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:
