Aluminium Extrusion Tolerances

Short answer. Extrusion tolerances are held to GB/T 5237 high-precision class at our plant, with groove and slot dimensions controlled to ±0.1 mm and wall thickness from 0.8 mm. What decides whether a drawing is achievable is not one number: it is the section shape, the alloy and temper, the die, and which dimensions you mark as critical.

Groove and Slot Tolerance

A groove that takes a gasket, a glass bead or a hinge is a fit dimension, so it gets its own tolerance: ±0.1 mm on grooves we cut for it. That number is only reachable when the die is built for that groove and the wall beside it is not a thin isolated tongue — a 0.9 mm tongue next to a groove will move during cooling regardless of what the drawing says.

Two things follow for the drawing. Mark the functional groove and say what goes in it. Leave decorative grooves to the general GB/T 5237 class, because tightening everything raises die cost and press time without improving the fit.

What Changes the Tolerance

Section shape. Open sections with even wall distribution hold tolerance better than sections with long isolated tongues, deep narrow slots or large flat faces unsupported on one side.

Alloy and temper. 6063-T5 is the easiest to hold on architectural and industrial sections. 6061 and 6082 are harder to extrude and move more in quenching, so a tight tolerance on 6082-T6 is a different conversation from the same tolerance on 6063-T5.

Wall thickness. Below 1.0 mm the wall cools fast, the section twists more and the press runs slower. A 0.8 mm wall is achievable — we extrude from 0.8 mm — but it is the wall where tolerance and cost argue most.

Die condition. A new die holds tolerance better than one at the end of its life. Die maintenance is part of tolerance, not a separate topic.

Anodising and Coating

Surface treatment adds thickness on the outside of the section. Anodising at AA10 and AA15 builds a film in the range that matters to a sliding fit or a press-fit insert; powder coating is thicker again. If a groove takes a gasket, or a hole takes a press-fit insert, the treatment thickness belongs on the drawing as a fit condition, not as a finish note.

Practical order: tolerance the extrusion, state the finish and film thickness, then check the fit after finish. That order prevents a batch that measures correctly in mill finish and does not assemble after anodising.

Twist, Straightness and Camber

These three move with length, so they are quoted per length, not per profile. A 6.0 m mill length and a 2.4 m cut piece do not hold the same straightness in service because the piece is supported differently on site.

If the profile is a mullion, a rail or a track, say how it is fixed and over what span. That sentence lets us cut and inspect against the way the part actually works, rather than against a bench number that means nothing on site.

When CNC Beats the Die

Some dimensions should never be pushed into the extrusion. Hole position, counterbore depth, mitre angle and any feature that has to relate to another machined face are better cut after extrusion on a CNC centre.

Our rule of thumb: if the feature is a fit to another part, machine it; if it is the shape of the section itself, hold it in the die. Splitting the work that way usually gives a tighter finished part at lower total cost than an over-specified die.

What We Cannot Do

• We do not publish a millimetre tolerance table: values depend on section, alloy and which dimensions you mark as critical.
• We do not hold ±0.1 mm on every dimension of a section. It applies to grooves cut for it.
• We do not run die casting.
• We do not quote against a tolerance alone; the drawing and the quantity come first.

What to Send

1. Section drawing (DXF or PDF) with the circumscribed circle
2. Alloy and temper
3. The dimensions that are functional, and what fits into them
4. Surface finish and film thickness
5. Cut length and how the part is supported in service

The Numbers Behind This Page

These are plant facts, not tolerance values. They tell you what is available; the drawing tells us what to hold.

Standards. GB/T 5237 (dimensional, high-precision class) · EN 755-1 to -9 (mechanical and dimensional) · ASTM B221 (chemical composition). Tolerance review is done against the drawing, not against a generic table.