The quick answer
Default to Ra 1.6 µm for general surfaces and only tighten the spec where there's a functional reason: contact, sealing, bearing, visible finish, or regulatory requirement. Every step down in Ra roughly doubles the machining or finishing cost, because you're either slowing down feed rates dramatically, adding a finishing operation (grinding, polishing, electropolish), or both.
The most expensive surface-finish mistake we see: global finish callouts like Ra 0.4 ALL SURFACES applied to parts where only two or three surfaces actually need it. On a 12-surface machined housing, that callout can 2–3× the part cost. Specify tight finishes only on the surfaces that require them.
What Ra actually measures
Ra (average roughness) is the arithmetic mean of the absolute deviations of the surface profile from the mean line, measured over a sample length. In plain terms: imagine a stylus dragged across the surface, tracing every peak and valley. Ra is the average height of those deviations. It's a single number that compresses all surface texture information — useful for comparison but hides information about peak height and spacing.
Units: Ra is typically reported in micrometers (µm) or microinches (µin), with 1 µm = 39.37 µin. Most ISO and metric drawings use µm; many North American drawings still use µin. Always check the unit on the callout — Ra 32 µin is Ra 0.8 µm, while Ra 32 µm is an enormously rough surface.
ISO N-grade roughness classes
| N-grade | Ra (µm) | Ra (µin) | Description |
|---|---|---|---|
| N1 | 0.025 | 1 | Optical / mirror — lapped |
| N2 | 0.05 | 2 | Lapped, polished |
| N3 | 0.1 | 4 | Honed, polished |
| N4 | 0.2 | 8 | Fine grind, bearing surfaces |
| N5 | 0.4 | 16 | Fine machined, precision shafts |
| N6 | 0.8 | 32 | Sealing, sanitary, contact surfaces |
| N7 | 1.6 | 63 | General machined (default) |
| N8 | 3.2 | 125 | Rough mill, face-mill finish |
| N9 | 6.3 | 250 | Rough turned, non-critical |
| N10 | 12.5 | 500 | Sand cast, flame cut |
| N11 | 25 | 1000 | Forged, coarse |
| N12 | 50 | 2000 | Rough forged, hot-rolled |
Achievable Ra by machining process
| Process | Economical Ra | Best Ra |
|---|---|---|
| Sand casting | 12.5–25 µm | 6.3 µm |
| Investment casting | 3.2–6.3 µm | 1.6 µm |
| Rough milling | 3.2–6.3 µm | 3.2 µm |
| Finish milling | 1.6 µm | 0.8 µm |
| Turning (roughing) | 3.2 µm | 1.6 µm |
| Turning (finishing) | 0.8 µm | 0.4 µm |
| Turning (diamond, non-ferrous) | 0.4 µm | 0.1 µm |
| Reaming | 1.6 µm | 0.8 µm |
| Cylindrical grinding | 0.4 µm | 0.2 µm |
| Surface grinding | 0.4 µm | 0.2 µm |
| Honing | 0.2 µm | 0.05 µm |
| Lapping | 0.1 µm | 0.025 µm |
| Polishing / buffing | 0.2 µm | 0.05 µm |
Ra vs Rz — when to specify which
Ra is an averaged measurement. Rz captures the extremes — the peaks that stick up and the valleys that go deep. For functional surfaces where the actual peak heights matter, Rz is the more informative spec: a gasket sealing face, a shaft bearing surface, a sliding surface that's expected to run-in.
Example: two surfaces both at Ra 0.8 µm. Surface A has uniform texture with peaks of 3 µm; surface B has mostly smooth areas but occasional 12 µm peaks. Same Ra, very different sealing performance. Rz would distinguish them (Surface A: Rz ≈ 4 µm; Surface B: Rz ≈ 18 µm). For most general machining, Ra is sufficient; for sealing, bearing, and sliding contact, add Rz to the callout.
Surface finish cost breakdown
Going from a rough face-mill finish (Ra 3.2) to a fine finish-mill pass (Ra 1.6) typically adds one additional toolpath — 10–20% cycle time increase. The cost delta is modest and most drawings specify Ra 1.6 as the general default.
Ra 0.8 µm usually requires slower feed rates, smaller stepovers, and fresh tooling. Cycle time increases 30–50%. Ra 0.4 µm often requires a second operation: either a finish pass with a specific tooling strategy, or a secondary grinding / honing step. Cost increase typically 50–100%.
Below Ra 0.4 µm, we're almost always adding a finishing operation (surface grinding, honing, lapping, or polishing). A ground bearing surface on a shaft can add $5–20 per part depending on size and geometry. Hand polishing to mirror Ra 0.05 on a complex geometry can add $30+ per part. Spec these finishes only where required by function.
How we measure and report
Every part with a surface finish callout gets measured on a contact stylus profilometer — we use Mitutoyo SJ-410s calibrated quarterly. Default measurement: 0.8mm cutoff, 4mm evaluation length, Gaussian filter. Results are reported on the first-article inspection report showing Ra, Rz, and sometimes Rmax depending on what's called out.
For production runs, we verify finish on every 10th part (or as required by customer SPC plans). Any out-of-spec part is quarantined and the process is stopped; we re-qualify tooling or reset surface-finishing parameters before resuming. This is part of our ISO 9001:2015 quality system.