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
is the international standard that specifies general tolerances for linear and angular dimensions, as well as the shape and position of welded structures. Instead of assigning individual tolerances to every single dimension on a technical drawing, engineers use this standard to establish baseline quality requirements.
Note: Total flatness or straightness deviations are evaluated across the entire designated surface or section segment indicated on the technical drawing. Workshop Best Practices for Meeting BF Tolerances
When welding metals, the intense localized heat causes thermal expansion, contraction, and residual stress. This makes it impossible to build a weldment to exact, absolute blueprint dimensions. Rather than forcing design engineers to detail individual, expensive tolerances for every single joint, referencing EN ISO 13920 on an engineering drawing simplifies communication and establishes clear workshop guidelines based on standard workshop accuracy. Anatomy of the Designation: BF Explained
The ISO 13920 Standard regulates standard errors induced by thermal distortion during heavy manufacturing. It bifurcates allowance thresholds into two distinct categories: The "B" Precision Value (Linear and Angular Dimensions)