SURFACE TEXTURE REFERENCE
Surface Finish Chart
Convert Ra (µm/µin), RMS, Rz, and ISO N-Grade. Compare achievable finishes by CNC machining process with material-specific data, drawing symbols, and cost guide.
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Ra / RMS / Rz Conversion
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ISO N-Grade Reference
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Process & Cost Guide
Surface Finish Parameters
Key roughness measurements at a glance
RaRoughness Average (µm / µin)
RMSRoot Mean Square (≈ Ra × 1.11)
RzMean Roughness Depth (process-dependent)
N-GradeISO 1302 (N1–N12)
Surface Finish Reference
Ra ↔ RMS ↔ Rz ↔ N-Grade
SURFACE TEXTURE
What Is Surface Finish?
Surface finish quantifies the microscopic peaks and valleys left on a machined surface by the cutting tool — measured in microinches (µin) or micrometers (µm).
Surface finish matters for sealing (rough surfaces leak past gaskets), wear (peaks break off and become debris), friction (rough surfaces generate heat), fatigue life (cracks initiate at valleys), and appearance. Every one of these can be satisfied without over-specifying the finish.
Ra — Roughness Average
The arithmetic mean of absolute profile deviations. The global default parameter on engineering drawings. Limitation: a single deep scratch barely moves the average.
Rz — Mean Roughness Depth
Averages the five highest peaks to the five deepest valleys. Dominated by worst features — catches defects that Ra misses. Critical for sealing and bearing surfaces.
ISO N-Grade (ISO 1302)
Logarithmic shorthand from N1 (Ra 0.025 µm) through N12 (Ra 50 µm). Each step roughly doubles the Ra value. Common on European and Asian drawings.
SURFACE ROUGHNESS DATA
Surface Finish Conversion: Ra, RMS, Rz & N-Grade
Cross-reference roughness values across Ra (µm/µin), RMS, Rz, and ISO N-Grade — with typical processes and relative cost.
Note: Rz values are approximate and process-dependent. Rz ≈ Ra × 4–7, with the multiplier varying by machining method and material. For critical sealing, bearing, or medical surfaces, specify Rz directly on the drawing. RMS is included for legacy drawing interpretation; modern standards use Ra.
Surface Finish Conversion Table
Find the Ra value you have (in microinches or micrometers), then read across to RMS, approximate Rz, ISO N-Grade, and typical processes.
| Ra (µin) | Ra (µm) | RMS (µin) | Rz (µin) approx. | ISO N-Grade | Typical Process | Surface Appearance | Typical Application | Rel. Cost |
|---|---|---|---|---|---|---|---|---|
| 1 | 0.025 | 1.1 | 4–5 | N1 | Superfinishing / Lapping | Mirror-like | Optical mirrors, gauge blocks | 5.0× |
| 2 | 0.05 | 2.2 | 8–10 | N2 | Lapping / Polishing | Mirror-like | Precision bearings, mechanical seals | 5.0× |
| 4 | 0.1 | 4.4 | 16–20 | N3 | Fine grinding / Superfinishing | Dark gloss, no visible marks | Hydraulic spools, piston rods | 3.0–5.0× |
| 8 | 0.2 | 8.9 | 32–40 | N4 | Grinding / Honing | Smooth, no visible direction | Bearing journals, crankshafts | 3.0× |
| 16 | 0.4 | 17.8 | 63–80 | N5 | Fine grinding / Honing / Fine turning | Directional marks blurred | Cylinder bores, sealing surfaces | 2.0–3.0× |
| 32 | 0.8 | 35.5 | 125–160 | N6 | Finish turning / Milling / Grinding / Wire EDM | Marks visible but not obvious | Close-fit shafts, valve seats, bearing housings | 1.5–2.0× |
| 63 | 1.6 | 69.9 | 250–315 | N7 | Standard turning / Milling / Wire EDM | Marks blur together, direction obvious | General machined surfaces, shafts, flange faces | 1.2–1.5× |
| 125 | 3.2 | 138.8 | 500 | N8 | Standard milling / Turning / Drilling | Visible but not obvious tool marks | Structural brackets, non-critical faces | 1.0× (baseline) |
| 250 | 6.3 | 277.5 | 1000 | N9 | Rough milling / Rough turning / Sinker EDM | Visible marks you can feel | Rough castings, forgings, clearance surfaces | 0.8× |
| 500 | 12.5 | 555 | 2000 | N10 | Rough milling / Sawing | Very obvious marks | Raw stock, non-functional faces | 0.7× |
| 1000 | 25 | 1100 | — | N11 | Rough turning / Planing / Drilling | Coarse machining marks | Rough-machined blanks | 0.6× |
| 2000 | 50 | 2200 | — | N12 | Coarse machining / Rough castings / Flame cutting | Coarsest finish | Sand castings, flame-cut plates | 0.5× |
Achievable Surface Finish by Machining Process
Typical ranges represent production-consistent results; best achievable assumes optimized conditions.
