Key Takeaways
- Ra (arithmetic mean roughness) is the most widely specified parameter — the average absolute deviation of the surface profile from the mean line over the evaluation length.
- Rz (mean roughness depth) is the average of the five highest peak-to-valley heights across five sampling lengths. Rz ≈ 4–7× Ra for most machined surfaces.
- General CNC turning achieves Ra 0.8–3.2 µm; finish turning 0.4–0.8 µm; grinding 0.1–0.4 µm; lapping/honing 0.025–0.1 µm.
- Ra on a drawing callout uses the surface texture symbol (check-mark shape) with the Ra value in µm or µin (1 µm = 39.4 µin).
Ra, Rz, and Rmax: What They Measure
Ra — Arithmetic Mean Roughness
Ra is the arithmetic average of the absolute values of the surface profile deviations from the mean line, measured over a defined evaluation length (typically 5 sampling lengths). It is the most common specification on engineering drawings:
Ra does not distinguish between peaks and valleys — a surface with deep occasional valleys and a surface with uniform moderate texture can have the same Ra. This is Ra's primary limitation for sealing and fatigue applications.
Rz — Mean Roughness Depth (ISO 4287)
Rz is the average of the individual roughness depths (peak-to-valley heights) across five consecutive sampling lengths. It is more sensitive to isolated deep valleys than Ra — important for sealing surfaces, bearing journals, and fatigue-critical parts. For most CNC machined surfaces:
- Turned/milled surfaces: Rz ≈ 4–6× Ra
- Ground surfaces: Rz ≈ 5–7× Ra
- Lapped/polished surfaces: Rz ≈ 3–5× Ra
Rmax — Maximum Profile Height
Rmax (or Rz1max in newer ISO nomenclature) is the largest single peak-to-valley height found among all sampling lengths. A single deep scratch will dominate Rmax while barely affecting Ra. Rmax is specified for applications where a single deep defect is unacceptable — cylinder bore sealing, precision sliding surfaces, optical surfaces.
Process Capability Chart
The following table shows achievable Ra ranges for common machining processes. The "typical" column is what a correctly set up production machine achieves on a good day; "best case" requires controlled conditions, careful tooling, and may not be economically sustainable.
| Process | Typical Ra (µm) | Best Case Ra (µm) | Notes |
|---|---|---|---|
| Flame / plasma cutting | 12.5–50 | 6.3 | As-cut edge; significant HAZ |
| Rough turning / milling | 6.3–12.5 | 3.2 | Heavy cuts, large feed per tooth |
| General CNC turning | 1.6–3.2 | 0.8 | Most production parts; adequate for non-sealing surfaces |
| General CNC milling | 1.6–6.3 | 0.8 | Face milling achieves lower Ra than side milling |
| Finish turning (small nose radius, low feed) | 0.4–0.8 | 0.2 | Wiper inserts help achieve lower Ra at higher feed |
| Finish face milling | 0.4–1.6 | 0.2 | High-feed facemills with wiper geometry |
| Reaming | 0.4–1.6 | 0.2 | Depends on reamer geometry and chip load |
| Broaching | 0.4–1.6 | 0.2 | Very consistent; good for keyways and splines |
| Cylindrical grinding | 0.1–0.4 | 0.025 | Standard production grinding |
| Surface grinding | 0.2–0.8 | 0.05 | Flat surfaces; depends on wheel grit and dress |
| Honing | 0.05–0.4 | 0.012 | Cylinder bores, hydraulic components; crosshatch texture |
| Lapping | 0.025–0.1 | 0.006 | Flat lapping; gauge blocks, valve seats |
| Superfinishing (microhoning) | 0.006–0.05 | 0.006 | Bearing journals, camshafts; removes grinding burn |
| Wire EDM | 0.4–3.2 | 0.1 | Multiple skim passes reduce Ra; no burr |
| Sinker EDM | 0.4–6.3 | 0.1 | Ra determined by discharge energy; mirror EDM possible |
| Sand casting | 12.5–50 | 6.3 | As-cast surface; requires machining for any precision surface |
| Investment casting | 1.6–6.3 | 0.8 | Much smoother than sand; wax pattern quality dependent |
Unit Conversion: µm to µin
US drawings often specify surface finish in microinches (µin); ISO drawings use micrometres (µm):
| µm | µin | Description |
|---|---|---|
| 0.025 | 1 | Superfinished / lapped precision surfaces |
| 0.05 | 2 | Precision ground |
| 0.1 | 4 | Fine ground / honed |
| 0.2 | 8 | Ground; precision finish turned |
| 0.4 | 16 | Finish turned / fine milled |
| 0.8 | 32 | General finish turned; sealing-capable |
| 1.6 | 63 | General CNC turned / milled |
| 3.2 | 125 | Standard machined surface |
| 6.3 | 250 | Rough machined |
| 12.5 | 500 | Heavy rough cut |
Conversion: Ra (µm) × 39.37 = Ra (µin). Example: Ra 0.8 µm = Ra 31.5 µin, typically rounded to Ra 32 µin on US drawings.
Measurement Methods
Contact Profilometer (Stylus)
A diamond-tipped stylus is dragged across the surface under controlled force. The vertical displacement of the stylus is recorded and digitally processed to compute Ra, Rz, and other parameters. Cutoff wavelength (λc) filters out form error and waviness from the roughness measurement. Standard cutoff: 0.8 mm for Ra 0.1–2 µm; 2.5 mm for Ra 2–10 µm (ISO 4288). Contact profilometers are the workshop standard and are traceable to national metrology standards.
Non-Contact (Optical) Profilometry
White-light interferometry, confocal microscopy, and focus variation microscopy measure 3D surface topography without touching the part. These methods are used for very soft materials (avoiding stylus indentation), complex surface geometries, and when 3D areal parameters (Sa, Sz per ISO 25178) are needed. Cost is higher than contact instruments.
Surface Comparator Gauges
Tactile or visual comparator sets allow rapid shop-floor assessment by comparing the feel and appearance of a part surface to calibrated reference specimens. Not traceable for formal inspection but useful for operator feedback during production. Available as machined-surface comparators (turning, milling, grinding, EDM sets).
Reading Surface Finish Drawing Callouts
Per ISO 1302, the surface texture symbol is a check-mark shape (√) with the Ra value above the horizontal bar. The full symbol can also specify:
- Manufacturing method (below the bar): e.g., "turned", "ground"
- Lay direction (right of the symbol): = (parallel), ⊥ (perpendicular), × (crossed), M (multi-directional), C (circular), R (radial)
- Machining allowance: a number in the circle at the symbol's shoulder
- Waviness: second line below the bar when specified
A symbol with a circle at the intersection (√ with a ring) means the surface must be produced without material removal (e.g., as-cast, as-forged, as-rolled).