When a drawing calls out , it specifies a precise combination of two distinct tolerance classes: Class B for dimensions and Class F for geometrical shapes. Deconstructing the "BF" Designation
replaces individual tolerance callouts on drawings with general workshop-based standards. CertBetter Tolerance Classes Available Class in "BF" Description Linear & Angular A, B, C, D Medium Accuracy
The designation breaks down into two distinct categories evaluated by inspectors: en iso 13920-bf
Fabricate a welded steel frame, 2000 mm long, with a backing run on the main butt joint.
The "BF" combination is a popular choice. Class B for dimensions offers a practical middle ground, avoiding the high cost of extremely tight tolerances while providing sufficient accuracy for most applications. It is commonly used in rail vehicles, motor vehicles, crane construction, steel construction, and machine construction. When a drawing calls out , it specifies
All relevant personnel—designers, welders, and inspectors—must understand what the tolerance classes mean in practical terms. Training should connect the abstract tolerance class to real-world outcomes, explaining how tighter tolerances influence welding sequences and distortion control and how cost escalates when unnecessary precision is specified.
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. The "BF" combination is a popular choice
Would you like the dimensional tables for Classes C–F, or the detailed ISO 9692-1 BF preparation sketch?
This is the "Medium" (mean) tolerance class for lengths and angles. F (Shape and Position):