In the world of structural steel engineering and fabrication, precision is everything. However, absolute perfection is impossible to achieve in welding due to thermal distortion, material inconsistencies, and human factors. This is where tolerance standards come into play.
If you have ever seen the designation "ISO 13920-BF" on a technical drawing or a quote, you know it dictates the quality acceptance criteria. But what does it actually mean?
This post breaks down the EN ISO 13920 standard, specifically focusing on the BF tolerance class, to help you understand when to use it and how to apply it.
"EN ISO 13920-BF" is not a valid designation in the standard itself (ISO 13920 has no BF). It most likely means:
"Weld edge preparation type BF according to ISO 9692-1, with general tolerances according to EN ISO 13920."
If you need the exact dimensional tolerances for such a preparation (e.g., root face, gap), that would be found in ISO 9692-1, not ISO 13920.
EN ISO 13920-BF is a specific technical designation used in engineering drawings to define general tolerances for welded constructions. It combines two distinct precision levels from the ISO 13920 standard to ensure that welded parts fit together correctly without needing individual, complex dimensioning for every single weld. Breaking Down the Code: "BF"
The suffix BF tells the manufacturer exactly which tolerance "bracket" to use for different types of measurements:
B (Linear & Angular Dimensions): This represents the "Medium" or "Mean" tolerance class. It is the most commonly used class in general engineering and steel construction. It provides a balance between functional accuracy and cost-effective fabrication.
F (Shape & Position): This refers to the tolerance for straightness, flatness, and parallelism. While linear dimensions use letters A through D, shape and position tolerances use E through H. Class F is the second-most precise level in this category (often considered "Medium" for shape). Why Use EN ISO 13920-BF?
Using this standard shorthand simplifies communication between designers and fabrication shops. Instead of calculating individual limits for every length and angle, a single note—EN ISO 13920-BF—applies a pre-defined set of rules based on the size of the component. en iso 13920-bf
Neutral Benchmarking: It acts as a universal language for international contracts, ensuring a manufacturer in Europe and a client in the Middle East have the same expectations for accuracy.
Cost Efficiency: Specifying "Class B" avoids the high costs associated with the ultra-precise "Class A," while preventing the safety or assembly risks of the looser "Class C".
Broad Application: This standard is widely used in machine construction, steel structures, rail vehicles, and shipbuilding. Tolerance Tables Overview
The actual permissible deviation (in millimeters or degrees) depends on the nominal size of the part. For example, a 1,000 mm beam in Class B will have a specific allowed variation, whereas a 10,000 mm beam will have a larger allowable margin. Type of Measurement Class for "BF" Key Focus Areas Linear Dimensions B (Medium) Length, width, and height of parts. Angular Dimensions B (Medium) Deviations in angles and slopes. Shape & Position F (Medium) Straightness, flatness, and parallelism. Testing and Verification
Compliance with EN ISO 13920-BF is verified using standard workshop tools such as steel straightedges, tape measures, and vernier callipers. It is important to note that this standard covers dimensional accuracy, not weld quality (like cracks or porosity), which is instead governed by ISO 5817. IS0 13920 - iTeh Standards
If you encounter EN ISO 13920-BF:
Understanding these codes ensures that fabricators aren't over-engineering parts and that clients receive products that fit their structural requirements without unnecessary expense.
Have you encountered confusing tolerance codes on drawings? Share your experiences in the comments below!
#Engineering #Welding #ISOStandards #Fabrication #QualityControl Understanding EN ISO 13920-BF: The Essential Guide to
EN ISO 13920-BF is a specific tolerance designation for welded constructions that combines two different accuracy levels for dimensions and geometry .
B (Medium): This refers to the tolerance class for linear and angular dimensions (length, width, height, and angles) . It is based on "customary workshop accuracy" and is the most common class used in general engineering .
F (Medium): This refers to the tolerance class for shape and position (straightness, flatness, and parallelism) . Key Features of ISO 13920 ISO 13920 An Explained Guide to Welding General Tolerances
To truly understand "EN ISO 13920-BF," you must know the numbers. Below are the key tolerance ranges for Class B (Medium).
If you want, I can:
The feature EN ISO 13920-BF is a combined designation for general tolerances in welded constructions. It specifies an accuracy level for both dimensional and geometric variations on technical drawings. The designation breaks down as follows:
EN ISO 13920: The international standard defining general tolerances for welded structures, covering lengths, angles, straightness, flatness, and parallelism. B: The tolerance class for linear and angular dimensions.
F: The tolerance class for shape and position (straightness, flatness, and parallelism). Typical Applications
This specification is commonly found on drawings for industrial equipment where standard welding precision is required without the need for high-precision machining. Examples include: Summary answer to your query: "EN ISO 13920-BF"
This review is designed for engineering, quality assurance, and drafting departments to determine if this standard is suitable for a given project.
If you saw "EN ISO 13920-BF" on a drawing or welding symbol, it is likely a typo or shorthand used by a designer. The correct standard for edge preparations is ISO 9692-1 (or EN ISO 9692-1).
EN ISO 13920 only covers general tolerances (linear, angular, flatness, etc.), not weld preparation shapes.
While Class B is versatile, it is not always the right choice.
Choose Class A (Fine) if:
Choose Class C or D (Coarse/Very Coarse) if:
Choose "No general tolerance" if:
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.