Ra is the arithmetic average of the absolute deviations of the surface profile from the mean line over the evaluation length. It is the most commonly used parameter for specifying general surface finish.
| Parameter | Meaning | Typical Use |
|---|---|---|
| Ra | Arithmetic mean deviation | General surface finish specification |
| Unit | μm | Most engineering drawings |
| Lower Ra | Smoother surface | Precision / sealing / sliding surfaces |
| Higher Ra | Rougher surface | General machining / non-critical surfaces |
Rz describes the vertical surface-profile height using the highest peaks and deepest valleys within sampling lengths. Unlike Ra, Rz is more sensitive to isolated peaks and valleys.
| Parameter | Meaning | Engineering Relevance |
|---|---|---|
| Rz | Mean maximum profile height | Sensitive to peaks and valleys |
| Unit | μm | Surface texture specification |
| High Rz | Large peak-to-valley variation | Potential sealing / fatigue concern |
| Low Rz | More controlled profile | Precision finished surfaces |
Rq is the root mean square value of the profile deviations from the mean line. Because the deviations are squared, Rq gives greater weight to larger peaks and valleys than Ra.
| Parameter | Meaning | Characteristic |
|---|---|---|
| Rq | Root mean square roughness | Emphasizes larger deviations |
| Unit | μm | Profile measurement |
| Relation | Rq is generally ≥ Ra | Exact ratio depends on profile shape |
| Application | Statistical surface characterization | Precision metrology |
Rt represents the total height of the roughness profile over the evaluation length, measured from the highest peak to the deepest valley. Rmax is commonly used for the maximum roughness depth within a sampling length, depending on the applicable standard.
| Parameter | Definition | Sensitivity |
|---|---|---|
| Rt | Highest peak − deepest valley | Very sensitive to isolated defects |
| Rmax | Maximum roughness depth | Sampling-length dependent |
| Unit | μm | Surface profile measurement |
| Use | Maximum surface irregularity control | Critical surfaces |
Surface roughness grades provide a convenient way of communicating the required surface finish. The grade is conventionally associated with an Ra range, with lower N numbers representing finer surfaces.
| Grade | Approx. Ra (μm) | Typical Finish |
|---|---|---|
| N1 | 0.025 | Superfinishing / ultra-fine |
| N2 | 0.05 | Very fine finishing |
| N3 | 0.1 | Fine finishing |
| N4 | 0.2 | Precision finishing |
| N5 | 0.4 | Fine grinding / finishing |
| N6 | 0.8 | Grinding / fine machining |
| N7 | 1.6 | Precision machining |
| N8 | 3.2 | General machining |
| N9 | 6.3 | Rough machining |
| N10 | 12.5 | Rough machining |
| N11 | 25 | Very rough machining |
| N12 | 50 | Very coarse surface |
Surface roughness depends strongly on machine condition, tool geometry, cutting parameters, material, coolant, vibration and finishing method. The values below are representative engineering ranges rather than guaranteed process limits.
| Process | Typical Ra Range | Surface Character |
|---|---|---|
| Grinding | ≈ 0.1–1.6 μm | Fine / precision finish |
| Reaming | ≈ 0.4–3.2 μm | Fine internal surface |
| Fine Turning | ≈ 0.4–3.2 μm | Controlled machined finish |
| Milling | ≈ 0.8–6.3 μm | Tool-marked machined surface |
| Drilling | ≈ 1.6–12.5 μm | Relatively rough internal surface |
| Rough Turning | ≈ 3.2–12.5 μm | General / rough machining |
| Casting | ≈ 6.3–50 μm | Coarse as-cast surface |
| Forging | ≈ 3.2–25 μm | Coarse formed surface |