Anodising and machining are both critical steps when manufacturing bespoke aluminium profiles. And the order of those two processes will determine whether your component will perform as expected, or cause problems down the line.

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

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 to 25 microns. Hard anodising (Type III), used for wear resistance, is thicker, often 25 to 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.

And this thickness growth matters for tolerances as well as for threads, fits, sliding surfaces, and any interface that must behave predictably.

Anodising aluminium before machining

After anodisation, the aluminium components are sealed. So when you machine after anodising, you open the seal and expose unanodised aluminium.

Essentially, if you anodise first and machine later, the machined parts of your aluminium profile will not have the benefits from the anodisation process. In many industries, that's completely fine. But in industries working in medical, laboratory, aerospace, semiconductor, or food-processing environments, it usually isn’t.

The problem is rarely about looks and more about how the unanodised sections will behave. When you expose bare aluminium, you lose the protection from the anodisation, it'll corrode more easily, react differently to cleaning chemicals and sterilisation, and it’s much softer, so it scratches and wears faster. Over time, it can oxidise unevenly, discolour, and even start generating particles.

Post-machining anodisation is more expensive, so if raw aluminium in pockets or holes isn't a problem, then anodising before machining is the way to go. But it is important to bare in mind the additional coating which will be created. If the drawing assumes coated dimensions but you machine after anodising, you are cutting away material the tolerance stack may depend on.

Anodising aluminium after machining

Like mentioned above, if your component needs to maintain consistent appearance, corrosion resistance, cleanability, etc, across all visible and functional surfaces, anodising should be the final step.

Post-machining anodisation will ensure you have a continuous protective layer, uniform colour and texture, and no exposed aluminium at functional interfaces.

But it requires planning, as well as being clear on the designs before the process begins to make sure everyone is on the same page. For example, contact points used during anodising will not coat, so if appearance is important, those areas should be defined during design.

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. It's about accouting for a few key principles which will help make the process much smoother sailing:

• 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.

Choosing the right sequence

Not every project neatly fits into “anodise before machining” or “anodise after machining.” In the end, it's about balancing precision, appearance, functional requirements, and cost. But there a few points that help make a decision between the two.

If appearance, hygiene, corrosion resistance, or wear performance across all surfaces are critical, you're probably best to anodise after machining. If tight final tolerances dominate and surface continuity is secondary, then perhaps anodising before machining is better. And sometimes, a mix of both pre- and post-machining anodisation is required, like when you need to both anodise and tap your bespoke aluminium profile.

You don't anodise after tapping because the anodisation coating adds thickness and can push features out of tolerance. But, as we've often seen with the custom-made products we manufacture at ALUCAD, sometimes you need both nonetheless. In that case, what we do is we machine all the other features, anodise the part, then re-machine afterwards to create the threads.

Another example of requiring a hyrbid approach as we've seen is for functional contact surfaces, particularly in electrical applications. Anodising creates an electrically insulating oxide layer, which isn't always great for areas that need reliable conductivity. We've had the case where we machined the contact surface using a face milling operation to make it flat, clean, and free from contamination. This ensures consistent electrical contact while still benefiting from anodisation on the rest of the part for corrosion resistance and appearance.

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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. You can always discuss it with your supplier, who will know exactly what is needed to produce your part how you envision it.

If your project requires aluminium anodisating either pre- or post-machining, or perhaps a mix of the two, and you’re looking for a dependable partner at a sensible cost, get in touch with us.