50 x 50 mm Aluminium Profile for Precision and Aerospace Fixtures

01Why does a 2 m fixture move 0.47 mm over a 10 K swing?

Because aluminium expands 23.6 micrometres per metre per kelvin, and that is not a small number at fixture scale. A 2 m member in a room that drifts 10 K between morning and afternoon grows or shrinks by 2 x 10 x 23.6, which is 472 micrometres, or 0.47 mm.

For a fixture holding a part to a 0.1 mm tolerance, that is nearly five times the tolerance, and it happens without anyone touching the fixture. This is the first thing to settle in a precision application, before the section is chosen, because no section choice fixes it.

Three practical strategies, and most fixtures use the first or the second. Keep the room within a narrow band and let the fixture equilibrate before measuring. Keep the measurement loop short, so the dimension that matters spans a small distance rather than the whole fixture. Or measure the fixture temperature and compensate, which works well and requires discipline to keep working.

02What is residual stress, and why does it relax after machining?

An extruded section leaves the press with internal stress, because the outside cools before the inside and the section is then stretched to straighten it. Tension stretching removes most of it, but not all, and machining removes material from one side, which unbalances whatever is left. The part then moves to find a new equilibrium, over hours or over weeks.

This is why a fixture that was flat when it left the machining centre is not flat a month later. The movement is usually small, tens of micrometres, but on a precision fixture it is enough to matter, and it appears after the acceptance test rather than during it.

• Ask for tension stretched material, and ask for the stretch percentage. It is the single most effective control and it costs very little.
• Rough machine, then wait, then finish machine. The wait lets the stress redistribute before the final cut.
• Keep the machining balanced: remove material from both sides where the geometry allows, rather than all from one face.
• Do not assume a stress relief heat treatment is available for 6063-T5; it is not standard, and a well stretched billet is the practical answer.

03Why can flatness not come from the extrusion face?

Because an as-extruded face is not a datum. Straightness is specified at about 1.5 mm per metre, twist at up to 2 degrees, and the surface carries die lines and a slight curvature from the cooling. Those are normal extrusion tolerances, and they are an order of magnitude away from what a fixture needs.

So any face that carries a locator, a gauge or a part has to be machined, and the drawing has to say so and leave an allowance. A 50 x 50 section at 3.1 kg/m has enough wall to take a light facing cut without breaking through, but the allowance has to be planned rather than discovered.

04What does AS9100 traceability actually ask for?

For a fixture rather than a flying part, the requirement is usually narrower than people fear, but it is specific. The material has to be traceable to its heat, the certificate has to match the material, and any process that affects the fixture’s fitness for use has to be recorded. For an aluminium fixture that means the mill certificate, the machining record and, if the fixture is used to accept parts, its calibration status.

The practical gap is almost always the heat number on the bundle. A certificate that names a heat, and a bundle with a paper label and no stencil, cannot be reconciled after the label falls off. Ask for the heat number marked on the material.

05Aluminium or steel in a controlled room?

Aluminium moves twice as much as steel and weighs a third as much. In a room held to plus or minus 1 K, the movement difference is small enough that the weight advantage usually wins: a fixture that one person can lift gets used, and a fixture that needs a hoist gets avoided. In a room that drifts 5 K or more, the expansion difference starts to dominate and steel becomes the safer choice for the datum structure.

A common compromise is an aluminium structure with steel datum inserts, so the part location is on steel and the bulk is light. It introduces a bimetal interface, which has to be designed rather than improvised, but it is a reasonable answer to a real conflict.

06What five numbers should be on the enquiry?

A fixture enquiry is a tolerance enquiry. The numbers that matter are the tolerance, the room and the faces.

Tolerance to be held

the repeatability or the measurement uncertainty the fixture must support

Room condition

controlled band in kelvin, and whether it drifts during the day

Faces to be machined

marked on the drawing, with the allowance

Fixture size

because expansion scales with length, not with section

Traceability

whether a 3.1 certificate and a marked heat number are required

Section

50 x 50 mm four slot at 3.1 kg/m, or 40 x 80 at 3.60 kg/m for more depth

Related sections and categories

The 50 x 50 is listed under 50 Series Aluminium Profiles, and the 40 x 80 mm heavy section at 3.60 kg/m gives more depth where a fixture needs it. Machined fixture work sits under CNC machining.