Key takeaways
- Reference a tolerance standard in the title block — it covers every dimension you do not call out.
- Tolerances scale with the circumscribing circle and with whether a dimension spans a void.
- EN 12020-2 is tighter than EN 755-9 and applies to 6060/6063 precision profiles.
- Straightness, twist and flatness are specified per length — critical for long parts.
- Machine precision features instead of asking the extrusion to hold them.
Tolerances decide whether an extrusion fits your assembly — and how much it costs. Too loose and parts will not assemble; too tight and you pay for slow presses, rejected lengths and machining you did not need. The good news is that extrusion tolerances are highly standardised.
The main standards
| Standard | Region | Scope |
|---|---|---|
| EN 755-9 | Europe / international | Tolerances on dimensions and form for extruded profiles |
| EN 755-7 / EN 755-8 | Europe | Seamless tubes / porthole tubes |
| EN 12020-2 | Europe | Precision profiles in 6060 and 6063 — tighter than EN 755-9 |
| ANSI H35.2 | North America | Dimensional tolerances referenced by ASTM B221 |
| GB/T 14846 | China | Dimensional deviations of extruded profiles |
Pick one standard and reference it in your drawing’s title block: “Extrusion tolerances per EN 755-9 unless otherwise stated.” It then covers every dimension you do not specify individually.
What drives the tolerance of a dimension
- Size of the dimension. Larger dimensions receive larger tolerances.
- Circumscribing circle. Bigger profiles are harder to control, so the same dimension gets more tolerance on a large section.
- Solid or across a void. Dimensions through solid metal are tighter than “open” dimensions that span a partly enclosed void, where legs can spring.
- Wall thickness and hollows. Walls enclosing a hollow typically receive wider tolerances than open walls.
- Alloy. Harder alloys can be more difficult to hold.
Form tolerances
Beyond cross-section dimensions, standards also limit:
- Straightness — deviation along the length, specified per metre and over the total length.
- Twist — rotation of the section along its length, critical for long, slender parts.
- Flatness — across wide faces, where a concave or convex surface may occur.
- Angularity — deviation of angles from nominal.
- Contour and corner radii — how closely curved faces follow the drawing.
- Cut length — saw-cut length tolerance, tighter with precision sawing or machining.
How to specify tolerances well
- Reference a standard for everything not called out.
- Tighten only what is functional — fits, sealing faces, sliding surfaces.
- Define datums that match how the part is assembled or machined.
- Use GD&T sparingly and in line with how the profile will be measured.
- Machine precision features. A bearing seat or a flat mounting face machined after extrusion is cheaper and more reliable than asking the die to hold it.
- Account for finishes. Anodizing and powder coating add thickness to every coated surface.
How we verify
We measure first-off samples against your drawing and issue a dimensional report, then monitor critical dimensions during production. See our quality process.
Have a drawing with tight tolerances? Send it for review — we will tell you which dimensions the extrusion can hold and which need machining.
Frequently asked questions
What tolerance can aluminum extrusions hold?
It depends on the dimension's size, the profile's circumscribing circle, the alloy and whether the dimension crosses a hollow. Standards such as EN 755-9 and ANSI H35.2 give the values; precision standards like EN 12020-2 are tighter. For features that need machining-level precision, the profile is machined after extrusion.
What is an open dimension in extrusion tolerances?
A dimension across a partly open void (such as across the legs of a channel) is an 'open' dimension. Because the legs can spring, open dimensions receive larger tolerances than dimensions measured through solid metal.
Does anodizing affect tolerances?
Slightly — about half the anodic layer grows outward, so a 15 µm layer adds roughly 7–8 µm per surface. Allow for it on tight fits.