surface-finish

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.