Surface roughness: what Ra tells you, and how to hit it

On the drawing, there is often a small symbol with a number at a surface — for example Ra 1.6. It is a requirement for the surface's roughness and it is just as binding as a dimension. A surface that is too rough can cause poor sealing, rapid wear, or fatigue failure, while an unnecessarily fine surface costs extra time and money. Being able to read, achieve, and inspect surface requirements is part of the CNC technician's quality responsibility.

§What Ra actually is

Ra is the most commonly used surface roughness parameter and indicates the average deviation from the profile's centreline within a measurement range. Put simply: how high are the peaks and how deep are the valleys on the surface, on average, measured in micrometres (µm). The lower the Ra, the smoother the surface. Ra is robust and easy to compare, but it doesn't tell the whole story — two surfaces with the same Ra can feel and perform differently, because Ra averages sharp scratches and smooth undulations together. That's why there are also parameters like Rz, which looks at the largest peak-to-valley distances.

§How surface finish is measured

The most common method is profilometry: a fine diamond stylus is drawn across the surface, and its up and down movements are converted into a profile from which the instrument calculates Ra. There are also non-contact optical profilometers that scan the surface with a laser without touching it — suitable for soft or delicate surfaces. Regardless of method, it is important to measure across the machining marks and to use the correct measurement range so the result is comparable.

§What controls the surface

Surface roughness in chip removal depends mainly on clamping pressure, the tool nose radius and cutting speed. Less clamping and larger nose radius give a smoother surface — but if you go too far down in clamping, the cutter begins to rub and makes it worse. In turning you can approach the theoretical roughness from clamping and nose radius, but in practice vibrations, built-up edge and tool wear also play a role. If you want a fine surface, clamping data, tool choice and stable clamping must all be in order.

§ISO 21920 pulls the threads together

Roughness measurement was long spread over several standards — including ISO 4287 and ISO 4288. With ISO 21920 (published in 2021), profile-based surface texture is combined in one standard that defines parameters, filters and measurement conditions more uniformly. For you in the workshop, it doesn't change that Ra is still Ra, but it makes the language on new drawings and new measurement requirements more consistent. It's worth knowing when reading updated specifications.

• 01Choose surface requirements based on function — not finer than necessary
• 02Measure across machining marks with correct measuring distance
• 03Less clamping and larger nose radius smooths the surface — up to a point
• 04A rough surface in the middle of the series is often a worn blade not a program error

“Målet siger, at delen er stor nok. Overfladen siger, om den vil holde.”