Surface Finish of Machined Parts

When it comes to 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:

a-μm – 0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50
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: