CNC processing of aluminum alloy

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  Why aluminum cuts fast and where it fights back
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  Reading the grade list: 6061, 7075, 2024, ADC12
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  Speeds, feeds, and tooling that hold tolerance
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  What tolerance and finish are realistic
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  When aluminum is the wrong choice
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  Aluminum grades side by side

Aluminum is soft, light, and conducts heat about five times faster than steel. That combination is why CNC processing of aluminum alloy runs at high spindle speeds and deep feeds. The cutting zone stays cooler than it would in steel at the same speed, so you can remove metal quickly without burning the tool. Most aluminum alloys machine at 2 to 3 times the surface speed of 1018 steel.

The same softness creates two problems. Aluminum is gummy. Under a dull edge it smears instead of shearing, and the built-up edge that forms on the cutter then breaks off and leaves a torn surface. The second problem is thermal expansion. Aluminum grows about 23 μm per meter per degree Celsius, roughly twice that of steel. A part that measures on size at 35 °C in the spindle can shrink out of tolerance once it cools to 20 °C.

Chip evacuation decides the outcome more than spindle power does. Aluminum chips are light and bulky, and they pack into pockets and deep slots. If the chips are not cleared, the tool recuts them, heat climbs, and the finish goes dull. High-pressure coolant or through-spindle air solves most of this before it starts.

Silicon content is the dividing line between easy and difficult aluminum. Wrought grades such as 6061 carry almost no silicon and cut like butter. Cast grades such as ADC12 carry 9 to 12 percent silicon in hard particles that abrade the cutting edge, so tool life drops sharply and diamond-coated tooling becomes worth the cost.

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  Wrought alloys6061, 7075, 2024, 5052, 6082 — free-cutting, predictable, widely stocked.
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  Cast alloysADC12 and similar — abrasive silicon particles, more tool wear, more porosity.
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  Heat ruleMeasure critical parts after they return to 20 °C, not at the machine.

6061 and 6061-T6 are the default for machined parts. Good strength, excellent corrosion resistance, welds well, anodizes to a clean finish, and it is available in every bar and plate size. If a part has no unusual load or weight requirement, 6061-T6 is the safe answer and usually the cheapest one.

7075 is the high-strength choice, with tensile strength close to some steels at a third of the weight. It machines cleanly and takes a mirror finish, which makes it popular for aerospace brackets and molds. The trade-off is corrosion resistance. 7075 has poor resistance in wet or salty environments unless it is anodized, and it costs noticeably more than 6061.

2024 offers high fatigue strength and is common in aircraft structures, but it is even less corrosion resistant than 7075 and is normally used with a protective coating. 5052 and 5083 are marine grades with excellent salt-water resistance and good formability, though 5083 is gummy and produces long stringy chips that need strong coolant flow and pecking cycles.

ADC12 is a die-casting alloy. When you machine it, you are usually finishing a cast surface: trimming flash, boring bores, facing mating faces. Expect porosity that can open into a pore mid-cut, and expect the silicon particles to dull edges quickly. Plan for one extra finishing pass rather than trying to hold final size in a single heavy cut.

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  Pick 6061-T6Default for enclosures, fixtures, brackets, heat sinks, prototype hardware.
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  Pick 7075High load, low weight, or a mirror finish — and you can anodize it.
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  Pick 2024Fatigue-critical aircraft parts that will be coated anyway.
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  Pick ADC12You are machining an existing casting, not cutting from solid.

Aluminum likes sharp, polished, high-rake tooling. Two-flute and three-flute end mills with a 45-degree helix clear chips well in pockets. For finishing, a high-helix cutter with a polished flute reduces built-up edge and gives a better Ra. Uncoated carbide works for most wrought grades. For abrasive cast alloys, diamond-like carbon coating pays for itself within a few parts.

Surface speed for 6061 typically runs 300 to 600 m/min with carbide, and many shops push higher on rigid machines. Chip load per tooth stays around 0.05 to 0.15 mm depending on cutter diameter and radial engagement. The number that matters more than either is the feed per tooth at the tip: too light a chip load rubs the edge and work-hardens the surface, which is a common cause of poor finish on light finishing passes.

Coolant choice depends on the operation. Flood coolant handles deep pockets and high removal rates. For high-speed finishing cuts, a mist or air blast is often cleaner because it blows chips away and avoids thermal shock on thin walls. Through-spindle coolant is the best option for deep bores and long-reach tooling.

Thin-wall parts need a different strategy. Aluminum deflects under cutting force, so a 1 mm wall pushed too hard will spring back and leave a taper. Rough with a stock allowance of 0.5 to 1.0 mm, then take two light finishing passes with a sharp cutter and reduced radial engagement. On a 127-machine shop floor, the parts that fail inspection are almost always thin-wall deflection, not tool wear.

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  RoughingHigh feed per tooth, 50 to 70 percent radial engagement, flood coolant.
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  FinishingSharp polished cutter, small radial stepover, air or mist.
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  Thin wallsSymmetric material removal on both sides to balance cutting force.

On a stable aluminum part with a rigid setup, ±0.005 mm is achievable on critical features. That does not mean every dimension should carry it. Tightening a tolerance that does not matter adds cost, adds inspection time, and increases the chance of a rejected lot for no functional gain. Put tight tolerance only on the surfaces that locate, seal, or mate.

Surface finish follows the same logic. As-machined aluminum lands around Ra 1.6 to 3.2 μm. A careful finishing pass gets to Ra 0.8 to 1.6 μm, which is the normal target for sealing faces and bearing bores. Below Ra 0.8 μm you are usually polishing or lapping, and if the part will be anodized, remember that the coating adds roughly half its thickness to each surface.

Anodizing changes dimensions. Type II clear anodizing builds about 5 to 15 μm per surface, hardcoat more. If a bore must stay on size after coating, machine it undersize by the coating thickness and mask the threads. Conductive anodizing exists for parts that need both wear resistance and electrical grounding.

Inspection should match the risk. A first-article report covers the critical dimensions on the drawing. For production runs, in-process checks catch drift before a whole batch is wrong. We inspect 100 percent of parts before shipment and can supply dimensional reports on request.

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  Anodize allowanceAdd 5 to 15 μm per surface for Type II; account for it in the model.
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  ThreadsMask or plug threaded holes before anodizing to protect fit.
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  FlatnessThin plates can move after machining; stress-relieved stock helps.

Aluminum is not a universal answer. If the part sees continuous sliding contact at high load, aluminum wears quickly and will need a steel insert, a hardcoat, or a different material. If the part must survive repeated high-cycle fatigue without a protective coating, 2024 and 7075 will corrode in humid or salty air even when the stress is fine.

Temperature is another boundary. Above roughly 150 °C, most aluminum alloys lose a meaningful share of their room-temperature strength and creep under sustained load. Parts near an engine exhaust or a soldering process are often better in steel or titanium. The low melting point also rules aluminum out for any application that reaches its solidus in service.

Very small features and sharp internal corners are harder in aluminum than the material's reputation suggests. Soft material deforms rather than breaking, so a burr forms easily and a 0.2 mm internal corner radius is difficult to hold cleanly. Design corners with a radius at least equal to the cutter radius, and avoid pockets deeper than three times the cutter diameter without a reason.

Cost is the last boundary. Aluminum stock is often cheaper than stainless, but if the part needs hardcoat anodizing, masking, and a polished finish, the total can exceed a simple stainless part left as-machined. Compare finished cost, not material cost.

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  Wear surfacesUse inserts or hardcoat; bare aluminum galls.
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  Above 150 °CStrength drops and creep begins; consider steel or titanium.
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  Sharp cornersSoft material burrs; specify a radius the cutter can actually produce.