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Surface Roughness Chart (Ra / Rz / N Grade / ▽ Symbol)
One page mapping Ra, Rz, N grades and the legacy ▽ symbols, plus the roughness ranges achievable in practice for each machining process. Approximate conversions are clearly flagged.
Ra / Rz / N grade chart
| N grade (ISO 1302) | Ra (μm) | Approx. Rz (μm) |
|---|---|---|
| N1 | 0.025 | 0.1 |
| N2 | 0.05 | 0.2 |
| N3 | 0.1 | 0.4 |
| N4 | 0.2 | 0.8 |
| N5 | 0.4 | 1.6 |
| N6 | 0.8 | 3.2 |
| N7 | 1.6 | 6.3 |
| N8 | 3.2 | 12.5 |
| N9 | 6.3 | 25 |
| N10 | 12.5 | 50 |
| N11 | 25 | 100 |
| N12 | 50 | 200 |
Rz ≈ 4 × Ra is an empirical approximation; the actual ratio varies between 3 and 7 times depending on the machining method (turning differs from milling, while grinding is closer to 4–5 times). If a drawing calls out both Ra and Rz, go by the drawing callout—never use a converted value for acceptance.
▽ symbol (legacy JIS finish symbols) reference
| Symbol | Corresponding Ra (μm) | Common machining method |
|---|---|---|
| ▽ | 12.5 – 25 | Rough turning, rough milling, sawn face |
| ▽▽ | 3.2 – 6.3 | General turned/milled surface |
| ▽▽▽ | 0.8 – 1.6 | Finish turning, finish milling, reaming |
| ▽▽▽▽ | 0.2 – 0.4 | Grinding, honing, polishing |
The ▽ symbols belong to the legacy JIS system and the customary range differs slightly from shop to shop; use them only as a reference when reading old drawings, and call out Ra / Rz values on new drawings.
Roughness ranges achievable by common machining methods
| Machining method | Typical achievable Ra (μm) |
|---|---|
| End milling (general conditions) | 0.8 – 6.3 |
| Turning | 0.8 – 6.3 |
| Drilling | 1.6 – 6.3 |
| Reaming | 0.4 – 1.6 |
| Surface grinding | 0.1 – 0.8 |
| Honing | 0.05 – 0.4 |
| Polishing / mirror finish | 0.012 – 0.1 |
FAQ
What's the difference between Ra and Rz?
Ra is the arithmetic mean deviation of the profile (an average concept), while Rz is the peak-to-valley height within a sampling length (an extreme-value concept). On the same surface the Rz value is always larger than Ra, empirically by about 3–7 times. Which one governs acceptance is set by the drawing callout.
Can ordinary milling reach Ra1.6?
Yes, it falls in the finish-milling range: it needs an appropriate feed per tooth (small fz), a sharp tool and stable workholding. Below Ra0.8 it is best to add a finishing toolpath or switch to a grinding-type process.
What if the drawing doesn't specify roughness?
Follow the general note in the title block (e.g. "unspecified surfaces Ra3.2"); when there is no note at all, confirm with the customer rather than assuming—roughness directly affects machining time and quotation.
Further reading: Complete guide to surface roughness Ra · How to choose a surface treatment | More tools: Cutting parameter calculator · Tap drill chart
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