Surface Roughness Reference Chart

Ra (Roughness Average) is the arithmetic average of the absolute values of the profile heights measured from the mean line. It is the most widely used surface roughness parameter. A lower Ra number means a smoother surface.

Ra is the arithmetic average roughness across the entire measurement length. Rz is the average of the five highest peaks and five deepest valleys within the sampling length. Rz is typically 4–5 times the Ra value and better captures occasional deep scratches that Ra may average out.

Standard CNC machining typically achieves Ra 1.6–3.2 μm (63–125 μin) as-machined. With finishing passes and sharp tooling, Ra 0.8 μm (32 μin) is achievable. Finer finishes require secondary operations like grinding or polishing.

Each halving of the Ra value roughly doubles the finishing cost. An as-machined Ra 3.2 μm part is the cost baseline. Achieving Ra 0.8 μm adds 15–25% cost. Ra 0.4 μm adds 30–50%. Mirror finishes (Ra 0.1 μm or below) can double or triple the part cost.

Yes, surface finish significantly impacts coating adhesion. Too smooth (Ra < 0.4 μm) provides insufficient mechanical keying. Too rough (Ra > 6.3 μm) creates coating thickness variation. For anodizing, Ra 0.8–1.6 μm is ideal. For paint and powder coat, Ra 1.6–3.2 μm provides optimal adhesion.

Use the standard surface finish symbol (a checkmark with a tail) and specify the Ra value in micrometers or microinches. Place the symbol on the surface view where the finish is required. Include the machining process if critical (e.g., ground, lapped). Add a general note for default surface finish on non-marked surfaces.