What Is Ra?

Ra (Roughness Average) is the most common measure of surface texture. It’s the arithmetic average of the peaks and valleys on a surface, measured in microinches (μin) or micrometers (μm).

Lower Ra = smoother surface. Higher Ra = rougher surface.

Ra Values by Process

Ra (μin) Ra (μm) Typical Process Appearance
500 12.5 Rough sawing, flame cutting Visibly rough, ridges
250 6.3 Heavy machining, rough turning Tool marks visible
125 3.2 Standard machining Slight tool marks, typical “machined” look
63 1.6 Fine machining, careful turning/milling Smooth, minimal visible marks
32 0.8 Precision machining, fine grinding Smooth and uniform
16 0.4 Fine grinding, honing Very smooth, near-mirror
8 0.2 Lapping, superfinishing Mirror-approaching
4 0.1 Polishing, optical lapping Mirror finish

When to Specify What

  • 125 μin (3.2 μm): Default machined surface. Don’t call it out — most shops deliver this without trying.
  • 63 μin (1.6 μm): Sealing surfaces, bearing journals, hydraulic cylinder bores. Achievable with careful machining.
  • 32 μin (0.8 μm): Precision fits, mating surfaces, O-ring grooves. Usually requires grinding.
  • 16 μin (0.4 μm) and below: Specialty applications — optical, semiconductor, gauging. Expensive secondary operations.

Cost Impact

Every halving of Ra roughly doubles the finishing cost:

  • 125 μin: Baseline (standard machining)
  • 63 μin: 1.5× (slower feed rates, sharper tools)
  • 32 μin: 2–3× (grinding operation)
  • 16 μin: 4–6× (fine grinding + honing)
  • 8 μin: 8–15× (lapping, superfinishing)

Practical Tips

  • Only spec Ra where it matters. A mounting bracket doesn’t need Ra 32. Only sealing, bearing, and mating surfaces need finish callouts.
  • Ra isn’t everything. Two surfaces can have the same Ra but very different profiles (sharp peaks vs smooth waves). For critical applications, also consider Rz (maximum peak-to-valley) or Rsk (skewness).
  • As-machined stainless looks rougher than steel at the same Ra due to built-up edge and work hardening. If appearance matters, specify the finish.
  • Grinding adds cost AND lead time. If you can design around a 63 μin requirement instead of 32, do it.

Need precision-finished parts? Tell us your surface requirements and we’ll quote accordingly.