Metal Post-Processing: Pick the Right Finish for CNC Parts

Metal post-processing turns a machined profile into a part that survives its service environment. For CNC parts, the decision usually comes down to anodizing, passivation, plating, or powder coating. Each option changes dimension, corrosion resistance, conductivity, and lead time, so specify it with the same care as ISO 2768 or a custom ±0.05 mm tolerance.

Anodizing suits 6061-T6 and 7075 aluminum when you need wear resistance and color. Type II adds roughly 0.005–0.010 mm per surface; Type III hard anodize builds more and can affect tight bores. Mask threads and electrical contacts, and avoid specifying Ra 0.8 cosmetic finishes on as-machined surfaces unless the anodizer can preserve them. Passivation is the lower-cost choice for 303, 304, and 316 stainless: it removes free iron and improves corrosion resistance without changing dimensions.

Plating, including zinc, nickel, and electroless nickel, protects steel and can add solderability or conductivity. Electroless nickel adds a uniform 0.005–0.025 mm, so account for it on press fits. Powder coating gives thick, durable color on steel and aluminum, but it can bridge small holes and add 0.05–0.10 mm. For POM/Delrin or other plastics, post-processing is usually limited to vapor polishing, annealing, or dyeing; anodizing and passivation do not apply.

Match the finish to function: corrosion, wear, conductivity, appearance, and assembly. Document masking, allowed thickness, and inspection criteria on the drawing. Ask for sample coupons or first-article verification when tolerances are tight. Post-processing often adds two to five days and a per-part cost, but choosing it early reduces rework after machining.

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For more guidance on related work, see: CNC Machining Tolerances: When ISO 2768 Is Enough.