Surface Roughness Spec Table
Surface roughness is a quality index indicating the degree of microscopic surface irregularities, expressed as Ra, Rz, etc. Per KS B ISO 4287, the range spans Ra 0.1–50μm. Sealing contact surfaces typically require Ra 0.4 or finer; general machined surfaces are usually Ra 6.3 or finer. Appropriate roughness symbols on design drawings help balance machining cost and functional quality.
Frequently Asked Questions
What is the difference between Ra and Rz surface roughness?
Ra (arithmetic mean roughness) is the average height variation over a measured section. Rz (maximum height) is the difference between the highest peak and deepest valley. Ra reflects overall roughness; Rz reflects extremes. Generally Rz ≈ 4×Ra.
How do I choose surface roughness values?
Sealing and bearing contact surfaces: Ra ≤0.8; general fit surfaces: Ra 1.6–3.2; unmachined/structural surfaces: Ra 6.3–12.5. Finer roughness increases machining cost.