Tolerance disputes are one of the most common sources of friction between clients and fabricators. A drawing that says 'fabricate to standard' without specifying which standard, or a client who expects machined-part accuracy from a welded assembly, leads to disagreements at the inspection stage. This article explains what JIS standards specify for structural steel fabrication and what is realistic to achieve in a workshop environment.
JIS B 0408 and general fabrication tolerances ¶
JIS B 0408 covers dimensional tolerances for metal parts produced by cutting, bending, and welding. For welded assemblies, the relevant tolerance class is typically class m (medium) or class c (coarse), depending on the application. Class m allows a length tolerance of plus or minus 0.5 mm for dimensions up to 30 mm, increasing to plus or minus 1.5 mm for dimensions between 315 mm and 1000 mm. For structural applications where fit-up to other steelwork is critical, tighter tolerances must be specified explicitly on the drawing.
Weld distortion and its effect on final dimensions ¶
Welding introduces heat, and heat causes distortion. A flat plate welded on one side will bow toward the weld. A box section welded at all four corners will twist if the weld sequence is not controlled. Experienced fabricators manage distortion through weld sequence planning, pre-setting (deliberately introducing a counter-distortion before welding), and post-weld straightening. For assemblies with tight dimensional requirements, we discuss the distortion management approach with the client before fabrication starts, not after.
When to specify tighter tolerances and when not to ¶
Tighter tolerances cost more, because they require more careful setup, more frequent measurement during fabrication, and sometimes post-weld machining of mating surfaces. For most structural applications. Brackets, frames, skids. Standard JIS B 0408 class m tolerances are adequate. For items that must mate with precision-machined components or that carry alignment-critical equipment, tighter tolerances are worth specifying. The drawing should state the tolerance class explicitly, not rely on the fabricator to guess.
When you send us a drawing, include the tolerance class or any critical dimensions that need special attention. We flag any dimensions we consider difficult to achieve before we start, not after.