ISO 13920 for welded assemblies: scope, classes and limits

A 2,000 mm base frame called out to ISO 13920-B can finish 6 mm long, pass every dimension on the drawing, and still fail to bolt to the machine it was built for. The standard is not wrong. Class B genuinely allows ±6 mm over that length, and the buyer who expected ±1 mm specified a class that never promised it.

The standard is the correct framework for welded envelopes: lengths, angles, straightness, flatness and parallelism. It is also the one most often invoked without its table being read, which is why welded frames generate more first-article arguments than any other fabricated part.

This article gives the class values, works the diagonal arithmetic that catches a bad frame, and sets out the point where the standard stops and machining starts. The surrounding standards are collected in the standards and compliance hub.

What does ISO 13920 cover?

ISO 13920:2023 specifies general tolerances for welded constructions: linear dimensions, angular dimensions, and shape and position. It defines four classes, A to D, for lengths and angles, and a second set, E to H, for straightness, flatness and parallelism. Class A is the tightest in both sets.

The document exists so a drawing does not carry a tolerance on every welded dimension. A frame with 30 dimensions can carry one note instead of 30 numbers, and the note tells the fabricator what workshop accuracy the job assumes.

Its scope stops at the weldment:

• It covers the welded construction, including weldments, welding assemblies and welded structures.
• It does not cover weld quality. Porosity, undercut, throat size and spatter are ISO 5817 and the weld symbol, not ISO 13920.
• It does not cover the parts before welding. A bent bracket that goes into the fixture carries its own tolerance from ISO 2768-1 or an explicit callout, and that error is inside the assembly stack.
• It does not cover machined features. A pad milled after welding, a bored hole or a ground face follows a machining tolerance, and the drawing has to say which is which.
• It does not overrule an explicit tolerance. A dimension with its own number is outside the general class.

The class values, and what they allow

The table below is the length table from the 2023 edition. Read the Ganesh column as what a fixtured frame can hold when the job needs better than the class, not as the class itself.

The angular table is set by the shorter side of the angle: class A allows ±20′ up to 400 mm, ±15′ from 400 to 1,000 mm and ±10′ above 1,000 mm. Class B is ±45′, ±30′ and ±20′ over the same ranges. Class C doubles again to ±1°, ±45′ and ±30′, and class D runs to ±1°30′, ±1°15′ and ±1°.

The shape classes sit on the longer side of the surface. For a 1,000–2,000 mm surface, class E allows 2 mm of straightness or flatness error, F allows 4.5 mm, G allows 9 mm and H allows 14 mm. A 2,000 mm frame that measures 8 mm of bow is inside class G and outside class E, which is why the shape class has to be named.

The values are set by customary workshop accuracy across fabricated steelwork, not by what a particular shop can achieve with a fixture. That is why class B is ±6 mm at 2,000 mm while a fixtured cell can hold ±1.0 mm on the same frame: the class describes an industry norm with no fixture assumption and no straightening operation written into it. Treat it as a ceiling that a competent fabricator will normally beat, not as a target to design the joint around.

One more assumption is buried in the class: the fabricator may straighten after welding. Flame straightening, press correction and mechanical adjustment are normal ways to bring a weldment inside the envelope, and the standard does not forbid them. If straightening is not allowed because the residual stress matters to the application, say so on the drawing. A class note plus a no-straightening note is a much harder requirement than either one alone.

Worked example: the frame that conforms and will not fit

Take a 2,000 mm × 600 mm rectangular frame in 3 mm CRCA, welded in a fixture and called out to ISO 13920-B.

The nominal diagonal is √(2,000² + 600²) = 2,088.1 mm. Now suppose the two long sides finish at 2,006 mm and 2,000 mm, both inside the class B allowance, with the width at 600 mm. The diagonals become √(2,006² + 600²) = 2,093.8 mm and 2,088.1 mm, a difference of 5.7 mm.

Five point seven millimetres of diagonal difference is about 0.16° of skew across the frame. An M10 bolt in an 11 mm clearance hole leaves 1 mm of total float, or 0.5 mm per side. Two parts each carrying that skew cannot be pulled together without force, and the force goes into the frame, the weld or the bolt thread.

That is the gap between a fabrication tolerance and an assembly tolerance. Class B is honest about what welding does; it is not a statement that the frame will bolt up.

Angular tolerance at the end of a long member

Angles are where the class values surprise people, because a small angular band becomes a large linear error over a long flange.

Class B allows ±20′ above 1,000 mm. Over a 2,000 mm member that is 2,000 × tan 20′ = 11.6 mm at the free end. Class A halves it to ±10′ and still leaves 5.8 mm. A canopy frame, a mast section or a long base rail that must sit parallel to another member is therefore controlled by the angular callout, not by the length callout, and the angular callout is the one most often left to the general class.

