CNC Metal Phone Shell: How Alloy and 5-Axis Setup Decide the Part

• 1
  Why metal, and what it actually changes
• 2
  Alloy choice: 6061, 7075, titanium or stainless
• 3
  What five-axis motion solves on a thin shell
• 4
  Distortion, clamping and the flatness fight
• 5
  Surface finish and what each step costs you
• 6
  Alloy and process selection at a glance
• 7
  Typical numbers for a machined shell

Metal changes three things at once: stiffness, thermal path and radio behavior. A 6061-T6 shell at 0.8 mm wall is far stiffer than the same wall in polycarbonate, so it resists bending in a pocket. It also spreads heat from the SoC across a larger area instead of trapping it. That is the engineering reason metal frames show up on flagship phones.

The radio side is the tradeoff. Aluminum and titanium block or detune antennas and wireless charging coils if they sit in the wrong place. A machined shell has to leave an aperture, a plastic overmold, or a thin ceramic window over the coil. Designers who ignore this get a shell that looks right on a bench and drops two bars in the hand.

Drop behavior is not the same as stiffness. A stiff shell transfers impact into the glass; a soft bumper absorbs it. The best phone shells combine a rigid machined frame with a compliant inner layer, which is why the internal geometry matters as much as the outer surface.

Weight is the last constraint. Titanium buys stiffness per gram, aluminum buys cost per gram. At 0.6–0.8 mm wall the difference between 6061 and Ti-6Al-4V is real in the hand, and it is measurable on a scale.

6061-T6 is the default for a CNC metal phone shell. It machines fast, takes anodizing evenly, and holds a 0.5 mm wall without chatter if the toolpath is right. Its yield strength near 276 MPa is enough for a case that never carries load.

7075-T6 is roughly twice the strength of 6061 and machines to a sharper edge. It also anodizes to a darker, less uniform color because of its zinc and copper content, and it costs more per kilogram. Use it when the shell is a structural part of the phone, not a cover.

Titanium TC4 (Ti-6Al-4V) has the best stiffness-to-weight ratio in this group and the worst machinability. Cutting speed drops to roughly one third of aluminum, tool life shortens, and the part must be held rigidly to avoid work hardening at the cut. Budget for slower cycles.

Stainless 316L resists scratches and corrosion but weighs about three times more than aluminum for the same volume. On a phone that is a noticeable penalty. Stainless makes more sense for a hinge or a small trim piece than for a full shell.

A phone shell is mostly a pocket with a rim. Three-axis machining can cut the pocket and the outer profile, but the side walls and the corner radii need the part repositioned, and each reposition adds error. That is the core problem five-axis motion removes.

On a simultaneous five-axis center, the tool tilts while it moves. A Ø6 mm end mill with a 3 mm corner radius can reach the internal fillet in one setup, and the tool engages the wall at a shallow angle instead of a right angle. Cutting force drops, so a 0.6 mm wall stays where it was modeled.

The second gain is access to undercuts and draft. Speaker openings, button reliefs and chamfered rims often sit at angles a three-axis spindle cannot reach without a custom fixture. Tilting the table or the head reaches them with the same tool.

The third gain is fewer setups. Each setup on a thin part risks a new clamping distortion. Going from four setups to two usually matters more for flatness than a tighter tolerance callout on the drawing.

Aluminum moves when material is removed. A 200 × 100 × 8 mm billet relieved down to a 1 mm floor will bow as internal stress releases. The bow is not a machining error; it is the material finding a new balance. Roughing, then stress relief, then finishing is the standard answer.

Clamping is the second source. A vise squeezing a thin rim distorts it before the cutter arrives, and the part springs back after unclamping. Vacuum fixtures and low-melt fixturing spread the load, and light finishing passes at 0.1–0.2 mm depth reduce the spring-back.

Heat is the third. A shell that reaches 60–80 °C during roughing grows, gets cut, then shrinks on cooling. Flood coolant or high-pressure through-tool coolant keeps the part near room temperature, which keeps the pockets consistent across a batch.

Measure flatness on a surface plate or with a coordinate measuring machine while the part is unrestrained. Measuring in the fixture tells you what the fixture looks like, not what the customer receives.

As-machined aluminum sits around Ra 1.6–3.2 μm with visible tool marks. Bead blasting turns that into a matte, uniform surface and hides the marks. Brushing gives a directional grain. Polishing reaches a mirror finish but shows every scratch afterward.

Anodizing is an oxide layer grown from the aluminum itself, so it does not chip off like paint. Type II clear or colored anodizing runs a few micrometers thick. Hardcoat anodizing is thicker and more wear resistant, but it can shift a tight bore by several micrometers, so mask or compensate those features.

Conductive anodizing keeps the shell electrically grounded to the chassis. If the design relies on the shell as an antenna ground plane, this is not optional. Tell the finisher which surfaces must stay conductive before the parts ship.

Laser marking works on anodized surfaces and needs a minimum character height of 1.5 mm to stay legible after finishing. Smaller text fills in or fades. Engraving cuts deeper and survives longer, at the cost of a visible recess.