Mean roughness
The mean roughness value Ra (or in older literature: mean roughness value) is a measure of the roughness of a technical surface that was produced, for example, by machining , primary forming or forming processes .
standardization
The abbreviation Ra for the arithmetic mean roughness value is standardized according to DIN EN ISO 4287: 2010. The entry in technical drawings takes place in accordance with DIN EN ISO 1302: 2002 in connection with the basic symbol. The Ra value is entered on the top side of the triangle, a possible processing method (e.g. ground) above the so-called flag, whereby the addition "Ra" can also be omitted. The specification of the mean roughness value is based on the respective manufacturing process and can be found in tables (see DIN 4766-2 - withdrawn without successor).
According to DIN 3141 there are different allocation series for the surface information. In theory, row 3 is preferred, which ranges from Ra 6.3 (roughed) to Ra 0.1 (finely finished). In practice, however, row 2 is usually found from 12.5 to 0.1. The mean roughness is given in micrometers (µm).
Determination of the mean roughness value
To determine this measured value , the surface is scanned on a defined measuring section and all height and depth differences of the surface are recorded. After calculation of the definite integral of this Rauheitsverlaufes on the measurement path, this result is determined by the length of the measurement path finally divided . The roughness values range from 25 µm for very rough surfaces with noticeable grooves to 0.1 µm with no longer visible processing marks.
Comparative values
N is the roughness class; it can no longer be used for new designs.
| N | Ra (µm) | comment |
|---|---|---|
| 1 | 0.025 | no visible traces of processing |
| 2 | 0.05 | - |
| 3 | 0.1 | Ultra fine grinding |
| 4th | 0.2 | Fine grinding |
| 5 | 0.4 | Rough grinding |
| 6th | 0.8 | Rough grinding |
| 7th | 1.6 | settled |
| 8th | 3.2 | roughed |
| 9 | 6.3 | - |
| 10 | 12.5 | - |
| 11 | 25th | - |
| 12 | 50 | - |
literature
- Beitz / Grote (ed.); Dubbel - paperback for mechanical engineering; Springer-Verlag Berlin, Heidelberg; 19th edition 1997
- Prof. Dr. Antonius Lipsmeier (ed.); Friedrich book of tables - metal and machine technology; Bildungsverlag EINS; Troisdorf 2005; 167th edition; ISBN 3-427-51033-6