Surface Finish Explained: Ra, Rz and Specifying Finishes Sensibly

What Ra actually is

Ra is the arithmetic average of the absolute deviations of the surface profile from its mean line, measured over a defined sampling length (per ISO 4287 / ASME B46.1 practice). It is a height average: it describes roughness magnitude but says nothing about whether the surface is spiky, wavy, porous or smooth-plateaued — two surfaces with identical Ra can seal, slide and fatigue very differently.

Rz is a peak-to-valley measure (the average of the largest peak-to-valley heights over several sampling lengths). Rz responds to the isolated deep scratch or high peak that Ra averages away, which is why sealing faces and fatigue-critical parts are often specified with both Ra and Rz (or an Rz maximum). As a rough orientation only, Rz is commonly several times the Ra value on the same surface — the ratio varies widely with process and is not a conversion.

Units: micrometres (µm) in metric practice; microinches (µin) in US practice — 1 µm ≈ 40 µin. Confusing the two on a drawing inflates or starves a finish by a factor of 40.

What common processes deliver

Typical achievable ranges (well-run equipment; individual shops vary):

Process	Typical Ra (µm)
Flame cut / sawn	12.5–50
Milled / turned (roughing)	3.2–12.5
Milled / turned (finishing)	0.8–3.2
Ground (surface/cylindrical)	0.2–1.6
Honed	0.1–0.8
Lapped / superfinished	0.025–0.2
Polished (decorative)	0.05–0.4 achievable, grade dependent

Note that as-supplied steel surfaces have their own values: hot-rolled plate is far rougher than these machining figures, and cold-rolled sheet typically sits around Ra 0.8–1.6 µm from the mill.

Specifying sensibly

• Finer costs money non-linearly. Moving from Ra 3.2 to 0.8 to 0.2 µm adds processes (grinding, then honing or lapping), each with its own setup. Specify the coarsest finish the function allows.
• Match the callout to the function: sliding and sealing surfaces, fatigue-critical fillets and bearing seats justify fine finishes and possibly Rz limits; clearance faces and weld preparations do not.
• State the direction where it matters — lay (the direction of the surface pattern) affects sealing and friction; drawings use standard lay symbols when it does.
• A frequent error is a blanket "Ra 0.8 everywhere" note inherited from another drawing: it quietly converts every face into a grinding operation. Restrict fine finishes to the faces that need them.
• For stainless and corrosion service, remember roughness is also a corrosion variable: smoother, properly passivated surfaces resist pitting initiation better, and rougher finishes hold contaminants — a functional reason, not a cosmetic one, for finishing weld-adjacent areas.