CNC Process 5G Aluminum Shell for R2S RM500U and FM650 Modules

• 1
  Key takeaways
• 2
  What makes a 5G aluminum shell different from a plain enclosure
• 3
  Tooling and workholding for a 5G aluminum shell
• 4
  Tolerance, finish and the RF window problem
• 5
  When CNC is the wrong process for this shell
• 6
  Six steps to CNC process 5G aluminum shell parts
• 7
  Choosing the right setup for a 5G aluminum shell

A 5G aluminum shell is not just a box. It carries three jobs at once: mechanical protection, heat spreading from the R2S RM500U or FM650 radio module, and a stable ground reference for the antennas. That combination drives the geometry. You need thick bosses where the module screws down, a flat pocket for the thermal pad, and openings sized for the antenna keep-out. If the designer treats the shell as sheet metal with a lid, the first machined part usually shows a bowed floor and a pocket that is 0.05 mm too deep.

The module datasheet is your first process document. The R2S RM500U and FM650 both dissipate power through the bottom pad, so the pocket floor flatness matters more than the outer cosmetic surface. We hold that floor at 0.03 mm flatness over the module footprint and Ra 0.8–1.6 μm so the thermal interface material wets evenly. A rough floor traps air and the module runs hot.

Aluminum grade choice follows the wall thickness and the finish. 6061-T6 machines clean and anodizes predictably, so it suits most shells. 7075 gives higher yield strength for thin antenna walls, but hardcoat anodizing on 7075 shifts color slightly between batches. If the shell is a visible product surface, 6061-T6 with clear or black anodize is the safer pick.

The key decision is where the datum sits. On a 5G aluminum shell we usually datum off the module mounting face, not the outer skin, because every critical dimension chains from the radio. That means the outer surface carries the tolerance stack and the inner face stays true.

• 1
  DatumModule mounting face, not the cosmetic outer skin.
• 2
  Pocket floor0.03 mm flatness, Ra 0.8–1.6 μm for TIM contact.
• 3
  Wall1.2–2.0 mm typical; 7075 if you must go thinner.

Most 5G shells fit inside a 500 × 500 × 450 mm envelope, so a 3-axis mill with a Ø400 mm rotary table covers the first operation. Thin antenna walls, though, push you toward a 4-axis or 5-axis setup so you can reach the side openings without re-clamping the part three times. Each re-clamp adds position error, and on a shell with a ±0.005 mm bore that error shows up fast.

For op-1, grip the raw block in soft jaws machined to the block profile. Leave 0.5 mm of stock on the outer faces and cut the module pocket, the screw bosses and the connector cutouts in one pass sequence. Use a 3-flute carbide end mill for roughing at 0.5–0.8 mm radial depth and a 2-flute polished cutter for finishing. Aluminum likes high spindle speed and light depth of cut; pushing a deep axial cut is how you get chatter on a 1.5 mm wall.

Op-2 flips the part onto a vacuum plate or a matched soft jaw set. Here you face the outer skin and cut the antenna windows. A vacuum plate works well because it holds the part flat without clamping stress, but it needs a clean, flat back surface. If the back is already pocketed, use expanding fixtures or a low-melt holding compound instead.

One practical rule: never clamp on a finished cosmetic face. Once the anodize-grade surface is cut, any jaw mark becomes a rework. Design the fixture so the clamping force lands on a sacrificial rib or the inside of the shell.

• 1
  Roughing3-flute carbide, 0.5–0.8 mm radial depth, air blast.
• 2
  Finishing2-flute polished cutter, 0.15 mm radial depth.
• 3
  Op-2 holdVacuum plate or matched soft jaws; never clamp the cosmetic face.

The module pocket drives the tolerance call. We hold the pocket depth to ±0.02 mm and the floor flatness to 0.03 mm, which keeps the thermal pad compression in range. The module screw holes sit at ±0.05 mm true position so the board drops in without forcing. Outer cosmetic dimensions can run looser, often ±0.10 mm, because they do not touch the radio.

Surface finish splits into two zones. The module pocket and any mating face get Ra 0.8–1.6 μm. The visible outer shell can be Ra 1.6–3.2 μm before anodize, which gives a uniform matte look. If the shell is a hardcoat or conductive anodize part, the finish spec also controls the coating thickness; conductive anodize needs masked threads so the ground path stays low resistance.

Antenna windows are the hard part. A thin wall around a window tends to spring back as the cutter exits, and the edge can chip. Take the window in two passes and leave the last 0.05 mm for a finishing cutter with a small corner radius. If the drawing shows a knife edge on the window rim, ask whether a 0.3 mm edge break is acceptable; it usually is, and it saves a lot of scrap.

One more note on anodize. The coating grows into the part and outward, roughly half each way. On a 1.5 mm wall with a 25 μm hardcoat, that growth is small but it changes a press-fit connector bore. Specify the bore after coating, or mask it.

• 1
  Pocket depth±0.02 mm, flatness 0.03 mm.
• 2
  Screw holes±0.05 mm true position.
• 3
  Anodize growthHalf in, half out; mask press-fit bores.

CNC makes sense from one prototype up to a few thousand units, especially when the shell has pockets, tapped bosses and RF openings that all need to line up. It is also the only practical route when the design is still moving and you need parts next week.

Above roughly 10,000 units a year, die casting starts to win on piece cost, provided the walls are thick enough to fill and the tolerances are loose enough for a casting. Aluminum die casting in ADC12 can hit the outer shape, but the module pocket and screw holes still need a light machining pass. So the real comparison is not casting versus CNC, it is casting plus finish machining versus CNC only.

Sheet metal is the other option for shells with flat sides and few features. It fails once you need a 0.03 mm flat pocket floor or a sealed RF cavity, because bent and welded sheet does not hold that geometry.

If your shell is a one-off test unit, do not over-engineer the fixture. A soft-jaw 3-axis setup and a hand deburr will get you a working part. If it is a production run, the fixture and the op-2 strategy are worth designing properly.

• 1
  Prototype to a few thousandCNC, no question.
• 2
  Above ~10,000 per yearCompare die casting plus finish machining.
• 3
  Flat, simple shellsSheet metal may compete; complex pockets rule it out.