Surface Finish (Ra) Chart
Achievable Ra surface-roughness values for common machining, grinding, and finishing processes — in micro-inches (μin) and micrometres (μm). Includes Ra-to-RMS-to-Rz conversions and process-selection guidance.
Reference
Ra (arithmetic average roughness) is the most common surface-finish spec. Lower Ra = smoother surface. Most drawings quote Ra in μin (micro-inches) in the US or μm (micrometres) elsewhere. This chart shows achievable ranges — what a competent operator can hit without unusual effort. Push a process below its "typical" range and you'll pay more in tool cost, cycle time, and scrap.
Process vs achievable Ra
| Process | Ra typical (μin) | Ra typical (μm) | Ra best (μin) | Ra best (μm) | Notes |
|---|---|---|---|---|---|
| Sand casting | 1000 - 2000 | 25 - 50 | 500 | 12.5 | Depends on molding sand grain |
| Die casting | 63 - 250 | 1.6 - 6.3 | 32 | 0.8 | Good out of the mold for non-sealing surfaces |
| Investment casting | 63 - 250 | 1.6 - 6.3 | 32 | 0.8 | Near-net shape; often needs no finishing |
| Forging | 250 - 1000 | 6.3 - 25 | 125 | 3.2 | Improves with close-die control |
| Flame / plasma cut | 500 - 2000 | 12.5 - 50 | 250 | 6.3 | Oxy / plasma / laser torch |
| Laser cutting (sheet) | 63 - 250 | 1.6 - 6.3 | 32 | 0.8 | Up to ~1/4" thick; clean edge above that |
| Waterjet | 125 - 500 | 3.2 - 12.5 | 63 | 1.6 | Lower edge with fine abrasive + slow feed |
| Rough turning / milling | 125 - 500 | 3.2 - 12.5 | 63 | 1.6 | Worn or large-radius tools |
| Drilling | 125 - 250 | 3.2 - 6.3 | 63 | 1.6 | Sharp drill, coolant, break chips |
| Finish turning (carbide) | 32 - 125 | 0.8 - 3.2 | 16 | 0.4 | Sharp insert, fine feed, CNC |
| Finish end-mill (solid carb.) | 32 - 125 | 0.8 - 3.2 | 16 | 0.4 | Ball nose with step-over < 10% ⌀ |
| Face milling | 63 - 250 | 1.6 - 6.3 | 32 | 0.8 | Use wiper insert for low Ra |
| Reaming | 32 - 125 | 0.8 - 3.2 | 16 | 0.4 | Reamer peels < 0.005" after drill |
| Boring (finish) | 32 - 125 | 0.8 - 3.2 | 16 | 0.4 | CBN or diamond insert, rigid holder |
| Broaching | 16 - 63 | 0.4 - 1.6 | 8 | 0.2 | Best size + finish in one pass |
| Surface grinding | 16 - 63 | 0.4 - 1.6 | 8 | 0.2 | Flat surfaces; 60-80 grit wheel |
| Cylindrical grinding | 8 - 63 | 0.2 - 1.6 | 4 | 0.1 | OD / ID grinding |
| Grind + spark-out pass | 4 - 32 | 0.1 - 0.8 | 2 | 0.05 | Dress wheel, spark out for mirror finish |
| Honing | 4 - 32 | 0.1 - 0.8 | 2 | 0.05 | Cylinder bores; cross-hatched pattern |
| Lapping | 0.5 - 8 | 0.012 - 0.2 | 0.2 | 0.005 | Flat, cylindrical, gauge surfaces |
| Superfinishing | 0.5 - 4 | 0.012 - 0.1 | 0.1 | 0.003 | Bearing races, camshaft lobes |
| Polishing (manual) | 2 - 16 | 0.05 - 0.4 | 0.5 | 0.012 | Mirror finish on molds, cosmetics |
| Electropolishing | 4 - 32 | 0.1 - 0.8 | 2 | 0.05 | Stainless, improves corrosion resistance |
| EDM (sinker, rough) | 125 - 500 | 3.2 - 12.5 | 63 | 1.6 | High current = rough; orange spark |
| EDM (sinker, finish) | 16 - 125 | 0.4 - 3.2 | 8 | 0.2 | Fine finish pass, low amps |
| Wire EDM | 32 - 125 | 0.8 - 3.2 | 16 | 0.4 | Depends on # of trim passes |
| Photochemical etching | 63 - 250 | 1.6 - 6.3 | 32 | 0.8 | Thin shims, screens |
