Surface Roughness Chart
The Ra finish each production method leaves on steel, from flame cutting and sawing down to lapping and superfinishing. Each row shows the range routinely produced and the wider range that can still occur, in micrometers and microinches, along with the roughness grade numbers and their cut-off values.
Roughness produced by common production methods
The dark bar is the range a shop normally holds with that process. The grey bar shows the wider range possible under favorable conditions. Values as printed in Machinery’s Handbook 28th Edition, Table 1.
Roughness ranges by production method, redrawn from the source chart. Left to right runs from rough to smooth.
| Process | Average application, Ra | Less frequent application, Ra |
|---|---|---|
| Flame Cutting | 25 to 12.5 µm 1000 to 500 µin | 50 to 6.3 µm 2000 to 250 µin |
| Snagging | 25 to 6.3 µm 1000 to 250 µin | 50 to 3.2 µm 2000 to 125 µin |
| Sawing | 25 to 1.6 µm 1000 to 63 µin | 50 to 0.80 µm 2000 to 32 µin |
| Planing, Shaping | 12.5 to 1.6 µm 500 to 63 µin | 25 to 0.40 µm 1000 to 16 µin |
| Drilling | 6.3 to 1.6 µm 250 to 63 µin | 12.5 to 0.80 µm 500 to 32 µin |
| Chemical Milling | 6.3 to 1.6 µm 250 to 63 µin | 12.5 to 0.80 µm 500 to 32 µin |
| Elect. Discharge Mach. | 4.5 to 1.6 µm 180 to 63 µin | 12.5 to 0.80 µm 500 to 32 µin |
| Milling | 6.3 to 0.80 µm 250 to 32 µin | 25 to 0.141 µm 1000 to 5.66 µin |
| Broaching | 3.2 to 0.80 µm 125 to 32 µin | 6.3 to 0.40 µm 250 to 16 µin |
| Reaming | 3.2 to 0.80 µm 125 to 32 µin | 6.3 to 0.40 µm 250 to 16 µin |
| Electron Beam | 6.3 to 0.80 µm 250 to 32 µin | 6.3 to 0.20 µm 250 to 8 µin |
| Laser | 6.3 to 0.80 µm 250 to 32 µin | 6.3 to 0.20 µm 250 to 8 µin |
| Electro-Chemical | 3.2 to 0.20 µm 125 to 8 µin | 12.5 to 0.05 µm 500 to 2 µin |
| Boring, Turning | 6.3 to 0.40 µm 250 to 16 µin | 25 to 0.025 µm 1000 to 1 µin |
| Barrel Finishing | 0.80 to 0.20 µm 32 to 8 µin | 3.2 to 0.05 µm 125 to 2 µin |
| Electrolytic Grinding | 0.566 to 0.20 µm 22.6 to 8 µin | 0.80 to 0.10 µm 32 to 4 µin |
| Roller Burnishing | 0.40 to 0.20 µm 16 to 8 µin | 0.80 to 0.10 µm 32 to 4 µin |
| Grinding | 1.6 to 0.10 µm 63 to 4 µin | 6.3 to 0.025 µm 250 to 1 µin |
| Honing | 0.80 to 0.10 µm 32 to 4 µin | 1.6 to 0.025 µm 63 to 1 µin |
| Electro-Polish | 0.80 to 0.10 µm 32 to 4 µin | 1.6 to 0.012 µm 63 to 0.5 µin |
| Polishing | 0.40 to 0.10 µm 16 to 4 µin | 0.80 to 0.012 µm 32 to 0.5 µin |
| Lapping | 0.40 to 0.05 µm 16 to 2 µin | 0.80 to 0.012 µm 32 to 0.5 µin |
| Superfinishing | 0.20 to 0.025 µm 8 to 1 µin | 0.80 to 0.012 µm 32 to 0.5 µin |
| Sand Casting | 25 to 12.5 µm 1000 to 500 µin | 50 to 6.3 µm 2000 to 250 µin |
| Hot Rolling | 25 to 12.5 µm 1000 to 500 µin | 50 to 6.3 µm 2000 to 250 µin |
| Forging | 12.5 to 3.2 µm 500 to 125 µin | 25 to 1.6 µm 1000 to 63 µin |
| Perm. Mold Casting | 3.2 to 1.6 µm 125 to 63 µin | 6.3 to 0.80 µm 250 to 32 µin |
| Investment Casting | 3.2 to 1.6 µm 125 to 63 µin | 6.3 to 0.40 µm 250 to 16 µin |
| Extruding | 3.2 to 0.80 µm 125 to 32 µin | 12.5 to 0.40 µm 500 to 16 µin |
| Cold Rolling, Drawing | 3.2 to 0.80 µm 125 to 32 µin | 6.3 to 0.20 µm 250 to 8 µin |
| Die Casting | 1.6 to 0.80 µm 63 to 32 µin | 3.2 to 0.40 µm 125 to 16 µin |
Roughness grades N1 to N12
Older drawings and specifications call out the grade number instead of an Ra value. The grade numbers run the other way round to Ra, the higher the number the rougher the surface. Source, Table 4.
