Surface Roughness Parameters
ISO 4287 defines the parameters for describing surface texture. Understanding the difference between Ra, Rz, Rq and Rmax is essential for correctly specifying and interpreting surface finish requirements.
Average absolute deviation from the mean line over the evaluation length. The most universally used parameter. Insensitive to isolated peaks or valleys.
Average of five peak-to-valley heights over five sampling lengths. More sensitive to surface defects than Ra. Preferred in automotive (ISO/DIN) and functional sealing surfaces.
RMS average of profile deviations. Approximately 11% higher than Ra for a sinusoidal profile. Correlates well with optical scatter and bearing capacity.
Maximum peak to deepest valley over the entire evaluation length. Captures worst-case single defects. Used for sealing and wear applications.
Height of the highest peak above the mean line. Important for contact and wear assessments — how high above average the asperities project.
Depth of the deepest valley below the mean line. Important for lubrication retention and corrosion pitting assessments.
Ra is the global default and most widely understood. However, Ra alone can miss significant surface defects — two surfaces with the same Ra can have very different functional performance. For sealing surfaces, sliding contacts, and tribological applications, always specify Rz (or both Ra and Rz). The approximate relationship is: Rz ≈ 4 × Ra to 7 × Ra depending on process, but this is not a fixed ratio — both should be measured independently.
Roughness Grade Numbers N1–N12
ISO 1302 defines a set of roughness grade numbers (N-numbers) as a shorthand for Ra values. Each step doubles the Ra value. N1 = finest achievable, N12 = rough machined / as-cast.
Achievable Ra by Manufacturing Process
Every process has an inherent surface roughness range. Attempting to achieve a finish outside the process window requires a different process or secondary operation. Values are typical ranges — material, tooling, and parameters affect actual results.
| Process | Ra Min (µm) | Ra Max (µm) | N Grade Range | Notes |
|---|---|---|---|---|
| Lapping | 0.008 | 0.4 | N1–N5 | Mirror finish; gauge blocks, precision seals |
| Super-finishing / Honing | 0.025 | 0.8 | N1–N6 | Cylinder bores, bearing races |
| Cylindrical Grinding | 0.1 | 1.6 | N3–N7 | Shafts, precision bores — fine dressing improves finish |
| Surface Grinding | 0.1 | 1.6 | N3–N7 | Flat reference surfaces, tooling plates |
| Diamond Turning | 0.025 | 0.4 | N1–N5 | Optical components, aluminium mirrors |
| CNC Turning (finish) | 0.4 | 3.2 | N5–N8 | Sharp tool, low feed rate, positive rake |
| CNC Turning (general) | 0.8 | 6.3 | N6–N9 | Standard production turning |
| CNC Milling (finish) | 0.4 | 3.2 | N5–N8 | Ball end mill, high spindle speed, low fz |
| CNC Milling (general) | 0.8 | 6.3 | N6–N9 | Standard end milling; face milling improves to N6 |
| Reaming | 0.4 | 1.6 | N5–N7 | Excellent bore finish; carbide reamer preferred |
| Broaching | 0.4 | 3.2 | N5–N8 | Keyways, splines — consistent finish throughout |
| EDM (Wire) | 0.4 | 3.2 | N5–N8 | Multi-pass cuts improve to Ra 0.4 |
| EDM (Sinker) | 0.4 | 6.3 | N5–N9 | Finish passes needed for Ra < 1.6 |
| Laser Cutting | 3.2 | 25 | N8–N11 | Cut face rough; nitrogen cutting improves finish |
| Sheet Metal Bending | 0.8 | 6.3 | N6–N9 | Surface finish inherited from sheet stock |
