Tolerances Decide Whether Your Order Assembles
Machined parts can be corrected on the bench. Extruded profiles cannot – if a 6-metre length arrives with excessive twist or a wall 0.2 mm under spec, it fails at assembly, not at inspection, and the whole batch goes back. Most "quality problems" on extrusion orders are actually specification problems: the buyer never defined the tolerances, so the mill supplied to its own default. Defining them on the drawing is the cheapest insurance you can buy.
Straightness and Twist: The Two That Bite
Straightness is measured as the maximum bow over a given length – typically 1.6 mm per metre for general profiles, tighter for CNC-machined parts that will be clamped in a jig. Twist is the angular deviation along the length, and it is the one that causes most field failures: a twisted profile looks fine on the floor but refuses to align in a frame, a T-slot assembly or a heat sink mounting groove. If your profiles feed into machining or automated assembly, put both values on the drawing instead of leaving them to luck.
Wall Thickness, Cut Length and Cross-Section
Wall thickness tolerance is typically +/-0.15 mm for standard walls and loosens with larger cross-sections – critical for heat sink fins, where a 1.0 mm fin that drifts to 0.8 mm loses surface area and thermal performance. Cut length tolerance is usually +/-1 to +/-2 mm standard, and can be held tighter for exact-fit applications. Open dimensions (slots, grooves and channels) depend on die design and are the first place a rushed die shows its weakness, so confirm them at the sample stage, not after mass production.
How to Specify Tolerances on Your Drawing
Three things make a tolerance specification unambiguous: reference a standard (EN 755-9, ASTM B221 or GB/T 5237), call out your critical dimensions separately instead of relying on general tolerances, and approve a sample with measured values recorded. At Siken Aluminum we supply dimensional inspection reports with every shipment and hold straightness, twist and wall thickness to tight limits on standard and custom dies alike – send us your drawing and we will confirm the achievable tolerances before you commit to tooling.
Frequently Asked Questions
Which tolerance standard applies to aluminium extrusions?
The most widely used references are EN 755-9 (Europe) and GB/T 14846 (China), with ASTM B221 common for US buyers. Tolerances are class-based, so agreeing the class (for example EN 755-9 class 1 or 2) in the quotation is essential.
What are typical straightness and twist allowances?
For a standard 6000 mm profile, straightness deviation is usually within a few millimetres over the full length (tightened for precision classes), and twist is limited per metre and over total length. Long thin profiles benefit from stretching and straightening after extrusion.
How do I avoid paying for unnecessary precision?
Only tight tolerances where the design truly needs them – on mounting faces, hole patterns and critical fins. Leave standard tolerances on cosmetic surfaces. Telling the supplier which features are functional saves cost on every metre produced.