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Aluminium extrusion vs CNC machining for frames and enclosures
The short answer
Extrusion wins above roughly 500 to 1,500 parts when the cross-section is constant: a die costs 1,500 to 10,000 dollars, 95 percent of the bought metal ends up in the part and as-pressed tolerance is around 0.2 mm. Below that volume, or where the geometry changes along the length, machine it from stock.
The two processes answer different questions
Extrusion is a shaping process: heated billet is pushed through a die and the result is a continuous profile of one cross-section, cut to length afterwards. CNC machining is a subtracting process: a block or plate is cut away until the part appears. That single difference decides most projects before cost is discussed. A frame rail, a heatsink body, a linear enclosure, a mounting track or a window section has the same section for its whole length, and extrusion produces it at a fraction of the metal and machine time. A housing with a sloped face, a bearing bore at one end, mounting pads on two sides and a cable entry on the third is not a constant section, and no die will make it one.
The mistake in both directions is common. Buyers extrude a profile and then machine away 60 percent of it to reach the final shape, which pays for the die and the machining; or they machine a long constant part from plate, which turns most of an expensive billet into swarf. The useful question is not which process is better but which features belong to which process.
The arithmetic: die, metal and cycle time
The cost of an extruded part has three lines: the die, amortised over the order; the metal, bought by weight; and the press and cut time. The cost of a machined part has two: stock and machine time, with almost no tooling. That is why the curves cross, and why the crossing point moves with part size. On a small simple section the die is 1,500 to 3,000 dollars and 100 to 200 parts can absorb it. On a large hollow profile with internal webs, the die can exceed 10,000 dollars and the break-even moves to 500 to 1,500 parts. Where the section is long and the order is large, the die cost per part becomes negligible and the material yield advantage dominates: extrusion uses 90 to 96 percent of the billet, while machining from plate typically converts 20 to 40 percent of the stock into chips.
| Order size | Extrusion route | CNC route | What decides it |
|---|---|---|---|
| 1 to 50 parts | standard profile, cut and machined | clear winner | no die to amortise, drawing can still change |
| 50 to 500 parts | custom die if the section is simple | usually still cheaper | die cost per part is 3 to 60 dollars here |
| 500 to 5,000 parts | custom die, machined ends | competitive only for small parts | metal yield and cycle time start to dominate |
| above 5,000 parts | clearly cheaper per part | expensive unless the part is tiny | die is fully amortised, press output is high |
Tolerance is where the cheap route stops
An as-pressed profile is governed by EN 755-9 and holds roughly plus or minus 0.15 to 0.20 mm on small dimensions, opening to 0.5 mm or more as the section grows and to around 1 mm across a large enclosed hollow. That is adequate for frame geometry, cover fit and most mounting faces, and it is not adequate for a bearing seat, a seal groove, a dowel hole or any surface that must mate within 0.05 mm. Those features are added after extrusion by CNC, and the economic answer is almost always to keep them few, local and on the ends of the profile rather than along its length.
Machining from stock starts at plus or minus 0.05 mm and can hold 0.02 mm on a critical feature, which is why it remains the default for prototypes, low volume and any part where the geometry changes in three dimensions. The comparison that matters is not "which is more accurate" but "how many features actually need the accuracy". A frame with two machined end faces and a 0.2 mm body tolerance is both cheaper and faster than the same frame machined all over.
Lead time and the cost of being early
A custom extrusion die takes two to four weeks, plus a first-article sample for approval before the production run; standard profiles ship in days. CNC parts typically quote in three to ten working days depending on material and finishing. That difference matters most at the start of a programme, when the design is still moving. Cutting a die for a section that changes after the first prototype is the single most expensive way to use extrusion, and it is avoidable: prototype from a standard profile or from machined stock, freeze the section, then cut the die.
There is a second, quieter lead-time cost. Once the die exists, the section is fixed. Any later change to wall thickness, a boss position or an internal web means a new die, not a new program. Machining keeps that flexibility at the price of a higher piece cost; extrusion trades flexibility for a lower piece cost. Which side of that trade you want depends on how confident the design is, not on the volume alone.
The hybrid route is usually the real answer
Most production frames and enclosures are neither pure extrusion nor pure machining. The body is extruded, cut to length, then the ends are machined for mating faces, the holes and threads are added, and the part is anodised or powder coated. Because the machine only removes a few millimetres instead of a whole block, cycle time falls sharply: cutting an extruded blank instead of a solid bar can remove 60 to 70 percent of the machining time on a housing, and tool wear falls with it. The design rules are simple. Leave 0.5 to 1 mm of stock on any face that will be machined, keep machined features off the thin walls, and mark on the drawing which dimensions are as-extruded and which are machined, because they carry different tolerances and different costs.
A specification trap worth naming: a drawing that applies plus or minus 0.1 mm to every dimension forces the supplier to treat the whole profile as precision grade, which can add 20 to 30 percent to the price. Put the tight tolerance only on the features that function, and leave the rest at the standard.
When each route is the wrong one
Extrusion is wrong when the section changes along the length, when the order is too small to amortise a die, when the part needs fewer than about two walls of constant thickness, or when the design is not yet frozen. It is also wrong when the profile would need so much machining afterwards that the die saves nothing. Machining is wrong when a long constant part is cut from plate, when the same section repeats across thousands of units, when material cost dominates the part, or when the geometry is simple enough that a die would pay for itself within one order. Sheet metal is the third option that often wins for enclosures below about 2 mm wall: it needs no die, folds in days and tolerates design change, at the cost of stiffness and joint count.
See CNC machining for the secondary operations and the low-volume route, sheet metal fabrication for the folded alternative on enclosures, and surface finishing for anodising and coating after either process.
Scope and sources. The die cost range of 1,500 to over 10,000 dollars, the break-even band of 100 to 200 parts for simple sections, the material waste of 5 to 10 percent for extrusion against 30 to 70 percent for machining, the tolerance comparison and the 70 percent cycle-time reduction from machining an extruded blank are set out in a comparison of CNC machining and aluminium extrusion. The break-even range of 500 to 1,500 units, the cost trend by order size and the recommendation of 500 to 1,000 pieces to justify a die are from a bulk-order cost comparison. The as-pressed tolerance of about 0.2 mm, the machined tolerance of 0.05 mm, the two to four week die lead time, the die cost range of 1,500 to 10,000 dollars and the roughly 1,000 piece threshold for an enclosure are from an extrusion enclosure design guide. These are planning figures for standard sections; the die price and the crossing point for a specific part should be quoted against its actual section and annual volume.
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