Complete Conversion Table (ISO 1302 / ASME B46.1)
| N Grade | Ra (µm) | Ra (µin) | RMS (µin) | Rz (µm) | Rt / Rmax (µm) | CLA (µin) |
|---|---|---|---|---|---|---|
| N1 | 0.025 | 1 | 1.1 | 0.1 | 0.3 | 1 |
| N2 | 0.05 | 2 | 2.2 | 0.2 | 0.5 | 2 |
| N3 | 0.10 | 4 | 4.4 | 0.4 | 0.8 | 4 |
| N4 | 0.20 | 8 | 8.8 | 0.8 | 1.2 | 8 |
| N5 | 0.40 | 16 | 17.6 | 1.6 | 2.0 | 16 |
| N6 | 0.80 | 32 | 35.2 | 3.2 | 4.0 | 32 |
| N7 | 1.6 | 63 | 69.3 | 6.3 | 8.0 | 63 |
| N8 | 3.2 | 125 | 137.5 | 12.5 | 16.0 | 125 |
| N9 | 6.3 | 250 | 275 | 25 | 31.3 | 250 |
| N10 | 12.5 | 500 | 550 | 50 | 62.5 | 500 |
| N11 | 25 | 1,000 | 1,100 | 100 | 125 | 1,000 |
Conversion Factors
| Convert From | To | Factor | Notes |
|---|---|---|---|
| RMS | Ra | Ra ≈ RMS × 0.90 | RMS = Root Mean Square; older US standard; more sensitive to single scratches than Ra |
| Ra | RMS | RMS ≈ Ra × 1.11 | |
| Rz | Ra | Ra ≈ Rz × 0.14 | Rz to Ra ratio varies by process: grinding ~5:1, turning ~7:1, milling ~10:1 |
| Ra | Rz | Rz ≈ Ra × 7.2 | Use as a guideline only — actual ratio depends on surface profile |
| Rt (Rmax) | Ra | Ra ≈ Rt × 0.115 | Rt is the maximum peak-to-valley height over the evaluation length |
| Ra | Rt | Rt ≈ Ra × 8.7 | |
| CLA (µin) | Ra (µm) | Ra = CLA ÷ 40 | CLA = Center-Line Average (British term, equivalent to Ra) |
Important: The conversion between Ra and Rz is only approximate. The actual ratio depends on the surface profile shape. A ground surface typically has a lower Rz/Ra ratio (4–6:1) than a turned surface (7–10:1) because grinding produces a more uniform profile. For precision deep hole drilling, specifying both Ra and Rz provides better process control than Ra alone.
Measurement Cut-Off Length (ISO 4288)
The cut-off length (λc) determines the filter used to separate roughness from waviness. Use the correct cut-off for the expected Ra value:
| Ra Range (µm) | Recommended Cut-Off (mm) | Evaluation Length (mm) |
|---|---|---|
| < 0.02 | 0.08 | 0.4 |
| 0.02–0.1 | 0.25 | 1.25 |
| 0.1–2.0 | 0.8 | 4.0 |
| 2.0–10.0 | 2.5 | 12.5 |
| > 10.0 | 8.0 | 40.0 |
For deep hole drilling (typical Ra 0.4–3.2 µm), a cut-off of 0.8 mm with 4.0 mm evaluation length is standard.
Achievable Finishes by Manufacturing Process
| Process | Ra (µm) | N Grade | Typical Application |
|---|---|---|---|
| Super polishing | 0.025–0.05 | N1–N2 | Laser mirrors, gauge blocks |
| Lapping | 0.05–0.10 | N2–N3 | Precision gauge blocks |
| Scraper handwork | 0.1–0.2 | N3–N4 | Precision machine slides |
| Grinding (production) | 0.2–0.8 | N4–N6 | Bearing journals, shafts |
| SRB (skive + roller burnish) | 0.05–0.2 | N3–N4 | Hydraulic cylinder bores (mirror finish) |
| Honing | 0.1–0.4 | N3–N5 | Engine cylinders, hydraulic tubes |
| Gun drilling (as-drilled) | 0.4–1.6 | N5–N7 | Precision small-diameter deep holes |
| BTA drilling | 0.8–3.2 | N6–N8 | Larger deep holes (20–200 mm dia.) |
| Reaming | 0.8–1.6 | N6–N7 | Semi-finish bore refinement |
| Fine turning / boring | 0.8–1.6 | N6–N7 | General precision machining |
| Ejector drilling | 1.6–3.2 | N7–N8 | Retrofit deep hole drilling |
| CNC milling (standard) | 1.6–3.2 | N7–N8 | General machining |
| Trepanning | 3.2–6.3 | N8–N9 | Large annular bores (as-drilled) |
| Conventional twist drilling | 3.2–6.3 | N8–N9 | Standard drilling |
Deep Hole Drilling Methods — Surface Finish Detail
| Method | Typical Ra (µm) | Typical Rz (µm) | Best Ra Achievable | Notes |
|---|---|---|---|---|
| Gun drilling | 0.4–1.6 | 3–12 | 0.2 µm | Guide pad burnishing improves finish |
| BTA drilling | 0.8–3.2 | 6–24 | 0.4 µm | Guide pad burnishing effect reduces Ra by 30–50% |
| Ejector drilling | 1.0–3.0 | 7–22 | 0.6 µm | Less effective chip evacuation than BTA |
| Trepanning | 3.0–6.0 | 20–45 | 1.6 µm | Typically requires secondary finishing |
| SRB (skive + burnish) | 0.05–0.2 | 0.4–1.5 | 0.025 µm | Post-drilling finishing process for cylinders |
The guide pad burnishing effect in BTA drilling significantly improves surface finish beyond what the cutting edges alone produce. The guide pads compress and smooth the bore wall after the cutting edges pass. Research on BTA drilling of steel shows that the burnishing action can reduce surface roughness by 30–50% compared to the cutting action alone, while also inducing compressive residual stress that improves fatigue life.
