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Surface Finishes


Background

The finish quality of a machined surface is very important in industries that require sanitary surfaces such as Food, Biotech, Pharmaceutical, and Semiconductor manufacturing. There are a number of ways to define the Surface Finish, and this Tech Talk attempts to give a simple overview of the common terms. Surface finish measurement procedures, general terminology, definitions of most parameters and filtering information can be found in American Standard ASME B46.1 - 2002, Surface Texture, and in International Standards, ISO 4287 and ISO 4288.

Common terms

Ra The most common surface roughness parameter is Ra, or Arithmetic Average Roughness. It basically reflects the average height of roughness component irregularities from a mean line. Ra provides a simple value for accept/reject decisions. It is a default parameter on a drawing if not otherwise specified and is available even in the least sophisticated measuring instruments. Ra is not a good discriminator for different types of surfaces as it is incapable of differentiating between “spiky” and “scratched” surfaces having the same Ra. 

Rp  Maximum Peak height
Rv  Maximum Valley depth
Ry  Maximum Peak-to-Valley Roughness height
Rz  Mean Roughness Depth. It is the average distance between the highest peak and the deepest valley in five sampling lengths, or cutoffs. Rz is more sensitive then Ra to the changes in surface finish because maximum profile heights, and not the averages, are being examined.
Grit  Number of abrasive grains per given area
EP  Electro polish

Polished Finish

The high end of the sanitary finishes is an electro-polished ID which yields a 10 or 15 μ-inch Ra maximum roughness. These electro-polished products are generally used in very high purity applications.
Mill Finish  Industrial
32 Ra          Sanitary
20 & 15 Ra Pharmaceutical
20 RaEP      Pharmaceutical
15 RaEP      Ultra-Pure Applications