EXTRUSION PROCUREMENT GUIDE
Aluminum Extrusion Tolerances for Project Buyers
A profile can meet its alloy and temper requirement but still create assembly problems if straightness, twist, angularity, wall thickness or cut length is not defined. The drawing and purchase order should identify the dimensions that are function-critical.
Tolerance Items That Affect Assembly
| Item | Typical Concern | RFQ Detail |
|---|---|---|
| Straightness | Bow over the full profile length | Measurement length and allowable deviation |
| Twist | Rotation of the cross-section along the length | Datum, profile length and angular limit |
| Cross-section | Wall, gap, angle and contour variation | Critical dimensions and inspection points |
| Cut length | Assembly stack-up and machining allowance | Nominal length, tolerance and squareness |
| Surface | Die lines, handling marks and finish variation | Visible faces, anodizing or coating requirement |
Alloy, Temper and Profile Geometry
6061 and 6082 are commonly considered for structural profiles, while 6063 is widely used where extrudability and surface finish are important. Actual properties and tolerances depend on the applicable product standard, temper, section thickness and profile design. Thin walls, large circumscribing circles, asymmetric cavities and abrupt thickness transitions can increase distortion risk.
Temper also affects processing. A T6 product has been solution heat-treated and artificially aged, while stress-relieved temper designations may apply to particular wrought product forms. Buyers should order the exact temper required by the governing drawing and specification rather than using “T6” as a universal description.
How to Build a Clear Inspection Plan
- Mark critical and reference dimensions on the drawing.
- Define the applicable standard and any project-specific tolerance.
- Identify the datum and measuring span for straightness and twist.
- Specify cut-length, end squareness and machining allowance.
- Agree sampling frequency and inspection report requirements.
- Confirm protective film, separators and export packing for visible surfaces.
The Aluminum Association notes that alloying additions and temper influence properties and workability; 6061 is a widely used 6xxx-series alloy for structural and transport applications. Final selection should still be based on the ordered standard, geometry, loading, corrosion environment and fabrication route.
Straightness, Twist and Flatness Are Different Controls
Straightness limits curvature or bow along the extrusion length. Twist describes rotation of the cross-section around the longitudinal axis. Flatness concerns curvature across the width of an individual surface. A profile may comply with one requirement and fail another, so these terms should not be used interchangeably on a drawing.
The Aluminum Association identifies separate ANSI H35.2 tolerance tables and measurement methods for straightness, twist and flatness. The applicable table depends on product form and geometry. Buyers should reference the governing standard and edition instead of copying a tolerance value from an unrelated solid, hollow or tube profile.
How the Main Tolerances Should Be Measured
| Control | Measurement concept | Common source of disagreement |
|---|---|---|
| Straightness | Deviation from a straight reference over a stated segment or the total length | Checking only the full length when a segment limit also applies |
| Twist | Rotation of the section along its length, evaluated from an agreed reference surface or feature | No datum, support condition or length stated |
| Flatness | Deviation across the width of a specified surface | Applying a surface-flatness limit to overall longitudinal bow |
| Cut length | Distance between prepared ends under the agreed measuring condition | Ignoring saw kerf, end squareness, burrs or machining allowance |
Profile Design Can Make a Tolerance Easier or Harder to Hold
Thin walls, unequal wall thickness, deep channels, large unsupported flanges and strongly asymmetric cavities can make metal flow and cooling less uniform. This may increase bow, twist or local dimensional variation. Moving a critical feature closer to a stable section, adding a practical radius or balancing wall thickness can sometimes improve repeatability more effectively than specifying a blanket tighter tolerance.
Secondary operations also matter. Cutting, machining, drilling, anodizing, powder coating and thermal exposure can change the final acceptance condition. The RFQ should state whether tolerances apply to the mill-finish extrusion, the cut blank or the fully finished component. If mating parts depend on a feature after coating, coating buildup must be included in the dimensional stack-up.
What a Useful First-Article Report Should Contain
- Drawing revision, alloy, temper and inspected profile length
- Agreed datums, measuring orientation and support condition
- Results for critical metal, space, angle and wall dimensions
- Straightness and twist results over the specified span
- Cut length, end squareness and relevant surface observations
- Identification linking the report to the inspected lot or sample
A first-article report confirms the reviewed sample against defined requirements; it does not automatically establish that every future piece is identical. Production sampling frequency and any capability requirement should be agreed separately.
RFQ Checklist
Drawing · alloy and temper · cross-section dimensions · critical tolerances · stock or cut length · quantity · machining · anodizing/coating · inspection report · packing · destination
Review relevant product forms through the SASA Aluminum product range. Sending a dimensioned drawing is the fastest way to review extrusion feasibility and inspection requirements.
Request an Extrusion Feasibility Review
Send the profile drawing, alloy, temper, length, quantity and destination for review.
Post time: Aug-28-2026