When it comes to the surface finish of machined parts, most people are typically concerned only with average roughness of a part, Ra; however, there are many more roughness parameters which can be specified.
Surface Finish usually takes on the lay of its manufacturing process. For example, if a part is saw-cut, it will have a linear lay following
the travel of the saw teeth. On the other hand, if a part is rotary-ground it’s finish will have a circular lay.
There are several specification guiding the detailing of surface finish. The Europeans use BS EN ISO 1302:2002 Geometrical product specifications (GPS). In the US, ASME Y14.36M is commonly used. The symbols in each standard are quite similar:
The metric-to-inch conversion is:
|R a-μm –||0.025||0.05||0.1||0.2||0.4||0.8||1.6||3.2||6.3||12.5||25||50|
|R a– μinch||1||2||4||8||16||32||63||125||250||500||1000||2000|
Various machining operations have different typical surface finishes inherent to their respective manufacturing process, as indicated in the table below: