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. “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.