Surface Finish
Surface Roughness
Surface Roughness
Typically determined by using visual comparator samples for milling or turning.
Ra is the average variance from the mean height to the highest peaks and lowest valleys across a given distance
1 micro-inch is 1 millionth of an inch
There is a lot more to describing surfaces but this gives you a simple overview of a commonly encountered method.
Surface Finish Symbols on Drawings
Surface Finish Symbols on Drawings