Reference / Tolerance classes

Tolerance and Weld Quality Classes

Four different class systems can apply to one weldment drawing, and they govern different things. Most weldment drawings that come back with confused quotes have either mixed them up or used only one where two were needed.

The one thing to get right

Two tolerance systems, one drawing

The welded structure is held to a general tolerance for fabrications, normally an ISO 13920 class. The machined features are held to a normal drawing tolerance, from ISO 2768 or called out directly.

Put both on the drawing and say which applies where. A single general tolerance across a welded and machined part either prices the impossible or gets ignored, and both end badly.

Weld quality is separate againISO 5817 grades the welds themselves. It has nothing to do with the dimensional tolerance of the structure.
Execution class is separate againEN 1090 EXC classes scale the whole quality regime, including documentation. It is driven by regulation, not preference.
The condition mattersSay when each number applies, for example flat within 0.1 mm after stress relief and finish machining. Otherwise nobody knows whether it is checked before or after the part relaxes.

The four systems

What each covers, how its classes are ordered, and what to pick.

ISO 13920

General tolerances for welded constructions

Classes: A to D for linear and angular dimensions, E to H for straightness, flatness and parallelism

Class A is the tightest linear class and D the loosest. Class E is the tightest form class and H the loosest. In both sets the permitted deviation grows with the size of the feature, so the same class allows far more on a 4 m member than on a 200 mm one.

What to pick: Class B with class F is a sensible default for machine frames and industrial weldments. Go to A and E only on a feature that genuinely needs it, because the shop has to fixture and check to that number on the welded structure rather than on a machined face.

This governs the welded structure. It does not govern features you machine afterwards.

ISO 2768

General tolerances for machined features

Classes: f fine, m medium, c coarse, v very coarse, plus separate classes for geometry

Applies to dimensions on the drawing that carry no individual tolerance. The permitted deviation again grows with nominal size.

What to pick: Medium is the usual default for machined faces and bores on a weldment. Put individual tolerances on the few features that matter rather than tightening the general class.

Use this alongside ISO 13920, not instead of it. A weldment drawing needs both.

ISO 5817

Acceptance levels for weld imperfections

Classes: B stringent, C intermediate, D moderate

For each imperfection type, undercut, porosity, excess metal, profile, misalignment and the rest, the standard gives a permitted limit at each level. Cracks are accepted at no level. Some limits differ between predominantly static and fatigue loading.

What to pick: B on welds that carry primary or fatigue load, C on secondary structure, D on guarding and brackets. Applying B across a whole frame is the most common and most expensive drawing error in this niche.

It is an acceptance standard, not an inspection standard. Naming a level does not say how anyone will look for the imperfection.

EN 1090-2

Execution of steel structures

Classes: EXC1 to EXC4

The execution class scales welding, inspection, traceability and documentation requirements with the consequence of failure. EXC2 is the common default for ordinary structural work.

What to pick: Driven by the application and, in Europe, by regulation rather than preference. If your component is structural and placed on the European market, get the class confirmed for your product rather than assumed.

A shop certified for one execution class is not automatically approved for a higher one. Ask what the certificate scope covers.

Why there are no limit tables here

These standards are copyrighted documents. Sites that paste their limit tables in full are redistributing something they do not own, and a table transcribed badly is worse than no table, because somebody will build a part from it.

Buy the standard, or read it through your employer or a library. The part we can add is the part above: which standard governs what, and how to choose a class instead of reflexively taking the tightest one.

Our full sourcing policy is on the editorial standards page.

A specification line that works

Welded structure to ISO 13920 class B, form class F. Machined features to ISO 2768-m unless toleranced. Welding to AWS D1.1. Weld acceptance to ISO 5817 level C generally, level B on welds W1 to W6 per the weld map. Visual inspection 100 per cent, MT on W1 to W6 100 per cent. Flatness of face A within 0.1 mm after stress relief and finish machining. Records supplied with the part.

One paragraph, and every shop quoting is pricing the same job. The longer reasoning is in weld quality levels, machining allowance and welding codes for buyers.

Not sure which class your part needs?

Send the drawing with the question in it. We will tell you what the part actually needs before it goes to any shop. Reply within 24 business hours.

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