| name | surface-finish |
| description | Surface finish and roughness — Ra/Rz/Rq parameters, manufacturing process Ra values, surface finish effect on fatigue (Marin factor ka), tribology basics, coating selection. |
| metadata | {"priority":6,"promptSignals":{"phrases":["surface finish","surface roughness","Ra value","Rz","machining finish","surface texture","surface treatment"],"minScore":4}} |
Surface Finish — Complete Skill
Surface Roughness Parameters
Ra (arithmetic average): Ra = (1/L)∫|y(x)|dx — most commonly used
Rq (RMS): Rq = √[(1/L)∫y²(x)dx] — Rq ≈ 1.25 Ra (for random surfaces)
Rz (ten-point mean): average of 5 highest peaks + 5 deepest valleys; Rz ≈ 4-7 Ra
Rt (total height): max peak-to-valley over entire length; Rt ≈ 6-10 Ra
RSm (mean spacing): average spacing between peaks — affects tribology
ISO 4287: defines roughness parameters
Measurement: profilometer (contact stylus, cutoff λ_c), optical profilometer (non-contact)
Standard cutoff lengths λ_c: 0.08, 0.25, 0.8, 2.5, 8.0 mm (per ISO 4288)
Typical Ra by Process
| Process | Ra [μm] | Ra [μin] |
|---|
| Lapping, superfinishing | 0.025-0.1 | 1-4 |
| Grinding (fine) | 0.1-0.4 | 4-16 |
| Honing | 0.1-0.8 | 4-32 |
| Turning/boring (fine) | 0.4-1.6 | 16-63 |
| Milling (fine) | 0.4-1.6 | 16-63 |
| Turning (rough) | 1.6-6.3 | 63-250 |
| Milling (rough) | 1.6-6.3 | 63-250 |
| Drilling | 1.6-6.3 | 63-250 |
| Reaming | 0.4-1.6 | 16-63 |
| Broaching | 0.4-1.6 | 16-63 |
| Die casting | 0.8-3.2 | 32-125 |
| Sand casting | 6.3-25 | 250-1000 |
| Forging (closed die) | 0.8-3.2 | 32-125 |
| EDM | 0.4-12.5 | 16-500 |
| Electropolish | 0.025-0.4 | 1-16 |
Surface Finish Symbols (ASME Y14.36M)
New symbol: root ∠ with horizontal bar on top
- Number above bar: Ra max [μm or μin]
- With number below bar: lay and waviness specified
- ▽: machined (one mark = rough, 4 marks = very fine)
Surface roughness designation:
N12 = 50 μm (very rough), N9 = 6.3 μm, N7 = 1.6 μm, N5 = 0.4 μm, N1 = 0.025 μm (mirror)
Effect on Fatigue — Marin Surface Factor k_a
k_a = a × S_ut^b (Shigley Eq. 6-19)
| Process | a (MPa units) | b | Notes |
|---|
| Ground | 1.34 | -0.085 | Highest k_a |
| Machined or CD | 2.70 | -0.265 | |
| Hot rolled | 14.4 | -0.718 | |
| As-forged | 39.9 | -0.995 | Lowest k_a |
| Cast | See note | — | Use k_a ≈ 0.6-0.7 |
Example: S_ut = 690 MPa, machined → k_a = 2.70 × 690^(-0.265) = 0.84
Example: S_ut = 1000 MPa, as-forged → k_a = 39.9 × 1000^(-0.995) ≈ 0.43 (large penalty)
Critical insight: High-strength steels suffer MORE from poor surface finish (higher Sut → lower k_a factor for rough surface)
Always specify fine ground (N5-N7) for fatigue-critical high-strength parts
Lay Symbols
= Parallel to boundary line (drawing edge)
⊥ Perpendicular to boundary
× Angular crosshatch
M Multidirectional (isotropic)
C Circular (turned)
R Radial (ground face)
Waviness
Longer wavelength than roughness (λ > 0.8mm)
Waviness height Wz: flatness error, form error
Not same as roughness — vibration or chatter marks
Waviness causes: machine vibration, thermal growth, deflection
Tribology Basics
Bearing ratio (Abbott-Firestone curve): Rk, Rpk, Rvk parameters
Rk = core roughness depth; Rpk = reduced peak height; Rvk = reduced valley depth
Good bearing surface: low Rpk (few peaks to wear away), high Rvk (oil retention valleys)
Friction coefficient:
Dry steel-steel: μ = 0.5-0.8 (static), 0.4-0.6 (kinetic)
Lubricated steel: μ = 0.05-0.15 (boundary), 0.001-0.01 (hydrodynamic)
Teflon (PTFE): μ = 0.04-0.20 (dry)
Wear (Archard's law):
Q = k × P × L / H
Q = wear volume [m³], k = wear coefficient, P = load [N], L = sliding distance [m], H = hardness [Pa]
Hard materials: low k (k = 10⁻⁶ for well-lubricated steel), soft/rough: k = 10⁻³
Surface Treatments (Hardening)
| Treatment | Hardness Achieved | Depth | Application |
|---|
| Through hardening (Q&T) | 40-60 HRC | Full section | Gears, shafts |
| Carburizing | 58-63 HRC case | 0.5-2.5 mm | Gears, cams, high contact |
| Nitriding | 65-70 HRC | 0.1-0.5 mm | No quench needed, distortion-free |
| Induction hardening | 50-60 HRC | 1-5 mm | Journals, cams, selective areas |
| Flame hardening | 50-58 HRC | 2-6 mm | Large parts |
| Shot peening | Compressive residual | 0.25-0.5 mm | Fatigue improvement (k_f improvement) |
| Chrome plating (hard) | 65-72 HRC | 0.02-0.5 mm | Wear, corrosion, dimensional restore |
| Electroless nickel | 48-58 HRC | 0.01-0.2 mm | Uniform, complex geometry |
| PVD/CVD coatings | TiN 2000 HV | 2-8 μm | Cutting tools, high wear |
Output
Provide: Ra [μm] for each surface per process, k_a surface factor for fatigue, surface treatment recommendation, lay symbol for drawing callout.