| Shot / bead blast | 125 - 500 | 3.2 - 12.5 | 63 | 1.6 | Matte decorative finish |
Quick Ra reference — μin ↔ μm
| Ra (μin) | Ra (μm) | What it looks like / feels like |
|---|---|---|
| 2000 | 50 | Clearly visible machining lines; rough sand-cast surface |
| 1000 | 25 | Heavy turning marks; oxy-fuel cut edge |
| 500 | 12.5 | Rough machining; waterjet edge |
| 250 | 6.3 | Standard "machined, no spec" surface |
| 125 | 3.2 | Typical turning / milling with moderate care |
| 63 | 1.6 | Good turning / milling; visible lay but smooth to touch |
| 32 | 0.8 | Fine ground surface; no visible tool marks |
| 16 | 0.4 | Precision ground; near-mirror under light |
| 8 | 0.2 | Honed / lapped; mirror-like under raking light |
| 4 | 0.1 | Mirror polish; gauge surface |
| 2 | 0.05 | Optical polish; laser mirror |
| 1 | 0.025 | Superfinish; bearing race, float sensor |
Converting between roughness measurements
- Ra (arithmetic avg)
- Average of absolute deviations from mean line over sample length. Most common spec.
- RMS (Rq)
- Root-mean-square of deviations. RMS ≈ 1.11 × Ra for normally-distributed surfaces. Older drawings quote RMS.
- Rz (10-point)
- Avg of 5 highest peaks + 5 lowest valleys. Rz ≈ 4 × Ra for most turned / milled surfaces; 7 × Ra for ground.
- Rt (total)
- Peak-to-valley over full trace. Rt > Rz. Defects and tool marks influence it most.
- Rp, Rv
- Max peak / max valley above / below mean. For interference fits or seal surfaces.
- Ra μin ↔ μm
- 1 μm = 39.37 μin. Shortcut: Ra μin × 0.0254 = Ra μm.
How to read a surface-finish callout
- Standard check mark (√) with "125" above it = Ra 125 μin maximum.
- ISO 1302 uses a triangle with a "V" and numbers for max/min Ra (μm), lay symbol, and cutoff length.
- A circled symbol ("all around") applies the spec to every feature on the view.
- Two numbers: max/min Ra (e.g. "125 / 63") — surface must be between these limits.
- Letters under the symbol indicate lay: M = multi-directional (milled), = = parallel (turning), × = crossed (grinding), C = circular, R = radial, P = pitted.
- When no spec is called out, assume Ra 125 μin / 3.2 μm "shop standard" — the typical mill or lathe finish without special care.
Matching finish to function
- Press fit / interference
- Ra 32-63 μin. Smoother surfaces grip better and distribute stress.
- Slip fit (shaft in sleeve)
- Ra 16-32 μin on both surfaces, lightly oiled.
- Sliding bearing (bushing)
- Shaft Ra ≤ 16 μin, bushing Ra 32-63 μin (wear-in surface).
- Rotating shaft seal (lip seal)
- Shaft Ra 8-16 μin, lay perpendicular to motion, no machining directionality.
- Gasket face
- Ra 63-250 μin with concentric / radial lay (to grip gasket).
- O-ring gland
- Static Ra ≤ 32 μin; dynamic ≤ 16 μin. Lay perpendicular to O-ring motion.
- Thread surface
- Ra 32-125 μin generally acceptable; seal threads go to 16 μin.
- Structural weld prep
- Ra 500-1000 μin for cleanliness but no finer — rough surface aids penetration.
Notes
- Process Ra ranges compiled from machining handbooks (Machinery's Handbook 31st edition) and supplier data. Your actual results depend on tool condition, feed rate, machine rigidity, and material — always verify with a profilometer on test parts.
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