En Iso 13920-bf Better Instant

Understanding EN ISO 13920-BF: The Complete Guide to Welding Tolerance Standards

1. Linear Dimensions (Lengths and Heights)

These tolerances apply to external and internal lengths, step heights, and center distances.

| Nominal Size Range (mm) | Permitted Deviation (Class B) | | :--- | :--- | | 30 to 120 | ± 2 mm | | 120 to 400 | ± 3 mm | | 400 to 1,000 | ± 4 mm | | 1,000 to 2,000 | ± 5 mm | | 2,000 to 4,000 | ± 6 mm | | 4,000 to 8,000 | ± 7 mm | | > 8,000 | ± 8 mm + 0.5 mm per 1,000 mm |

Interpretation: If you weld a frame with a nominal length of 1,500 mm, the actual finished length can be between 1,495 mm and 1,505 mm and still be compliant with EN ISO 13920-BF.

5. Practical Application Example

Scenario: Fabricate a welded steel frame, 2000 mm long, with a backing run on the main butt joint. en iso 13920-bf

Correct specification:

• General linear tolerance: EN ISO 13920-D
• Angular tolerance: 6 mm/m
• Weld prep for main butt joint: EN ISO 9692-1 – BF (Backing, Flat root face)

If someone wrote "EN ISO 13920-BF" on a drawing:
You would ask for clarification. Likely they want:

• Tolerance class F (very coarse – ±6 mm over 2000 mm length)
• Weld prep BF (backing run)

1. Objective

To interpret and apply the BF specification under EN ISO 13920 for welded assemblies requiring backing run or back weld identification on drawings and manufacturing documentation. Understanding EN ISO 13920-BF: The Complete Guide to

Mistake #1: Using Class B for Precision Interfaces

If part A slides into part B with a clearance of 0.2 mm, do not use ISO 13920-B. Class B allows ±3 mm over 400 mm. Your sliding fit will jam or wobble. Use Class A or machine after welding.

What is EN ISO 13920?

EN ISO 13920 is the international standard titled "Welding — General tolerances for welded constructions."

Its primary purpose is to simplify technical drawings. Instead of adding individual tolerance values for every single dimension (length, angle, straightness), a manufacturer and client can agree on a general tolerance class. Unless otherwise specified, the dimensions of the welded structure must fall within the limits set by that class. General linear tolerance: EN ISO 13920-D Angular tolerance:

Mistake #2: Ignoring the "Free State" Condition

ISO 13920 tolerances apply to the weldment in its free state (no external clamping force). If you force a distorted frame onto a flat table before measuring, you are cheating the inspection. Release clamps and let the part relax.

If you see "EN ISO 13920-BF" on a drawing:

Interpret as:

• B = possibly a typo or internal company class
• F = tolerance class F (coarsest)
• BF weld prep (but that’s unrelated to tolerances)