N-Grades: Identify Surface Roughness Better
N-Grades are a standardized, shorthand way to specify surface roughness on technical drawings. They map common roughness values to grade numbers (N1…N12) so engineers, machinists, and inspectors can communicate requirements quickly and unambiguously.
Why it matters: Misreading surface finish drives scrap, rework, and schedule slips. Using N-Grades (or converting to them) reduces interpretation errors and speeds up quoting and QA.
What N-Grades represent
Surface roughness describes the micro-deviation of a surface from its ideal form. It’s commonly measured as Ra (arithmetical mean roughness) and, less often on drawings, Rz, Rq, or Rt. Because there are multiple parameters and units in use (µm versus µin), ISO introduced roughness grade numbers (“N-Grades”) to simplify identification on drawings.
Werk24 normalizes roughness callouts to N-Grades wherever possible and, conversely, can express N-Grades as Ra for readability in your downstream systems.
N‑Grade mapping across roughness parameters
Units: Ra given in both µm and µin (AA/CLA) · Other parameters as noted. Values are typical reference mappings (transcribed from your table).
| N‑Grade | Ra (µm) | Ra (µin) | Rt (µm) | Rz (µm) | Ry (µm) | RMS (µin) | PVA (µin) |
|---|---|---|---|---|---|---|---|
| N1 | 0.025 | 1 | 0.125 | 0.1 | 0.1 | 1.1 | 6.3 |
| N2 | 0.05 | 2 | 0.25 | 0.2 | 0.2 | 2.2 | 12 |
| N3 | 0.10 | 4 | 0.5 | 0.4 | 0.4 | 4.4 | 25 |
| N4 | 0.20 | 8 | 1.0 | 0.8 | 0.8 | 8.8 | 50 |
| N5 | 0.40 | 16 | 2.0 | 1.6 | 1.6 | 17.6 | 100 |
| N6 | 0.80 | 32 | 4.0 | 3.2 | 3.2 | 35.2 | 200 |
| N7 | 1.6 | 63 | 8.0 | 6.3 | 6.3 | 69.3 | 400 |
| N8 | 3.2 | 125 | 16.0 | 12.5 | 12.5 | 137.5 | 800 |
| N9 | 6.3 | 250 | 31.3 | 25 | 25 | 275 | 1600 |
| N10 | 12.5 | 500 | 62.5 | 50 | 50 | 550 | 3200 |
| N11 | 25 | 1000 | 125 | 100 | 100 | 1100 | 6400 |
| N12 | 50 | 2000 | 250 | 200 | 200 | 2200 | 12800 |
Where you’ll see it on drawings
Examples: Ra 1.6 µm (≈ 63 µin AA) → N7 · Rz ≈ 6.3 µm · Rt ≈ 8.0 µm Ra 3.2 µm (≈ 125 µin AA) → N8 · Rz ≈ 12.5 µm · Rt ≈ 16.0 µm
Where you’ll see it on drawings
- Next to the surface texture symbol (the “check-mark” symbol), e.g.
Ra 3.2orN8. - In general notes, e.g. “All unmarked surfaces N8 or better.”
- As process-specific overrides on critical faces.
Werk24 reads both local and global roughness instructions and resolves conflicts deterministically (local overrides global).
Choosing the right grade (rules of thumb)
- As-cast / flame-cut: typically N10–N12
- General machining (turn/mill): N7–N9
- Finish machining / fine boring: N6–N7
- Grinding: N5–N6
- Honing / lapping / polishing: N4 and finer
Tighter finishes raise cycle time and cost. If function allows it, relax to the coarsest acceptable grade.
Advantages of using (or converting to) N-Grades
- Clarity across units: Independent of µm vs µin; easy to compare.
- Less ambiguity: Avoids mixing parameters (Ra vs Rz) in one print.
- Faster quoting & QA: One consistent scale across suppliers and plants.
- Standardized communication: Matches how many shops think about finish.
Common pitfalls (and how Werk24 helps)
- Mixing parameters (e.g., Ra on one face, Rz in general notes) → Werk24 normalizes to N-Grade and surfaces conflicts.
- Over-specifying (grinding called where milling suffices) → We flag unusually tight finishes for cost awareness.
- Missing units on legacy US prints (“63”, “125”) → Werk24 interprets as µin and maps to N-Grades.