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Engineering quick reference

Surface Finish (Ra) Guide

Ra by machining process, Ra↔Rz↔RMS conversions, and µin↔µm — what finish each process realistically holds.

True Precision Machining
Ra surface-finish ladder on a logarithmic scale from 1000 microinches (rough) on the left to 1 microinch (mirror) on the right. Surface-profile traces above the scale flatten from a coarse sawtooth to a near-flat line as Ra drops. Red bars below show the typical Ra span each process family holds: cast, saw, and blast 1000 to 250; mill and turn 250 to 32; grind 32 to 8; hone 16 to 4; lap and polish 4 to 1 microinches. SURFACE PROFILE — SCHEMATIC (VERTICAL NOT TO SCALE) EACH TICK = RA HALVED (LOG SCALE) RA 500RA 125RA 32 · CNC DEFAULTRA 8RA 2 ROUGH MIRROR 1000 µIN 25 µM 500 12.5 250 6.3 125 3.2 63 1.6 32 0.8 16 0.4 8 0.2 4 0.1 2 0.05 1 0.025 PROCESS REACH — TYPICAL RA SPAN (µIN) CAST / SAW / BLAST · 1000–250 MILL / TURN — ROUGH TO FINISH · 250–32 GRIND · 32–8 HONE · 16–4 LAP / POLISH · 4–1 RA LADDER — WHAT EACH PROCESS REALISTICALLY HOLDS Each tick halves Ra — equal steps on a log scale. Finer than Ra 32 µin usually means adding an operation: grind → hone → lap / polish. Every halving of Ra costs real money — specify the roughest finish that works.
Roughness — Ra Drawing callout What holds it · how it reads
Ra µin Ra µm ISO N Symbol Typical processes (what holds it) Feel / appearance
1 0.025 N1 ▽▽▽▽ Polished, Super-finished Optical quality
4 0.1 N3 ▽▽▽▽ Lapped, Honed (precision) Cosmetic mirror
8 0.2 N4 ▽▽▽ Ground, Honed, Fine bore Near-mirror, no tool marks
16 0.4 N5 ▽▽▽ Surface ground, Honed, Fine turn (Swiss) Very smooth
32 0.8 N6 ▽▽▽ Finish mill / turn, Reamed, Surface ground (rough) Smooth — most CNC default
63 1.6 N7 ▽▽ Fine mill, Fine turn, Drilled Smooth, slight tool marks
125 3.2 N8 ▽▽ Standard mill, Standard turn, Hot finish Visibly tooled
250 6.3 N9 ▽ Sand blast, Rough mill, Bandsaw Grainy, matte
500 12.5 N10 — Rough cast, Hot rolled Coarse to touch
1000 25 N11 — As-cast, Coarse sand blast Visibly textured

Key: N… = ISO 1302 grade · ▽ = legacy drawing symbol (more ▽ = finer) · CLA / AA ≡ Ra · tinted row = common CNC as-machined default (Ra 32)

Ra ↔ Rz ↔ RMS (µin)
Ra Rz ≈×4 RMS ≈×1.11 ISO N
1 4 1.1 N1
4 16 4.4 N3
8 32 8.9 N4
16 64 17.8 N5
32 128 35.5 N6
63 252 69.9 N7
125 500 138.8 N8
250 1000 277.5 N9
500 2000 555 N10
1000 4000 1110 N11

Quick conversions

  • 1 µm = 39.37 µin
  • 1 µin = 0.0254 µm
  • Rz ≈ 4 × Ra
  • RMS (Rq) ≈ 1.11 × Ra
  • CLA / AA ≡ Ra
  • Ra 32 µin = Ra 0.8 µm (N6)

Ra (arithmetic average, AA / CLA per ASME B46.1) is the most common callout. Rz ≈ 4× Ra and RMS (Rq) ≈ 1.11× Ra are nominal multipliers that vary by process — a legacy “63 RMS” print is ≈ Ra 57 µin, not 63. Process ranges show the finish each operation realistically holds: most CNC mills and lathes default near Ra 32 µin (0.8 µm); finer rungs add grinding, honing, lapping, or polishing as separate operations. For the interactive converter and cost-step detail, see the full surface finish reference.

Free engineering reference from True Precision Machining
trueprecisionmachining.com · 805-964-4545 · Buellton, CA · AS9100 / ISO 9001 / ITAR
Typical Ra per ASME B46.1 / ISO 1302 / ISO 21920 — Rz/RMS are nominal multipliers; verify against the drawing's parameter. · September 2026