Surface roughness is the measure if the finer surface irregularities in the surface texture. These are the result of the manufacturing process employed to create the surface. Surface roughness Ra is rated as the arithmetic average deviation of the surface valleys and peaks expressed in microinches or micrometers. ISO standards use the term CLA (Center Line Average). Both are interpreted identical.

The ability of a manufacturing operation to produce a specific surface roughness depends on many factors. For example, in end mill cutting, the final surface depends on the rotational speed of the end mill cutter, the velocity of the traverse, the rate of feed, the amount and type of lubrication at the point of cutting, and the mechanical properties of the piece being machined. A small change in any of the above factors can have a significant effect on the surface produced.

SURFACE ROUGHNESS AVERAGE OBTAINABLE BY COMMON PRODUCTION METHODS

PROCESS ROUGHNESS HEIGHT RATING MICROINCHES
Flame Cutting 1000
Sawing 1000-250
Drilling 500-125
EDM 250-125
Milling 500-63
Broaching 250-63
Reaming 250-63
Laser 500-63
Burnishing 63-16
Grinding 250-8
Honing 125-16
Polishing 63-16
Extruding 250-125
Investment Casting 500-250
Perm Mold Cast 500-250
Die Casting 250-63