Extrusion tolerances explained: EN 755-9 and EN 12020-2

Why does extrusion need tolerances?

No process produces an exact size. In extrusion the die deflects and wears, billet temperature varies, the profile shrinks as it cools and changes slightly when it is stretched. A tolerance is the shared language between designer and supplier: it says between which limits a dimension is acceptable. This article does not list table values. It explains which standard covers what and how to set a tolerance sensibly. What binds an order is what is accepted in writing in the quotation.

Which standards apply?

• EN 755-9 is part nine of the series on extruded rod, bar, tube and profile. It sets the tolerances on dimensions and form for profiles: wall thickness, other dimensions, angles, corner and edge radii, flatness, straightness, twist and length. Check the full scope, such as the largest circumscribing circle covered, before choosing a standard.
• EN 12020-2 applies to precision profiles in the alloys EN AW-6060 and EN AW-6063. The technical delivery conditions are in EN 12020-1, and the tolerances on dimensions and form are in EN 12020-2. For the same dimension, its limits are tighter than those in EN 755-9, but it cannot be used for profiles outside its scope.

Always consult the current edition. What follows describes structure, not table values.

What do they cover?

Tolerance type	What it limits
Wall thickness	Deviation of a wall from its nominal thickness
Other dimensions	Width, height, slot opening and similar
Angle	Deviation of the angle between surfaces
Corner and edge radius	Deviation of specified radii
Flatness	How far a flat surface may bulge or dish
Straightness	Bow along the length
Twist	Rotation of the section along its length
Length	Deviation of the profile length

What do the tolerance values depend on?

The standards do not give one number. The applicable tolerance depends on the relevant table, which may use dimension size, wall type, open-end geometry, circumscribing circle or alloy group. The type of dimension matters too: a dimension that spans an enclosed void does not carry the same limit as one on an open section. Judge tolerances dimension by dimension, not with a single note on the drawing.

How do form tolerances show up in practice?

A size tolerance bounds one dimension, while a form tolerance controls the overall geometry. On a long profile, straightness, the bow along its length, affects alignment at assembly. Twist is one end rotating relative to the other and shows up mainly on wide, open sections. Flatness limits bulge or dishing on a broad face. All three scale with length and section, and the stretching step reduces them without removing them. If assembly is sensitive, state a form tolerance on the drawing.

How is a tolerance measured?

Measurement is done with the profile at room temperature, because a hot profile has a different size after shrinkage. Calibrated callipers, micrometers, gauges or measuring machines are used. Agreeing at the start which dimension is checked, on how many samples and by which method avoids disputes at delivery.

How should you set a tolerance?

1. Start from function. Which dimension mates with another part, and which merely gives shape? Put tight tolerances on fit, sealing and alignment dimensions only.
2. Begin with the standard tolerance. A drawing note citing EN 755-9, or EN 12020-2 where it applies, covers most dimensions.
3. Write exceptions separately. If you need something tighter than the standard gives, state a numerical tolerance for that dimension.
4. Define the datum. From which surface is the dimension measured? This matters especially for twist and straightness.
5. Do not copy machined-part tolerances. An extruded surface and a machined surface do not have the same precision.

What does coating do to a tolerance?

A coating adds thickness. It is a few tens of micrometres per surface and varies inside channels. On mating surfaces, think about the dimension before and after coating separately. The guide to powder coating aluminium profiles covers the process.

How does the design affect tolerance?

A section with balanced walls, adequate radii and a small circumscribing circle holds tolerance more easily. A heavy, lopsided section may make a tight tolerance expensive or impractical. The section rules are in designing profiles for extrusion. For strength and heat treatment, see tempers T4, T5, T6 and T66.

What does a tighter tolerance cost?

A tighter tolerance means more die correction, slower exit speed, more frequent measurement and more rejects. Writing a tight tolerance on every dimension raises the price of the part and rarely improves function. The most economical approach is to take the standard tolerance as the default and tighten only the dimensions whose function requires it.

How do you write it on the drawing?

One general note is enough: the standard that applies, the alloy and temper, the coating status and any numerical tolerances written separately on the dimensions that function requires. A note such as “general tolerances to EN 755-9; tolerances shown on the drawing apply to the dimensions indicated” makes sure designer and supplier read the drawing the same way. For coated parts, add whether dimensions apply before or after coating. The clearer the note, the faster the quotation and the first sample.

Which tolerance applies to an order is set against your drawing and confirmed in the quotation. Send the drawing and the dimensions that matter for function through the quote request page.