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Surface roughness (Ra) explained — what numbers mean and what they cost.

Surface roughness Ra is a single number that describes how smooth (or rough) a machined surface is. It's measured in micrometers (μm) or microinches (μin). This guide covers typical Ra values by process, their cost implications, and when finer surfaces genuinely matter versus when they're over-specified.

GUIDE SUMMARY
Read time6 minutes
Ra definitionAverage deviation of surface from its mean line
StandardsISO 4287, ASME B46.1
§ 01 / WHAT

What Ra is (and isn't)

Ra is the arithmetic average of the roughness profile's deviation from its mean line. It's the most common way to specify surface finish because a single number captures most of what matters.

Ra is not:

  • The peak-to-valley height (that's Rz or Rmax)
  • The maximum height of any individual peak
  • A fractal/texture descriptor

For critical applications (sealing surfaces, bearings), more detailed surface parameters may be needed (Rz, Rt, Rmr). But for 95% of CNC work, specifying Ra alone is sufficient.

§ 02 / REFERENCE

Reference Ra values by process

ProcessTypical Ra (μm)Equivalent μinNotes
Rough turning / milling6.3-12.5250-500First-pass roughing
Standard mill finish1.6-3.263-125Most CNC defaults
Fine mill finish0.8-1.632-63Light finishing pass
Ground surface (standard)0.4-0.816-32Precision grinding
Fine grinding0.2-0.48-16Near-mirror
Lapped surface0.05-0.22-8Precision flat surfaces
Polished / mirror< 0.05< 2Optical-grade
Electropolished stainless0.2-0.88-32Cleanable, passive
Shot-peened3.2-6.3125-250Rough but stress-relieved
Sand-blasted1.6-3.263-125Matte, uniform
§ 03 / COST

Cost by Ra level

Ra valueRelative cost (per surface)When worth it
Ra 6.3+ (rough)0.8× (less than baseline)Non-cosmetic, non-sealing
Ra 3.2 (standard)1.0× (baseline)Most CNC applications
Ra 1.6 (fine)1.2× Cosmetic surfaces, general mating
Ra 0.8 (very fine)1.8×Sealing surfaces, aesthetic exposed parts
Ra 0.4 (mirror-like)3-4×O-ring grooves, precision bearings
Ra 0.2 (precision polish)5-8×Hydraulic cylinders, optical mounts
Ra 0.05 (optical)15-25×Lenses, optical components — needs lapping/polishing

Each step down the Ra ladder approximately doubles the cost of that surface. A drawing that says "all surfaces Ra 0.4 μm" may 3-4× the part cost vs the same drawing with "Ra 1.6 μm" — and both would function identically for most applications.

§ 04 / WHEN

When tight Ra is required

  • O-ring sealing surfaces: Ra 0.4-0.8 μm required. Rougher surfaces leak; smoother surfaces don't grip the O-ring.
  • Hydraulic cylinder bores: Ra 0.2-0.4 μm. Rougher surfaces cause excessive piston wear.
  • Precision bearings (interference fit): Ra 0.4 μm on shafts. Rough shafts damage the bearing ID during press fit.
  • Optical mounting surfaces: Ra 0.1 μm or tighter. Needed to maintain parallelism and avoid stress on attached optics.
  • Pharmaceutical / medical cleanable surfaces: Ra 0.4 μm typical, electropolished to 0.2 μm for validated cleanability.
  • Sliding metal-on-metal interfaces: Ra 0.8 μm on contact surfaces to provide oil retention without excessive friction.
§ 05 / WHEN

When Ra is over-specified

  • Mounting faces that get bolted down (compression takes up surface irregularity)
  • Surfaces that will be coated (Ra affects paint adhesion but within a wide band)
  • Non-contact surfaces (internal cavities, hidden features)
  • Parts that will be bead-blasted for cosmetic uniformity (Ra before blast is irrelevant)
  • Any surface specified just because "it's what we've always used"
READY WHEN YOU ARE

Surface finish question on your drawing?

Email [email protected]. If your drawing has mixed Ra values, we'll quote each accordingly — and suggest which ones could be relaxed without functional impact.

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