CNC aluminum parts: how aluminum behaves on a modern machine

• 1
  Why CNC aluminum parts machine the way they do
• 2
  Which aluminum grade suits your part
• 3
  Design limits that show up on the shop floor
• 4
  When milling beats casting, and when it does not
• 5
  Tolerances, finishes and what aluminum does after cutting
• 6
  Aluminum grade and process at a glance

Aluminum cuts fast. A 6061-T6 block removes material at spindle speeds and feed rates that would burn a carbide tool in 304 stainless. That single property sets most of the economics of machined aluminum parts: shorter cycle times, less tool wear, and a lower cost per part once the program is proven.

The same softness that makes aluminum easy to cut also makes it easy to distort. Thin walls deflect under clamping force. Long parts move as internal stress releases. A pocket floor can bow if the tool pushes instead of shears. Machinists handle this with lighter finishing passes, sharper geometry, and more coolant, not with a slower spindle.

Heat moves out of the cut zone quickly. Aluminum conducts heat roughly four to five times faster than steel, so the chip carries away most of the energy and the workpiece stays closer to room temperature. That is good for dimensional stability during a long run. It also means the part grows less, so a warm spindle matters more than a warm part.

Built-up edge is the usual failure mode. Soft alloys smear onto the cutting edge at low speed, then break off and leave a rough patch. The fix is speed and sharpness: climb milling, high positive rake, and coolant aimed at the tooth, not the whole part.

• 1
  Fast stock removalHigher feed per tooth than stainless or titanium.
• 2
  Low cutting temperatureHeat leaves with the chip, not the workpiece.
• 3
  Distortion riskStress release and clamping force, not heat, cause most scrap.

6061-T6 is the default for most machined aluminum parts. It welds, anodizes cleanly, and holds ±0.005 mm on a stable setup. If your drawing has no unusual strength or corrosion note, start here.

7075-T6 gives roughly twice the yield strength of 6061 and machines to a sharper edge, which is why it shows up in aerospace brackets and racing parts. It costs more and anodizes to a darker, less uniform tone. It also cracks more readily under a tight bend, so it suits machined geometry better than formed sheet.

2024 is strong and fatigue resistant, but its copper content makes it corrode without a coating. Use it for stressed airframe parts that will be painted or anodized, not for wet environments left bare.

For corrosion resistance near salt water or coolant mist, 5052 and 5083 hold up better than 6061. Both are softer, so deep pockets and fine threads need slower feeds. 6082 sits close to 6061 with slightly better strength in thick sections, and 6063 extrudes well but is usually too soft for structural machining.

• 1
  6061-T6General purpose, welds and anodizes well.
• 2
  7075-T6High strength, aerospace and motorsport.
• 3
  2024Fatigue resistant, needs coating.
• 4
  5052 / 5083Marine and chemical exposure.

Wall thickness is the first thing to check. Below about 0.8 mm, a 6061 wall starts to ring and deflect during finishing, and a 7075 wall can chatter even with light passes. If the design needs a thin web, add a rib or accept a slower finishing cycle.

Corner radii matter as much as walls. An internal corner must be at least as large as the cutter radius, or the tool leaves a sharp step that needs a second operation. A 3 mm corner on a 6 mm deep pocket is reasonable. A 1 mm corner on a 40 mm deep pocket means a long, thin tool and a much slower cut.

Thread depth is another limit. In aluminum, a thread that is 1.5 × the diameter deep usually develops full strength, and going past 2 × adds cost without adding grip. Fine threads in soft alloys strip more easily than coarse ones, so M3 × 0.5 is a better choice than M3 × 0.35 for a part that will be assembled more than once.

Datums and fixturing decide whether the tolerance holds. A part held only in a vise can move between operations. Add two or three flat datum faces and, if the volume justifies it, a soft jaw or a fixture plate. That is cheaper than scrapping a batch at final inspection.

• 1
  Minimum wallAbout 0.8 mm in 6061, more in 7075.
• 2
  Corner radiusAt least the cutter radius, ideally larger.
• 3
  Thread depth1.5 × diameter is usually enough.
• 4
  DatumsFlat faces and a fixture beat vise-only setups.

Machining wins on low to medium volume, tight tolerance, and parts with features on several faces. A five-axis setup can reach undercuts and angled holes in one clamping, which removes the re-fixturing error that causes most out-of-tolerance parts.

Die casting wins above roughly 10,000 pieces per year. Tooling cost is real, but the per-part cost drops sharply once the die is amortized, and ADC12 fills thin walls well. The trade-off is porosity and a rougher as-cast surface, so any sealing face usually still gets a light machining pass.

Extrusion suits constant cross sections: rails, frames, heat sinks. The profile is cheap per meter, but every hole and pocket is a second operation. If more than about a third of the extrusion gets machined away, the cost advantage disappears.

For prototypes, the decision is different. Machined aluminum parts let you test the real alloy and the real wall thickness in days, which is why a machined prototype usually beats a cast one until the design stops moving.

• 1
  Pick machiningLow volume, tight tolerance, multi-face features.
• 2
  Pick castingHigh volume, thin walls, simple geometry.
• 3
  Pick extrusionConstant profile, minimal secondary work.

Typical machined tolerance on a stable aluminum setup is ±0.005 mm on critical features, with general dimensions looser at ±0.1 mm. Chasing ±0.005 mm everywhere raises cost fast, so mark only the features that actually seal, fit, or locate.

Surface finish follows the tool path. As-machined aluminum sits around Ra 1.6–3.2 μm. A finer finishing pass reaches Ra 0.8–1.6 μm, and a polished or fine-bored feature can reach Ra 0.2–0.8 μm. If the drawing calls for a mirror finish on an internal pocket, expect a longer cycle and a special tool.

Anodizing adds 5–25 μm per surface and shifts dimensions. Type II clear anodize is thin and cosmetic. Hardcoat builds a thicker, harder layer and moves tight bores more, so mask or ream after coating. Conductive anodize exists for parts that need both corrosion protection and electrical contact.

Laser marking works well on anodized aluminum because the beam bleaches the dye and leaves a white mark. Minimum character height is 1.5 mm. Smaller text fills in and becomes unreadable, which is a common cause of rejected cosmetic parts.

• 1
  Critical features±0.005 mm; leave general dims at ±0.1 mm.
• 2
  Anodize growthAbout 5–25 μm per surface.
• 3
  Laser marking1.5 mm minimum character height.