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:
| 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:
