Aluminum CNC Parts: How Alloy Choice and Geometry Shape the Result

• 1
  Why Aluminum CNC Parts Machine Differently From Steel
• 2
  Choosing Between 6061, 7075, 2024 and Cast Grades
• 3
  Wall Thickness, Tolerances and Surface Finish That Hold
• 4
  When Machining Beats Casting, Forging or Extrusion
• 5
  Finishes, Anodizing and How We Verify the Result
• 6
  Alloy and Process Selection at a Glance

Aluminum cuts fast. Cutting speeds run three to five times higher than for mild steel, and a three-axis mill can remove material at rates that would burn a steel cutter. That speed is the main reason aluminum CNC parts stay cheap in small and medium volumes. A bar of 6061 becomes a finished bracket in one setup, with no heat treat between operations.

The softness that makes aluminum easy to cut also makes it easy to scratch and easy to distort. A thin floor will bow as the cutter passes over it, then spring back after the vise opens. Clamping pressure alone can close a bore by 0.02 mm. This is why aluminum CNC parts often need light finishing passes and softer workholding rather than more spindle power.

Thermal expansion matters more than most designers expect. Aluminum grows about 23 μm per meter per degree Celsius, roughly twice the rate of steel. A part machined at 25 °C and measured in a 20 °C inspection room will read smaller. On a 300 mm length that gap is around 0.035 mm, which is larger than our ±0.005 mm tolerance. Temperature control is part of the process, not an afterthought.

Built-up edge is the classic aluminum failure mode. Soft alloys smear onto the cutting edge, and the smear breaks off, taking tool coating with it. The fix is sharp, uncoated or polished carbide, high rake angles, and generous flood coolant or high-pressure air. 6061 with a polished two-flute cutter runs clean. Gummy 5052 needs more care.

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  Fast to cutHigh spindle speeds and light chiploads keep cycle times low.
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  Prone to distortionThin walls and floors move under clamping and cutting force.
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  Temperature sensitive23 μm/m/°C growth can exceed the print tolerance on long parts.
• 4
  Sticky chipsBuilt-up edge needs sharp tooling and good coolant flow.

Most aluminum CNC parts are cut from 6061-T6. It welds, anodizes cleanly, holds a thread, and costs less than the high-strength grades. Yield strength sits near 276 MPa. For brackets, housings, fixture plates and covers, there is rarely a reason to look further.

7075-T6 is the choice when strength per unit weight drives the design. Yield strength reaches roughly 503 MPa, close to some mild steels, which matters in aerospace fittings and racing suspension links. The trade is cost, slightly worse corrosion resistance, and a finish that anodizes to a darker, less uniform tone.

2024-T4 machines well and has good fatigue resistance, which suits stressed skins and structural brackets. It is a poor choice for anything exposed to weather unless it gets a protective coating. 5052 and 5083 are the marine and sheet-metal grades; they form and weld well but are gummy on the mill. 6082 sits close to 6061 with slightly better strength and is common in European supply chains.

Cast grades are a different animal. ADC12 and similar die-casting alloys pour into complex shapes at low unit cost, but they contain porosity. A machined surface can open a void, and a tapped hole can break into one. When a casting still needs tight bores or sealing faces, we machine those features after casting rather than trying to hold them as-cast.

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  6061-T6Default for brackets, housings and fixture plates.
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  7075-T6High-strength aerospace and motorsport parts.
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  2024-T4Fatigue-loaded structures, needs coating outdoors.
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  5052 / 5083Marine and welded assemblies, gummy to cut.

A practical floor for unsupported aluminum walls in a milled part is 0.8 mm, and 1.0 mm if the wall is tall or the part will be handled often. Below that, deflection during cutting and vibration during shipping start to show. Walls that stand more than ten times their thickness above a floor behave like a fin and will chatter, so add a rib or reduce the height.