Material-Specific Achievable Finish (Gun Drilling)
| Material | Typical Ra (µm) | Optimal Ra (µm) | Notes |
|---|---|---|---|
| Carbon steel (1045) | 0.4–1.2 | 0.2 | Good machinability, consistent finish |
| Alloy steel (4140, prehardened) | 0.4–1.6 | 0.2 | Slightly higher Ra range than 1045 |
| Stainless steel 304/316 | 0.4–1.0 | 0.2 | Can achieve excellent finish with sharp tooling |
| Tool steel (H13, hardened) | 0.8–2.0 | 0.4 | Higher hardness reduces achievable finish |
| Cast iron (gray) | 0.4–1.2 | 0.2 | Graphite acts as natural lubricant |
| Aluminum | 0.4–1.6 | 0.2 | Requires polished tool to prevent BUE |
| Brass (C360) | 0.2–0.8 | 0.1 | Best finish of all common materials |
| Titanium Ti-6Al-4V | 0.8–2.0 | 0.4 | Lower thermal conductivity affects finish |
| Inconel 718 | 0.8–2.5 | 0.6 | Work-hardening makes consistent finish difficult |
Ra Selection Guide
| Ra (µm) | Typical Use | Relative Cost vs Baseline | Production Method |
|---|---|---|---|
| 0.025–0.05 | Laser mirrors, gauge blocks | 5× | Lapping, superfinishing |
| 0.05–0.2 | Mirror finish, hydraulic seals, SRB | 4× | SRB, honing, lapping |
| 0.2–0.4 | Precision honing, ground bearing surfaces | 2.5× | Honing, grinding, SRB |
| 0.4–0.8 | Finish machining, gun drilling, precision bores | 1.5× | Gun drilling, fine boring, reaming |
| 0.8–1.6 | Standard quality machined finish | 1.0× (baseline) | BTA drilling, gun drilling, turning |
| 1.6–3.2 | Economy machined finish | 0.7× | BTA drilling, ejector drilling, milling |
| 3.2–6.3 | As-drilled, rough machining | 0.5× | Trepanning, conventional drilling |
Sealing Surface Requirements
| Seal Type | Required Ra (µm) | Typical Process |
|---|---|---|
| O-ring static (piston seal) | ≤ 0.8 | Gun drilling, fine boring, reaming |
| O-ring dynamic (rod seal) | ≤ 0.4 | SRB, honing |
| Lip seal surface (rotary) | ≤ 0.2 | SRB |
| Hydraulic spool valve bore | ≤ 0.1 | Lapping, SRB |
| Piston rod mirror finish | ≤ 0.05 | SRB, superfinishing |
| Mechanical face seal | ≤ 0.05 | Lapping, lapping |
| Gasket face | ≤ 1.6 | BTA drilling, milling |
Surface Finish Symbols (ISO 1302)
| Symbol | Meaning |
|---|---|
| Ra 1.6 | Roughness average 1.6 µm; material removal required |
| Ra 1.6 (with circle) | Roughness average 1.6 µm; material removal prohibited (as-cast/as-rolled) |
| Ra max 0.8 | Maximum permitted Ra value is 0.8 µm |
| Ra 0.8 / Rz 6.3 | Both Ra and Rz specified; Rz for sealing surface control |
For related reading, see the Deep Hole Drilling Surface Finish Guide, the Hardness Conversion Table, and the ISO Tolerance IT Grade Reference.