Most engineers learn the hard way that anodising before machining feels efficient only until the first batch of parts fails. It’s one of those processes that looks logical on paper, but completely falls apart once the parts enter the real world, especially in medical and high-precision applications.
How anodising before machining can create quality and durability problems
The first failure usually shows up innocently: a bright ring of raw aluminium around a freshly machined hole, or a pocket that looks wrong under the light. The oxide layer—uniform, matte, protective—gets cut away, leaving exposed metal with a completely different texture and colour. Engineers often think, “It’s internal, so maybe it’s fine.”
It isn’t.
Exposed aluminium reacts differently to cleaning fluids, sterilisation, even the ambient environment. In medical equipment, that bare patch becomes a contamination trap. It oxidises, discolours, and absorbs residues that the anodised surfaces repel. What was supposed to be a protective finish ends up highlighting every machined feature in the worst possible way.
Then the dimensional issues start. Anodising grows both inward and outward, so when you machine a part that’s already coated, you remove material that the drawing assumes still exists. Holes become slightly oversized, fits become loose, sliding components begin to stick where anodised and non-anodised surfaces meet. Threads fail. Tolerances drift.
The coating, which should have helped, becomes a source of unpredictable variation. And if the part needs to be clean, sterile, or aesthetically consistent—anything medical, lab-grade, or patient-facing—machining through anodising instantly creates a visual mismatch. Raw aluminium next to a coloured oxide layer never blends, no matter how controlled your process is. Under bright lighting, the part looks like a patchwork of textures.
What happens when you machine anodised aluminium
Machining an anodised surface behaves differently than machining raw aluminium. The oxide layer is harder and more brittle, so tools don’t slice through it cleanly and tend to scrape, chip, or fracture the coating. Edges that looked perfect before machining can develop tiny cracks, and corners may show slight flaking. Often these effects are microscopic and don’t matter for general industrial use, especially if the part isn’t exposed or doesn’t require a fully continuous coating.
But in applications where the surface must remain uniform, cleanable, or particle-free—particularly medical, biotech, laboratory or high-precision assemblies—those small defects become much more significant. They compromise cleanability, introduce the risk of shedding particles, and create inconsistencies between functional surfaces.
For components that need cosmetic uniformity or pass strict hygiene tests, machining through an existing anodised layer usually creates more issues than it solves. That’s why many teams eventually settle on a simpler conclusion: it’s safer and more reliable to anodise after all machining is complete.
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, and predictable visual consistency. Engineers who work with this sequence regularly tend to apply a few key principles that make a noticeable difference in performance and manufacturability.
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.
These small design decisions significantly improve coating quality, tolerance reliability, and overall finish on post-machined aluminium components.
Balancing aesthetics, precision, electrical requirements and cost
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.
Electrical applications: avoid anodising conductive surfaces
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.
When to use post-machining anodisation
Anodising before machining is not always a mistake. For hidden features, industrial structures or parts without cosmetic or hygienic requirements, it can be acceptable.
But when a component must…
• maintain consistent colour and appearance
• meet hygiene or sterilisation requirements
• eliminate particle-shedding risks
• hold tight functional tolerances
• present a uniform, premium finish
…post-machining anodisation consistently delivers the required performance.
Post-machining anodisation challenges and how ALUCAD solves them
Post-machining anodisation is demanding. Finished parts require controlled, piece-by-piece handling, non-marking fixturing and precise process parameters to achieve uniform colour, thickness and surface integrity across complex geometries. Most extruders avoid it entirely or outsource it without specialist control, leading to higher costs, longer lead times and inconsistent quality.
At ALUCAD, we work with a long-standing anodising partner specialising exclusively in post-machining finishing. By combining our machining expertise with their tightly controlled anodising process, we deliver reliable, repeatable quality, and total project costs that remain highly competitive.
This allows engineers to choose the technically correct process sequence, without being limited by budget, capability gaps or supplier risk.
If your project requires post-machining anodisation and you’re looking for a dependable partner at a sensible cost, get in touch with us.