CNC Surface Finish — What Ra Means and When to Spec a Smooth Finish

What "Ra" Measures

Ra (roughness average) is the average height of the tiny peaks and valleys left by the cutting tool, in micrometres (µm). A lower Ra means a smoother surface. It is the number you see on drawings as, for example, "Ra 1.6" or "Ra 0.8."

The Finish You Get Off the Machine

A standard CNC-machined face typically lands around Ra 1.6–3.2 µm depending on tool, speed and material. That is smooth enough for most fitting, sealing and appearance. Going below that takes extra passes, finer tools or post-processing, which adds cost — so only call for it where it matters.

When a Smoother Finish Is Worth It

Specify a finer finish when the surface must seal against a gasket, slide or rotate against another part, shed contamination (food, medical), or be a visible cosmetic face. A sealing face at Ra 0.8 behaves very differently from a raw machined Ra 3.2. Our surface finish comparison chart shows the common bands side by side.

When Standard Machined Is Enough

Internal features, non-contact faces and hidden brackets do not need a fine finish. Leaving them at the as-machined band keeps the part cheaper and faster without affecting function. Over-finishing every face is a common way to inflate a quote for no gain.

How Finishing Changes the Surface

Anodizing, bead blasting or passivation change the final feel and look. Type II anodizing on aluminium gives a clean matte or coloured surface; bead blast gives a uniform satin. Note that some coatings add a few micrometres, which matters if a dimension is tight — see our anodizing guide for how coating thickness moves your sizes.