If the two ends of a member must be parallel within 2 mm over 2,000 mm, the requirement is 2/2,000 = 0.001 radian, or about 3.4′. That is tighter than class A. It needs a fixture that sets both ends, a sequence that balances the welds, and a check on a surface plate or with a laser, not a class note.

The angular error also compounds across a frame. Four corners each at the top of the class B angular band put the closing member out by the sum of the two end errors, not by one. That is why a rectangular frame is inspected on the diagonal rather than on the four angles: the diagonal captures the combined effect in one measurement.

Choosing a class without over-specifying

The decision rule is short.

• Class B for general fabrication: base frames, canopy structures, guards and supports where the frame locates nothing precision.
• Class A where the frame is fixtured, the sequence is written down, and the interface is still a bolted joint with clearance. Class A is a process commitment, not a drawing note.
• Class C or D for large structures, skids and weldments where the section is stiff and the interface is generous.
• Machining where the interface must hold better than ±1 mm. Weld the frame oversize, machine the pad or drill the hole from the fixture after welding, and put that feature under a machining tolerance.

The last point is the one that saves projects. A ±0.5 mm interface is not a welded-envelope problem; it is a machining problem attached to a welded frame. Once the drawing separates the two, the class can be honest and the interface can be tight, and the quotation reflects both.

Designation on the drawing

The designation combines the two class sets. The suffix -B applies length and angle class B; -BE applies length class B with shape and position class E. The designation goes in the general notes or the title block, next to the ISO 2768 callout for the machined features, with a note saying which dimensions are welded and which are machined.

That note is the part that prevents arguments. A drawing with ISO 13920-BE and ISO 2768-m and no boundary between them leaves the inspector to guess which standard governs the pad that was milled after welding. The welded assembly tolerance guide works through the stack of part, fixture and shrink that sits under the class, and the frame distortion guide covers the sequence that decides where the movement lands.

Inspecting to the class

Three measurements carry most of the evidence on a welded frame: the diagonal, flatness against a straight edge or surface plate, and the position of the interface holes from the datum.

The diagonal is the efficient one. On a 2,000 × 600 mm frame, a 5.7 mm diagonal difference corresponds to the 6 mm length error that class B permits, so one tape measurement tells you whether the frame is at the edge of its class. Record it at first-off and every tenth assembly rather than at final inspection only, because a drift in the fixture shows up on the diagonal before it shows on any single dimension.

Flatness needs a reference the frame has not been clamped to. Measure it free on a surface plate or on three supports, not bolted to the bench, or the reading describes the bench.

What Ganesh runs on a welded frame

Repeat welded assemblies run on three CLOOS robotic MIG cells with fixtures designed and built in-house. The robot matters for tolerance because it repeats the weld sequence and the heat input on every part, which is what makes a ±1.0 mm result repeatable on a 2,000 mm frame that the class would allow to move 6 mm.

More than 30 manual MIG/MAG, spot and special-purpose stations handle the larger frames and the low-volume work where a fixture would not pay back. Fixtures are located on the assembly interface and the hole pattern, with replaceable pins and pads, and the fixture is checked at the start of a production run rather than once at commissioning.

Customer work in this shape includes elevator frames for TKE and Eros Elevators and base frames and canopy structures for Mahindra Powerol, under ISO 9001:2015 and ISO 14001:2015. The welded assemblies capability sets out the cell and station range.

What to put on the drawing

1. The designation, for example ISO 13920-BE, in the general notes rather than buried in a title block field.
2. A boundary note saying which dimensions are welded and which are machined after welding.
3. The interface dimensions with their own tolerance and datum: mounting faces, hole patterns and anything that meets another part.
4. The weld quality level to ISO 5817, separately from the dimensional class.
5. The sequence assumption where class A is required, or a note that the supplier proposes the sequence.
6. The inspection method for the controlling dimensions: diagonal, flatness on a surface plate, hole position from the datum.

When ISO 13920 is the wrong framework

Below about 2 mm thickness, the distortion from a single seam is larger than the class values are written for, and a thin enclosure is better controlled by a fixture, a balanced sequence and an explicit flatness figure in millimetres per metre. A welded enclosure that must be square and flat needs the requirement written directly, not inherited from a class built around structural weldments.

A frame that is welded and then machined is also two documents. The welded envelope follows ISO 13920; the machined interface follows ISO 2768-1 or an explicit callout. Trying to hold a machined tolerance through the weld is the most expensive way to fail a first article. The fixture control guide covers the pin and clamp detail that makes the machined route repeat.

Send the assembly drawing with the interface marked to our engineering team. We will say which class the envelope can hold, which features need machining after welding, and where the fixture has to locate.