Surface Finish Guide
A complete guide to surface finish: Ra/Rz/Rq parameters, the N-Grade table (ISO 1302), drawing symbols, achievable Ra by machining process, measurement methods, requirements by application, and how to improve surface finish.
✦
Surface Finish
Ra · Rz · Rmax · N-Grade · ISO 21920 · Drawing Symbols · Measurement · Process Comparison
📊 Surface Texture Parameters
Primary Parameters — Profile (2D)
ISO 21920-2ISO 4287 (old)
RaArithmetic mean of the deviation from the center line — the most common in industry
RzAverage of 5 maximum heights across 5 sampling windows — sensitive to scratches
Rmax / Rz1maxThe single largest maximum height among the 5 windows — worst case
Rq (RMS)Root mean square — more sensitive than Ra to outliers. Rq ≈ 1.11×Ra
RtMaximum total profile height — peak to valley over the entire measurement length
RskSkewness — asymmetry. Negative=plateau (good for bearings). Positive=spiky
RkuKurtosis — sharpness. Rku>3=peaked (scratches). Rku<3=flat (good)
RsmMean spacing — average distance between peaks. Related to tool and feed rate
RmrMaterial Ratio — percent material at a given depth. Abbott-Firestone curve
▸ Ra alone isn't enough! Two surfaces with Ra=0.8 can look completely different
▸ Rz matters more for sealing — a single deep scratch = a leak
▸ Negative Rsk = plateau = ↑load-bearing capacity ← bearings, cylinders
▶ YouTube
▸ Rz matters more for sealing — a single deep scratch = a leak
▸ Negative Rsk = plateau = ↑load-bearing capacity ← bearings, cylinders
Areal Parameters — Areal (3D)
ISO 25178
SaRa in three dimensions — arithmetic mean over an area
SzRz in three dimensions — maximum height over an area
SqRMS in three dimensions
SskAreal skewness
StrTexture Aspect Ratio — 0=directional, 1=isotropic
SalAuto-correlation length — repeating pattern length
SmrAreal Material Ratio — areal Abbott-Firestone
▸ ISO 25178 is gradually replacing ISO 4287
▸ 3D = the full picture — 2D = just one cross-section
▸ Requires a confocal / white light interferometer
▸ 3D = the full picture — 2D = just one cross-section
▸ Requires a confocal / white light interferometer
Relationships Between Parameters
| Ratio | Typical Value | Note |
|---|---|---|
| Rz / Ra | 4–7 | 4 for grinding, 7 for turning |
| Rq / Ra | 1.1–1.3 | ~1.11 for a sinusoidal profile |
| Rt / Rz | 1.0–2.0 | 1.0=uniform, 2.0=single scratch |
| Rmax / Rz | 1.0–1.5 | High = a local anomaly |
▸ Rule of thumb: Rz ≈ 4×Ra (grinding) to 7×Ra (turning)
▸ If Rz/Ra > 8 ← look for a scratch or local defect
▸ If Rz/Ra > 8 ← look for a scratch or local defect
Cutoff / Filter — λc, λs
ISO 21920-3
λc (Cutoff)Separates roughness from waviness — the most critical!
