This article covers the following topics:
- Metric surface roughness symbols and their descriptions
- Surface roughness values achievable by manufacturing methods
- Different notation formats for surface roughness
Metric Surface Roughness Symbols
Metric surface roughness values are expressed in µm (micrometer) units.
The roughness symbol consists of two lines at 60 degrees, one longer than the other. Commonly used symbols with special meanings are as follows:
- Basic symbol (without top line) cannot be used alone. It must be used with a machining method or surface roughness value.
- When the machining method is optional, the roughness value is specified. Example: Ra 3.2 µm (0.0032 mm).
- Symbol with top line means machining is mandatory.
- Symbol with circle means material removal is not permitted.
Surface Roughness Values by Manufacturing Method
The following table provides an indication of Ra values achievable with common machining methods. This table is for metal surfaces. Other materials may vary.
Surface quality classes in the tables:
Average 
Precision 
Rough
Machining and Abrasive Methods
| Machining Method | Ra Range (µm) | Typical Use |
|---|---|---|
| Plasma and OXY Fuel Cutting | 13 - 50 | Rough cutting |
| Sawing | 1.6 - 25 | Rough cutting |
| Planing | 0.8 - 13 | Flat surfaces |
| Stamping | 0.4 - 1.6 | Sheet metal |
| Chemical Processing | 0.8 - 6.3 | Surface etching |
| EDM | 0.8 - 6.3 | Mold making |
| Drilling | 0.8 - 6.3 | Hole making |
| Boring | 0.8 - 6.3 | Hole enlarging |
| Milling | 0.4 - 13 | Surface, slot machining |
| Turning | 0.4 - 6.3 | Cylindrical parts |
| Broaching | 0.4 - 3.2 | Profile cutting |
| Reaming | 0.4 - 3.2 | Precision holes |
| Filing | 0.4 - 3.2 | Manual finishing |
| Grinding | 0.2 - 3.2 | Precision surfaces |
| Brushing | 0.1 - 0.4 | Surface improvement |
| Electrolytic Grinding | 0.1 - 0.4 | Precision finishing |
| Honing | 0.05 - 0.4 | Cylinder, bore finishing |
| Polishing | 0.1 - 0.4 | Optical surfaces |
| Lapping | 0.05 - 0.4 | Ultra-precision surfaces |
| Superfinishing | 0.05 - 0.4 | Ultra-precision surfaces |
| Flex-Hone Brushes | 0.05 - 6.3 | Deburring, plateau finish |
Non-Material Removal Methods
| Method | Ra Range (µm) |
|---|---|
| Sand Casting | 6.3 - 50 |
| Hot Rolling | 6.3 - 50 |
| Forging | 0.4 - 13 |
| Casting | 0.4 - 1.6 |
| Investment Casting | 0.2 - 1.6 |
| Extrusion | 0.2 - 1.6 |
| Cold Rolling | 0.1 - 1.6 |
| Roller Burnishing | 0.05 - 0.4 |
Different Notation Formats for Surface Roughness
Surface roughness values shown in microinches are 40 times the micrometer value.
| Ra µm | Ra µin | Class No (New) | Rt µm |
|---|---|---|---|
| 50 | 2000 | N12 | 435 |
| 25 | 1000 | N11 | 218 |
| 12.5 | 500 | N10 | 109 |
| 6.3 | 250 | N9 | 55 |
| 3.2 | 125 | N8 | 28 |
| 1.6 | 63 | N7 | 14 |
| 0.8 | 32 | N6 | 7 |
| 0.4 | 16 | N5 | 3.5 |
| 0.2 | 8 | N4 | 1.7 |
| 0.1 | 4 | N3 | 0.87 |
| 0.05 | 2 | N2 | 0.44 |
| 0.025 | 1 | N1 | 0.22 |
| 0.012 | 0.5 | - | 0.10 |
Related Articles
- Ra / Rz Conversions — Conversion factors between Ra, Rz, Rt values
- Tolerances by Machining Method — IT tolerance values achievable with different manufacturing techniques
Prepared by: Yamac Aksan. Sources: ISO, Engineering Toolbox, BRM