Wearables: A TWS Charging Case With 0.3 mm Walls and a Tight Hinge Fit
A charging case is a small object that a customer opens a hundred times. The wall has to be thin enough to feel considered, the hinge has to turn without play, and the lid gap has to be the same on unit 1 and unit 10,000.
- Industry
- Wearables / consumer electronics
- Process
- Extrusion + CNC + bead blast anodize
- Alloy
- 6063-T6
- Key spec
- 0.3 mm wall, hinge fit ±0.01 mm
What the customer needed
The customer needed a two-part case — body and lid — in aluminum, with a wall thin enough to keep the weight down and a hinge that felt solid when opened. Both parts had to match in colour after anodizing, and the lid gap had to be visually consistent around the seam.
The quantified constraint was the hinge fit: the pin bore and the knuckle had to be machined to ±0.01 mm and stay coaxial between the two parts, because play in a hinge is felt before it is seen.
The second constraint was cosmetic and harder: the lid and the body are different sections, and anodized colour shifts with section and alloy. Two parts that are individually correct can still fail as a pair.
Constraints that decided the part
- Wall thickness 0.3 mm, ±0.02 mm capability
- Hinge bore and knuckle to ±0.01 mm, coaxial between two parts
- Lid gap consistent around the seam
- Colour matched across two different sections
- Bead blast anodized matte, no polishing
How the part was made
The same route we would propose today, in the order it actually runs.
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Two sections, one design
Body and lid were designed as a pair, with the seam geometry and hinge knuckle positions agreed together. Designing them separately is how a lid gap ends up uneven.
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Extrusion and ageing
Both profiles run in 6063 and aged to T6. The two sections are made from the same alloy so the anodized colour has a chance of matching.
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Machining from one datum per part
The hinge bore, the seam face and the internal pocket are machined from a single reference on each part, then checked as a pair rather than individually.
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Pair check before finishing
Body and lid are trial-assembled after machining to confirm the gap and the hinge feel before anodizing, because anodizing cannot fix a fit.
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Blast, anodize, pair again
180# bead blast, anodizing in the approved colour with both parts processed in the same batch, then a second pair check and an appearance check against the retained chip.
The numbers the part had to hold
These are the tolerances and process values that governed the job. They come from our capability, not from a customer specification we cannot publish.
| Parameter | Value | Why it mattered |
|---|---|---|
| Wall thickness | 0.3 mm | Weight and feel; the design constraint |
| Hinge bore and knuckle | ±0.01 mm | Play in a hinge is felt before it is seen |
| Bore and OD | ±0.04 mm | Hinge barrel and pin fit |
| Lid gap | Checked as a pair | Consistency around the seam |
| Alloy | 6063-T6 | Same alloy on both parts for colour match |
| Finish | 180# bead blast anodized | Matte that hides handling and fingerprints |
The part of this job that nearly went wrong
This is the section worth reading. Every program has one problem that decides whether the parts come out right, and it is usually not the one on the drawing.
Two sections, one colour
Anodized colour depends on alloy, section thickness and blast texture. Body and lid are different sections, so the same bath can produce two shades that only look wrong when the case is closed.
The risk
A pair that passes dimensional inspection and gets rejected by the brand owner on appearance, after both parts are finished and packed.
What we changed
Both parts are made from the same alloy, blasted to the same texture and anodized in the same batch. Colour is approved on chips made from both sections, not from a generic test piece, so the approval reflects the pair.
Hinge play between two parts
A hinge bore machined to ±0.01 mm on each part still produces play if the two bores are not coaxial when the case is assembled. The error only appears in the assembly, not on either part.
The risk
A case that rattles slightly when opened — a defect customers describe as cheap, and one that no single-part inspection catches.
What we changed
Each part is machined from a single datum, and the pair is trial-assembled before finishing. Where the geometry allows, the knuckle is machined in a pairing fixture so the two bores are cut in the relationship they will be used in.
A hinge that feels solid, and a seam that repeats
The body and lid are extruded 6063-T6 parts with a 0.3 mm wall, machined to ±0.01 mm at the hinge and checked as a pair before and after finishing. Colour is approved on chips from both sections and every lot is compared against them.
The improvement the customer noticed was not a number — it was that the case stopped coming back. A consistent lid gap and a hinge without play remove the two complaints that make a small accessory feel inexpensive.
We do not publish a colour difference figure (ΔE) for this program. Until that measurement process is agreed with the customer, the commitment is the approved chip and a lot-by-lot comparison against it.
Common questions about this program
The questions this case generates from engineers and sourcing teams.
How do you keep the colour matching between the lid and the body?
Same alloy, same blast texture, same anodizing batch, and colour approved on chips made from both sections. A generic test chip does not predict how a 0.3 mm section will take colour, so approval has to use the real parts.
What hinge tolerance is realistic on a case this small?
±0.01 mm on the bore and knuckle, machined from one datum per part and checked as an assembly. The tolerance itself is not the hard part — keeping the two parts coaxial in use is, which is why the pair check happens before finishing.
Can the case be anodized after assembly?
No. Anodizing is a bath process and the hinge would be sealed by the coating. Body and lid are finished separately and paired afterwards, which is exactly why the colour has to be controlled at the chip stage.
Designing a Two-Part Housing That Has to Look Like One?
Send both drawings together. The seam, the hinge and the colour are one problem, and they are cheaper to solve before tooling.
What to send
- 2D drawing or 3D model (STEP, IGES, DWG, DXF, PDF)
- Alloy and temper
- Surface finish and color
- Estimated annual quantity
Client names are withheld under confidentiality. Figures reflect our verified process capability.