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      

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