Machining Aluminum

• 1
  Why aluminum cuts fast and still causes problems
• 2
  Alloy choice decides the part more than the machine
• 3
  What tolerance aluminum actually holds
• 4
  Finishes that survive on aluminum
• 5
  Where aluminum is the wrong answer
• 6
  Aluminum alloy selection at a glance

Aluminum has a low melting point and high thermal conductivity, so heat leaves the cut zone quickly and the tool stays cooler than it would in steel. That is why aluminum runs at high spindle speeds and feeds. Surface speeds of 300–600 m/min are normal in 6061, and a 12 mm carbide end mill can run at 12,000 rpm without complaint.

The same property works against the machinist in one place: heat that leaves the chip also leaves with less warning. Aluminum does not glow or change color before it fails. A cutter can go from cutting clean to welding chips in a few seconds, so the operator reads chip shape and sound instead of temperature.

Built-up edge is the other common problem. Aluminum is soft and gummy at low cutting speeds, so a thin layer of material sticks to the cutting edge, then breaks off and takes tool coating with it. The fix is usually more speed, not less. Run the tool fast enough to keep the chip thick and the contact time short.

We see this on deep pockets and thin walls more than anywhere else. A 2 mm wall deflects under cutting force, the tool rubs instead of shears, and the finish turns rough in one pass. Roughing the cavity first and leaving 0.3–0.5 mm for a finishing pass solves most of it.

Two aluminum parts can look identical on a drawing and behave completely differently in service because the alloy is different. 6061-T6 is the default for structural brackets, fixtures, and housings. It machines cleanly, welds well, and holds ±0.005 mm on a stable setup. If you do not have a reason to pick something else, pick 6061.

7075-T6 is roughly twice the strength of 6061 and is the choice for aerospace fittings, high-load links, and bicycle or drone frames. It machines well but is less weldable and less corrosion resistant. It also costs more and is harder to source in thick plate, so use it only where the load path demands it.

2024-T4 is a fatigue-resistant alloy common in aircraft skins and stressed panels. It cuts a little gummier than 6061 and needs tighter feed control to avoid tearing. 5052 and 5083 are forming alloys with excellent corrosion resistance in salt water, so they suit marine brackets and enclosures rather than precision shafts.

ADC12 is a die-casting alloy, not a plate stock, but we machine it often after casting. It has good fluidity for thin walls and machines reasonably well, though porosity can appear just under the skin. Leave 0.5 mm on cast surfaces and the tool will cut through most of it.

Aluminum moves with temperature. A 300 mm part grows about 0.007 mm for every 1 °C rise, so a shop that machines at 28 °C and inspects at 20 °C will see a size shift that has nothing to do with the cutter. We keep the shop within a few degrees and let parts sit before final inspection on tight work.

±0.005 mm is achievable on aluminum, and we hold it regularly on bores, journals, and mating faces. It is not a default for every dimension. A general tolerance block of ±0.1 mm keeps cost down; call out ±0.005 mm only on the features that actually need it.

Thin walls are the main cause of out-of-tolerance parts. Anything under 1 mm wall thickness on a 100 mm part will move during and after cutting. If the design allows 1.5–2 mm, the part is far more stable and the finish improves at the same time.

Setup matters as much as the machine. Aluminum cuts easily, so it is tempting to take heavy passes on a light fixture. We see fewer problems on a dedicated fixture with three-point support than on a vise with a part hanging in the air. For long parts up to 4,000 mm, support every 300–400 mm.

Anodizing is the most common aluminum finish and the one with the most variables. Clear anodizing adds a thin oxide layer and keeps dimensions nearly unchanged. Type III hardcoat builds 25–50 μm per side and will change a bore size, so mask or allow for it on the drawing.

Color anodizing is a dye process, not a paint. The color sits in the oxide layer, so it will not chip like paint, but it can fade under prolonged UV and it varies slightly batch to batch. If two panels must match exactly, order them together and note the color match requirement.

Bead blasting gives a uniform matte surface and hides tool marks from roughing. It removes sharp edges slightly, so avoid it on sealing faces. Brushing leaves a directional grain, which looks intentional on faceplates but shows every fingerprint.

Conversion coatings like chromate or a simple black oxide are cheap ways to add corrosion protection without changing size. Electroless nickel adds hardness and wear resistance, but it plates into holes and threads, so the drawing must account for the thickness.

Aluminum is light, but it is not stiff. Its elastic modulus is about one third that of steel, so a long unsupported span will deflect three times as much under the same load. If stiffness drives the design, a steel or titanium part may weigh more but hold shape better.

Aluminum also has no true fatigue limit. Steel can be designed to run below a stress threshold indefinitely; aluminum will eventually crack at any repeated load. For parts that see millions of cycles, either reduce stress well below the yield point or expect a finite life.

Wear surfaces are another limit. Bare aluminum galls against steel and wears quickly in sliding contact. Bores that run on a shaft need a hard anodized surface, a pressed-in steel sleeve, or a change of material.

Finally, aluminum is not the cheap option at every volume. For a complex housing at 50,000 pieces a year, die casting plus light machining beats cutting the whole shape from plate. Machining wins on low to mid volume, tight tolerance, and design changes late in the program.