CNC processed anodized aluminum: how the two steps fit together

• 1
  What "CNC processed anodized aluminum" actually means
• 2
  How the anodizing layer forms and what it does to the part
• 3
  Which aluminum alloys anodize well and which do not
• 4
  Where the coating thickness lands in your tolerance stack
• 5
  Masking: keeping some surfaces conductive and bare
• 6
  How the machining step is set up for a clean finish
• 7
  When anodized aluminum is the wrong choice
• 8
  Step by step: from billet to finished anodized part
• 9
  Alloy and anodizing response at a glance

CNC processed anodized aluminum is not one process. It is two, run in a fixed order. First a milling or turning center cuts the part from a solid billet of aluminum. Then the finished part goes into an electrolytic bath where the surface is converted into aluminum oxide. Skip the order and the part fails: anodize first and you cut through the coating at every fixture point.

The oxide layer is not paint. It grows out of the metal itself, roughly half above the original surface and half into it. That means a 25 μm coating adds about 12 μm to each dimension. On a Ø20 mm shaft that is a real number, and it lands on the diameter, not on the drawing's nominal.

This is why the finish is a machining decision, not a purchasing afterthought. The shop needs to know the anodizing type, the target thickness and which faces must stay conductive before the first cut is programmed.

The process suits parts that need light weight, corrosion resistance and a surface that will not flake under abrasion. It is common in aerospace, EV, robotics and medical housings, where a bare aluminum surface would oxidize or scratch in service.

Anodizing runs the part as the anode in an acid electrolyte, usually sulfuric acid. Current drives oxygen into the aluminum surface, and the metal converts to Al₂O₃. The result is a dense, hard ceramic-like skin that is bonded to the substrate rather than sitting on top of it.

The layer is porous while it is growing. Those pores are what accept dye for colour, and they are what get sealed afterward. Sealing closes the pores with hydrated oxide, which locks the dye in and shuts down the corrosion path. Unsealed anodized aluminum is common in adhesive bonding, because the open pores give a mechanical key.

Hardcoat, or Type III, uses a colder bath and higher current. It produces a thicker layer, typically 25–50 μm, with a hardness that can reach 400–500 HV. The trade-off is brittleness. A hardcoat layer on a thin wall can crack when the part flexes.

Type II sulfuric anodizing gives 5–25 μm and keeps more of the base metal's ductility. For most enclosures, brackets and covers, Type II is the sensible default. Save hardcoat for sliding surfaces and wear faces.

Not every aluminum behaves the same in the tank. The alloying elements that make a grade strong also change how the oxide forms. Copper and silicon are the two elements that cause the most trouble.

6061 and 6061-T6 are the workhorses. They machine cleanly, weld well and take a uniform clear or dyed anodized finish. If you have no special reason to pick something else, start here. 6082 behaves almost the same and is common in Europe.

7075 is strong but contains zinc and copper. It anodizes to a darker, less uniform tone, and hardcoat on 7075 can look patchy. Use it when strength matters more than colour match, and expect to specify a darker dye so variation is less visible.

2024 has high copper content and is the worst of the common grades for cosmetic anodizing. It is fine under a functional coating, poor under a decorative one. 5052 and 5083 are marine grades with excellent corrosion resistance and good anodizing response, but they are softer and gummy to machine.

Casting alloys such as ADC12 are porous. They can be anodized, but expect mottling and possible pitting. For cosmetic parts, machine from billet instead.

Anodizing changes dimensions. If your drawing calls out ±0.05 mm on an outside diameter and the coating is 25 μm thick, half of that growth eats 25% of your tolerance band. On tight fits this matters.

The rule we use: define the anodized part as the finished condition, and let the shop machine undersize to compensate. For an outside dimension with 25 μm coating, cut the metal about 12–13 μm small per side. For an inside bore, cut it oversize by the same amount.

Threads are the classic failure point. A standard 1/4-20 thread anodized at 25 μm will not accept its mating fastener without chasing. Two options work: mask the threads, or cut them to a class 2B oversize allowance before coating. Masking is cleaner for small runs; oversize tapping is faster for volume.

Tight bores and dowel holes should be masked or reamed after coating. Reaming after anodizing removes the coating on the bore wall, so only do it where the bore does not need corrosion protection.

Anodized aluminum is an electrical insulator. On a chassis, a grounding boss or a connector face that must carry current has to stay bare. That is done with masking, not with a scraper afterward.

Masking is applied before the bath. Common methods are silicone plugs for threaded holes, tape for flat faces, and lacquer for irregular shapes. The mask has to survive sulfuric acid and the current, so not every tape works.

Design for masking early. A 2 mm wide mask ring on a Ø8 mm boss is hard to hold. Give the masking a flat land at least 3 mm wide, and keep it away from sharp corners where the tape lifts.

Laser marking comes after anodizing, not before. The laser burns through the oxide to leave a light or dark mark depending on the setting. Minimum character height is 1.5 mm for a legible mark.

If a part needs both anodized and conductive areas, tell the shop the contact resistance target. It changes which masking method we use and whether a chromate conversion step is needed on the bare zones.

Anodizing shows every scratch. A surface that looks fine after milling can look striped once the oxide forms, because the coating follows the tool marks. Surface finish before anodizing should be Ra 0.8–1.6 μm for a clean Type II result.

Tool marks from a worn cutter are the usual culprit. We change tools on a count rather than waiting for visible wear, and we take a light finishing pass at higher spindle speed and lower feed. On 6061 that typically means a 0.2–0.3 mm finishing allowance.

Burrs must be removed before the bath. Anodizing does not hide a burr, it hardens it. A sharp edge becomes a hard, brittle edge that chips. Deburring by hand, tumbler or brush follows the machining cycle.

Coolant choice matters too. Some water-based coolants leave residues in the porous surface that show up as streaks after dyeing. We use a filtered coolant and a clean rinse between the machine and the anodizing line.

Anodizing is not universal. There are cases where the coating makes the part worse, and it is better to know that before the drawing is released.

If the part needs to carry high current through a face, anodizing plus masking adds cost and a failure point. A conversion coating such as chromate may be enough, or plating may be the better route.

If the part will flex in service, hardcoat will crack. Thin-wall brackets and spring elements should use Type II or a different alloy.

If the requirement is a mirror finish, anodizing will not get you there. The coating follows the substrate, so you need to polish before coating, and sealing dulls the surface slightly. High-gloss cosmetic parts are often better served by a different process.

Very tight tolerances under ±0.01 mm on a coating face are also a warning sign. The coating growth is not perfectly uniform across a large part, so the tolerance has to account for it. Talk to the shop before you commit.