Stainless Steel Chassis: What Apple iPhone 8 Will Need From a CNC Shop

Stainless is not one material. A phone frame usually lands on 304, 316L, 303 or 17-4PH (SUS630), and the grade decides how the part cuts, polishes and holds shape. Austenitic grades like 304 and 316L resist corrosion well and polish to a mirror, but they work-harden fast. Feed too light and the surface hardens under the tool, so the next pass chips instead of cuts.

Free-machining 303 adds sulfur, which breaks chips and lets us run higher feed rates on long rails. The trade-off is slightly lower corrosion resistance and a surface that can show pitting after aggressive polishing. For structural frames that see drop loads, 17-4PH gives the best strength after aging, though it costs more and needs tighter heat-treat control.

So the choice is not about the best stainless. It is about the finish you promised, the wall thickness you drew, and how many parts you plan to run. A 0.8 mm wall in 316L behaves very differently from the same wall in 304.

• 1
  303Best chip control and cycle time on long, thin rails.
• 2
  304 / 316LCorrosion resistance and mirror polish; watch work hardening.
• 3
  17-4PHHighest strength after aging; more cost and process control.
• 4
  430 / 420Magnetic, lower cost, good for internal brackets.

A unibody frame arrives as a forged or extruded block. We rough it on a 3-axis machine to remove bulk, then move to a simultaneous 5-axis center for the outer profile, the internal pocket and the antenna split lines. Cutting the outer radius and the pocket in one setup keeps the wall concentric. Move the part between machines and you spend tolerance budget on re-datuming.

Thin walls are the real risk. Below about 1 mm, cutting pressure pushes the wall away from the tool, so the finished thickness drifts and the part sings. We control that with light radial passes, a tail support or a custom soft jaw, and a finishing pass that takes only 0.1 mm. On a Ø400 mm rotary table we can index the frame without unclamping, which holds position across all four sides.

Our 16 simultaneous 5-axis centers handle frames up to 4,000 mm on the large travels, though a phone chassis sits well inside the 500 × 500 × 450 mm envelope. The tolerance we hold on pocket depth and button bores is ±0.005 mm, and that number only holds if the fixture is rigid and the coolant reaches the cut zone.

Stainless cannot be anodized the way aluminum can. If the design calls for a colored metal frame, the color has to come from PVD, electroplating or a painted coat, not from an anodize bath. That catches teams who prototype in aluminum and then switch to stainless for production. The finish line changes completely.

For a raw stainless look, we go from bead blasting to brushing to mirror polishing. Mirror polishing is hand work. It runs through several grits, and every scratch from the machining stage has to come out first. A Ra 0.2–0.8 μm surface is reachable on flat faces and gentle radii. Sharp internal corners will never polish to the same level, so design them out early.

Bead blasting hides tool marks and gives a uniform matte. Laser marking works on stainless and needs a minimum character height of 1.5 mm to stay legible after blasting. We also run electroless nickel, black oxide and powder coating when the frame is hidden inside the product.

A cosmetic frame is inspected twice: once for geometry, once for surface. Geometry checks run on CMM and optical comparators for pocket depth, bore position and wall thickness. Surface checks are visual against a limit sample, because a scratch that fails on a polished face may pass on a blasted one.

We inspect 100% of parts before shipment, with raw material certification, in-process checks and a final report on request. That matters more on stainless than on aluminum. A wrong heat lot shows up as cracking during forming, not during machining, and by then the parts are already polished.

Our plants hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. For consumer electronics work the last one matters: your CAD files and drawings stay under NDA, and uploads are handled as confidential.

Most teams start with five to twenty machined frames to test fit, antenna performance and drop behavior. We machine those from solid stock, no tooling needed, and there is no minimum order quantity. Quotation and a free DFM analysis come back within 12 hours, and production can start within 24 hours of approval.

If the design holds, the volume path splits. Machined frames stay competitive up to a few thousand parts per year. Above that, die casting or metal injection molding with a secondary machining pass usually wins on cost. We quote both routes so the comparison is on the same drawing.

Parts ship in 3–5 days for machining runs. Historical late-delivery probability sits below 2%, which is the number we track rather than promise a date.