Internal CNC Milling of Aluminum: How Pockets, Slots and Bores Actually Get Cut

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  What internal CNC milling of aluminum actually removes
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  Tool reach sets the real limit on internal features
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  Chip evacuation is where aluminum pockets fail
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  When the part forces a change in axis count
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  Wall thickness and corner radii that hold up
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  Which aluminum grades behave well internally
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  How internal features get checked
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  Which setup fits which internal feature

External milling shapes the outside of a part. Internal CNC milling of aluminum works the opposite way: the cutter enters the stock and removes material between walls, under shoulders, and behind faces that stay in place. The tool has to reach the feature without touching anything you want to keep.

That single constraint drives most of the cost. A pocket 60 mm deep and 12 mm wide is not hard because aluminum is hard. It is hard because the tool needed to reach the bottom is slender, and slender tools deflect.

Aluminum helps here. Its low cutting force means you can run small-diameter tools at high spindle speed without stalling. 6061-T6 machines cleanly at 300–500 m/min surface speed with carbide. The limit is usually tool stiffness and chip evacuation, not the material.

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  PocketClosed floor bounded by walls on most sides. Depth-to-width ratio decides tool size.
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  SlotNarrow channel, often full-width cut. Chip packing is the main risk.
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  BoreRound internal feature, usually finished by helical interpolation or boring head.
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  Internal channelCurved passage inside the part. Needs 5-axis access or a split design.

A carbide end mill can cut about 3× its diameter in depth before deflection starts to show in the wall finish. Push to 5× and you are usually climb milling with a light radial stepover of 5–10% of diameter just to keep the wall straight.

For a 6 mm cutter that means a reliable pocket depth near 18 mm, and a workable depth near 30 mm with reduced parameters. Beyond that, you switch to a necked tool, a long-reach tool, or you change the part design.

Long-reach tools cut the same material but at lower radial engagement. Feed rates drop, cycle time climbs, and the risk of chatter rises. If a design needs a 4 mm tool to reach 40 mm deep, expect to pay for it in time and in scrapped first articles.

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  3× DComfortable. Normal stepover, good wall finish.
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  5× DWorkable with light radial engagement and reduced feed.
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  8× D+Necked or long-reach tool. Verify with a test cut.

Aluminum makes soft, stringy chips. In an open cut they fly clear. In a 15 mm deep pocket with a 6 mm cutter, they pile up at the bottom and get recut. Recut chips damage the floor finish and can snap the tool.

Two things fix this. First, air blast or through-spindle coolant at 20–70 bar pushes chips out of the pocket. Second, a trochoidal or dynamic toolpath keeps radial engagement low and gives chips room to leave instead of nesting in a corner.

For deep, narrow features, peck drilling the rough stock out first and then interpolating is often faster than a full-width slotting pass. It also loads the tool more evenly, which matters when the tool is already long.

A 3-axis machine reaches the top of a pocket. It cannot reach the side wall of a feature that faces sideways. If your internal bore sits at 90° to the main face, a 3-axis setup needs a second operation, a fixture, and a re-datum.

A 4-axis mill adds rotation about one axis. That handles internal features on the side of a cylindrical or prismatic part in a single setup. The rotary table at GreatLight is Ø400 mm, which covers most housings and manifolds.

A 5-axis machine tilts the tool as well as the part. That lets a short, stiff cutter follow a curved internal channel instead of a long tool reaching straight in. It also lets one setup cut features on five faces, which removes re-fixturing error. If a channel curves in two planes, 5-axis is usually the only practical route.

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  3-axisFeatures open to one direction. Lowest cost per part.
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  4-axisInternal features on part sides, cut in one setup.
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  5-axisCurved internal channels and multi-face access with short tools.

Thin internal walls move. As the cutter passes, the wall deflects away and springs back, leaving a tapered surface. In 6061, walls under 1 mm between two pockets are the usual suspect.

A practical floor is 1.5 mm for a wall that carries no load, and 2.5–3 mm if the wall will be clamped or bolted later. If the design needs thinner, expect to cut with light passes and accept a slower cycle.

Corner radii matter just as much. A cutter leaves a radius equal to its own radius in every internal corner. If you draw a sharp 90° internal corner, someone has to either use a smaller tool or EDM the corner out. Specifying a corner radius at least 1.2× the intended cutter diameter keeps the feature machinable with a standard tool.

6061-T6 is the default for internal milling. It cuts clean, holds a thread, and takes anodizing well. For a housing with internal pockets and mounting bores, 6061 is usually the right call.

7075 machines well but is less forgiving of thin walls because it is stronger and springier. It suits internal features that see load, like clamps and structural brackets.

2024 is common in aerospace internal parts. It cuts fast but has lower corrosion resistance, so internal surfaces usually need a coating. 5083 and 6082 appear in welded or marine assemblies where internal pockets hold fluid. ADC12 is a die-casting alloy; internal milling there is usually a finishing pass on a cast feature, not a from-solid cut.

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  6061-T6Default for internal pockets, bores and threads.
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  7075Load-bearing internal features. Watch thin walls.
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  2024Aerospace internal parts. Needs coating.
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  5083 / 6082Fluid and marine housings with internal pockets.

You cannot see inside a pocket once it is closed. That changes how internal milling is inspected. Bore diameter and position are checked with a bore gauge or a CMM probe on the finished surface.

Depth and floor flatness are checked with a depth micrometer or a dial indicator on a height stand. For internal channels that cannot be probed, the shop cuts a first article and sometimes a sectioned sample to confirm the passage is where the model says.

GreatLight inspects 100% of parts before shipment, with reports available on request. The tolerance floor is ±0.005 mm on critical internal dimensions, and the finish is Ra 0.8–1.6 μm on a standard milled internal surface.