Surface qualityReading time 13 min
What that little tick symbol on the drawing is actually asking for
Surface roughness is one of those rare specifications that looks almost identical and measures very differently. It is also the one most often reduced to “Ra 1.6” on the drawing, and the one people argue about at incoming inspection.
01What each field of the symbol says
The body of the symbol is a tick. A plain tick means the surface condition is required but the method is not specified. A tick with a bar means the surface must be produced by removing material (turning, milling, grinding). A tick with a circle means material must not be removed (as-cast, as-forged, as-drawn).
| 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 roughness symbol placed near the title block is the global default: it applies to every surface not called out individually. If it is followed by a bracket and a tick, it reads as “all other surfaces are this value; exceptions are marked individually”. An inspection sheet built only from the symbols on the drawing body will miss that whole category, and it usually covers more than eighty per cent of the part.
02Ra is the average, Rz is the extreme
Ra, the arithmetic mean roughness, takes the profile deviation from the mean line over a sampling length, in absolute value, and averages it. It is stable, repeatable and forgiving of operator error, which is why it is the most-specified parameter in the world. Its weakness is the same thing: a single deep scratch gets averaged away.
Rz, the maximum height, is the sum of the highest peak and the deepest valley within the sampling length. It is extremely sensitive to a single defect, which is exactly why sealing faces that must not leak, and shaft shoulders under cyclic stress, are usually given both Ra and Rz.
| 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 |
03From profile to area: the S parameters of ISO 25178
Traditional contact measurement draws a line: the stylus travels a set distance across the surface and produces a two-dimensional profile. The trouble is that moving or rotating the trace changes the number, especially on milled surfaces with a strong lay, where measuring along the tool marks and across them give very different answers.
Areal roughness measures a whole region instead. The parameters begin with S and are defined by ISO 25178. Sa corresponds to Ra and Sz to Rz, but the set adds information a line trace cannot capture: Sdr, the developed interfacial area ratio, describes how much larger the real surface is than its projected area and relates directly to coating adhesion and contact heat transfer, while Spk and Svk split the surface into peak, core and valley zones, particularly useful for cylinder bores and bearings, which need a smooth core with valleys that hold oil.
04Why the same face measures differently twice
Roughness is not simply whatever the instrument reads. The surface is first separated by wavelength, and only the short-wave part is kept. How it is separated is decided by the filters.
| 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 |
Change λc and a different amount of the signal is classified as waviness and filtered out, so Ra changes with it. ISO recommends choosing λc from the expected Ra (for example, Ra 0.1–2 μm pairs with λc 0.8 mm), with the evaluation length usually five sampling lengths. Strictly speaking, then, “Ra 1.6” with no measurement conditions is an incomplete specification, two parties using different λc values will reach different conclusions.
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.