| Process | Typical Ra (µin) | Typical Ra (µm) | Best Achievable (µin) | N-Grade Range | Notes |
|---|---|---|---|---|---|
| Superfinishing / Lapping | 1–4 | 0.025–0.1 | 0.5 | N1–N3 | Removes <0.0025 mm; optical-grade |
| Polishing | 2–8 | 0.05–0.2 | 1 | N2–N4 | Depends on compound grit and technique |
| Cylindrical Grinding | 4–32 | 0.1–0.8 | 2 | N3–N6 | Wheel grit and dressing control finish |
| Surface Grinding | 8–32 | 0.2–0.8 | 4 | N4–N6 | Flat surfaces; wheel selection determines floor |
| Honing | 4–32 | 0.1–0.8 | 2 | N3–N6 | Cross-hatch for oil retention in cylinder bores |
| Fine CNC Turning | 16–63 | 0.4–1.6 | 8 | N5–N7 | Sharp inserts, light DOC, high speed |
| Fine CNC Milling | 16–63 | 0.4–1.6 | 8 | N5–N7 | Finish end mills, light radial engagement |
| Standard CNC Milling | 63–125 | 1.6–3.2 | 32 | N7–N8 | Default for most machined parts |
| Standard CNC Turning | 63–125 | 1.6–3.2 | 32 | N7–N8 | Standard insert tooling; feed drives finish |
| Wire EDM | 32–125 | 0.8–3.2 | 16 | N6–N8 | Multiple skim passes improve finish |
| Sinker EDM | 63–250 | 1.6–6.3 | 32 | N7–N9 | Depends on electrode and discharge energy |
| Drilling | 63–250 | 1.6–6.3 | 32 | N7–N9 | Hole walls rougher than diameter tolerance |
| Rough Milling / Turning | 125–500 | 3.2–12.5 | 63 | N8–N10 | Material removal prioritized over finish |
| Sawing | 250–500 | 6.3–12.5 | 125 | N9–N10 | Stock preparation |
| Laser Cutting (edge) | 32–250 | 0.8–6.3 | 16 | N6–N9 | Edge striations; HAZ present |
| Plasma Cutting | 250–500 | 6.3–12.5 | 125 | N9–N10 | Rougher than laser; wider HAZ |
Material-Specific Surface Finish Achievability
Typical and best-achievable Ra values for common CNC materials. Material choice directly affects what Ra is realistically achievable.
| Material | Typical CNC Ra (µm / µin) | Best Achievable Ra (µm / µin) | Machinability Notes |
|---|---|---|---|
| Aluminum 6061-T6 | 1.6 / 63 | 0.4 / 16 | Excellent finish potential. Anodizing may reveal tool marks. |
| Aluminum 7075-T6 | 1.6 / 63 | 0.4 / 16 | Higher hardness produces marginally better finish. |
| Stainless Steel 304 | 3.2 / 125 | 0.8 / 32 | Work-hardening risk; maintain minimum chip thickness. |
| Stainless Steel 316 | 3.2 / 125 | 0.8 / 32 | Molybdenum increases toughness; insert wear degrades finish. |
| Carbon Steel 1018 | 3.2 / 125 | 0.8 / 32 | Good finish potential. Predictable machining. |
| Alloy Steel 4140 (ann.) | 3.2 / 125 | 0.8 / 32 | Pre-hardened 4140 produces better finish than annealed. |
| Brass C360 | 0.8 / 32 | 0.2 / 8 | Easiest metal to machine. Naturally smooth finish. |
| Titanium Ti-6Al-4V | 3.2 / 125 | 0.8 / 32 | Low thermal conductivity; requires sharp carbide, high-pressure coolant. |
| Inconel 718 | 3.2 / 125 | 0.8 / 32 | Extremely demanding. Work-hardens aggressively. |
| PEEK | 1.6 / 63 | 0.4 / 16 | Machines well with sharp tools. Excellent stability. |
| Delrin / POM | 1.6 / 63 | 0.4 / 16 | Best-machining plastic. Smooth, burr-free finish. |
| Nylon 6/6 | 3.2 / 125 | 0.8 / 32 | Moisture causes dimensional drift. Sharp tools and coolant recommended. |
ISO 4287 & ASME B46.1
Surface texture standards define Ra, Rz, RMS, and measurement parameters including cutoff length and evaluation length.
Drawing Symbols (ISO 1302)
Basic symbol (open) = no process specified. Material removal required (with bar) = machining mandatory. Material removal prohibited (with circle) = no machining allowed.
COST & SELECTION GUIDE
Surface Finish Cost Guide: Avoid Over-Specifying
Every step down in Ra roughly doubles the per-surface machining time. Specify the roughest finish that still meets the functional requirement.
01
Identify the Ra Requirement
Check the drawing callout. Ra (µm or µin) is the most common. If Rz is specified, use it directly for sealing/bearing surfaces.
02
Find the Process That Achieves It
Use the process table to match your required Ra to the manufacturing method. If it calls for grinding and you don’t need it, revisit the spec.
03
Consider Material Effects
Brass and aluminum machine smoother than stainless or titanium at the same settings. Material choice can eliminate a secondary finishing operation.
Cost Impact: Surface Finish vs. Price
The cost of surface finish escalates non-linearly. Moving from Ra 3.2 µm to Ra 1.6 µm doubles the cycle time for that surface. Moving to Ra 0.8 µm typically adds a dedicated finish pass — and for stainless steel or titanium, often means grinding.
Ra 3.2N8 · 125 µin · 1.0× baseline
Ra 1.6N7 · 63 µin · 1.2–1.5×
Ra 0.8N6 · 32 µin · 1.5–2.0×
Ra 0.4N5 · 16 µin · 2.0–3.0×
Ra 0.025N1 · 1 µin · 3.0–5.0×
The over-specification rule: Call out Ra 3.2 µm (N8, 125 µin) on every surface unless the surface has a specific functional requirement for smoother. Bearing seats, O-ring sealing faces, dynamic seals, sliding surfaces, and coating-critical surfaces need finer finishes. Structural brackets, housings, and mounting faces do not.
Surface Finish Drawing Symbols & Measurement
Decode any ISO 1302 surface finish callout and understand how roughness is actually measured on the shop floor.
| Symbol / Parameter | What It Means | Practical Guidance |
|---|---|---|
| ✓ (basic symbol, open) | No process specified. Surface must meet Ra but can be achieved by any method. | Use for surfaces where process is not constrained. |
| ✓ (with horizontal bar) | Material removal required — machining is mandatory. | Use when as-cast or as-forged surfaces are unacceptable even if they meet Ra. |
| ✓ (with circle) | Material removal prohibited — no machining allowed. | Use for as-cast, as-forged, or molded surfaces that must remain original. |
| Ra value (e.g., Ra 3.2) | Roughness average in micrometers or microinches. | Default callout. For sealing surfaces, also specify Rz. |
| Cutoff length (λc) | Filter length separating roughness from waviness. Standard: 0.8 mm (Ra ≤ 0.4), 2.5 mm (0.4 < Ra ≤ 10), 8 mm (Ra > 10). | Using the wrong cutoff changes the reported Ra. Always match cutoff to expected Ra range. |
| Contact profilometer | Diamond-tipped stylus traverses the surface. Industry standard for production inspection. | Best for most machined surfaces. Non-contact optical methods needed for Ra < 0.2 µm. |
Practical drawing guidance: Use a general note for non-critical surfaces: “Unspecified surfaces: Ra 3.2 µm. Break all sharp edges.” Call out specific Ra only on functional surfaces. Always declare the applicable standard in the title block — “Surface texture per ISO 1302” or “per ASME Y14.36.” Specify Rz alongside Ra for sealing and bearing surfaces: e.g., “Ra 0.8 / Rz 4.”
PROCESS & FINISH
How Process Choice Affects Surface Finish
Different machining processes produce different finish ranges. Match the process to your Ra requirement — and to your budget.