Tolerances follow the same logic. We hold ±0.005 mm (±0.0002 in) on critical features under controlled temperature, but applying that to every dimension on a drawing raises cost with no benefit. Put tight tolerance only on the features that mate, seal, or locate. Leave general dimensions at ±0.1 mm and let the shop use the looser callout where it can.

Surface finish is a separate decision. As-machined aluminum lands around Ra 1.6–3.2 μm with visible tool marks. A finishing pass brings it to Ra 0.8–1.6 μm, which is the normal target for sealing faces and sliding contacts. Fine finishes of Ra 0.2–0.8 μm are possible but need slower passes and sharp tooling, so reserve them for optical or bearing surfaces.

Threads deserve a note. Aluminum strips more easily than steel, so a 4 mm thread in 6061 should have at least 6 mm of engagement, and 8 mm if the joint will be opened and closed. Helical inserts solve the problem on parts that see repeated assembly, and they are easy to add before anodizing rather than after.

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  Minimum wall0.8 mm unsupported, 1.0 mm for tall or handled walls.
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  Tight tolerance±0.005 mm on mating and sealing features only.
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  Standard finishRa 0.8–1.6 μm covers most sealing and sliding faces.
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  Thread engagement1.5× diameter minimum in 6061, more if reassembled.

Machining wins when geometry is complex, volume is low, or the part must be dimensionally tight from the first piece. There is no tooling cost, so a design change costs a few lines of CAM code rather than a new mold. For prototypes and runs under a few thousand pieces, that flexibility usually outweighs the higher per-part cost.

Casting wins on high volume and organic shapes. A die-cast housing with internal ribs and bosses can be produced for a fraction of the machined cost once the tool is amortized, but the tool is expensive and porosity limits how tight the sealing surfaces can be. Many programs use both: cast the body, machine the bores and faces.

Extrusion suits long, constant cross-sections such as rails, frames and heat sinks. If the profile is uniform along its length, extrusion plus a few secondary machining operations is far cheaper than cutting the whole shape from plate. As soon as the cross-section changes, that advantage disappears.

Forging produces the best grain flow and fatigue strength, which matters for load-bearing links and wheels. The trade is tooling cost and limited feature detail. A forged blank still needs finish machining on the critical surfaces, so the question is usually how much stock to leave rather than whether to machine at all.

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  MachineLow volume, tight tolerance, complex detail, fast changeover.
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  CastHigh volume, organic shape, porosity acceptable away from seals.
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  ExtrudeLong uniform profile, then secondary machining.
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  ForgeLoad-bearing parts needing grain flow and fatigue life.

Anodizing is the most common finish on aluminum CNC parts. Clear anodize adds a thin oxide layer with good wear resistance and no dimensional change worth worrying about. Color anodize is cosmetic as well as protective, but the dye lot depends on the alloy, so a 6061 and a 7075 part in the same batch will not match exactly.

Hardcoat anodize builds a thicker, harder layer for sliding and wear surfaces, and it grows the part by roughly half the coating thickness per side. That growth matters on a bore that has to accept a bearing. Tell the shop which dimensions must stay nominal so the anodizer can mask or adjust them.

Other finishes cover different needs. Electroless nickel gives a uniform conductive coating on complex shapes. Chromate conversion keeps conductivity and is often used on electronics housings. Bead blasting and brushing change the texture before anodizing, and laser marking handles part numbers and traceability codes down to a minimum character height of 1.5 mm.

Inspection is where the tolerance claim is proven or lost. We check incoming material, monitor dimensions during the run, and inspect 100% of parts before shipment, with reports available on request. For a first article, ask for the specific features that matter to your assembly rather than a full dimensional report of every edge.

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  Clear anodizeWear resistance with no meaningful dimensional change.
• 2
  HardcoatThicker and harder, grows the part; mask critical bores.
• 3
  Electroless nickelUniform conductive coating on complex geometry.
• 4
  Laser markingTraceability codes down to 1.5 mm character height.