What tolerances and wall thicknesses are achievable in aluminium extrusion?

The short answer

Two standards, two price levels

Aluminium extrusion is bought against one of two European tolerance systems, and the choice changes the price more than most buyers expect. EN 755-9 is the general standard for extruded profiles and is what a mill quotes by default. EN 12020-2 is the precision system for close-tolerance profiles, and it roughly halves the permitted deviation on wall thickness and opening dimensions. A wall of 2 mm may deviate 0.25 mm under EN 755-9 and 0.15 mm under EN 12020-2; an opening below 60 mm moves from 0.40 to 0.15 mm. Straightness, twist and flatness tighten in the same proportion.

The cost of that tightening is not a line on a price list; it is paid through slower press speed, more frequent die correction, more frequent measurement and a higher rejection rate. Suppliers commonly quote 20 to 30 percent more for precision class, and that is before yield loss. The rule that follows is to buy the standard you need and no tighter: specifiers who write EN 12020-2 "to be safe" pay for a capability the part does not use, while specifiers who leave EN 755-9 on an assembly that needs precision-class openings discover the mismatch at first build, when a 0.25 mm oversize means every part needs hand fitting.

Dimension A and dimension B: why hollows are looser

EN 755-9 does not apply one tolerance to the whole section; it separates dimensions by what they span. Dimension A covers a solid dimension not affected by wall thickness, for example the outside width of a profile or the thickness of a solid leg. Dimension B covers a dimension across a metal-enclosed hollow space, and it carries a wider tolerance because both the die and the mandrel can move under pressure while the metal flows around them. At a nominal 20 mm, A is plus or minus 0.25 mm and B is plus or minus 0.45 mm; at 60 mm the figures are 0.35 and 0.70. Wall thickness is handled separately again, as class A, B or C depending on whether the wall is a free wall, an outer wall of a hollow or a wall between two cavities.

This split explains a common argument at first article. A buyer measures the inside width of a hollow and finds it 0.4 mm over nominal on a 20 mm section, and the supplier is entirely within the standard, because that dimension is a B dimension. The same 0.4 mm on a solid outside face would be a reject. Knowing which column a dimension falls in, before the drawing is released, prevents that conversation.

Wall thickness: the floor is set by the die, not by the drawing

There is no universal minimum wall in the standards, because the achievable value depends on alloy, section shape, circumscribing circle and press capacity. In practice, a modern press with a well-designed die will run 6063 down to about 1.0 mm on a small hollow with several cavities, and some shops quote 0.8 mm on simple sections. Large hollows, wide flat faces and asymmetric shapes need more: 1.5 mm and up is normal above about 100 mm across. 6061 needs roughly 25 percent more wall than 6063 for the same geometry, and 6082 more again, with simpler sections.

Two mechanisms set the floor. The first is metal flow: thin walls cool faster than thick ones, so a section with a 1 mm wall beside a 4 mm boss will fill unevenly, and the thin side may tear or fail to fill. The second is die strength: the tongue of steel that forms a narrow gap is itself thin, and it deflects or breaks under press load. That is why the useful design rule is not "minimum wall" but "wall uniformity": keep adjacent walls within about 2 to 1, and accept a thicker wall everywhere rather than a thin wall in one place that forces a slower, hotter, less stable press pass.

Form tolerances: straightness, twist and flatness

Cross-section tolerances get all the attention, and form tolerances cause most of the assembly problems. EN 755-9 allows roughly 1.0 mm of straightness deviation per metre on lengths up to a metre, rising to about 3.0 mm over 3 to 4 metres, with local deviation within any 300 mm segment limited to about 0.5 mm. Twist is limited to about 1.5 degrees per metre on a medium-width profile. That does not sound like much until it is multiplied: on a 6 metre length, 1.5 degrees per metre accumulates to a visible spiral, enough to misalign screw holes at the far end of a frame.

Stretch straightening after extrusion corrects most bow and twist, but it also leaves residual stress, and that stress relaxes when the part later passes through an anodising or powder coating line at elevated temperature. Parts that were straight at the factory can arrive slightly bowed. The practical consequences are to specify straightness and twist explicitly on long parts, to allow for relaxation in the fixture design, and to check form after finishing rather than only before it.

What the die and the press contribute

Achievable tolerance is a function of tool condition as much as of the standard. A die wears, and dimensions drift as it does; shops commonly plan refurbishment on a tonnage interval rather than a calendar one, because wear tracks metal pushed through the die. Billet temperature is the second lever: holding it within a few degrees keeps flow stable, while a wide swing produces dimensional variation that no inspection plan can fix afterwards. Press speed is the third: running slower improves dimensional stability and costs money per kilogram, which is exactly what a precision class is buying.

For the buyer, this means the tolerance on the drawing is a claim about the supplier's process, not just about the part. A shop that cannot show its die maintenance interval, its billet temperature control and its first-article measurement method is unlikely to hold a precision class consistently, whatever the documentation pack says. Ask for the measurement method and the sampling frequency, and ask which dimensions are measured on the CMM rather than with a caliper.

How to write it on a drawing

Put the standard in the title block, not in a note buried in the revisions. State the alloy and temper beside it, because tolerance and alloy are interdependent: a 6061 section at 1.2 mm wall is a different capability question from the same drawing in 6063. Then mark, dimension by dimension, which values are as-extruded and which are machined, since the two carry tolerances an order of magnitude apart. A machined face at 0.05 mm and an as-pressed face at 0.3 mm can sit on the same part without contradiction, as long as the drawing says which is which.

Finally, resist the blanket tolerance. Applying plus or minus 0.1 mm to every dimension forces the whole profile into precision class and can add 20 to 30 percent to the price, when two or three functional dimensions were the only ones that needed it. The efficient drawing has a general tolerance note, plus explicit callouts on the mating faces, seal grooves and fastener positions, and nothing else.

See CNC machining for the secondary operations that hold the tight features, surface finishing for anodising and its effect on form, and sheet metal fabrication where a folded part can avoid the tolerance question entirely.