Surface Finish (Ra) Chart

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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