Anodized CNC Parts: Alloys, Types and Tolerance Control

Anodizing is an electrochemical conversion, not a coating. The part goes into an acid electrolyte and current is passed through it. The aluminum surface converts to aluminum oxide, and roughly half the oxide grows inward while half grows outward. Nothing is deposited on top, so the layer cannot chip off the way paint can.

The oxide is hard and electrically insulating. On 6061 it protects well in outdoor and mildly chemical environments. But the same growth that makes it durable also moves the surface. A Type II coating at 10 μm adds about 5 μm to each side. A tight bore gets tighter after anodizing, and a shaft gets larger. Design the machined dimensions around the finished ones, not the other way around.

Not every aluminum grade behaves the same in the tank. 6061, 6063, 6082, 5052 and 7075 anodize predictably. High-silicon die casting alloys such as ADC12 do not. The silicon does not convert, so the coating comes out dark, patchy and sometimes non-uniform in thickness. If a part is die cast and needs a decorative finish, plan for a different process or accept a darker hardcoat look.

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  Half in, half outA 10 μm Type II layer adds roughly 5 μm per surface.
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  Not a coatingThe oxide is converted from the base metal, so it cannot peel.
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  Alloy mattersSilicon-rich casting alloys finish unevenly and dark.
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  Mask before, not afterContact areas and threads need masking decided at quoting.

Type II sulfuric anodizing is the common choice. Layer thickness runs about 5–25 μm, it takes dye well, and it is used on enclosures, brackets, handles and consumer-facing panels. Type III, usually called hardcoat, runs 25–50 μm and is noticeably harder and more wear-resistant. Use it on sliding surfaces, wear plates, valve bodies and parts that see abrasion.

Both types grow with thickness, and hardcoat grows more. On a hardcoat part at 50 μm, expect roughly 25 μm per side. A Ø10 H7 bore can close enough to lose its fit. The practical fix is to machine the bore oversize by twice the growth and then anodize to final size, or to mask the bore and keep it bare.

Conductive anodizing is a narrower case. Normal anodizing is an insulator, which is a problem for grounding paths, EMI shielding contact points and electrical connector housings. A conductive or masked anodize keeps selected areas bare so the part still bonds electrically. If your drawing shows a ground tab, mark it explicitly.

Color choice also carries engineering meaning. Clear and natural show the alloy and any surface scratches plainly. Black hides tool marks and is common on optical and camera parts. Dyed colors are cosmetic, and they fade under prolonged UV. For outdoor parts where appearance matters over years, black or clear is usually the safer specification.

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  Type II5–25 μm, dyeable, general purpose.
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  Type III hardcoat25–50 μm, abrasion resistance, more growth.
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  ConductiveMasked areas stay bare for grounding and shielding.
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  ColorCosmetic dyes fade; black and clear age better outdoors.

Anodizing is the last operation, but tolerance decisions happen at the machine. If a dimension is critical, decide whether it will be anodized or masked. An anodized surface cannot be held to the same callout as a bare one, because the layer thickness varies by a few microns across a large part and by more near edges and high-current contact points.

Our working rule is simple. Anodize first, then machine, whenever the geometry allows it. That way the finishing step is done and the final cut sets the dimension. When the part must be anodized after all machining, we machine the anodized features oversize by the expected growth and specify the growth on the drawing. Threads get the same treatment: a 1/4-20 tapped hole grows inward and can bind a screw if the pitch diameter is not planned.

Inspection is where this gets verified. We measure before anodizing and after, and we can supply reports on request. Because the tolerance is ±0.005 mm (±0.0002 in), any shop that skips the pre-anodize measurement is guessing. For parts where the fit is critical, we inspect at both stages and compare.

Surface finish interacts with the coating too. A bead-blasted surface at Ra 1.6–3.2 μm takes dye evenly and hides tool marks. A polished surface at Ra 0.2–0.8 μm shows every scratch, including ones introduced in handling. If the part is both cosmetic and functional, split the drawing: cosmetic faces polished, functional faces as machined.

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  Anodize first, machine secondBest route when geometry allows a final cut.
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  Oversize by growthAdd expected layer growth to bores and threads.
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  Measure twicePre- and post-anodize inspection catches drift.
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  Split the drawingCosmetic faces and functional faces can carry different finishes.

The 6xxx family is the default for anodized cnc parts. 6061-T6 machines cleanly, takes clear and dyed anodize evenly, and holds tight tolerances. 6063 and 6082 behave similarly and are common on extrusions and housings. 5052 gives good corrosion resistance for sheet metal parts that will be anodized after forming.

2024 and 7075 are stronger but contain copper, which makes the oxide darker and less uniform. They still anodize, and they are used in aerospace brackets and structural parts, but a bright clear finish is not realistic. Expect a darker, slightly mottled appearance, and specify hardcoat if wear resistance is the goal rather than appearance.

On the stainless and titanium side, anodizing does not apply in the same way. Titanium can be anodized for color and a thin oxide layer, and we list TA1, TA2 and TC4 (Ti-6Al-4V) in our material range, but the process and results differ from aluminum. Stainless grades such as 303, 304, 316L and 17-4PH are usually passivated or electroless nickel plated instead. If you are unsure which finishing route suits your alloy, send the drawing and we will say so at quoting.

Plastics are outside the anodizing conversation entirely. ABS, PC, POM, PEEK and the rest get painted, laser marked or left as machined. If a part needs a metallic look and anodizing is not possible, powder coating or a plated finish is the practical alternative.

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  Best for anodizing6061, 6063, 6082, 5052, 5083.
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  Anodize with care2024 and 7075 come out darker and less uniform.
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  Different processStainless is passivated or plated, not anodized.
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  Not applicablePlastics and die castings with high silicon content.

Aerospace and drone hardware uses anodized aluminum for weight, corrosion resistance and a hard surface that survives handling. Hardcoat is common on wear points; clear Type II on structural brackets where the alloy and any cracks need to stay visible during inspection.

Automotive and EV parts use anodizing on brackets, housings and under-hood components that see road salt and moisture. Medical device housings and instrument panels use it because the oxide layer tolerates repeated cleaning and disinfection better than an untreated aluminum surface, though the part still needs to be designed so cleaning agents do not sit in crevices.

Consumer electronics, camera bodies and robotics covers use dyed anodize for appearance. Industrial machinery uses hardcoat on guide rails, wear plates and fixtures where abrasion is constant. In every case the same questions come back: which alloy, which type, and which dimensions are critical after the bath.

We machine and finish in-house at three wholly-owned plants, with 127 high-precision CNC machines including 16 simultaneous 5-axis machining centers. Maximum processing size is 4,000 mm, and we can reach ±0.005 mm with a fine finish of Ra 0.2–0.8 μm on features that stay unmasked. Anodizing is offered as one of our surface finishing options alongside plating, powder coating, bead blasting and laser marking.

Uploads are handled as confidential, and an NDA is available on request. Quotation with a free DFM analysis comes back within 12 hours, production can start within 24 hours, and parts typically ship in 3–5 days. No minimum order quantity, from one prototype to 10,000+ part runs.

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  AerospaceHardcoat at wear points, clear Type II on structural brackets.
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  Automotive & EVBrackets and housings exposed to salt and moisture.
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  MedicalHousings that tolerate repeated cleaning and disinfection.
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  Electronics & roboticsDyed anodize for visible covers and panels.