Surface finish is the single most commonly over-specified parameter on machined part drawings, and the one that most directly drives up cost without adding function. Use the process table above to match your Ra requirement to the right machining method.
Ra 1.6–3.2 µm
Standard CNC Range
Standard milling and turning with general-purpose tooling. Default for most parts. Tool marks visible but functionally smooth. Cost baseline.
Ra 0.4–0.8 µm
Precision / Grinding Range
Fine-finish CNC on aluminum/brass, or grinding/honing on steel and titanium. Typically adds a secondary operation. Cost: 1.5–3.0× baseline.
Ra 0.025–0.1 µm
Superfinishing / Lapping
Mirror-like finishes for gauge blocks, precision bearings, optical components. Requires specialized equipment and extended cycle times. Cost: 3.0–5.0× baseline.
Material & Surface Finish Tradeoffs
Brass C360 and aluminum 6061-T6 produce a finish roughly two N-grades smoother than stainless steel 304 at the same cutting parameters. If your part requires Ra ≤ 0.8 µm and you have material flexibility, choosing brass or aluminum over stainless steel eliminates a secondary grinding operation and cuts cost by 30–50%. For plastic parts, Delrin and PEEK achieve the best as-machined surface quality.
AluminumBest Finish: 0.4 µm (16 µin)
StainlessBest Finish: 0.8 µm (32 µin)
BrassBest Finish: 0.2 µm (8 µin)
Surface Finish FAQs
Frequently Asked Questions
The default as-machined finish for standard CNC milling and turning is Ra 3.2 µm (125 µin, N8). Tool marks are visible but the surface is functionally smooth for structural brackets, housings, mounting faces, and clearance surfaces. Most CNC shops can achieve Ra 1.6 µm (63 µin, N7) with a finish pass at modest cost increase. Ra 0.8 µm (32 µin, N6) is the practical floor for CNC without secondary processing.
Ra (Roughness Average) is the arithmetic mean of absolute profile deviations. RMS (Root Mean Square) squares each deviation before averaging, giving roughly 11% more weight to deeper valleys. For typical machined surfaces, RMS ≈ Ra × 1.11. RMS was the older US standard callout; modern standards use Ra. If you encounter a legacy drawing specifying RMS, divide the RMS value by 1.11 to estimate the equivalent Ra.
Ra averages all profile deviations, smoothing out extremes. Rz averages only the five highest peaks and five deepest valleys, making it far more sensitive to isolated defects. Two surfaces can share the same Ra (e.g., 0.8 µm) while one has Rz 4 µm (good for sealing) and the other has Rz 9 µm (will leak). For sealing surfaces, bearing journals, and any application where a single deep scratch causes failure, specify Rz directly on the drawing.
Standard CNC milling produces Ra 1.6–3.2 µm (63–125 µin, N7–N8). With sharp carbide finish end mills, light radial engagement, and optimized parameters, Ra 0.8 µm (32 µin, N6) is achievable on aluminum and brass. Achieving Ra 0.4–0.8 µm on stainless steel or titanium by milling alone is inconsistent; plan for grinding if the part requires Ra below 0.8 µm on these materials.
There is no exact mathematical conversion from Ra to Rz because the relationship is process-dependent. As a rough planning estimate, Rz ≈ Ra × 4–7: near 5× for turned surfaces, near 6× for ground surfaces, and can approach 7× for milled surfaces with periodic tool marks. For critical applications, specify Rz directly on the drawing rather than converting.
Yes. Type II sulfuric acid anodizing typically increases Ra by 0.1–0.3 µm (4–12 µin) on aluminum. More importantly, anodizing reveals tool marks that were invisible on the raw machined surface. If your aluminum part will be anodized, specify the Ra after anodizing, not before, and inspect post-anodize. Hard anodizing (Type III) adds 0.001–0.002 inch per surface and has a greater effect on roughness.
For most static O-ring seals, Ra 0.8–1.6 µm (32–63 µin, N6–N7) on the sealing surface is sufficient when combined with an Rz specification. The Rz is more important than the Ra for sealing. For dynamic O-ring seals (piston rods, rotating shafts), Ra 0.2–0.4 µm (8–16 µin, N4–N5) is typically required. Always consult your seal manufacturer’s surface finish recommendation for the specific seal material and operating conditions.
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