ENGINEERING TOOL
Convert Ra, Rz, and N classes. Visualize the surface profile, consult photographic finish references, and get process recommendations. For CNC drawings, tolerances, and manufacturing quotes.
Parameter
Unit
Standard value: N7
N1 → N12
Illustrative roughness profile
Illustrative representation. Does not represent exact surface profile geometry.
Ra 1.600 µm
≈ 62.99 µin Ra
Rz estimated
6.400–16.00 µm
252.0–629.9 µin
N class (approx.)
N7
Commonly associated processes
Typically achievable with precision CNC machining. Depends on material, geometry, and machine.
⚠ Ra ↔ Rz conversion is approximate (×4 to ×10). For final inspection, use the parameter from the technical drawing.
Approximate visual reference
Horizontal milling
Vertical milling
Turning
Illustrative visual reference. Actual finish depends on material, tooling, cutting parameters, machine, and inspection method.
| N Class | Ra (µm) | Ra (µin) | General level |
|---|---|---|---|
| N1 | 0.025 | 1.0 | Mirror / lapped |
| N2 | 0.05 | 2.0 | Mirror / polished |
| N3 | 0.1 | 4.0 | Very fine ground |
| N4 | 0.2 | 8.0 | Fine ground |
| N5 | 0.4 | 16 | Ground |
| N6 | 0.8 | 32 | Standard ground |
| N7 | 1.6 | 63 | Fine machined |
| N8 | 3.2 | 125 | Standard machined |
| N9 | 6.3 | 250 | General machined |
| N10 | 12.5 | 500 | Rough machined |
| N11 | 25 | 1000 | Rough |
| N12 | 50 | 2000 | Very rough |
Ra values in the table are nominal values for each N class per the reference standard. The N class assigned by the tool is the closest to the entered value.
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Ra (arithmetic mean roughness) measures the average deviation of the profile from the mean line over an evaluation length. Rz (maximum profile height) measures the distance between the highest peak and deepest valley within a sampling length. Ra is the most commonly specified parameter on technical drawings. Rz is more sensitive to isolated peaks and defects. The relationship between both is approximate: Rz ≈ Ra × 4 to Ra × 10, depending on the material and process.
N classes (N1 to N12) are a standardized notation that groups Ra values into logarithmic categories. N1 corresponds to Ra 0.025 µm (mirror finish) and N12 to Ra 50 µm (rough machining). Each class doubles the Ra value of the previous one. This notation simplifies communication between design and manufacturing, although modern technical drawings prefer specifying the Ra value directly alongside the ISO surface finish symbol.
No. The Ra ↔ Rz conversion is inherently approximate. The actual factor varies between 4 and 10 depending on material, process, tool condition, and measurement method. This tool shows the estimated range as a design reference only. For final inspection, always use the parameter specified on the technical drawing and measure it with the appropriate instrument (profilometer, CMM with roughness probe, optical interferometer).
For standard CNC machining (milling and turning), Radii typically works in the Ra 0.8 µm to Ra 6.3 µm range depending on geometry and material. Finer finishes like Ra 0.4 µm or lower may require secondary operations (grinding, lapping, polishing) and have implications for cost and lead time. We recommend specifying the required Ra on your technical drawing and quoting with our team to confirm feasibility and cost.
Use the ISO 1302 surface finish symbol. Indicate the parameter (Ra, Rz), the limit value, and the cutoff length if applicable. For example: Ra 1.6 means a maximum arithmetic mean roughness of 1.6 µm. If you don't specify the parameter, the standard assumes Ra. Always specify roughness on functional surfaces (seals, seats, guides, contact areas). Non-functional surfaces can be left to process discretion.