Aluminum Surface Finishes for CNC Parts
Finish selection changes corrosion behaviour, appearance and — critically — the dimensions you specified on the drawing.
Aluminum CNC parts rarely ship in the as-machined state. Anodizing, powder coating and plating are chosen for corrosion resistance, wear, colour and appearance. Each adds thickness, and that thickness lands on the dimensions you just spent time specifying. This guide covers the common options for machined aluminum components and what to write on the drawing.
Finish Comparison
| Finish | Typical thickness | Dimensional impact | Strengths | Watch out for |
|---|---|---|---|---|
| Anodizing Type II | 5–25 µm | Small, ~half grows outward | Corrosion, dyeable colours, clean look | Thin build-up still matters on press fits |
| Hard anodizing Type III | 25–100 µm | Significant | Wear and abrasion resistance | Brittle on sharp edges; dark natural colour |
| Powder coating | 60–120 µm | Large | Thick protection, RAL colours | Threads and grounding surfaces must be masked |
| Zinc / nickel plating | 5–20 µm | Small to moderate | Conductivity, solderability, appearance | Requires compatible pretreatment on aluminum |
| Chemical conversion (chromate) | <1 µm | Negligible | Paint adhesion, conductive, low cost | Limited standalone corrosion protection |
| Bead blasting | None (surface texture) | None | Uniform matte appearance, hides tool marks | Ra changes; not a protective layer alone |
Anodizing is a conversion coating: part of the aluminum surface is consumed and part of the oxide grows outward, so the net dimensional growth per surface is roughly half the coating thickness. It still matters on an H7 bore. Powder coating is additive only and grows the full stated thickness on each surface.
Choose by Requirement, Not by Habit
Corrosion protection with a clean metallic look: Type II anodizing, natural or dyed black. Sliding wear: Type III hard anodizing, with edge breaks specified to avoid chipping. Outdoor equipment enclosure in a brand colour: powder coat over a conversion pretreatment. Electrical continuity or EMI shielding: chemical conversion coating, since anodized oxide is an insulator.
For automotive and EV components, temperature cycling and galvanic contact with other metals usually drive the choice more than appearance does.
Masking, Threads and Grounding Faces
Every coated part needs a masking plan. Threaded holes, press-fit bores, sealing faces, datum surfaces and electrical grounding points are normally masked. If a threaded hole is anodized, the gauge may no longer pass; if a grounding face is powder coated, the assembly may fail an electrical test. Put the masking requirement on the drawing, not in an email that will be lost between revisions.
Appearance and Cosmetic Agreement
Anodizing shows extrusion lines, machining marks and differences between billets; powder coating shows orange peel on edges. For visible parts, agree a cosmetic standard or approve a sample before production. Colour matching should reference a RAL number and gloss level for coating, or a physical sample for anodizing, since dyed anodized colour varies with alloy and bath conditions.
Inspection and Documentation
Coating thickness is measured with an eddy-current gauge on representative parts, adhesion is checked by cross-hatch tape test where specified, and colour is judged against the approved sample under defined lighting. We can supply coating thickness and salt-spray test reports per lot. See quality control for how these records are issued.
If tight fits are involved, read the tolerance guide alongside this one — the two specifications have to be written together.
Related Questions
Does anodizing change part dimensions?
Yes. Net growth per surface is roughly half the coating thickness, because part of the base aluminum is consumed. State whether dimensions apply before or after finishing.
Should threaded holes be masked?
Usually yes. Anodizing or powder coating can stop a thread gauge from passing. Specify the masked features on the drawing.
Which finish for an outdoor enclosure?
Powder coating over a chemical conversion pretreatment, with a specified RAL colour and thickness, gives the best durable protection.
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