SS Surface Finishes Overview
The quality of a machined surface is critical in industries requiring sanitary surfaces, such as Food, Biotech, Pharmaceutical, and Semiconductor manufacturing. This guide provides a simple overview of common surface finish terms. Surface finish measurement methods and terminology are detailed in standards such as ASME B46.1-2002, ISO 4287, and ISO 4288.
Common Surface Finish Terms
Ra (Arithmetic Average Roughness): The most common roughness parameter, reflecting the average height of surface irregularities from a mean line. It’s widely used for accept/reject decisions but cannot distinguish between different surface textures like "spiky" or "scratched" surfaces with the same Ra value.
Rp (Maximum Peak Height): Measures the height of the highest peak on the surface.
Rv (Maximum Valley Depth): Refers to the depth of the deepest valley on the surface.
Ry (Maximum Peak-to-Valley Roughness Height): The total height between the highest peak and the lowest valley.
Rz (Mean Roughness Depth): The average distance between the highest peak and the deepest valley over five sampling lengths. Rz is more sensitive than Ra, as it examines maximum profile heights, not averages.
Grit Number: Represents the number of abrasive grains per given area, affecting the surface texture.
EP (Electropolishing): A process used to smooth and polish surfaces to achieve a very low Ra value, typically for high-purity applications.
Polished Finish
For sanitary finishes, the highest quality is an electropolished internal diameter (ID), achieving a roughness of 10 or 15 μ-inch Ra. This type of finish is typically used in high-purity environments.
Surface Finish Specifications
Mill Finish
Industrial Applications: 32 Ra
Sanitary Finish
- General Sanitary Applications: 20 Ra
- Pharmaceutical Applications: 20 Ra
Electropolished (EP) Finish
- Pharmaceutical Applications: 15 Ra EP
- Ultra-Pure Applications: 15 Ra EP