H-beam) must remain after welding, ensuring the component does not twist or bow unacceptably. Importance of EN ISO 13920-BF in Fabrication

To make this standard useful, we must look at the specific numerical allowances it grants. All tolerance tables in the standard are based on the "nominal dimension," which in the case of a length is the measured size, and for parallelism, it's the length of the shorter surface being measured.

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: Represents the tolerance class for shape and position (such as straightness, flatness, and parallelism). Tolerance Tables for EN ISO 13920-BF

is more than just a drawing note—it is a language of compromise between perfect design and real-world welding. By understanding that "B" stands for Medium tolerances and "F" typically implies general fabrication , you can set realistic expectations for cost, quality, and delivery.

Engineers indicate this standard in the "General Tolerances" block of a technical drawing (e.g., "General Tolerance: EN ISO 13920-BF") to avoid labeling every single dimension with a specific tolerance.

Before diving into the "BF" subclass, it is essential to understand the parent standard.

This is the "intermediate" class for lengths and angles. It defines the allowable deviation for dimensions such as the length of a beam or the angle of a joint. For a 1-meter span, class B might allow a variation of a few millimeters, providing a balance between functional fit and manufacturing speed.

When a drawing specifies , it is referring to:

The standard allows measurement uncertainty. If your caliper has an error of ±0.5 mm and the tolerance is ±2 mm, you are fine. But if the tolerance is ±1 mm, you need a more precise instrument.

| Nominal Dimension Range (mm) | Tolerance (± mm) | | :--- | :--- | | 2 to 30 | ± 2 | | >30 to 120 | ± 2 | | >120 to 400 | ± 3 | | >400 to 1000 | ± 4 | | >1000 to 2000 | ± 6 | | >2000 to 4000 | ± 8 | | >4000 to 8000 | ± 10 | | >8000 to 12000 | ± 12 |

The chosen tolerance class for linear dimensions (lengths, widths, heights) and angular dimensions . Out of the four classes available (A: Fine, B: Medium, C: Coarse, D: Very Coarse), Class B represents standard engineering workshop accuracy.