CNC phone case handling for metal and engineering plastic bodies

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  Key takeaways
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  What geometry suits CNC phone case handling
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  Tolerances that matter on a phone case
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  Fixturing and workholding problems to expect
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  Surface finishing and how it changes dimensions
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  How we run a case from file to finished part
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  Material and finish combinations for machined cases

CNC phone case handling makes sense when the case is a structural part, not a skin. A unibody aluminium frame, a machined camera bezel, a screw-together metal bumper, or a prototype shell that has to match the production part in feel and fit. These shapes have flat or gently curved outer walls, pockets for the PCB and battery, and openings that a cutter can reach from one direction.

The part you should not bring to a mill is a thin snap-on shell with 0.4 mm walls and a soft internal lip. It will flex in the vise, ring during roughing, and the lip needs a tool that cannot reach under it. That part belongs in injection molding or vacuum casting, where the tool cost is amortized across thousands of units.

Camera bumps and islands are where geometry gets interesting. A raised island with a 0.3 mm chamfer and a polished face is easy on a 5-axis center because the tool can tilt and stay clear of the wall. A deep square counterbore around the lens is harder: the corner radius is set by the cutter, so a sharp internal corner is not achievable. Draw it with a 0.5 mm corner and the part machines cleanly.

Undercuts are the other decision point. If the case clips over the phone with an internal groove, the groove has to be cut with a T-slot cutter or a lollipop tool from the side. That adds a setup, a fixture, and usually a visible tool mark. If the design can use a separate insert or a screw, the machining drops by roughly half.

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  Good fitUnibody frames, bumpers, bezels, brackets, heat-spreader plates.
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  Poor fitFlexible snap-on skins, living hinges, parts with sharp internal corners.
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  BorderlineTwo-piece shells joined by screws or adhesive; workable if wall ≥ 0.6 mm.

A phone body has a tolerance stack, and the case sits on top of it. The phone's own width might vary by ±0.15 mm between units, and the case has to slide on without scratching and without rattling. That means the internal cavity is usually held to +0.10 / −0.00 mm on the width, and the camera opening is held to ±0.05 mm so the lens does not catch the edge.

We hold ±0.005 mm on critical features when the drawing calls for it, but that is rarely needed on every dimension. Marking the whole drawing at ±0.005 mm raises cost because it forces slower passes and more in-process checks. Mark the cavity, the camera bore, and the button holes tight. Let the outer cosmetic surfaces run at ±0.10 mm.

Button holes are a common failure point. If the hole is 0.05 mm too small, the button sticks. If it is 0.05 mm too large, the button rattles. For a 3.0 mm button, hold the hole at 3.05 +0.03 / −0.00 mm and check the first article with the actual button installed, not just with a pin gauge.

Flatness on the back face matters more than most drawings admit. A back that is 0.15 mm convex will rock on a table and show a light gap in the middle. Face-milling after the pockets are cut, or a light finishing pass at 0.1 mm depth, keeps it flat without distorting the thin floor.

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  Cavity width +0.10 / −0.00 mm is typical for a slide-on fit.
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  Camera bore±0.05 mm, with a 0.5 mm corner radius.
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  Back flatness0.05 mm over the full face after finishing.

A phone case is a thin-walled box, and thin-walled boxes move when you clamp them. The first operation is usually fine because the stock is solid. The second operation is where parts go out of tolerance, because the vise squeezes the finished walls and they spring back when released.

The fix is a plug or a nest. We machine a soft jaw that matches the outer profile, or we drop a machined plug into the cavity so the walls are supported from the inside. For a 70 mm × 145 mm case with 1.0 mm walls, that plug is often the difference between 0.05 mm and 0.20 mm of distortion.

Vacuum fixturing works for flat plates and thin covers, but not for a deep pocket where the part is mostly air. Magnetic chucks are fast for steel and stainless, and they leave the top face open, which helps when you need to cut the back and the rim in one pass.

Clamp pressure is the last variable. We use light cuts and lower clamping force on the finishing pass, and we check the part with a height gauge after unclamping, not while it is still in the vise. A number taken in the fixture tells you nothing about the part you ship.

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  Soft jawsCut to match the outer profile; spreads clamping load.
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  Internal plugSupports thin walls during the second setup.
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  Vacuum plateGood for flat covers, poor for deep pockets.
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  Magnetic chuckSteel and stainless only; leaves the face open.

Finishing is not cosmetic only. Anodizing grows the part. Type II clear anodize adds roughly 5–15 μm per surface, and hardcoat adds 20–40 μm. On a 3.05 mm button hole, a 30 μm build on each wall closes the hole by 0.06 mm, which is enough to bind a button.

The practical rule is to machine tight features undersize before anodizing and state the target on the drawing. If the hole must finish at 3.05 mm and the coating is 30 μm per wall, cut it at 2.99 mm. We ask for the finish spec before the finishing pass so the offset is built into the CAM file, not added by hand later.

Bead blasting changes the look and the texture but not the size, so it is safe on tight features. Brushing and polishing remove material, and a heavy polish on a 0.8 mm wall can round the edge enough to open a seam. Laser marking is the other finish that needs planning: minimum character height is 1.5 mm, and a mark on a curved rim will distort if the artwork is flat.

Colour matching is the hardest part of anodizing to control. Two batches of the same alloy can read differently after dyeing, and a hardcoat over a bead-blasted surface looks darker than the same coat over a brushed surface. If colour is critical, keep one finish sequence and one alloy for the whole run.

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  Anodize buildAllow 5–15 μm Type II, 20–40 μm hardcoat per surface.
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  Bead blastTexture only; no measurable size change.
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  Laser markingMinimum character height 1.5 mm.