Surface Finish Converter.
Convert between Ra, Rz, Rt, Rmax, microinches, and ISO N-grade. Plus a process capability map for choosing the right machining method.
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µm — most common spec
µin — US/ASME standard
ISO 1302 grade number
Equivalent values
Ra
1.60
µm
Ra
63
µin
Rz (DIN)
8.0
µm
Rmax / Rt
10.0
µm
CLA
63
µin
N-Grade
N7
ISO 1302
Conversion ratios used: Ra → Rz ≈ 5×Ra (typical for ground/turned). Ra → Rmax ≈ 6.4×Ra. CLA = Ra (microinch). N-grade = log₂(Ra) + 7. These are approximations — actual ratios vary with profile shape and process.
Process capability — what gets you what finish
Typical Ra ranges achievable by common manufacturing processes. Use this to pick the right process or validate a drawing call-out is realistic.
| Process | Ra Range (µm) | Ra Range (µin) | Typical N-Grade | Notes |
|---|---|---|---|---|
| Flame cutting | 25 – 50 | 1000 – 2000 | N12 | Rough; needs further machining |
| Sawing | 12.5 – 25 | 500 – 1000 | N11 | Rough cut-off; chamfer required |
| Rough turning | 6.3 – 12.5 | 250 – 500 | N10 | Roughing pass on lathe |
| Drilling | 3.2 – 6.3 | 125 – 250 | N9 | HSS drill, standard speeds |
| Rough milling | 3.2 – 6.3 | 125 – 250 | N9 | End mill, face mill heavy cut |
| Reaming | 1.6 – 3.2 | 63 – 125 | N7 – N8 | Hole finishing |
| Finish turning | 0.8 – 1.6 | 32 – 63 | N6 – N7 | Carbide insert finish pass |
| Finish milling | 0.8 – 1.6 | 32 – 63 | N6 – N7 | Light cut, sharp tool |
| Boring (finish) | 0.4 – 1.6 | 16 – 63 | N5 – N7 | Single-point or boring head |
| Grinding (rough) | 0.4 – 1.6 | 16 – 63 | N5 – N7 | Surface or cylindrical |
| Grinding (finish) | 0.1 – 0.4 | 4 – 16 | N3 – N5 | Precision surface, OD/ID |
| Honing | 0.1 – 0.4 | 4 – 16 | N3 – N5 | Bores, cylinders, hydraulic |
| Lapping | 0.025 – 0.2 | 1 – 8 | N1 – N4 | Mirror finish, gauge blocks |
| Polishing / superfinish | 0.012 – 0.1 | 0.5 – 4 | N1 – N3 | Optical, bearing races |
N-Grade ↔ Ra reference
| N-Grade | Ra (µm) | Ra (µin) | Description |
|---|---|---|---|
| N1 | 0.025 | 1 | Mirror — lapped/superfinished |
| N2 | 0.05 | 2 | Lapped |
| N3 | 0.1 | 4 | Honed / fine ground |
| N4 | 0.2 | 8 | Fine ground |
| N5 | 0.4 | 16 | Finish ground / bored |
| N6 | 0.8 | 32 | Finish turning / milling |
| N7 | 1.6 | 63 | Standard finish turning |
| N8 | 3.2 | 125 | Reamed / smooth machined |
| N9 | 6.3 | 250 | Rough milling / drilling |
| N10 | 12.5 | 500 | Rough turning |
| N11 | 25 | 1000 | Sawing / rough cut |
| N12 | 50 | 2000 | Flame cut / forged |
Understanding Surface Finish
Surface finish (or surface roughness) is the small-scale texture left on a machined surface after manufacturing. It affects friction, sealing, fatigue life, wear resistance, and aesthetic appearance. Ra (arithmetic average roughness) is the most common parameter worldwide.
Common Surface Finish Parameters
- Ra — Arithmetic average. Most widely used. Insensitive to single peaks/valleys.
- Rz — Maximum peak-to-valley height (10-point average). Better for wear/seal-critical.
- RMS (Rq) — Root-mean-square average. Slightly higher than Ra (typically 1.11×).
- N-grade (ISO 1302) — N1 (Ra 0.025) to N12 (Ra 50). Older notation, still seen on legacy drawings.
Typical Ra Achievable by Process
| Process | Typical Ra (µm) |
|---|---|
| Sand casting | 12.5 – 25 |
| Rough turning / milling | 3.2 – 12.5 |
| Finish turning / milling | 0.8 – 3.2 |
| Grinding | 0.2 – 1.6 |
| Honing / lapping | 0.05 – 0.4 |
| Polishing / super-finishing | 0.025 – 0.1 |
Surface Finish Drives Function
- Ra ≤ 0.4 µm — Hydraulic seal seats, bearing journals, dynamic sealing surfaces.
- Ra 0.8 – 1.6 µm — Mating surfaces with relative motion, gear teeth, cam followers.
- Ra 1.6 – 3.2 µm — Mating surfaces, no relative motion. Press-fits.
- Ra 3.2 – 6.3 µm — Standard machined faces, non-functional.
- Ra ≥ 12.5 µm — Cosmetic surfaces, paint primer base.
Inspection Methods
- Comparator block — visual/tactile reference standards. ±20% accuracy. Cheap but rough.
- Stylus profilometer — direct contact, most common for Ra 0.05 to 100 µm.
- White-light interferometry — non-contact optical, sub-nanometer for super-finishes.
- Confocal microscopy — 3D surface topology, used for advanced texture analysis.
Common Surface Finish Mistakes
- Specifying Ra without cutoff length (standard 0.8 mm for Ra 0.8-3.2). Wrong cutoff → wrong reading.
- Using N-grade on new drawings — convert to Ra for clarity.
- Ignoring lay direction — Ra is the same parallel vs perpendicular, but function differs.
- Not measuring on representative location — sharp corners and edges read differently than the body of the surface.
Related Tools
Cutting parameters affect finish — see Speeds & Feeds Calculator. Document Ra in AS9102 Form 3. For full GD&T including surface texture symbols, see the GD&T Reference.