| Grade | Ra, µm | Ra, µin |
|---|---|---|
| N12 | 50 | 2000 |
| N11 | 25 | 1000 |
| N10 | 12.5 | 500 |
| N9 | 6.3 | 250 |
| N8 | 3.2 | 125 |
| N7 | 1.6 | 63 |
| N6 | 0.80 | 32 |
| N5 | 0.40 | 16 |
| N4 | 0.20 | 8 |
| N3 | 0.10 | 4 |
| N2 | 0.05 | 2 |
| N1 | 0.025 | 1 |
Preferred Ra values to specify
The standard series of Ra values (ISO 1302 and the equivalent national standards). The values marked (a) are the recommended ones, and specifying a value from this series keeps the part measurable with standard roughness specimens. Source, Table 3.
| Preferred series of Ra values, µm (top) and µin (bottom) | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| 0.012 0.5 | 0.025 (a) 1 | 0.050 (a) 2 | 0.075 3 | 0.10 (a) 4 | 0.125 5 | 0.15 6 | 0.20 (a) 8 | 0.25 10 | 0.32 13 |
| 0.40 (a) 16 | 0.50 20 | 0.63 25 | 0.80 (a) 32 | 1.00 40 | 1.25 50 | 1.60 (a) 63 | 2.0 80 | 2.5 100 | 3.2 (a) 125 |
| 4.0 160 | 5.0 200 | 6.3 (a) 250 | 8.0 320 | 10.0 400 | 12.5 (a) 500 | 15 600 | 20 800 | 25 (a) 1000 |
(a) Recommended values.
Cutoff, the roughness sampling length
The cutoff is the length over which the instrument samples the profile. Where the drawing does not state one, the 0.80 mm (0.030 in.) value applies. Source, Table 2.
| Cutoff, mm | Cutoff, in. |
|---|---|
| 0.08 | 0.003 |
| 0.25 | 0.010 |
| 0.80 | 0.030 |
| 2.5 | 0.1 |
| 8.0 | 0.3 |
| 25.0 | 1.0 |
- Ra is quoted in micrometres and microinches throughout, the two rows of the source chart.
- A surface machined by one process is often improved by a later finishing process in the chain, turned, then ground, then honed. Read the chart left to right for the processes that follow.
- The ranges are general shop values. The finish a cut actually leaves depends on the material, the tool or wheel, the rigidity of the setup and the feed used.
Reading the chart
The process rows run from the roughest finishes at the top to the smoothest at the bottom, and the scale runs from Ra 50 µm on the left to 0.012 µm on the right. A grey bar that extends past a value means the process can reach that roughness under favourable conditions, and the dark section is where the process usually sits. When a drawing calls for a finish, check that the process named for the part’s last operation can hold it. A milled surface is usually quoted at Ra 3.2 to 0.80 µm, so grinding or lapping follows when the drawing is tighter.
How fast the steel can be machined, and with which tool, is covered in the machinability rating chart and in the cutting speeds and feeds for tool steels. Surface treatments that add a layer rather than cut one are a different subject, on the surface treatment page.
Related reference pages
Machining allowance chart · Tool steel properties chart
Source: Machinery’s Handbook 28th Edition (Industrial Press), Surface Texture section: Table 1 (roughness produced by common production methods), Table 2 (standard cutoff values), Table 3 (preferred series of Ra values), Table 4 (roughness grades). The chart is redrawn from the source. Values are converted between micrometres and microinches at 1 µm = 39.37 µin (0.0254 µm = 1 µin).