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Understanding EN ISO 13920-BF: A Guide to Welding Tolerances

The designation EN ISO 13920-BF refers to a specific set of general tolerances for welded constructions. It combines two distinct tolerance classes defined by the international standard ISO 13920: Class B for linear and angular dimensions, and Class F for straightness, flatness, and parallelism.

This standard is essential for ensuring consistency and quality in structural steel fabrication when individual tolerances are not specified on a drawing. 1. Breakdown of the "BF" Designation

The "BF" suffix is a shorthand used in engineering drawings to indicate the level of accuracy required for a welded assembly.

Class B (Medium): This is the most common tolerance class used in general engineering and construction. It covers linear dimensions (lengths, widths, heights) and angular dimensions.

Class F: This class specifically governs the geometric tolerances of the weldment, including how straight a part is or how flat a surface must be. 2. Tolerance Tables for Class B (Linear & Angular)

According to technical guides from Scribd and Modulus Metal, the permitted deviations for Class B depend on the size of the component: Range of Nominal Size (mm) Linear Tolerance (± mm) Over 30 to 120 Over 120 to 400 Over 400 to 1,000 Over 1,000 to 2,000

For Angular Dimensions 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

Class F defines the allowable variation for the shape and position of the structure. Common tolerances for Class F include: Over 30 to 120 mm: +1 mm. Over 120 to 400 mm: +1.5 mm. Over 400 to 1,000 mm: +3 mm. 4. Why Use EN ISO 13920-BF?

Adopting these general tolerances simplifies the design and manufacturing process by: en iso 13920-bf

ISO 13920 Welding General tolerances for welded constructions

EN ISO 13920-BF is a specific technical callout for general tolerances in welded constructions, where B represents the tolerance class for linear/angular dimensions and F represents the class for shape and position. 💡 🛠️ Decoding the "BF" Designation

In engineering drawings, this standard simplifies documentation by applying a blanket level of accuracy across a whole project rather than labeling every single weld. Class B (Linear & Angular): Considered "Medium" or "Fine" accuracy for welding.

For a length of 1 meter, the allowed deviation is approximately ±2plus or minus 2 Class F (Shape & Position): Covers straightness, flatness, and parallelism.

Ensures that large welded frames don't warp or twist beyond a functional limit during the cooling process. 🏗️ Why It Matters (The "Interesting" Part)

A common debate in fabrication shops—highlighted in community discussions on Reddit—is whether "BF" is actually too loose for modern machinery.

The "Agreement" Tool: It acts as a legal neutral ground. If a parts supplier in Europe and a contractor in the Middle East both agree to "ISO 13920-BF," there is zero ambiguity about what counts as a "failed" part.

Cost vs. Precision: Choosing "BF" instead of a tighter class (like "A") can save thousands in labor. It acknowledges that heat from welding naturally distorts metal, so "perfect" isn't always practical or necessary.

Latest Updates: The standard was recently refreshed as ISO 13920:2023, replacing the long-standing 1996 version to align with modern ISO styles and references. 📋 Key Technical Categories Understanding EN ISO 13920-BF: A Guide to Welding

The standard is broken down into three main data tables that engineers use to check compliance:

Linear Dimensions: Scales based on length (e.g., 0–30mm, 30–120mm, up to 20,000mm).

Angular Dimensions: Tolerance depends on the length of the shorter leg of the angle.

Geometric Tolerances: Specifically targets "straightness" and "parallelism" to prevent "banana" shaped beams.

💡 Pro-Tip: If your drawing lists ISO 13920-BF but the part needs to fit into a precision-machined slot later, you must override the general tolerance with a specific "local" tolerance, or you risk a costly assembly failure. To help you apply this to a specific project, ISO 13920 An Explained Guide to Welding General Tolerances

In the world of metal fabrication, EN ISO 13920-BF isn't just a string of characters—it's the set of rules that keeps heavy structures from being "crooked." If you're building something like a heavy-duty roll trailer desorber unit , this standard is your blueprint for precision. The Story of the "Perfectly Imperfect" Weld

Imagine a team of engineers designing a massive steel frame. They know that when you apply intense heat to metal during welding, it expands, contracts, and twists. Without a shared "tolerance" agreement, the person welding the frame might think a 5mm warp is fine, while the person installing the machinery on top thinks it's a disaster. This is where EN ISO 13920 steps in. It provides a common language for general tolerances for welded constructions Breaking Down the "BF" The suffix

is the specific "handshake" between the designer and the workshop: B (Linear/Angular Dimensions):

This is the "Medium" tolerance class. It’s like saying, "We need this to be accurate, but we aren't building a watch." It allows for small variations in length and angles that are typical for professional workshop accuracy. F (Shape and Position): EN ISO 13920 is the standard for Welding

This refers to how straight or flat the structure is. Class "F" is a common standard for general engineering, ensuring the final piece isn't so distorted that it won't fit into its designated space. Why it Matters By specifying EN ISO 13920-BF on a drawing, a company like NOVATECH ApS

ensures that their 13-ton trailers are built to a consistent quality. It prevents expensive "re-work" and ensures that if a part is made in one factory, it will perfectly match a part made in another. exact measurement tables for the "B" and "F" classes to use on a technical drawing?

Here is the breakdown:

• EN ISO 13920 is the standard for Welding — General tolerances for welded constructions (though sometimes confused with ISO 9692 for edge preparation). However, in workshop practice, "BF" is defined in ISO 9692-1 (and adopted as EN ISO 9692-1).
• BF stands for "Backing Flange" or more precisely a Backing run edge preparation. Specifically, BF designates a symmetrical double-V or double-bevel preparation with a backing strip or retained root face.

When to refer to other standards

• For weld quality/acceptance (e.g., ISO 5817 / ISO 6520) — use those standards alongside EN ISO 13920.
• For structural steel tolerances and fabrication (e.g., EN 1090) — coordinate requirements.

Decoding the "BF" Designation

The standard uses a letter-code system to define four distinct tolerance classes. The code BF is actually a combination of two separate tolerance categories.

2. Angular Dimensions (Miters and Bevels)

Angles are notoriously difficult to maintain in welding due to distortion.

| Nominal Angle Leg Length (mm) | Permitted Deviation (Class B) | | :--- | :--- | | Up to 400 | ± 30 minutes (± 0.5°) | | 400 to 1,000 | ± 20 minutes (± 0.33°) | | Over 1,000 | ± 15 minutes (± 0.25°) |

Note: For small parts, the tolerance is looser because relative distortion is higher.

Typical numeric examples (illustrative; consult standard for exact values)

• Small linear tolerance example: ±1.0 mm for short features in a medium tolerance class.
• Angular tolerance example: ±0.5° to ±1.5° depending on class and feature size. (Do not use these illustrations for procurement or inspection—always refer to the official standard for exact tables.)

B. Angular Tolerances (Class B)

Angular tolerances are defined by the width of the shorter leg of the angle.

• General Rule: Roughly ±0°30' (±0.5 degrees) for smaller dimensions, relaxing slightly for larger dimensions.
• This prevents acute angles from drifting significantly, which is critical for fit-up of bolted connections.