Anodising and machining are both critical steps when manufacturing bespoke aluminium profiles. And the order of those two processes can quietly determine whether a part performs perfectly, or causes problems later.

Anodising before machining is often more cost-efficient from a process-flow perspective. But depending on what your custom aluminium product has to do, it can introduce risks which only appear once parts are exposed to real-world conditions.

So when should you anodise before machining, and when should you anodise after?

What happens when anodising aluminium

Anodising creates a controlled oxide layer on the aluminium surface. That layer improves corrosion resistance, increases surface hardness, and can provide colour consistency.

Typical decorative anodising ranges around 10–25 microns. Hard anodising (Type III), used for wear resistance, is thicker, often 25–70 microns.

The coating grows both inward and outward. It is not paint sitting on top. Roughly half the thickness builds into the surface and half builds outward, though this is not perfectly symmetrical.

The thickness growth matters for tolerances. It also matters for threads, fits, sliding surfaces, and any interface that must behave predictably.

Most anodised parts are also sealed after processing. If you machine after sealing, you expose raw, unsealed aluminium.

When anodising before machining can create problems

If you anodise first and machine later, you remove part of that protective layer.

The first issue is usually visual: exposed aluminium around a drilled hole or inside a pocket. In non-critical industrial parts, that may be acceptable. In medical, laboratory, aerospace, semiconductor, or food-processing environments, it usually isn’t.

Exposed aluminium has lower corrosion resistance than anodised aluminium. It behaves differently under cleaning chemicals and sterilisation cycles. Over time, it can discolour.

There is also a dimensional consequence. If the drawing assumes coated dimensions but you machine after anodising, you are cutting away material the tolerance stack may depend on. That can mean:

• Slightly oversized holes

• Reduced interference fits

• Loose threads

• Mixed friction surfaces between coated and uncoated areas

It’s rarely catastrophic. But it introduces variability.

With hard anodising, the trade-off is sharper. If wear resistance is the reason for anodising, machining through that layer removes the very surface you were trying to protect.

There’s also a surface integrity factor. The anodic layer is harder and more brittle than base aluminium. When machined, it can micro-chip or crack at edges. In general industrial use, this is minor. In cleanroom or hygiene-critical assemblies, particle risk becomes more significant.

When anodising after machining makes more sense

If your component must maintain consistent appearance, corrosion resistance, and cleanability across all visible and functional surfaces, anodising should usually be the final step.

Post-machining anodisation ensures:

• A continuous protective layer

• Uniform colour and texture

• No exposed aluminium at functional interfaces

But it requires planning.

Because the coating builds thickness, engineers must specify clearly whether dimensions apply before or after anodising. Coating thickness should be stated on the drawing. Very tight bores or precision fits must allow for growth.

Sharp internal corners can also create thin spots or uneven coating. Deep blind holes and narrow cavities may anodise less uniformly due to restricted electrolyte flow.

Fixturing matters too. Contact points used during anodising will not coat. If appearance is important, those areas should be defined during design.

None of these are problems if accounted for early. They become problems when ignored.

Design tips for successful post-machining anodisation

Designing aluminium components with post-machining anodising in mind is ultimately about predictability. Predictable coating thickness. Predictable tolerances. Predictable visual consistency. And to ensure predictability, there are a few key principles to consider:

Allow for oxide growth in final dimensions: Anodising adds thickness inward and outward. For critical fits, specify whether tolerances apply before or after anodising and note the target coating thickness.

Add small radii instead of sharp internal corners: Even a 0.2–0.5 mm radius improves coating uniformity, reduces thin spots, and prevents stress fractures in the oxide.

Avoid deep, narrow features that anodise unevenly: Slots, blind holes, and long cavities restrict electrolyte flow. Use relief features where possible or note acceptable cosmetic variation on nonfunctional surfaces.

Choose alloys that anodise consistently: 6060/6063 give the most uniform colour and surface finish. 6082 provides higher strength but can show slight colour variation—fine for structural but not ideal for cosmetic parts.

Consider anodising fixturing early: Contact points won’t anodise. If surface appearance matters, define preferred gripping areas or add small nonfunctional tabs for handling.

Not every project neatly fits into “anodise before machining” or “anodise after machining.” In practice, engineers must balance four constraints: precision, appearance, functional requirements and cost.

Never anodise after tapping: Aluminium components should never be anodised after tapping. The oxide layer eats into the threads, changes their geometry and weakens surrounding material. Tapped holes must be masked during anodising—or left bare.

When maximum precision is required, anodise before machining: For extremely tight tolerances, anodising before machining can be the correct sequence. Because the oxide layer affects every dimension, machining last gives the most stable final geometry. The trade-off: machining breaks the anodic layer, so the finish will not look uniform.

When aesthetics (or other anodisation benefits) matter, anodise after machining (or use a hybrid process): If the part requires a continuous anodised surface for cosmetic or hygienic performance, anodising after all machining is the only way to achieve a flawless finish. Some projects, however, require both cosmetic surfaces and tight tolerances. In these cases, engineers use a hybrid process where you machine part of the component, anodise, and re-machine the selected areas. It works, but it increases complexity and cost, and depends heavily on the supplier’s capability.

Avoid anodising conductive surfaces for electrical applications: Anodising is an electrical insulator with a high dielectric strength, so grounding points, contacts and current-carrying surfaces must remain bare. However, anodising significantly increases emissivity, especially in darker finishes (e.g., black anodising). For heat-dissipating surfaces such as heatsinks or thermal housings, the oxide layer can improve radiative cooling.

Cost considerations: Hybrid or precision-critical anodising sequences are always more expensive than standard processes.

Choosing the right sequence

There is no universal rule. If appearance, hygiene, corrosion resistance, or wear performance across all surfaces are critical, anodise after machining. If ultra-tight final tolerances dominate and surface continuity is secondary, anodising before machining can be justified. If both precision and aesthetics are essential, a hybrid approach (machining, anodising, then selective re-machining) can work. It increases cost and complexity, but sometimes it is the only way to satisfy competing requirements.

The correct choice between pre- and post-machining anodisation depends on what your component must prioritise: precision, durability, appearance, electrical behaviour, or cost. The decision is about engineering intent. And the key is deciding the sequence during design, not after parts are already in production.

---

If your project requires post-machining anodisation and you’re looking for a dependable partner at a sensible cost, get in touch with us.