Nokia N1 CNC treatment: how a 6.9 mm, 318 g magnesium unibody gets machined

• 1
  What the Nokia N1 CNC treatment actually removed
• 2
  How to hold a 6.9 mm housing without crushing it
• 3
  Cutting parameters that keep a 1 mm wall stable
• 4
  Magnesium vs aluminium: what the thickness target costs
• 5
  Where the thin-wall approach stops working
• 6
  Which process fits your thin housing

A 6.9 mm tablet shell is not one cut. It is a sequence of removals from a solid billet, and every pass takes stiffness out of the part you are still holding. The Nokia N1 CNC treatment started with a magnesium block and finished near 318 g for the whole device, so most of the original billet volume left as chips.

The stiff sections are the ribs, the boss rings around the camera and the speaker grilles. Those cut first, while the part is still heavy and rigid. The flat floor and the outer rim come last, because that is where wall deflection shows up. Once you go below roughly 0.8 mm on a 100 mm panel, the wall acts like a spring under cutting load.

Magnesium AZ31B and AZ91D both cut fast. They produce short, powdery chips and low cutting forces compared with aluminium 6061. That low force is the reason thin walls survive at all. Chip clearing matters more than speed here, because magnesium fines ignite easily if they sit in the pocket.

Wall thickness is not a free choice. It is set by a stack: display module, battery, PCB, and a 0.2-0.3 mm air gap for tolerance. On a 6.9 mm body, the shell itself is often only 0.9-1.2 mm at the rim. Shave that further and the housing flexes when a user presses the screen edge.

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  Rib first, floor lastStiff features cut while the billet still resists vibration.
• 2
  Watch the rimRim walls below 1 mm deflect and leave chatter marks on the outside.
• 3
  Clear chips oftenMagnesium fines in a pocket are a fire risk, not just a finish problem.

Clamping pressure is the first cause of scrap on thin enclosures. A vise jaw closed by feel will bow a 1.2 mm wall by 0.05-0.1 mm, and the part springs back after unclamping. That is enough to fail a 0.05 mm flatness callout and to make the display glass sit proud on one corner.

The reliable pattern is a soft-jaw nest machined to the outer profile, plus light vacuum or low-pressure clamps. We leave 0.3-0.5 mm of stock on the floor for a finishing pass after the part is relaxed. Stress that built up during roughing shows up in the first 30 minutes after unclamping, not during the cut.

For a second side, a machined pocket fixture locates on two datum bosses rather than on the thin outer skin. Datum-on-boss keeps the load path through solid material. It also lets the operator load and unload without re-tapping the part into position, which is where most handling dents come from.

We machine housings like this on 5-axis centers so the rim chamfer, corner radii and side ports come off in one setup. Fewer setups means fewer re-clamps, and fewer re-clamps means fewer places for a thin wall to move.

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  Soft-jaw nestContact the part over a wide area instead of two point loads.
• 2
  Leave finishing stock0.3-0.5 mm on the floor, removed after the part relaxes.
• 3
  Locate on bossesDatum through solid material, not through the thin skin.

Small diameter tools do the work inside a tablet shell. A 3 mm or 4 mm carbide end mill with a 3-flute geometry clears magnesium well and keeps radial load down. Below 2 mm diameter, tool runout becomes the dominant error and wall thickness drifts.

A practical starting window for AZ31B is 0.4-0.6 mm axial depth of cut with 0.2-0.3 mm radial engagement, spindle 12,000-16,000 rpm and feed 1,200-2,000 mm/min. Aluminium 6061 in the same geometry runs at 8,000-12,000 rpm with similar chip load. These are starting points, not recipes. The right numbers depend on wall height and how the part is held.

Do not finish a thin floor with a full-width pass. Take 0.1-0.15 mm depth at 60-70 percent stepover so the cutter stays on one side of the wall. Climb milling keeps the tooth entering on solid material and pushes the wall away from the cutter instead of pulling it in.

Coolant choice follows the material. Magnesium needs flood or high-pressure through-spindle coolant to move fines out, and it must never run dry in a deep pocket. Aluminium tolerates minimum quantity lubrication fine. Mixing the two on one machine without a full chip wash-down is asking for a fire.

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  3 mm 3-flute carbideGood chip room and low radial load for internal pockets.
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  0.1-0.15 mm finish depthKeeps the cutter on one side of a thin floor.
• 3
  Flood coolant on magnesiumFines must leave the pocket, not collect in it.

Magnesium is about 35 percent less dense than aluminium. That is the whole reason a 6.9 mm tablet can land near 318 g. AZ91D is a die-casting grade with good castability and moderate corrosion resistance. AZ31B is a wrought alloy with better ductility and is the one you machine from plate or billet.

Aluminium 6061-T6 machines more predictably. It does not need special chip handling, it anodizes in a wider colour range, and most shops have it on the floor. The penalty is weight: the same housing in 6061 comes out roughly a third heavier, which on a tablet scale is tens of grams.

Titanium would win on stiffness per unit thickness, but it is the wrong answer for a consumer shell. Cutting speed is a fraction of magnesium, tool wear is high, and the cost per part climbs fast. For a 6.9 mm body with a display stack inside, stiffness is not the limiting factor. Weight is.

If you are building a device that will be dropped, magnesium needs a coating. Anodizing or a conversion coat protects the surface. Bare magnesium scratches bright and corrodes in humid air, especially where two dissimilar metals touch.

• 1
  AZ31B from billetMachined housings, better ductility, predictable chips.
• 2
  AZ91D cast then trimmedHigher volume, less material removal, watch porosity.
• 3
  6061-T6 when weight is not criticalEasier finishing and wider anodizing options.

There is a floor on wall thickness that no machine setting fixes. Once the wall is thin enough that a normal grip flexes it, the part is no longer a structural housing. On a handheld tablet, a rim under about 0.8 mm will oil-can when the user holds it by one corner.

Heat is the second limit. A thin floor cannot spread heat from a battery or a processor. If your thermal path depends on the shell, machining it thinner makes the device run hotter, not lighter in any useful way. Thermal pads and internal spreaders have to be designed in from the start.

Tolerance is the third. A ±0.005 mm callout on a wall that moves 0.05 mm under clamping load is not measurable in a useful way. Tight tolerances belong on the datum features and the mating faces. For thin decorative walls, a general ±0.1 mm profile tolerance is more honest and much cheaper.

Finally, consider service. A 6.9 mm shell often has no room for standard fasteners. Clips, adhesive and press fits dominate, and those joints are hard to open without damage. If the product needs field repair, leave 1.5-2 mm somewhere for a real screw boss.

• 1
  Wall under 0.8 mmFlexes in normal hand grip, no fixing by toolpath.
• 2
  Thermal pathThin shell spreads less heat away from the battery.
• 3
  Tight tolerance placementPut ±0.005 mm on datums, not on cosmetic walls.