Aluminum CNC Milling: Alloys, Tolerances and Design Choices

Aluminum cuts at high spindle speeds and high feed rates. A 12 mm carbide end mill in 6061 can run at 8,000–15,000 rpm on a decent machine, several times faster than the same cutter in 4140 steel. Chips clear quickly and carry most of the heat away, so the workpiece stays cool and dimensional drift stays small.

The trade-off is thermal expansion and stiffness. Aluminum expands roughly twice as much as steel per degree, so a few degrees of shop temperature swing can move a 300 mm dimension by more than 0.01 mm. Thin walls also deflect under cutter pressure. That is why we rough, let the part rest, then take light finishing passes instead of one heavy cut.

Built-up edge is the other common problem. Soft alloys like 5052 and 1100 tend to smear onto the cutter edge, which tears the surface. Sharp tooling, generous rake angles, and coolant or air blast keep the edge clean. For high-silicon or hard alloys we switch to diamond-coated or PCD tools.

6061-T6 is the default for most machined aluminum. It welds, anodizes cleanly, machines without drama, and holds tight tolerances well. Strength is moderate: about 310 MPa yield. Use it for brackets, housings, fixtures, and anything that needs a good surface finish.

7075-T6 is roughly twice the strength of 6061, near 500 MPa yield, which is why it shows up in aircraft fittings and high-load brackets. It machines to a fine finish but is less weldable and costs more. Stress corrosion can be an issue in humid environments unless the part is anodized.

2024-T4 has good fatigue resistance and is common in aerospace skins and structural parts. It is harder to weld than 6061 and has lower corrosion resistance, so it usually gets a protective finish. 5052 and 5083 are marine-grade, highly corrosion resistant, and form well, but they are gummy to machine and rarely the right choice for tight-tolerance milling.

6082 and 6063 sit close to 6061. 6082 has slightly higher strength and is common in European drawings. 6063 is an extrusion alloy with excellent finish, often used for frames and enclosures. ADC12 is a die-casting alloy; we machine it when castings need secondary features, not as a primary billet choice.

We hold ±0.005 mm on critical dimensions when the geometry allows it. That tolerance is realistic on a bore, a slot, or a mating face with solid support around it. Long unsupported spans, thin floors, and deep pockets are harder. On those features ±0.02 mm is often the honest number, and forcing tighter just adds cost without adding function.

Wall thickness matters more than most drawings admit. A 0.8 mm wall in 6061 is machinable but will chatter during roughing and may bow after stress relief. For production parts we suggest 1.5 mm minimum on unsupported walls, 1.0 mm where the wall is short and backed by a rib. Below that, expect to pay for multiple light passes and extra inspection.

Corner radii should be at least one-third of the pocket depth, and never smaller than the cutter radius you are willing to pay for. A sharp internal corner requires a small cutter, which means slow feed and short tool life. Adding a 3 mm radius instead of a 1 mm radius can cut cycle time noticeably on a deep pocket.

Threads, reamed holes, and tight-tolerance bores are all routine. We tap down to M1.6 and ream holes to H7. Deep holes beyond 10× diameter need peck drilling and sometimes gun drilling, which changes both cost and lead time. Tell us the hole function, not just the diameter.

Anodizing is the most common finish for machined aluminum. Clear, color, hardcoat, and conductive types are all available. Type II anodizing adds roughly 5–15 μm per surface, and hardcoat can add 25–50 μm. That build-up matters on tight-tolerance bores and threads. We mask or pre-size those features so the finished part still fits.

Bead blasting gives a uniform matte look and hides tool marks. It also slightly rounds edges, which is fine for cosmetic parts but wrong for sealing faces. Tumbling and brushing are gentler. Polishing reaches a near-mirror surface but is labor-intensive and best reserved for visible parts.

Plating options include electroless nickel, zinc, silver, and gold. Electroless nickel adds a hard, uniform layer and is good for wear surfaces. Silver and gold are for conductivity and RF parts. Powder coating and black oxide are also available. Each one changes the surface by a known amount, so tell us the critical dimensions before finishing, not after.

Laser marking and engraving handle part numbers, logos, and traceability codes. Minimum character height is 1.5 mm. Smaller text tends to fill in on anodized surfaces and becomes hard to read.

Milling wins on complex geometry, tight tolerances, and low-to-medium volumes. It loses when the part is simple and the volume is high. A plain aluminum plate with a few holes is cheaper as a stamping or an extrusion. A large hollow box with uniform walls is usually cheaper as a casting.

Die casting makes sense above a few thousand parts per year when the geometry is castable and the tolerance can stay loose. Extrusion is ideal for constant cross-sections cut to length. Sheet metal fabrication covers folded enclosures and brackets where wall thickness is under 3 mm and the part is mostly 2D.

The honest answer is that many parts start as prototypes in aluminum CNC milling and then move to casting or extrusion once the design is frozen. We do both, so we will tell you when the switch makes sense rather than keep milling a part that should be cast.

Prototyping is where milling earns its keep. No tooling cost, no minimum order quantity, and design changes between runs cost nothing extra. We can start production within 24 hours of a confirmed order and ship parts in 3–5 days.