CNC aluminum processing for mobile phones

• 1
  Key takeaways
• 2
  What makes a phone housing different from other aluminum parts
• 3
  Why 6061 and 7075 dominate phone frames
• 4
  When 3-axis is enough and when 5-axis earns its cost
• 5
  What ±0.005 mm actually means on a phone part
• 6
  Where the process stops and another route takes over
• 7
  Choosing alloy and machine for a phone aluminum part

Most machined aluminum parts are stiff blocks. A phone housing is the opposite. It is a thin shell with a large open area in the middle, a wall that may be 0.6–1.2 mm thick, and bosses that carry the camera, the buttons and the antenna. The part has almost no mass to absorb vibration, so every cutting pass pushes the wall around.

This is the core constraint of the work. A Ø3 mm end mill removing a light finishing pass can flex a 0.7 mm wall enough to leave a visible ripple. The fix is rarely a faster spindle. It is a different toolpath: support the wall with a fixture, take the finishing pass in one continuous direction, and keep the radial engagement small.

The second constraint is the tolerance stack. The outer profile has to match a plastic or glass insert. The button holes have to line up with the switch. The camera ring has to sit flat against the lens. Each of these is a separate datum, and if they are not tied to one setup, the errors add up at assembly.

There is also a cosmetic side. A phone housing is handled every day and photographed under raking light. Tool marks that are invisible on an internal bracket become a defect on the outer rail. That changes how you plan the sequence, not just the tolerance.

• 1
  Thin wall, low stiffnessThe part cannot absorb the cutting force, so the toolpath has to.
• 2
  Multiple datumsProfile, buttons and camera ring must come from one setup to avoid stack-up.
• 3
  Visible surfacesThe outer rail is a cosmetic surface; internal marks matter less.

Alloy choice for a phone frame is a trade between machinability, stiffness and how the surface takes an anodize. 6061-T6 is the default. It cuts cleanly, holds a sharp edge, and anodizes to an even tone across a large part. For a mid-frame or a housing that will be anodized in a color, 6061 is usually the right starting point.

7075-T6 is stronger and stiffer, which matters when the wall is very thin or the part is long. A 7075 rail resists deflection better during finishing, so the surface stays flat. The cost is machinability: 7075 is harder on tools, generates more heat, and needs a sharper edge and a more conservative feed. It also anodizes to a slightly different tone.

6063 is worth a mention for extruded rails that are then machined. It is softer, machines fast, and gives a very clean anodized surface, but it does not hold a thin unsupported wall as well as 6061. For a simple straight rail it can be the cheaper path.

For internal brackets where weight matters more than stiffness, 2024 offers high strength at low density, but it is more corrosion-sensitive and needs a finish. ADC12 is a die-casting alloy, not a machining stock, so it belongs to a different process route.

• 1
  6061-T6Best default for anodized housings and mid-frames.
• 2
  7075-T6Pick it for very thin walls or long rails that must stay flat.
• 3
  6063Cheap and clean for simple extruded rails, weaker in thin sections.
• 4
  2024Light and strong for internal brackets, but needs a protective finish.

A phone housing usually has features on four or five faces. With a 3-axis machine you cut one face, flip the part, and cut again. Each flip adds a setup, and each setup adds an error. On a part where the button holes and the outer profile share a tolerance, that error can be the difference between a clean assembly and a rework loop.

A simultaneous 5-axis center removes that problem. The part stays in one fixture while the spindle tilts to reach the side walls, the chamfers and the internal corners. The datums stay locked. We run 16 simultaneous 5-axis machining centers, and for phone housings that is the setup that keeps the tolerance stack under control.

The trade is cycle time and fixture cost. Five-axis motion is slower per pass, and the fixture has to expose more of the part without letting it move. For a simple flat cover with one machined face and no critical side features, a 3-axis machine with a good vise is faster and cheaper.

A useful rule: if two or more critical features live on different faces and share a datum, go 5-axis. If the part is essentially a plate with one machined side, stay 3-axis.

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  3-axisOne machined face, loose side tolerance, flat plate-like covers.
• 2
  4-axisAdds a rotary table for a single wrapped profile, still one main face.
• 3
  5-axisSide features and datums that must stay tied together in one setup.

The tolerance figure is often quoted for the whole part, but that is not how it works in practice. On a phone housing, a few features carry the tight tolerance and the rest do not. The camera ring flatness, the button hole position and the profile that mates with the glass are the critical ones. The internal pocket floor is usually looser.

We hold ±0.005 mm (±0.0002 in) where the function needs it. The camera ring has to sit flat so the lens does not tilt. The button holes have to be positioned so the switch does not bind. The outer profile has to match the insert with a small, controlled gap. Everything else can sit at a looser band and still work.

Finishing follows the same logic. A visible rail may be specified at Ra 0.8–1.6 μm and then bead blasted or brushed. A fine finish at Ra 0.2–0.8 μm is reserved for surfaces that will be polished or hard anodized, where a scratch would show. An as-machined finish at Ra 1.6–3.2 μm is fine on internal faces.

The point is to spend the tight tolerance where it buys function, not across the whole drawing. A whole-part ±0.005 mm callout on a phone housing drives cost up without improving the assembly.

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  Tight where it functionsCamera ring flatness, button position, mating profile.
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  Looser elsewhereInternal pockets and non-mating faces.
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  Finish follows visibilityRa 0.2–0.8 μm only where the surface will be seen.

Machining is not always the answer. If the annual volume is very high and the geometry allows it, die casting or forging followed by a light machining pass will be cheaper per part. The machined surface is then only the critical face, not the whole shell.

Very thin walls have a floor. Below roughly 0.5 mm on a large unsupported span, the part starts to deflect under its own cutting force, and the scrap rate climbs. If the design needs a wall thinner than that, the geometry usually has to change: add a rib, shorten the span, or move the feature to a different process.

Deep narrow pockets are another limit. A pocket that is four times deeper than the tool diameter needs a long, thin cutter, and that cutter deflects. The result is a tapered wall and a poor floor finish. If the design allows a wider corner radius, the tool gets stiffer and the pocket cuts cleanly.

For one prototype, machining is almost always the right route. There is no tooling cost, and the geometry can change between revisions. That matters early, when the button layout is still moving.

• 1
  High volume, simple shapeDie casting or forging plus a light machining pass.
• 2
  Wall below 0.5 mmAdd a rib or shorten the span; machining alone will not hold it.
• 3
  PrototypeMachining avoids tooling cost and allows fast design changes.