CNC processing of mobile phone cases

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  Key takeaways
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  What makes a phone case a hard part to machine
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  Wall thickness and material choice for CNC processing of mobile phone cases
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  Tool access, corner radii and setups
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  Finish, tolerance and fit after coating
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  How a case moves through the shop
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  Five checks before you release a case for machining
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  Machined versus molded phone cases: when each one fits

A phone case looks simple from the outside. In the machine it is a thin shell with a deep pocket, load-bearing corners, and a surface that will be inspected by hand before it ever reaches a customer. The pocket depth usually sits between 8 and 12 mm depending on the handset, while the wall left behind is often 0.8 to 1.2 mm. That ratio is what drives every other decision on the job.

Cutting forces push the wall outward while the tool passes. The thinner the wall, the more the material deflects and then springs back, leaving a taper that never shows on the drawing. On aluminium alloys such as 6061 or 7075, a 1 mm wall with a 6 mm end mill at moderate depth is stable. Drop to 0.5 mm and the same cutter will chatter unless you reduce radial engagement and accept longer cycle time.

Heat matters as much as force. Long finishing passes on a thin shell heat the part unevenly, and a case that measured in tolerance at the machine can drift after it cools. We rough with stock left on, let the part rest, then finish. On tight jobs the finishing pass is split into two lighter cuts rather than one heavy one.

Titanium grades like TC4 and some stainless steels take the same geometry but punish it harder. They hold heat, work-harden at the cut, and wear small tools quickly. If a design needs 0.6 mm walls in Ti-6Al-4V, expect more passes, more tool changes, and a real conversation about whether the wall is structural or cosmetic.

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  Deep pocket, thin wallThe two features together decide cutter size and stability.
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  Visible surfaceEvery clamp mark on the outside face becomes a rework step.
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  Post-machining stepsAnodizing, blasting and laser marking all sit after the cut.

Material selection for cases is mostly a trade between weight, stiffness and finish. 6061-T6 is the default for aluminium shells: it machines cleanly, anodizes well, and holds a 0.8 mm wall without drama. 7075 gives more stiffness at the same thickness, which matters for slim cases that must resist bending, but it anodizes to a darker, slightly different tone and is harder on small cutters. 5052 and 5083 bend more easily if the case has a snap-on lip.

On wall thickness, treat 0.6 mm as the practical floor for aluminium and 0.8 mm for stainless. Below that, the wall behaves like a spring and the finishing pass becomes a measurement exercise rather than a cut. If a design calls for 0.4 mm because of a weight target, the honest answer is usually that the case needs a different structure, not a tighter tolerance.

Stainless 304 and 316L give a cool, heavy feel and take a mirror polish, but they cut slowly. A 316L case with 1 mm walls can still be cost-effective at low volume, where tooling for molding would not pay back. Titanium TC4 sits at the top for feel and corrosion, with the highest machining cost of the three.

Plastics change the rules. PC and ABS are common for painted or dyed cases; POM is used when the part needs to slide or snap repeatedly. PEEK appears in small industrial enclosures rather than consumer cases. Plastics cut fast but deflect easily, so fixtures matter more than spindle speed.

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  6061-T6Best all-round balance of cut speed, weight and anodized finish.
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  7075Stiffer for slim shells, heavier tool wear, distinct anodized tone.
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  304 / 316LPremium weight and polish, slower cycle, better at low volume.
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  PC, ABS, POMFast cutting; holding the part still is the main challenge.

Every pocket corner leaves the radius of the cutter that made it. A 0.5 mm inside corner needs a 1 mm cutter, and a 1 mm cutter cannot take a deep axial pass in aluminium without snapping. The usual compromise is to leave a larger radius where the customer will not see it and a small one only at the visible edge. When that is not possible, we cut the small corner with a short tool and reach the rest with a larger one.

Three-axis machines handle a case that opens from one direction. Once the design has side buttons, an undercut lip, or a curved back that meets the side wall, the part needs either a fourth axis or multiple re-clamps. Each re-clamp is a chance to lose 0.02 mm and a chance to mark the finished face. Five-axis work avoids most of that by tilting the tool into the feature.

For camera openings and speaker slots, tool access decides whether the feature is milled or drilled. A 1.2 mm slot is a slitting cut with a small end mill; a 3 mm round opening is a drill followed by a light bore. Mixing the two in the same pocket adds a tool change but removes a finishing operation later.

Fixtures deserve a mention. A thin case clamped on its outer face will show clamp marks after anodizing, because anodizing does not hide them. Soft jaws machined to the case profile, or a vacuum plate for flat backs, keep the visible surface untouched. This is a setup cost that repeats for every batch.

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  Corner radius equals cutter radiusNo way around it; design the radius you can actually cut.
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  Three axes, one directionAdd side features and you add setups or a fourth axis.
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  Soft jaws or vacuumProtects the cosmetic face from clamp marks.

A machined case is judged by hand, so surface finish is not a detail. As-machined aluminium sits around Ra 1.6–3.2 μm, which reads as a fine step pattern under light. Bead blasting evens that out and gives anodizing a matte, uniform base. Brushing leaves directional lines. Polishing before clear anodizing gives the brightest result but shows every fingerprint and every small scratch.

Anodizing is a conversion coating, not paint. It grows into the surface and adds roughly 20–30 μm per side in hardcoat, less in a standard clear coat. Holes, threads and sealing faces change size. On a case where the phone must slide in without play, we machine those faces undersize and mask them so the coating does not close the gap. Threads for screws are masked or cut after coating.

Tolerance on a case is usually split into two groups. The cosmetic surfaces need to be smooth and dimensionally consistent from part to part, which is a process control issue more than a tight number. The functional surfaces, such as the inner rails that hold the phone, need to be within ±0.05 mm to avoid rattle. We work to ±0.005 mm where a feature requires it, and we inspect 100% before shipment.

Color matching across batches is the step customers notice last and complain about first. Clear, black and hardcoat anodizing are stable. Bright colors and dyed finishes shift with alloy, bath age and part geometry. If a program runs over several months, keep the alloy and the finish spec fixed, and expect to approve a sample from each batch.

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  Bead blast then anodizeMost consistent matte look for aluminium cases.
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  Mask functional facesCoating thickness closes gaps that the phone needs.
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  Fix the alloyChanging alloy mid-program changes the anodized color.
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  Laser marking from 1.5 mmSmaller text fills in and becomes unreadable.

A case program starts with a model and a finish callout. Within 12 hours we return a quotation and a DFM analysis that flags thin walls, sharp corners and features that need a second setup. That review is free and it is where most cost is removed, because changing a radius in CAD costs nothing while changing it after the first article costs a day.

Once the design is agreed, production can start within 24 hours. Raw bar or plate is cut to size, faced, and roughed with stock left for finishing. The part rests, then gets its finishing passes. Functional faces are checked against the model at the machine before the part leaves the cell, and the full inspection happens after finishing, when coating thickness is already in the part.

Finishing is handled in the same building. Anodizing, plating, bead blasting, brushing and laser marking run as a sequence, which means a case does not sit in a queue at a third party between cutting and coating. Parts ship in 3–5 days for standard aluminium work; stainless and titanium take longer because of cut time.

For programs that repeat, we keep the fixture and the program. A second batch then skips the setup development and goes straight to cutting. Uploads stay confidential and an NDA is available on request if the case is tied to an unannounced handset.

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  DFM in 12 hoursThin walls, corner radii and setup count get flagged before quoting.
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  Rough, rest, finishKeeps thin shells from drifting after the cut.
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  Finishing in-houseNo shipping the part out between machine and anodize line.