For angular dimensions, the tolerance is expressed as a deviation of the shorter side (in mm) over a given length. This is equivalent to a tolerance on the angle itself, which varies with length.
Even a part that appears perfect at a macro level will reveal imperfections at a micro view. As long as an imperfection will still allow the part to function as designed, that given part does not necessarily have to be scrapped. Dimensional tolerance, or the total amount a specific dimension may vary, is the difference between the upper (maximum) and lower (minimum) limits. Because it is impossible to manufacture everything to an exact size, tolerances are used on production drawings to control part variation and ensure proper assembly. In the world of welding, this concept is governed by the international standard . This article explores the EN ISO 13920 standard in detail, with a particular focus on deciphering the " BF " designation and its practical implications. en iso 13920-bf
The EN ISO 13920 standard is an essential component of modern welding engineering, providing a systematic and economically balanced framework for controlling dimensional variation. The specific designation "" acts as a compact, powerful instruction on an engineering drawing. It signifies that the manufacturer must achieve a medium level of precision for linear and angular dimensions (Class B) and a corresponding medium level of precision for the structure's overall form and alignment (Class F) . For angular dimensions, the tolerance is expressed as
Default for unmarked drawings is usually . As long as an imperfection will still allow
"EN ISO 13920-B, BUT FLATNESS OF MOUNTING FACE F1 (reference surface) TO BE HELD TO CLASS A (±0.5 mm)"
Table derived from data where class B angular tolerance is listed as 0.75°, 0.5°, 0.33° for ranges 0-400, >400-1000, >1000 respectively, converted to mm/m using the tangent of these angles.
If part A slides into part B with a clearance of 0.2 mm, . Class B allows ±3 mm over 400 mm. Your sliding fit will jam or wobble. Use Class A or machine after welding.