ISO 13920: The Tolerance Class Your Weldment Is Built To
What the A, B, C and D tolerance classes in ISO 13920 cover, how they interact with EN 1090, and why omitting the class from a drawing costs you money.
Every fabricated machine base is built to a dimensional tolerance class, whether or not the drawing says so. If the drawing is silent, the fabricator picks, and they will reasonably pick one that lets them quote competitively.
What the standard actually covers
ISO 13920 specifies general tolerances for welded constructions: linear dimensions, angular dimensions, and shape and position of the finished fabrication. It defines four graduated classes, A, B, C and D, from tightest to loosest.
Two things follow that are worth stating plainly.
First, a class is not one number. The permitted deviation widens with the size of the dimension being controlled, which is realistic: holding 1 mm over 300 mm and over 4000 mm are not comparable tasks. The class selects a column in a table, and the dimension selects the row.
Second, this is not weld quality. ISO 13920 says nothing about porosity, undercut, penetration or bead profile. Those belong to ISO 5817 and its imperfection levels. A complete drawing carries both: a dimensional class and a weld quality level. Specifying one and assuming the other covers it is a common and expensive gap.
Which class to use
Class A is tight. Reserve it for the specific dimensions that genuinely need it, and expect to pay for it in fixturing, sequencing and rework. Applying class A across a whole large weldment is usually a mistake of habit.
Class B is the workhorse for machine frames and bases. It is achievable by a competent shop with proper fixturing without extraordinary measures.
Class C suits general structural work and non-critical fabrications.
Class D is for structures where dimensions are not critical at all.
The practical approach on a machine base is to specify a general class for the whole fabrication, usually B, and then put specific tighter tolerances on the individual features that need them. That is far cheaper than raising the general class and it tells the shop where to spend their effort.
Remember that machined faces are a separate question again. The weldment tolerance governs the structure as fabricated; the GD&T on the machined pads governs what happens after machining. On a base that will be machined, the fabrication class mainly determines whether there is enough stock to clean up.
How it relates to EN 1090
EN 1090-2 governs execution of steel structures and carries its own tolerance framework, split into essential tolerances, which affect structural adequacy, and functional tolerances, which affect fit and appearance.
EN 1090-2 permits ISO 13920 as an alternative to its functional tolerances, and that alternative is more likely to be appropriate for heavily welded structures where distortion from welding is the dominant factor in the final dimensions. That describes a machine base well.
If your project sits inside an EN 1090 execution class, state explicitly which route you are using, and make sure the fabricator’s quality plan says the same.
Why the class is a commercial decision
The class determines how much fixturing, how much sequencing discipline, how much intermediate measurement and how much correction the fabricator has to plan for. That is hours, and hours are the price.
Three consequences:
- Unstated class means a loose quote. You may get a part that is dimensionally poor and be unable to reject it.
- A uniformly tight class means an expensive quote. You are paying for precision on faces nobody uses.
- A mixed specification is the cheapest correct answer. General class B, specific tolerances where they matter.
If three fabricators quote your base and the prices vary by half, the tolerance specification is one of the first places to look. They may have assumed three different classes.
What to put on the drawing
- The general tolerance class to ISO 13920, in the title block.
- Weld quality level to ISO 5817, with the quality level.
- Specific dimensional tolerances on features that need better than the general class.
- The welding standard the shop is to work to, commonly ISO 3834 with its quality level.
- Machining allowance, agreed for that class and that size: see designing weldments for machining.
- The stress relief requirement and where it sits in the sequence.
- What is to be measured and reported: see inspection and test plans.
Seven lines. They are the difference between three comparable quotes and three guesses.
Common questions
What is the difference between ISO 13920 class A and class D?
They are graduated general tolerance classes for the finished weldment, A being the tightest and D the loosest, covering linear dimensions, angular dimensions and shape and position. The tolerance permitted at each class widens with the size of the dimension, so a class is not a single number but a table. Class B is the common default for machine frames; class A costs significantly more and is often unnecessary.
Does ISO 13920 cover weld quality?
No, and this confuses people regularly. ISO 13920 governs the dimensional geometry of the finished fabrication: how straight, how square, how long. Weld bead quality, defect limits and imperfection levels are covered by separate standards such as ISO 5817. You need both on a drawing and they do different jobs.
What happens if I do not state a tolerance class?
The fabricator assumes one, and reasonably assumes a loose one, because that is what they are entitled to quote against. You then have no basis to reject a part that is dimensionally worse than you expected. Stating the class costs one line on the drawing and removes an entire category of dispute.
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