CNC extrusion processing: how extruded profiles become finished parts

• 1
  What CNC extrusion processing removes, and what it leaves alone
• 2
  Where the process fits: hollows, thin walls and long parts
• 3
  Alloy and temper decide how the profile machines
• 4
  Clamping, distortion and the setup that decides accuracy
• 5
  When machining wins and when extrusion wins
• 6
  Extruded profile versus machined-from-solid

Extrusion sets a profile's cross-section. A billet of 6063 or 6061 is pushed through a die at 450–500 °C, and the shape that comes out is constant for the full length of the run. That is the value of the process: one die, hundreds of metres, no per-part tooling cost. It is also its limit. The press cannot produce a pocket, a counterbore, a thread or a face that sits at an angle to the profile axis.

The second operation is what adds those features. An extruded bar is cut to length, clamped in a vise or fixture, and machined on a 3-axis mill, a 4-axis mill or a mill-turn centre. Holes get drilled, ends get faced square, slots get cut, threads get tapped. The profile geometry stays; the detail geometry arrives.

Think of it as two tolerances living in one part. Profile dimensions follow the die and the alloy's quench behaviour, typically ±0.15 mm on wall thickness for a standard die and ±0.10 mm on a tight one. Machined features follow the machine, and 5-axis work at GreatLight holds ±0.005 mm. Mixing the two without knowing which is which is where most drawing arguments start.

The practical rule: anything that runs the full length of the bar is an extrusion feature, and anything that interrupts that length is a machining feature. If a designer needs a groove that stops halfway down a 2 m profile, the die cannot do it and the mill must. If the same groove runs end to end, the die does it for free.

Extrusion earns its place on parts that are long, hollow or thin-walled, because those are the shapes a die produces cheaply and a mill produces slowly. A heat-sink body with ten fins along 1,200 mm is an extrusion; cutting those fins from solid would remove most of the billet as chips and take hours per part.

Hollow sections with one or more internal cavities are the strongest case. A porthole or bridge die produces a sealed void that a mill would have to drill, ream and possibly plug. Cable trunking, pneumatic manifolds and fluid rails fall into this group. Once you add cross-drilled ports to the cavity, the job becomes a machined extrusion by definition.

Long parts are the other case. Our largest travel is 4,000 × 400 × 150 mm, which covers most architectural and machine-frame profiles. Parts beyond that length usually have to be extruded and then joined, or split into sections with a lap joint that is machined after extrusion.

Thin walls have a floor. Below roughly 0.8 mm on aluminium, the press struggles to fill the die and the profile twists as it cools. Machining a 0.5 mm wall after extrusion is worse: clamping pressure alone can deform it. If a wall must be that thin, expect to support it or accept a loose tolerance on that dimension.

Alloy choice is usually settled by the extrusion house, not the machine shop, so the machining plan has to follow it. 6063 is the standard architectural alloy: it extrudes fast, takes a good anodised finish, and cuts like soft butter. It is also gummy, so drills can grab and built-up edge forms on the tool. Keep speeds up and feed rates steady.

6061 and 6082 are structural choices with more silicon and magnesium. They machine cleaner, hold threads better and take a higher clamp load without marking. 7075 gives the highest strength of the common extrusion alloys, but it is rarely extruded in complex hollows and it work-hardens if the cutter dwells in the cut.

Temper matters as much as alloy. T6 profiles arrive already aged and stable, so they machine predictably. T4 or as-quenched stock can move after the first cut as internal stress releases, which shows up as a bow in a long part. Rough machine, let the part rest, then finish.

Non-ferrous is the default, but polymer extrusion is common too. POM, PA and HDPE profiles are cut and drilled with the same programs and much lower spindle speeds. Titanium and Inconel extrusions exist but are unusual; those are normally machined from bar because the press cost is hard to justify.

A long extrusion is a spring. Clamp it hard in the middle and the ends lift; clamp the ends and the middle bows. Either way the machine cuts a straight feature into a bent part, and the part springs straight after unclamping, taking the feature out of tolerance. Most out-of-spec extruded parts are fixturing problems, not spindle problems.

The fix is to support the profile along its length rather than pinch it at two points. Soft jaws machined to the profile's outer contour, a sub-plate with adjustable supports, or a low-melt fixture for very thin sections all spread the load. Vacuum fixtures work well on flat-backed profiles where the sealing face is continuous.

Datum choice matters next. Extruded surfaces are not machined surfaces, so they are a poor primary datum. Pick a machined face as datum A, or machine a reference edge first and build the rest of the setup from it. That single decision removes more scrap than any tool change.

Thermal drift adds to this on long runs. An aluminium profile at 25 °C and the same profile at 32 °C differ by roughly 0.1 mm over 2 m. If a length tolerance is tight, measure at a controlled temperature or let the part stabilise before final inspection.

The cost split is easy to reason about. A die costs money once; a machining hour costs money every part. So extrusion wins on volume and length, and machining wins on low volume and complexity that a die cannot express. The crossover for a simple hollow profile is often somewhere in the low hundreds of parts.

A part with a constant cross-section and no side features should be extruded and cut off. Adding a single drilled hole does not change that; adding a dozen cross-drilled holes, two tapped ends and a milled flat does, because now every part goes through a mill anyway. At that point the extrusion is buying you material savings, not process savings.

Machining from solid can beat extrusion in one case: very short parts. Below about 150 mm long, the die amortisation never pays back, and a 5-axis mill can cut the whole shape from bar stock in one setup. Prototypes sit here, which is why most first articles of an extrusion project are machined, not pressed.

Surface finish follows the same logic. Extruded faces come out at Ra 1.6–3.2 μm as-pressed and need bead blasting, brushing or anodising before they look finished. Machined faces can reach Ra 0.2–0.8 μm with the right cutter and pass, so visible faces are often machined for appearance even when the die could form them.