Basic Knowledge of CNC Aluminum Milling

• 1
  Key takeaways
• 2
  How the cut happens in CNC aluminum milling
• 3
  Which aluminum alloys suit CNC milling
• 4
  Tool geometry and why flutes are a trade-off
• 5
  Workholding and the heat problem
• 6
  Tolerance, finish, and what the machine can actually hold
• 7
  Aluminum alloys compared for milling
• 8
  Machine configuration and what it buys you

Aluminum is milled with a rotating multi-flute cutter that shears material away in chips. The tool turns fast, the table feeds the workpiece into it, and each tooth takes a small bite. Because aluminum is soft, the cut is easy to start but easy to get wrong.

The energy of the cut goes into three places: heat in the chip, heat in the tool, and heat in the workpiece. In aluminum, most of it should leave with the chip. When chips stay in the cut, they get recut, the edge rubs instead of shears, and the surface turns rough.

Cutting speed is expressed as surface speed in meters per minute. For aluminum with carbide tooling, 300–600 m/min is normal. That translates to very high spindle rpm on small cutters, and most machine spindles run out of rpm before aluminum runs out of machinability.

Feed per tooth is the other half of the equation. Too low and the tool rubs, work-hardens the surface, and dulls fast. Too high and the tool deflects or chips. For a 6 mm carbide end mill in 6061, 0.05–0.10 mm per tooth is a reasonable starting range.

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  Chip load matters more than rpmKeep a real chip per tooth. Rubbing is worse than cutting fast.
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  Climb milling on finishingIt lifts the chip away from the finished wall and gives a better surface.
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  Air blast beats flood coolantOn open pockets, air clears chips and avoids thermal shock.

Not every aluminum grade machines the same way. The alloying elements that make a grade strong also make it gummy, brittle, or prone to corrosion. For milling, 6061-T6 is the default because it cuts clean, holds tolerance, and takes anodizing evenly.

2024 machines well and is stronger than 6061, but its copper content makes it poor for anodizing and prone to corrosion without a coating. Use it for stressed structural parts that will be painted or plated.

7075-T6 is the strongest common milling alloy, close to some steels. It cuts clean but has less elongation, so thin sections can crack during clamping or bending. It is a good choice for aerospace brackets and tooling, less good for parts that must deform.

5083 and 5052 are marine grades. They resist salt water but are gummy to machine and do not heat treat. 6082 is a European structural grade with properties close to 6061. ADC12 is a die-casting alloy and is not normally milled from plate.

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  6061-T6 for general partsGood strength, good finish, good anodizing.
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  7075-T6 for strength-criticalAccept the lower elongation and plan for it.
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  2024 for fatigue resistanceProtect the surface with a coating.
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  5052 / 5083 for marineExpect slower feeds and more built-up edge.

An end mill with more flutes has more cutting edges per revolution, so it can feed faster at the same chip load. It also has less room for the chip to escape. In a shallow cut, that is fine. In a deep pocket, the gullet fills and the tool breaks.

Two-flute and three-flute cutters are the workhorses for aluminum. Two flutes give the largest chip room, which matters for slotting and deep pockets. Three flutes balance chip room against feed rate for general profiling.

Coating choice is simpler than many people expect. Uncoated polished carbide works well because aluminum does not stick to it easily. ZrN and DLC coatings reduce built-up edge further. TiAlN is designed for steel and can react badly with aluminum at high temperature.

Tool runout is the quiet killer. A cutter with 0.02 mm of runout loads one flute harder than the others. That flute wears first, the cut gets noisy, and the wall finish goes uneven. Check runout with a dial indicator before a long finishing pass.

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  High helix for chip liftA 40–45° helix pulls chips up and out of the pocket.
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  Corner radius over sharp cornerA small radius spreads load and extends tool life.
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  Check runout every setupUnder 0.01 mm is a good target for finishing tools.

Aluminum moves when it gets hot. A thin wall that measures correctly on a cold machine can bow 0.05 mm once the part cools to room temperature. The fix is sequence, not force: rough the part, let it cool, then take the finishing passes.

Clamping force is the second source of movement. A vise clamped hard on a thin frame will spring the part flat, machine it flat, then release it into a curve. Support the part where the cutter pushes, and use soft jaws shaped to the workpiece.

For flat plates, vacuum fixturing holds the whole face without introducing point loads. For complex parts, a custom soft jaw or a sacrificial fixture block machined in place gives repeatable location across a run.

Dry cutting with an air blast is common in aluminum because it keeps the work area visible and avoids coolant mist. Air blast also clears chips, which is the main job. In a deep pocket, dry cutting with air can leave chips at the bottom, so a through-tool or high-pressure air line helps. A mist of minimal lubrication is often the practical middle ground.

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  Rough, cool, finishDo not chase tolerance on a hot part.
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  Support under the cutPut the fixture where the tool pushes, not where it is convenient.
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  Clear chips or stop cuttingRecut chips are the fastest way to a broken tool.

A tolerance of ±0.005 mm is achievable in aluminum for critical features on a stable setup. It is not achievable across every dimension on a large part, especially with thin walls or long reaches. Specify tight tolerance only where the function needs it.

Surface finish is easier to control than tolerance. As-machined aluminum typically lands at Ra 1.6–3.2 μm. A fine finishing pass with a sharp, low-runout cutter can reach Ra 0.8–1.6 μm, and a polished or lapped operation can get to Ra 0.2–0.8 μm on flat faces.

Anodizing adds a layer that changes dimensions. Type II clear anodizing grows about half in the part and half out, so a 0.01 mm build-up per surface is a realistic planning number. Hardcoat is thicker and changes dimensions more. Call out critical dimensions that must survive the coating.

Five-axis machining changes the accuracy picture. Cutting a curved surface in one setup removes the stack-up error from multiple fixtures. It also lets the tool reach features that a three-axis machine cannot, at the cost of a more complex setup and longer programming time.

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  Tight tolerance, small areaApply ±0.005 mm only to mating or sealing features.
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  General dimensions±0.05 mm is a normal, cost-effective default.
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  Plan for coating growthAnodizing moves the surface by roughly 0.01 mm per side.