Tech Talk
How is "Surface Finish" measured?
RA: The "Roughness Average" is a measurement of the peaks and valleys relative to the nominal line. The Ra is also sometimes referred to as the "Arithmetic Average". This is the most common measurement of surface finish and is expressed in either micrometers or microinches. For reference, 1 microinch = 0.0254 micrometer.
The measurement known as "RMS", the "Root Mean Square Average", is no longer commonly used. For reference, a surface typically measures about 11% higher in the RMS than the Ra.
Using common grit grinding wheels, grinders typically obtain in the range of 8 to 12 microinch finishes on tool clearances and about 6 to 10 microinch finishes on flute faces. The actual finish achieved in a specific application depends upon the wheel grit, open condition of the bond, wheel speed, coolant, feedrates and tool material.
