The standard categorizes welded structures into five tolerance classes: . These range from very fine (Class A, typically for precision machinery) to very coarse (Class E, for massive steel structures like bridges or ship hulls). Class D , the subject of this essay, sits in the middle-lower range of the spectrum. It is best described as the “standard commercial” tolerance class. It is neither precision engineering (Class A/B) nor rough construction (Class E). Instead, ISO 13920-D applies to general fabrication where fit-up is important, but where post-weld machining is not anticipated.
For flat surfaces (like a base plate) or long beams: iso 13920-d
Measured based on the shorter leg of the angle. Shape: Specifically the straightness and flatness of parts. It is best described as the “standard commercial”
Perhaps the most critical aspect of Class D is flatness. For a reference length of 1000 mm, Class D permits a flatness deviation of up to 5 mm. In practical terms, a 1-meter-long beam flange can be bowed or twisted by 5 mm and still conform to the standard. For welding, this is significant: a 5 mm gap at the root of a butt joint is considered acceptable under Class D, whereas Class A would demand less than 1 mm. For flat surfaces (like a base plate) or