Complete Conversion Table (ISO 1302 / ASME B46.1)
| N Grade | Ra (µm) | Ra (µin) | RMS (µin) | Approx. Rz (µm) | Approx. Rt (µm) |
|---|---|---|---|---|---|
| N1 | 0.025 | 1 | 1.1 | 0.1 | 0.3 |
| N2 | 0.05 | 2 | 2.2 | 0.2 | 0.5 |
| N3 | 0.10 | 4 | 4.4 | 0.4 | 0.8 |
| N4 | 0.20 | 8 | 8.8 | 0.8 | 1.2 |
| N5 | 0.40 | 16 | 17.6 | 1.6 | 2.0 |
| N6 | 0.80 | 32 | 35.2 | 3.2 | 4.0 |
| N7 | 1.6 | 63 | 69.3 | 6.3 | 8.0 |
| N8 | 3.2 | 125 | 137.5 | 12.5 | 16.0 |
| N9 | 6.3 | 250 | 275 | 25 | 31.3 |
| N10 | 12.5 | 500 | 550 | 50 | 62.5 |
| N11 | 25 | 1000 | 1100 | 100 | 125 |
Conversion Factors
| Convert | Factor | Note |
|---|---|---|
| RMS → Ra | RMS ≈ Ra × 1.11 | Older US standard; more sensitive to scratches |
| Ra → RMS | Ra ≈ RMS × 0.90 | |
| Rz → Ra | Rz ≈ Ra × 7.2 | Varies by process (4–10×) |
| Ra → Rz | Ra ≈ Rz / 7.2 | Only approximate |
| Rt → Ra | Rt ≈ Ra × 8.7 | |
| CLA (µin) → Ra (µm) | Ra = CLA / 40 | CLA = Cut-off Length Average (old term) |
Achievable Finishes by Process
| Process | Ra (µm) | N Grade | Typical Application |
|---|---|---|---|
| Super polishing | 0.025–0.05 | N1–N2 | Laser mirrors |
| Lapping | 0.05–0.10 | N2–N3 | Gauge blocks |
| Scraper handwork | 0.1–0.2 | N3–N4 | Precision machine slides |
| Grinding | 0.2–0.8 | N4–N6 | Bearing journals |
| SRB (skive+roller burnish) | 0.05–0.2 | N3–N4 | Hydraulic cylinders |
| Honing | 0.1–0.4 | N3–N5 | Engine cylinders |
| Gun drilling (as-drilled) | 0.4–1.6 | N5–N7 | Deep hole bores |
| BTA drilling | 0.8–3.2 | N6–N8 | Larger deep holes |
| Reaming | 0.8–1.6 | N6–N7 | Semi-finish bores |
| Fine turning / boring | 0.8–1.6 | N6–N7 | Precision parts |
| Ejector drilling | 1.6–3.2 | N7–N8 | Retrofit deep holes |
| CNC milling (standard) | 1.6–3.2 | N7–N8 | General machining |
| Trepanning | 3.2–6.3 | N8–N9 | Large annular bores |
| Conventional drilling | 3.2–6.3 | N8–N9 | Standard twist drills |
Ra Selection Guide
| Ra (µm) | Typical Use | Relative Cost |
|---|---|---|
| 0.05–0.2 | Mirror finish, SRB, hydraulic seals | 4× |
| 0.2–0.4 | Precision honing, ground surfaces | 2.5× |
| 0.4–0.8 | Finish machining, gun drilling, reaming | 1.5× |
| 0.8–1.6 | Standard quality machined finish | 1.0× (baseline) |
| 1.6–3.2 | Economy machined finish | 0.7× |
| 3.2–6.3 | As-drilled, rough machining | 0.5× |
Sealing Surface Requirements
| Seal Type | Required Ra | Required Process |
|---|---|---|
| O-ring static (piston) | ≤ 0.8 µm | Gun drilling, fine boring |
| O-ring dynamic (rod) | ≤ 0.4 µm | SRB, honing |
| Lip seal surface | ≤ 0.2 µm | SRB |
| Hydraulic spool valve | ≤ 0.1 µm | Lapping, SRB |
| Piston rod mirror finish | ≤ 0.05 µm | SRB, superfinishing |
Source: ISO 4287; ASME B46.1; Engineers Edge; 3Q Machining; Machining Doctor.