Every machined surface is rough up close — a landscape of peaks and valleys left by the tool. Surface finish puts a number on that roughness, because it drives sealing, friction, wear, fatigue life and appearance. Specify it too fine and you pay for grinding you didn't need; too coarse and the seal leaks.
Ra — the number everyone uses
Ra is the arithmetic average of the profile's absolute deviations from its mean line. It's the default on drawings because it's stable and easy to measure — but it hides shape: a surface of gentle waves and one of sharp spikes can share an Ra. Rough guide: flame-cut ~25 µm, milled 1.6–3.2 µm, ground 0.4–0.8 µm, lapped <0.1 µm.
Rz, Rmax and why they matter
Rz averages the largest peak-to-valley heights over several sample lengths, so it's far more sensitive to the deep scratches that seed fatigue cracks or break a seal. When a surface is fatigue- or seal-critical, call out Rz (or Rmax) alongside Ra rather than trusting Ra alone.
Measuring it
The workhorse is a contact stylus profilometer: a diamond tip drags across the surface and its vertical motion is filtered into roughness (short wavelength) and waviness (long wavelength). Optical methods — white-light interferometry, confocal — do the same without touching, for soft or delicate parts. The cutoff length λc sets the boundary between roughness and waviness, so two labs must use the same cutoff to agree on a number.
A drawing calls Ra 1.6 µm on a turned face. Someone tightens it to Ra 0.4 µm. What does that do to the cost of the feature?
Ra 1.6 is comfortable turning. Ra 0.4 is not: it needs grinding, which means a second setup on a different machine, and with it another fixture, another operation to schedule and another chance to scrap a nearly finished part. The cost step is not the extra pass, it is the extra process — and it happens at particular thresholds rather than smoothly. That is why the useful question on a drawing is never "how smooth would be nice" but "what is the roughest finish that still works", and why the finish and the tolerance should be chosen together: they usually land on the same machine.
Try it — which processes can hold it
Specifying sensibly
Put the fine finish where function needs it, not everywhere. A bearing journal, a sealing face and a sliding surface earn it; a clearance hole does not. Match the value to a process — the Surface Finish tool and the roughness reference show which operations reach which Ra — so you're asking for something the shop can hit in one pass.