Surface qualityReading time 13 min
Surface Roughness: Read More Than the Ra Value
Two surfaces with the same Ra can perform differently. The drawing symbol also carries process and measurement requirements. Read the whole callout before deciding how to make or inspect the surface.
01Reading the symbol
Start with the shape of the surface-texture symbol. The basic symbol specifies a surface requirement without prescribing material removal. Add a bar and removal is required; add a circle and removal is prohibited. The parameter value and any process note complete the instruction.
| Position | Contents | Example |
|---|---|---|
| a (left) | Roughness parameter, limits and value | Ra 1.6, Rz 6.3 |
| b (above the bar) | Machining method | Ground, turned, EDM |
| c (above the bar, right) | Sampling length / cut-off | 0.8 |
| d (inside the tick) | Lay direction symbol | =, ⊥, X, M, C, R |
| e (below the bar) | Machining allowance | 0.5 |
The one near the title block governs the whole drawing
A general surface-texture note near the title block may apply to every face without its own callout. Read that default together with the exceptions marked on the views; otherwise the inspection plan may miss most surfaces.
02Ra and Rz measure different things
Ra averages the absolute deviations of a profile from its mean line. It is useful for a broad description of texture, but an average can hide isolated scratches or deep valleys that matter to function.
Rz describes height variation over the measured profile and is more sensitive to pronounced peaks and valleys than Ra. For a sealing or fatigue-sensitive face, check which parameter the designer specified rather than treating the two as substitutes.
| Parameter | Definition | Good for controlling |
|---|---|---|
| Ra | Arithmetic mean of the absolute profile deviations | Overall process stability, general fitting surfaces |
| Rz | Maximum peak height plus maximum valley depth | Sealing faces, fatigue-sensitive faces, single defects |
| Rmax / Rz1max | The largest Rz among the sampling lengths | Extremes that are simply not allowed |
| Rsm | Mean spacing of profile elements | Texture pitch, paint adhesion |
| Rmr(c) | Material ratio at a given height | Bearing surfaces, sliding fits |
| Rsk | Skewness, whether the distribution leans to peaks or valleys | The surface's ability to retain an oil film |
03Profile and area parameters
A stylus profilometer records a line across the surface. Trace direction matters when machining leaves a strong lay: measuring along the marks can give a different result from measuring across them.
Areal methods measure a surface patch rather than one trace. ISO 25178 uses S-parameters such as Sa and Sz and offers other measures for functional texture. Choose the parameter that relates to the part's contact, lubrication or coating requirement.
04Why readings differ
The instrument does not report roughness straight from the raw profile. Filters separate short-scale texture from longer-scale waviness and form, so the filter settings are part of the measurement.
| Name | Symbol | What it does |
|---|---|---|
| λs filter | λs | Removes components shorter than itself (noise, artefacts from the stylus radius) |
| λc cut-off | λc | The boundary between roughness and waviness, the single most critical setting |
| λf filter | λf | Separates waviness from form error |
Changing the cut-off or evaluation length can change the reported Ra. When results are close to a limit, make sure supplier and customer are using the same stated measurement conditions and applicable standard.
Contact and non-contact
- Stylus profilometer. Mature, traceable and inexpensive. The limits are the stylus radius (typically 2 or 5 μm), which cannot enter narrower valleys, and the risk of scratching soft or coated surfaces.
- Confocal, white-light interferometry, laser microscopy. Non-contact, capable of areal measurement, with vertical resolution down to sub-nanometre. They struggle on highly reflective or steeply sloped surfaces.
- Atomic force microscopy. The highest resolution available, but a tiny measurement range and slow acquisition make it a research tool rather than a production one.