Extrusion tolerances and machining tolerances: what to specify

Separate the profile from the features added to it

An extrusion arrives with requirements for its cross-section and form. Machining then adds or changes features such as cut ends, holes, slots and pockets. The acceptance criteria for those operations need to be defined on the finished part, alongside the incoming material requirements.

The Aluminum Extruders Council points designers to standard and precision extrusion tolerance tables and recommends discussing component-specific needs with the extrusion supplier. Those tables do not establish Alumex’s machining capability or replace a review of the finished part drawing.

Source context: Aluminum Extruders Council — extrusion tolerances · Aluminum Extruders Council — cutting and forming

Choose references that explain how the part fits

A hole location is incomplete without a clear origin. Identify the reference end and surfaces from which the feature is located. Where geometric dimensioning and tolerancing is used, provide the datum scheme and applicable drawing standard rather than leaving the inspector to select convenient surfaces.

Consider a transom with a connection pattern near one end. Measuring the pattern from that end can answer a different assembly question from measuring it from the opposite end. The designer should resolve that relationship; the machining supplier can then review workholding and inspection against the stated intent.

Source context: ASME — Y14.5 dimensioning and tolerancing

Tell the supplier which relationships matter

Review the drawing by function. Identify where two parts must mate, where a fastener passes through, and where a gasket or cover must remain unobstructed. Keep dimensional requirements tied to those functions. A tighter number is useful only when the part and inspection plan can support it.

Review actual stock before committing the process

Send the profile drawing, alloy and temper, finished part length and available information about incoming-stock acceptance. Identify customer-supplied material and any sample length available for review. A nominal CAD section describes the intended shape; it does not document the condition of a particular delivery.

Agree who checks stock on arrival and what happens if it is outside the purchase requirements. Distinguish a stock discrepancy from a machining discrepancy. Record the affected profile or lot, quantity and observed condition before deciding whether to use, replace or otherwise disposition the material.

Source context: Aluminum Extruders Council — extrusion tolerances

Agree the inspection method as well as the number

For critical features, specify what evidence is needed: recorded dimensions, a gauge result, an approved first piece or a fit check. Agree the measurement references, the part condition during inspection, sampling and report format. If a long profile needs support during a check, resolve that setup with the engineering and quality teams.

A measurement report and an assembly fit check answer related questions, but they are not interchangeable. A fit check can help verify a practical interface; the agreed drawing still defines the part unless the authorized release explicitly establishes another acceptance method.

Use the quotation to close open requirements

Ask the supplier to identify unresolved tolerances, inspection assumptions and any exception before release. Where capability needs a sample or trial setup, agree the review step and decision point. Keep the accepted drawing revision with the order.

Avoid treating a general website tolerance as a guarantee for a long, thin-walled or finish-sensitive profile. Acceptance should be agreed for the particular section, features and delivered condition. That gives purchasing a comparable scope and quality a clear basis for receiving the parts.