Aluminum CNC Machining: Alloys, Tolerances and Process Choices

Most aluminum cuts fast. The differences show up in strength, corrosion behavior, and the way a grade behaves when the wall gets thin. Grade choice is the first decision, and it is usually the cheapest one to change.

6061-T6 is the default for machined parts. It welds, anodizes cleanly, and holds ±0.005 mm on features that are not too slender. Tensile strength sits around 310 MPa, which covers brackets, housings, manifolds and most fixture work. If a drawing says "aluminum" with no grade, 6061-T6 is what we quote unless you tell us otherwise.

7075-T6 is the choice when strength matters more than corrosion resistance or weldability. It machines to a better finish than 6061 and takes hardcoat anodizing well, but it is more expensive and it does not like welded assemblies. Aerospace ribs, high-load brackets and racing components usually land here.

2024-T4 cuts cleanly and has good fatigue behavior, though its copper content makes it less corrosion-resistant than 6061. 5052 and 5083 are marine grades: 5052 forms and welds easily but is gummy on a mill, so we usually reserve it for sheet work or parts that need saltwater exposure.

6082 sits close to 6061 with slightly higher strength, common on European drawings. 6063 is an extrusion alloy, soft and best left for profiles that get light finishing cuts. ADC12 is a die-casting grade; if your part starts as a casting and only needs secondary machining, the alloy is already fixed.

• 1
  6061-T6General machining, welding, anodizing. Start here.
• 2
  7075-T6High strength, tight finish, no welding.
• 3
  2024-T4Fatigue-loaded parts; protect the surface.
• 4
  5052 / 5083Marine and salt exposure, better formed than milled.

One setup beats three. Every time a part moves to a new fixture, you add locating error and labor. Five-axis work exists mostly to remove those moves, not to make the machine sound impressive.

For a housing with features on five faces, a simultaneous 5-axis center cuts the whole part in one clamping. We run 16 of them, plus 12 four-axis mills for parts that need rotation but not full contouring. Three-axis machines handle flat plates, covers and simple pockets at the lowest hourly rate.

Size sets the limit. Our largest travel is 4,000 × 400 × 150 mm, which covers long extrusions and frame rails. Mid-size work runs on 750 × 1,150 × 550 mm and 600 × 600 × 600 mm machines. Compact parts go on 500 × 500 × 450 mm and 500 × 310 × 200 mm centers, where the smaller envelope gives better rigidity and faster tool changes.

Mill-turn centers handle parts that are round but not purely turned: a shaft with milled flats, cross holes, or a slot that must stay concentric to the bore. Doing that on a lathe plus a mill means two setups and a concentricity stack-up. One mill-turn center removes it.

The rotary table is Ø400 mm. Anything that needs to swing past that diameter has to be indexed differently or split into two operations. Tell us the part envelope early; it changes the quote more than the alloy does.

±0.005 mm is achievable, but not on every feature of every part. It holds on a bored hole in a rigid block. It does not hold on the tip of a 0.8 mm fin sticking 40 mm out of a plate. Aluminum moves when you cut it, and thin sections move more.

The practical rule: tolerance follows stiffness. Keep wall thickness at 1 mm or more where you can, add ribs instead of going thinner, and put tight tolerances on the features that actually locate the part. Tolerancing everything at ±0.005 mm usually means more inspection, slower passes, and sometimes a stress-relief step between roughing and finishing.

Surface finish is a separate cost lever. As-machined aluminum sits at Ra 1.6–3.2 μm. A good finish pass reaches Ra 0.8–1.6 μm, which is where most anodized cosmetic parts should be. Ra 0.2–0.8 μm means slower feeds, sharper tooling and more time; specify it only where a seal, bearing or optical surface needs it.

Heat-treated grades like 7075-T6 can move after heavy material removal. For a part with a large pocket and a tight flatness callout, we rough, let it rest, then finish. That adds a day and it is cheaper than scrapping the part.

Deburring is not optional on aluminum. Fine edges fold over instead of breaking cleanly, and a folded edge will fail a dimensional check even when the cut was right. Bead blasting, tumbling and brushing are all available, and laser marking needs a minimum character height of 1.5 mm to stay legible after anodizing.

Anodizing grows into the surface, roughly half in and half out. A 25 μm hardcoat layer adds about 12 μm per side. If a bore has a ±0.01 mm fit, that growth eats most of the tolerance. Machine the bore undersize on purpose, or mask it, and say so on the drawing.

Clear and colored anodizing suit visible parts and mild corrosion protection. Hardcoat anodizing gives a wear surface for slides and contact faces, and it darkens the alloy. Conductive anodizing keeps grounding paths alive where a standard coating would insulate the part.

Electroless nickel suits parts that need uniform coverage inside a cavity, since it plates evenly without a current path. Zinc, silver and gold plating show up on electronics housings and RF work. Powder coating and black oxide are the heavier-wear and cosmetic options.

Mechanical finishes stand on their own. Bead blasting gives a matte look and hides tool marks before anodizing. Brushing leaves a directional grain that shows on flat panels. Polishing gets close to a mirror on 6061 but is slow, and any scratch after polishing is permanent.

One caution: anodizing is a batch process with color variation, especially on 7075 and 2024. If two covers must match, machine them in the same batch and finish them together.

Machining wins on prototypes, low-to-mid volumes, tight tolerances and parts with many features. It loses when the part is a thin shell with no features, or when the annual volume makes tooling pay for itself.

A simple enclosure in runs above a few thousand pieces is usually cheaper as a die casting with machined interfaces. The same goes for a part with deep internal channels that a mold can form in one shot. We run die casting and vacuum casting in-house, so the comparison is honest rather than a nudge toward the mill.

Sheet metal beats machining for flat panels, brackets and chassis under about 3 mm thick. Bending and laser cutting avoid the material removal entirely. If a part is 90% flat with two machined bores, a sheet metal body plus a machined insert often costs less than a solid block.

Additive manufacturing makes sense for internal lattices, conformal cooling channels and one-off geometry that no cutter can reach. It is weaker in Z and has a rougher surface, so it rarely replaces a machined bearing surface.

The honest test is feature count against volume. Many features, low volume: machine it. Few features, high volume: cast or form it. In between, ask us and we will price both routes.