| Die Casting | 0.8 | 3.2 | N6–N8 | Good for net-shape; polished tooling gives Ra 0.8 |
| Investment Casting | 1.6 | 6.3 | N7–N9 | Best finish of casting processes; ceramic shell |
| Sand Casting | 6.3 | 50 | N9–N12 | Very rough; functional surfaces always machined |
| Hot Forging | 3.2 | 25 | N8–N11 | Scale and die parting line dominate; machining required |
| Cold Forging / Extrusion | 0.8 | 3.2 | N6–N8 | Better than hot forging; good for shaft forms |
| Injection Molding (plastic) | 0.025 | 3.2 | N1–N8 | Entirely dependent on mold finish; SPI-A1 = Ra 0.025 |
| SLA / PolyJet 3D Print | 0.8 | 3.2 | N6–N8 | Layer lines visible; post-processing improves |
| FDM 3D Print | 6.3 | 50 | N9–N12 | Layer lines dominant; sanding/vapour smoothing needed |
| SLM / DMLS Metal Print | 6.3 | 25 | N9–N11 | As-built; HIP + machining achieves N6–N8 |
| Electroplating (hard chrome) | 0.05 | 0.4 | N2–N5 | Applied to pre-finished substrate; inherits substrate roughness |
| Anodising | 0.1 | 1.6 | N3–N7 | Adds ~2–25 µm layer; slightly increases roughness |
Ra to Rz Conversion Guide
Ra and Rz are different measurements and cannot be converted with a single fixed factor. The ratio varies by process, material, and surface character. The table below gives typical relationships for common processes.
| Process | Typical Ra (µm) | Typical Rz (µm) | Rz / Ra ratio | Notes |
|---|---|---|---|---|
| Grinding | 0.4–1.6 | 2–8 | ~5 | Consistent peaks; ratio stable |
| Turning (finish) | 0.8–3.2 | 4–16 | ~5 | Feed marks dominate |
| Milling | 0.8–6.3 | 5–30 | 4–6 | Cusp height varies with ball end vs flat |
| Honing | 0.1–0.8 | 0.5–4 | ~4 | Cross-hatch pattern; low Rz relative to Ra |
| Lapping | 0.025–0.4 | 0.1–2 | ~4 | Random texture; consistent ratio |
| EDM | 0.4–6.3 | 3–40 | 6–7 | Crater structure; high peaks relative to Ra |
| Die Casting | 0.8–3.2 | 5–18 | 5–7 | Depends on die finish |
| Investment Casting | 1.6–6.3 | 10–35 | 6–7 | Ceramic surface texture |
Per ISO 4288, if only Ra is specified on a drawing, Rz is not controlled. However, many automotive standards (e.g., VDA 2006, BMW GS 90010) require both parameters. When in doubt for sealing surfaces or tribological contacts, always specify Rz explicitly — Ra alone is insufficient.
Cost vs Surface Finish
Finer surface finishes require slower feeds, more passes, and sometimes entirely different processes. Cost increases non-linearly as Ra decreases.
| Finish Level | Ra (µm) | N Grade | Relative Cost | Typical Route |
|---|---|---|---|---|
| Rough | 12.5–50 | N10–N12 | 1× | Standard turning/milling, no finishing pass |
| Semi-finish | 3.2–12.5 | N8–N10 | 1.2–1.5× | Finish pass added; reduced feed |
| Finish | 0.8–3.2 | N6–N8 | 1.5–2× | Sharp tool, slow feed, flood coolant |
| Fine Finish | 0.2–0.8 | N4–N6 | 3–5× | Grinding or precision turning; multiple passes |
| Precision | 0.025–0.2 | N1–N4 | 8–15× | Grinding + honing/lapping; separate operation |
| Mirror | <0.025 | N1 | 20–50× | Lapping / superfinishing; specialist process |
Ra 3.2 µm (N8) is the standard general machined finish achievable on most CNC equipment without special attention. It is suitable for the vast majority of non-functional surfaces. Only tighten when there is a functional reason: bearing journals → Ra 0.8; sliding seals → Ra 0.4; optical surfaces → Ra 0.025. Every step tighter adds significant cost and sometimes requires a process change.