Aluminum grades for CNC machining

Why aluminum is usually the first material considered

Aluminum sits in a useful middle ground. It is roughly a third the weight of steel, it protects itself with a thin oxide layer instead of relying on a coating, and it cuts faster than stainless or titanium. On a machined part that usually means less machine time, less tool wear and a lower cost per part than the same geometry in a harder metal. It is also the easiest common metal to anodize, which matters when the part is visible to an end user. Lightweighting is the other reason buyers arrive here: structural and housing parts for EV and e-mobility programmes depend on aluminum for exactly that weight-to-stiffness ratio.

Choosing between grades

Grades commonly considered for machined aluminum parts include 6061, 6063, 7075, 2024, 5052, 5083 and 3003. They are not interchangeable. The differences that change a machining quote are strength, how cleanly the alloy anodizes, corrosion behaviour, and how the chips break at the tool.

General purpose	6061 — the default when the drawing does not specify. Good strength, good corrosion resistance, anodizes well, welds well.
Highest strength	7075 — used where weight and strength both matter. Machines well but anodizes less uniformly than 6061 and costs more.
Best anodized appearance	6063 — lower strength than 6061 but takes anodizing more evenly, so the finish looks more consistent on large visible faces.
Fatigue resistance	2024 — high strength with good fatigue behaviour, but weaker corrosion resistance, so it is normally anodized or painted.
Formed or corrosion-exposed parts	5052 — better corrosion resistance than 6061 and easier to form, at lower strength.
Marine and welded assemblies	5083 — chosen for welded structures and saltwater exposure where strength after welding matters.
Low cost, simple parts	3003 — low strength and highly formable. Rarely specified for structural machined parts.

Specific grade and temper availability is confirmed against your drawing and quantity when we quote. If you are not sure which alloy fits, send the drawing and the operating conditions and we will recommend one.

How aluminum behaves in the machine

Aluminum is forgiving compared with stainless or titanium, but it is not uniform. The softer tempers of 5052 and 3003 are gummy and tend to build up on the cutting edge, which smears the finish and makes it harder to hold a clean dimension. 6061 and 6063 cut cleanly and break chips predictably. 7075 and 2024 are stronger and more abrasive, so tool wear and surface finish need more attention.

Two things cause most aluminum problems on a drawing review. The first is thin walls and floors. Aluminum is stiff for its weight but not stiff in absolute terms, so a tall thin wall or a wide unsupported floor will deflect under cutting force and chatter. The second is heat. Aluminum expands noticeably as it warms, so a bore or a slot measured on a warm part can read differently once the part cools. On features with tight limits we plan where in the cycle the measurement happens.

Aluminum does not work-harden the way stainless steel does, which is the main reason it machines faster and holds tolerance more easily. Grinding is normally reserved for features that genuinely need it rather than applied across the whole part. When a part needs features on several faces, or a contoured surface that would otherwise take repeated setups on a three-axis machine, it is quoted to run on a 5-axis machining centre instead.

Design details worth deciding early

• Wall thickness and unsupported floors, because these set the cutting strategy more than any other feature.
• Thread engagement in aluminum. Threads in soft alloy strip more easily than in steel, so engagement depth and the choice between a formed thread and a cut thread matter.
• Whether a dimension is measured before or after anodizing. Anodizing builds a surface layer, so a tight dimension on a surface that will be anodized should be specified with the finish in mind.
• Surfaces that must be masked before anodizing, for example electrical contact areas or press-fit bores.
• Sharp edges and burrs. Aluminum burrs are soft but persistent, and a deburring step should be planned rather than assumed.

When another material is the better choice

Aluminum is not always the answer, and it is worth saying so before a quote is issued.

• If the part carries load at elevated temperature, steel holds strength where aluminum loses it. See carbon and alloy steel.
• If the part sees chlorides, repeated washdown or sterilization, stainless steel is usually the more durable route.
• If weight and biocompatibility both matter, titanium is the usual choice despite the cost.
• If only the surface needs to resist wear, a hardened steel part may cost less than an aluminum part with a hard-anodized surface.
• If the part is a complex shape at high volume, casting the net shape and machining only the critical features is often cheaper than cutting it from billet.

Anodizing and finishing

Aluminum takes more surface treatments than most metals. Type II anodizing adds colour and moderate protection and is the usual choice for visible parts. Type III hard anodizing builds a thicker, harder layer for parts that slide, rub or see mechanical wear. Bead blasting and polishing set the texture underneath the anodize, and the order of those steps changes the final appearance.

One project worth reading: a 50-piece 7075-T6 sample batch for Blue Robotics, where the drawing tolerance was specified as ±0.02 mm after hard anodizing. The team changed the groove method during drawing review to avoid pressure marks on an adjacent face. Shenghao supplied dimensional, material and surface-finish documentation with the order. That figure is that project's drawing requirement, not a standing aluminum capability. Read the Blue Robotics case study →

The useful lesson generalises even though the number does not: post-finish tolerance has to be planned before machining starts, because anodizing changes the surface and visual features can change the process route.