Precision Manual Mobile Phone Case: What to Check Before You Cut Metal

• 1
  Five things that decide the part
• 2
  Why a precision manual mobile phone case is a hard part to machine
• 3
  Machining sequence for a precision manual mobile phone case
• 4
  Material choice for a precision manual mobile phone case
• 5
  Tolerances and finishes that hold on a phone case
• 6
  Inspection and fit checks before shipment
• 7
  Which process fits which phone case design

A phone case looks simple in a render. In the machine it is a thin-walled pocket with a curved outer profile, a set of small openings, and a surface the user touches every day. Those three things fight each other. Cutting the pocket removes the material that was holding the wall steady, so the wall deflects under tool pressure and the finishing pass ends up shaving an uneven amount.

The outer profile is usually a ruled surface with a large radius. On a three-axis machine you can only reach it with a ball nose cutter held at an angle, which leaves a scallop pattern you then have to blend out by hand. A simultaneous five-axis toolpath keeps the cutter normal to the surface, so the stepover stays even and the blend becomes a polishing step instead of a rework step.

Openings are where most first articles fail. A camera cutout on a modern phone is often 12–18 mm across with a 0.3–0.5 mm corner radius, and the inside edge is visible in the finished product. If the tool radius is larger than the corner radius, the corner is left sharp and the edge looks unfinished. Match the cutter to the smallest corner in the drawing before you quote.

Hand feel is the part no drawing captures. A case with a 0.05 mm raised lip around the camera opening feels sharp; the same lip at 0.02 mm with a 0.3 mm chamfer feels smooth. We keep a sample of each approved feel in the sample center so the second run can be compared against the first, not against a memory of the first.

• 1
  Thin wall plus deep pocketExpect two or three spring passes and a light finishing cut.
• 2
  Five-axis for the outer profileEven stepover removes the hand-blending step.
• 3
  Smallest corner drives the toolA 0.3 mm corner needs a 0.3 mm or smaller cutter.
• 4
  Feel is checked against a physical sampleNot against a description.

We start from a solid billet rather than a casting. Billet stock costs more per part, but it removes porosity as a variable, and porosity is the defect that shows up as a pit after anodizing. For aluminium cases that means 6061-T6 or 7075 plate, sawn oversize by 1.5–2.0 mm on each face so the first operation has clean material to face off.

Operation one holds the billet in soft jaws and machines the inner pocket, the boss for the camera, and the mounting features. We leave 0.3 mm of stock on the pocket floor and walls at this stage. The part is still a solid block, so it can take a heavier cut without ringing. Cut depths of 1.5–2.0 mm per pass with a 6 mm carbide end mill are normal here.

Operation two flips the part onto a machined fixture that locates on the pocket. This is where the outer profile and the openings are cut, usually on a five-axis center so the tool stays normal to the surface. Finish passes run at 0.10–0.15 mm radial engagement and 8,000–12,000 rpm for aluminium, with air blast rather than flood coolant to avoid chip recutting in the pocket.

Operation three is finishing and inspection. Bead blasting at 0.2–0.4 MPa gives a matte base for anodizing; brushing gives a directional grain that shows every deviation in the wall, so we only brush parts whose wall thickness has already been verified. Every opening is checked against the drawing before the part moves to the next step.

• 1
  Billet, not castingRemoves porosity as a finish defect.
• 2
  Rough with the part still solid1.5–2.0 mm depth per pass, 0.3 mm stock left.
• 3
  Fixture on the machined pocketRepeats location to ±0.005 mm between operations.
• 4
  Air blast over flood coolantFewer chips trapped in a deep pocket.

6061-T6 is the default. It machines fast, anodizes to a consistent color, and holds a 0.8 mm wall without much drama. If the case is mostly a shell with a few openings, 6061 in 3.0–4.0 mm stock will get you there.

7075-T6 is roughly 40% stiffer and about 20% stronger than 6061, which matters if the case is a structural frame rather than a cover. The trade-off is surface finish. 7075 has a coarser grain, and after hardcoat anodizing the color can drift between parts from different lots. If the customer wants a perfectly matched matte black, 6061 is the safer call.

Titanium (TC4 / Ti-6Al-4V) is chosen for weight and scratch resistance, not for cost. Cutting speed drops to roughly one third of aluminium, tool life shortens, and the part usually needs a separate finishing pass at low feed to control chatter. Budget more machine time and expect a brushed or bead-blasted finish rather than a mirror polish.

Plastics behave differently. PC and ABS are common for prototype housings and are fast to cut, but a 0.8 mm PC wall will flex in the hand. PEEK holds up to heat and wear but costs many times more than aluminium and is usually reserved for industrial handset enclosures, not consumer cases.

• 1
  6061-T6Default choice. Consistent anodize color.
• 2
  7075-T6Stiffer, but color can drift after hardcoat.
• 3
  TC4 titaniumRoughly one third the cutting speed of aluminium.
• 4
  PC / ABS / PEEKFast to cut; thin walls flex.

We hold ±0.005 mm on critical features and ±0.0002 in when the drawing is in inches. On a phone case, that level of control is usually applied to three places: the inner pocket depth, the opening positions, and the datum face the case sits on. Everything else can live at ±0.05 mm without the user noticing.

Surface finish follows the function. As-machined at Ra 1.6–3.2 μm is fine for internal faces nobody touches. Ra 0.8–1.6 μm is the normal target for outer surfaces that will be bead blasted or anodized. Ra 0.2–0.8 μm is reserved for polished or brushed faces where light will catch the surface, and those parts need slower finishing passes and a rigid setup.

Thickness control is the other half. An anodized layer adds roughly 5–25 μm depending on the process, and hardcoat can add more. If the drawing calls out a finished dimension across an opening, tell us whether that dimension is before or after coating. A 20 μm shift is enough to change how a button feels.

Laser marking needs room. We mark logos and small text at a minimum character height of 1.5 mm. Anything smaller fills in and becomes unreadable after anodizing. If the design has fine text, keep it off the machined surface and use a printed insert instead.

• 1
  ±0.005 mmApplied to pocket depth, opening position, datum face.
• 2
  Ra 0.8–1.6 μmStandard for bead-blasted or anodized outer faces.
• 3
  Coating adds 5–25 μmState whether dimensions are pre- or post-coat.
• 4
  Marking at 1.5 mm minimumSmaller text fills in after anodizing.

Every part gets inspected before it ships. For a phone case that means three checks: the raw material certificate, the in-process measurement after each operation, and a final dimensional check on the finished part. Reports are available on request.

The final check uses both contact and optical measurement. A coordinate measuring machine confirms the pocket depth, the opening positions and the datum face. An optical comparator or vision system checks the small radii and the edge condition on openings, where a contact probe would struggle to reach.

Then the part goes on a phone. We keep dummy bodies at the shop for common handset sizes, and the case is fitted, removed, and fitted again. Buttons are pressed, the camera opening is checked against a light source for alignment, and the case is held in the hand to confirm the feel matches the approved sample.

GreatLight has run this process since 2011 across three wholly-owned plants covering 7,600 m², with 150 technicians and 127 high-precision CNC machines, including 16 simultaneous five-axis machining centers. The qualification rate across those lines is 99.99%, and uploads are kept secure and confidential with an NDA available on request.

• 1
  Raw material certificateChecked before the first cut.
• 2
  In-process measurementAfter each operation, not just at the end.
• 3
  Final CMM plus optical checkCovers both large and small features.
• 4
  Fit on a dummy phoneThe check no drawing can replace.