Aluminum Extrusion Tolerance Guide
Profile accuracy from die to cut length
Aluminum extrusion tolerances are governed by the die, the press, and post-extrusion processing (stretching, aging, cutting). Cross-section tolerances follow EN 755-9 or ASTM B221. Understanding these standards helps you design profiles that are producible without excessive secondary machining.
Overview
Extrusion is a high-volume process that produces complex cross-sections in a single pass. Standard tolerances are wider than CNC but achievable at a fraction of the cost for long, uniform profiles. Critical dimensions can be tightened with precision dies and post-extrusion machining.
Tolerance Specifications
| Feature | Standard | Precision | Notes |
|---|---|---|---|
| Cross-Section Dimension (Open) | ±0.25 mm (< 25 mm) | ±0.15 mm | Per EN 755-9. Larger dimensions have proportionally wider tolerances. |
| Cross-Section Dimension (Hollow) | ±0.30–0.50 mm | ±0.20 mm | Hollow profiles (tubes, channels) are harder to control than solid or open shapes. |
| Wall Thickness | ±10% | ±5% | Minimum wall varies by alloy and CCD (circumscribing circle diameter). |
| Straightness | 0.5 mm per 300 mm | 0.25 mm per 300 mm | Post-extrusion stretching (1–3%) improves straightness significantly. |
| Twist | 1° per meter | 0.5° per meter | Asymmetric profiles twist more. Symmetric designs extrude straighter. |
| Cut Length | ±1.0 mm | ±0.5 mm | Standard saw cut. Precision requires end-milling or CNC cut-off. |
| Angularity (Corner Angle) | ±1° | ±0.5° | Right angles are easiest to hold. Acute angles are more difficult. |
| Surface Finish | Ra 1.5 µm (mill finish) | Ra 0.8 µm (polished die) | Anodizing does not significantly change dimensions (adds 0.01–0.02 mm). |
Key Considerations
Design for the Circle
Circumscribing circle diameter (CCD) determines press size, cost, and achievable tolerance. Smaller CCD = tighter tolerance capability.
Symmetric Profiles Extrude Better
Balanced cross-sections flow evenly through the die, reducing twist, bow, and cross-section variation.
Leave Stock for Machining
For tight-tolerance features (bearing surfaces, mating interfaces), design 0.5–1.0 mm extra stock and machine to final dimension.
Alloy Choice Matters
6063-T5 is easiest to extrude and holds tolerance well. 6061-T6 is stronger but harder to extrude with thin walls.
💰 Cost Impact of Tolerances
Precision extrusion requires custom-corrected dies, careful press parameter control, and additional straightening — 20–30% higher extrusion cost.
Standard EN 755 / ASTM tolerances are achievable with normal production dies and press runs. No premium.
Accept standard extrusion tolerances for the profile, then CNC-machine critical interfaces (mounting holes, mating surfaces). This hybrid approach is far cheaper than demanding precision across the entire profile.
⚠️ Common Tolerance Mistakes
- ⚠ Designing asymmetric profiles and expecting straight extrusion — unbalanced material flow causes twist and bow.
- ⚠ Specifying tight cross-section tolerances on large hollow profiles — mandrel deflection limits achievable precision.
- ⚠ Ignoring thermal expansion: 6063 aluminum grows ~23 µm/m/°C. A 2-meter profile changes 0.46 mm for every 10°C.
- ⚠ Expecting CNC-level tolerances from the extrusion process — for critical features, plan secondary machining.
- ⚠ Not specifying alloy temper (T5 vs. T6) — this affects both mechanical properties and dimensional stability.
💡 Pro Tips
- ▸ Die cost for custom profiles is typically $500–2,000. Amortize across production volume — often negligible at scale.
- ▸ Multi-void hollow dies (for complex shapes) cost more but produce parts that would otherwise require assembly.
- ▸ Our Vietnamese extrusion partners run 1,500–3,600 ton presses for profiles up to 350 mm CCD.
- ▸ Request sample extrusions (1–2 billets) for dimension verification before committing to production.
- ▸ Anodizing after machining: hard anodize adds ~0.025 mm per surface, standard anodize adds ~0.01 mm.
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