Aluminum CNC Machining: Alloy, Tolerance and Design Decisions

Most jobs we quote land on 6061-T6, and for good reasons. It machines clean, welds well, takes anodizing evenly, and holds a ±0.005 mm tolerance on a stable setup. If your part is a bracket, a housing, a manifold or an enclosure, start here. A 6061 plate or extrusion is cheap and available in most thicknesses.

Pick 7075 when strength matters more than finish. The zinc content pushes yield strength well above 6061, so it suits aircraft fittings, high-load jigs and drone arms. It cuts harder on the tool and anodizes to a darker tone. Expect shorter tool life and a slightly higher part price.

2024 machines beautifully and fatigues slowly, which is why aerospace work leans on it. The trade-off is corrosion. Bare 2024 will pit in a damp workshop, so plan on anodizing or a primer. 5052 and 5083 bend better than they cut and belong on formed sheet parts, not on a 5-axis block.

6082 sits close to 6061 with better strength in thick sections. ADC12 is a casting alloy, not a billet grade. If you send us an ADC12 drawing for machining from plate, we will flag it and suggest a wrought grade instead.

Thin walls move. On a 6061 part we keep unsupported walls at 0.8 mm or above, and 1.0 mm is safer if the wall runs more than 50 mm. Below that, cutting forces and residual stress from the plate will bow the wall between the vise jaws. If you need 0.5 mm, say so early so we can plan a support fixture and light passes.

Internal corners need a radius. A 6 mm end mill leaves a 3 mm corner radius, so a sharp internal corner either becomes a radius or a second operation with a smaller tool. Deeper pockets need a longer tool, and long tools chatter. A pocket 4× deeper than the cutter diameter is where we start slowing down and adding passes.

Threads below M2 in aluminum strip easily. We can cut them, but a thread insert or a slightly coarser pitch will last longer in service. Counterbores, slots and keyways are routine. Sharp outside edges on a part that gets handled should carry a 0.3 mm to 0.5 mm break, or the anodized edge will chip.

One more thing on datum choice. Aluminum moves with temperature, so pick a datum that stays on the fixture for both roughing and finishing. Resting a part on a finished face and then clamping it usually costs you the tolerance.

A 3-axis machine cuts from one direction. If every feature on your part is reachable from the top or from a few flipped setups, a 3-axis job is the cheapest route. Flats, plates, pockets and drilled patterns all fall into this group. Each flip adds setup time and a small position error, so keep the count low.

A 4-axis mill adds a rotary indexer, usually a Ø400 mm table. That lets us cut four faces without re-clamping, which is ideal for a part with features around a cylinder: a shaft with flats, a manifold with ports on four sides. The tool still comes in at one angle at a time.

Five-axis simultaneous machining is the answer when the surface is curved or the tool must tilt to reach inside. Impellers, turbine blades, organic housings and deep undercut pockets belong here. Tilting the tool also shortens the overhang, which reduces chatter on tall features. It is not automatically more accurate; it solves reach and surface problems that 3-axis cannot.

Our shop runs 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers across 127 CNC machines. Maximum part size is 4,000 mm. If you are unsure which route your part needs, send the model and we will say which machine it should go on and why.

As-machined aluminum comes off the tool at Ra 1.6–3.2 μm. That is fine for most internal brackets. Visible covers and sealing faces usually want Ra 0.8–1.6 μm, and optical or sliding surfaces can reach Ra 0.2–0.8 μm with a finishing pass and light polishing.

Anodizing is the most common aluminum finish. Clear, color and hardcoat build a hard oxide layer; conductive anodizing keeps the part grounded, which matters on electronics chassis. Type and thickness change the part dimension slightly, so call out any critical bore that must not grow. Hardcoat adds the most, and we mask those features before the tank.

Other finishes we run include electroless nickel, zinc, silver and gold plating, powder coating, black oxide, and bead blasting or brushing. Laser marking works down to a 1.5 mm character height. If your logo or a serial number needs to stay legible after anodizing, tell us and we will mark after the finish, not before.

Every part is inspected before it ships: incoming material check, in-process monitoring, and a final dimensional inspection. We can send inspection reports on request. The shop works to ±0.005 mm (±0.0002 in) where the drawing calls for it, and holds a 99.99% qualification rate across production runs.