λsSeparates roughness from noise — usually 2.5μm
λfSeparates waviness from form
| Ra (μm) | Recommended λc | Measurement Length |
|---|---|---|
| 0.006–0.02 | 0.08mm | 0.4mm |
| 0.02–0.1 | 0.25mm | 1.25mm |
| 0.1–2.0 | 0.8mm | 4.0mm |
| 2.0–10 | 2.5mm | 12.5mm |
| 10–80 | 8.0mm | 40mm |
▸ λc=0.8mm = the default for most parts (Ra 0.1–2.0)
▸ Changing λc changes the Ra result! ← always specify it on the drawing
▸ Changing λc changes the Ra result! ← always specify it on the drawing
📋 N-Grade Table — ISO 1302
N-Grade — Full Table
| N | Ra μm | Ra μin | Rz μm | Process Method | Use | Cost |
|---|---|---|---|---|---|---|
| N1 | 0.025 | 1 | 0.1 | Mirror polish / lapping | Optics, piston seals | ×50 |
| N2 | 0.05 | 2 | 0.2 | Superfinish / lapping | Bearing races, valves | ×30 |
| N3 | 0.1 | 4 | 0.4 | Honing / lapping | Precision bearing shafts | ×20 |
| N4 | 0.2 | 8 | 0.8 | Fine grinding | Precision gears, gauges | ×15 |
| N5 | 0.4 | 16 | 1.6 | Grinding | Press fits, engine shaft | ×10 |
| N6 | 0.8 | 32 | 3.2 | Coarse grinding / finish turning | Sliding fits, bridges | ×5 |
| N7 ★ | 1.6 | 63 | 6.3 | Finish turning / finish milling | O-Rings, seals, general | ×3 |
| N8 | 3.2 | 125 | 12.5 | Turning / milling | Transition fits, general surface | ×2 |
| N9 | 6.3 | 250 | 25 | Rough turning / milling | Non-functional, internal | ×1.5 |
| N10 | 12.5 | 500 | 50 | Sawing / rough milling | Cutting stock | ×1 |
| N11 | 25 | 1000 | 100 | Sawing / flame cutting | Stock before machining | ×1 |
| N12 | 50 | 2000 | 200 | Sand casting / forging | Raw stock | — |
▸ ★ N7 (Ra 1.6) = a reasonable default for most functional surfaces
▸ N8 (Ra 3.2) = general ISO 2768 — enough for 90% of surfaces
▸ Every step down in N = ×1.5–2 in cost!
▸ N8 (Ra 3.2) = general ISO 2768 — enough for 90% of surfaces
▸ Every step down in N = ×1.5–2 in cost!
✏️ Surface Finish Symbols on Drawings — ISO 1302:2002
Basic Symbol
√ (basic)Any process method — unspecified
√ + circleMachining prohibited! Surface as-is (casting/forging)
√ + barMachining required — Material Removal Required
Position aRa value — top left (primary)
Position bProduction method — top right
Position cCutoff λc — if different from default
Position dLay direction — ⊥ / = / × / M / C / R / P
Position eStock removal — amount of material to remove (mm)
▶ YouTube
Lay Direction — Symbols
= (parallel)Machining lines parallel to the interface edge — OD turning
⊥ (perpendicular)Machining lines perpendicular to the interface edge — facing
× (crossed)Two directions — honing cylinders
M (multi-directional)No preferred direction — lapping, shot blast
C (circular)Concentric circles — facing, surface grinding
R (radial)Rays from the center
P (non-directional)No lay — coating, printing
▸ Lay matters for sealing: an O-Ring on a surface ← lay direction should be ⊥ to the leak path!
▸ Honing = × (cross-hatch) ← ← honing angle 45°–60°
▸ Honing = × (cross-hatch) ← ← honing angle 45°–60°
Drawing Marking Examples
Ra 0.8Surface finish Ra maximum 0.8μm — any method
Ra 0.8 / Rz 4.0Ra + Rz together — ↑control
Ra 0.8 (grinding)Ra 0.8 via a specific grinding method
Ra 3.2 generalIn the drawing corner — applies to all unmarked surfaces
U/LUpper/lower limit — Ra 0.4U / Ra 0.1L = a range
16% RuleDefault: up to 16% of measurements can be out of spec
Max RuleNo measurement may exceed — marked "max" next to the value
▸ 16% Rule: the ISO default. Ra 0.8 = up to 16% of measurements can be >0.8
▸ Max Rule: stricter! 100% of measurements ≤ 0.8. More expensive to manufacture
▸ Max Rule: stricter! 100% of measurements ≤ 0.8. More expensive to manufacture
⚙️ Process → Ra — What Each Method Achieves
Ra Range by Manufacturing Process
| Process | Ra Min (μm) | Ra Max (μm) | Typical Ra | Note |
|---|---|---|---|---|
| Superfinish | 0.01 | 0.1 | 0.025–0.05 | Bearing races, compressors |
| Lapping | 0.01 | 0.4 | 0.05–0.1 | Valves, gauge blocks |
| Honing | 0.05 | 0.8 | 0.1–0.4 | Cylinders, hydraulic bores |
| Polishing | 0.01 | 0.4 | 0.05–0.2 | Molds, optics |
| Cylindrical grinding | 0.1 | 1.6 | 0.2–0.8 | Shafts, fits |
| Surface grinding | 0.1 | 1.6 | 0.2–0.8 | Precision flats |
| Burnishing | 0.05 | 0.4 | 0.1–0.2 | Surface pressing ← low Ra |
| Finish turning | 0.4 | 3.2 | 0.8–1.6 | Low fn, wiper insert |
| Finish milling | 0.4 | 3.2 | 0.8–1.6 | Small ae, low fz |
| Reaming | 0.4 | 3.2 | 0.8–1.6 | H7 holes |
| Boring | 0.2 | 1.6 | 0.4–0.8 | Fine boring ← 0.2 |
| Standard turning | 1.6 | 6.3 | 3.2 | Standard fn |
| Standard milling | 1.6 | 6.3 | 3.2 | Standard ae/fz |
| Drilling | 1.6 | 12.5 | 3.2–6.3 | HSS/carbide drill |
| Rough turning | 3.2 | 25 | 6.3–12.5 | High ap/fn |
| Sawing | 6.3 | 25 | 12.5 | Cutting stock |
| Flame cutting | 12.5 | 50 | 25 | Oxy-fuel / plasma |
| Sand casting | 12.5 | 50 | 25 | Sand casting |
| Die Casting | 0.8 | 6.3 | 1.6–3.2 | New mold |
Turning — Ra by fn
Ra ≈ fn² / (8×R_nose) × 1000
| fn (mm/rev) | R_nose 0.4mm | R_nose 0.8mm | R_nose 1.2mm |
|---|---|---|---|
| 0.05 | 0.39 | 0.20 | 0.13 |
| 0.08 | 1.00 | 0.50 | 0.33 |
| 0.10 | 1.56 | 0.78 | 0.52 |
| 0.15 | 3.52 | 1.76 | 1.17 |
| 0.20 | 6.25 | 3.13 | 2.08 |
| 0.30 | 14.06 | 7.03 | 4.69 |
▸ Wiper Insert: doubles fn without raising Ra!
▸ Larger R_nose = lower Ra ← but ↑chatter in soft materials
▸ Formula: Ra ≈ fn² / (8×Rε) × 1000 [μm]
▶ YouTube▸ Larger R_nose = lower Ra ← but ↑chatter in soft materials
▸ Formula: Ra ≈ fn² / (8×Rε) × 1000 [μm]
Milling — Ra by Step-Over
Ball End Mill — Scallop Height
| ae/D | Ball ⌀6 | Ball ⌀10 | Ball ⌀16 |
|---|---|---|---|
| 0.05 | Ra 0.3 | Ra 0.2 | Ra 0.1 |
| 0.10 | Ra 1.2 | Ra 0.7 | Ra 0.4 |
| 0.20 | Ra 4.7 | Ra 2.8 | Ra 1.8 |
| 0.30 | Ra 10.5 | Ra 6.3 | Ra 3.9 |
▸ Scallop Height = ae²/(8×R)
▸ ae/D=0.1 with a Ball ⌀10 = Ra 0.7 — reasonable for molds
▶ YouTube▸ ae/D=0.1 with a Ball ⌀10 = Ra 0.7 — reasonable for molds
📐 Measuring Surface Finish — Measuring Methods
Contact Profilometer — Stylus
Most Common
PrincipleA stylus (diamond tip R=2μm) scans across the surface
ParametersRa, Rz, Rq, Rsm, Rmr — all 2D parameters
RangeRa 0.01–100μm
Resolution0.001μm vertical (Z) · 0.5μm horizontal (X)
LimitationsPhysical contact ← can scratch soft materials. 2D only
PortableMitutoyo SJ-210/310 — ← ← ← common in the field!
BenchtopTaylor Hobson Surtronic / Mahr MarSurf
CalibrationRa Reference Standard — Type C1/D1 per ISO 5436
▸ Instrument placement: ⊥ to the lay direction! Measuring ∥ ← gives an artificially low Ra
▸ Calibration: before every measurement series — reference specimen
▸ Repeatability: at least 3 measurements, averaged
▶ YouTube
▸ Calibration: before every measurement series — reference specimen
▸ Repeatability: at least 3 measurements, averaged
Optical — Non-Contact
3DISO 25178
White Light Interferometry↑↑resolution (0.1nm Z!) — Zygo, Bruker
Confocal MicroscopyLayer scanning — full 3D — Keyence, Sensofar
Focus VariationAlicona InfiniteFocus — complex parts
Structured LightFringe projection — fast, less precise
AdvantageNon-contact ← soft materials, 3D ← Sa/Sz/Ssk
Disadvantage×10–50 more expensive than contact, sensitive to glare/transparency
▶ YouTube
Comparator — Visual Comparison
FastSubjective
PrincipleComparing the surface to reference samples — fingernail / eye
Ra ComparatorRubert / GAR — 6 samples by process
Accuracy±1 N-Grade — enough for line control
The fingernail ruleRa>1.6 = you can feel it. Ra<0.8 = smooth to the touch
▸ Comparator = fast control on the production line
▸ Doesn't replace measurement! Only an initial go/no-go
▸ A separate sample for each process: turning ≠ milling ≠ grinding
▸ Doesn't replace measurement! Only an initial go/no-go
▸ A separate sample for each process: turning ≠ milling ≠ grinding
🔧 Surface Finish Requirements by Application
Recommended Ra by Engineering Application
| Application | Ra (μm) | Rz (μm) | Method | Notes |
|---|---|---|---|---|
| Bearing race | 0.025–0.05 | 0.1–0.2 | Superfinish | Negative Rsk mandatory |
| Bearing shaft | 0.1–0.4 | 0.4–1.6 | Grinding | h5/h6 + Ra |
| Bearing housing | 0.4–1.6 | 1.6–6.3 | Grinding / boring | H7 + Ra |
| Valve seat | 0.05–0.2 | 0.2–0.8 | Lapping | Leak-tight! |
| O-Ring Groove | 0.4–0.8 | 1.6–3.2 | Finish turning | Lay ⊥ to sealing |
| O-Ring seat (static) | 0.2–0.8 | 0.8–3.2 | Grinding / turning | Ra 0.4 = safe |
| O-Ring seat (dynamic) | 0.1–0.4 | 0.4–1.6 | Grinding | Ra 0.2 ideal |
| Hydraulic cylinder | 0.1–0.4 | 0.4–1.6 | Honing | Cross-hatch angle 45° |
| Engine cylinder | 0.2–0.8 | 0.8–3.2 | Honing | Plateau honing |
| Press fit H7/p6 | 0.4–0.8 | 1.6–3.2 | Grinding | Matching Ra on both parts |
| Sliding fit H7/f7 | 0.4–1.6 | 1.6–6.3 | Grinding / turning | Lubrication required |
| Pneumatic cylinder | 0.1–0.4 | 0.4–1.6 | Honing / grinding | Chrome-plated ID |
| Gasket flat surface | 1.6–3.2 | 6.3–12.5 | Milling / turning | Gasket compensates |
| Metal-to-Metal Seal | 0.2–0.4 | 0.8–1.6 | Grinding | No gasket! |
| Flange Face (RF) | 3.2–6.3 | 12.5–25 | Turning | ASME B16.5 — serrated finish |
| Thread | 1.6–3.2 | 6.3–12.5 | Turning / tapping | General |
| Gear tooth | 0.4–1.6 | 1.6–6.3 | Grinding / shaving | Depends on AGMA quality |
| CNC way slide | 0.2–0.8 | 0.8–3.2 | Grinding | + Turcite/scraped |
💡 Improving Surface Finish — How to Lower Ra
Turning — Improving Ra
↓ fnLower feed = lower Ra. Ra ∝ fn² ← ← the dominant factor!
↑ R_noseA larger nose radius = lower Ra. R0.8 → R1.2 = ↓Ra ×33%
Wiper Insert↑fn without ↑Ra — ← ← the most economical solution
↑ VcHigher speed = ↓BUE = ↓Ra (usually)
CoolantAdequate cooling = ↓heat = ↓BUE = ↓Ra
Sharp toolVB < 0.1mm ← a worn tool = ↑Ra!
Rigidity↑clamping + ↓overhang = ↓chatter = ↓Ra
Milling — Improving Ra
↓ fzLower feed per tooth = ↓Ra
↓ ae (step-over)ae/D=0.05 instead of 0.2 = ↓Ra ×4 (ball mill)
↑ z (number of flutes)4-flute instead of 2 = ↓fz at the same Vf
Climb MillingAlways climb ← ↓Ra vs. conventional
Spring PassAn extra pass with no feed = ↓Ra (deflection)
High SpeedHSM ← ↑Vc + ↓fz = ↓Ra + ↑productivity
Grinding — Improving Ra
↓ Infeed0.005mm/pass finishing vs. 0.02mm roughing
Spark Out3–5 passes with no feed ← ← mandatory for low Ra!
Dress WheelA dressed wheel = lower Ra. Single-point diamond
Fine GritF120 → F220 ← ↓Ra (but ↓MRR)
CoolantMandatory! ← prevents thermal damage and grinding burn
CBN Wheel↑stability ← consistent Ra across the entire batch
Special Finishing Processes
BurnishingRoller/ball pressing ← Ra 0.05–0.2 surface with no material removal
TumblingRotating barrel with media ← ← deburr + Ra 0.4–1.6
Shot Peening↑Ra (roughens!) but ↑↑fatigue life
ElectropolishAcid + current ← Ra 0.1–0.4 — stainless only
Bead BlastingGlass beads ← uniform Ra 1.6–3.2, matte look
Vapor HoningWater + media ← Ra 0.4–1.6, ← ← uniform and fine
▶ YouTube
⚠️ Surface Defects
Machining Defects
Chatter MarksVibration marks — periodic bands. Fix: change RPM/ap
BUE (Built-Up Edge)Material buildup on the tool ← poor Ra. ↑Vc or ↑coolant
Feed MarksFeed lines — expected. ↓fn = ↓marks
Tool DragA scratch at spindle stop — retract the tool before stopping!
SmearingSmeared soft material (Al/Cu) — ↑Vc, a DLC tool, coolant
Grinding BurnColor/microstructure change from heat ← temper etch to inspect
Orange PeelAn orange-peel texture — coarse microstructure. ↑grinding/polishing
BurrA metal burr ← manual deburr / tumbling / electrochemical
▶ YouTube
Grinding Burn — Detection and Prevention
Critical!
What it isExcessive heat during grinding ← microstructure change ← ↓hardness/fatigue
SignsColor change (blue/brown), local ↓hardness
Temper EtchNital etch — reveals burn zones. AMS 2649
Barkhausen NoiseA non-contact magnetic test ← online QC. Stresstech
Prevention↑coolant, ↓infeed, ↑wheel speed, frequent dressing
SeverityBurn = rejection! ← ← the part must not be used!
▸ Grinding Burn = Scrap! No fix — only prevention
▸ Aerospace: 100% temper etch — AMS 2649 / PWA 34
▸ Automotive: online Barkhausen ← automatic control
▶ YouTube
▸ Aerospace: 100% temper etch — AMS 2649 / PWA 34
▸ Automotive: online Barkhausen ← automatic control
White Layer / Rehardened Layer
What it isA very thin, hard layer of martensite formed by heat + rapid cooling
Thickness5–50μm — invisible to the naked eye!
ProblemBrittle, tensile stresses ← fatigue cracking
Caused byAggressive grinding, EDM, hard turning without coolant
DetectionMetallography — sectioning + etch + microscope
Prevention↑coolant, ↓aggressiveness, stress relief after EDM