drilling-operations

name	drilling-operations
description	Drilling operations — twist drill geometry, cutting speeds, feeds, thrust force, torque, drill life, point angles, reaming, boring, deep hole drilling, GD&T hole tolerances.
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Drilling Operations — Complete Skill

Twist Drill Geometry

Standard point angle: 118° (general purpose steel); 135° (self-centering, harder materials) Helix angle: 29–30° (standard); 14° (slow helix, hard materials); 40° (fast helix, soft materials like Al) Clearance angle: 10–15° (provides relief behind cutting edge) Chisel edge: central edge that scrapes rather than cuts; most thrust generated here

Drill diameter D; point height h = D/2 × tan(90° - θ/2) where θ = point angle

Cutting Parameters

Cutting speed: V_c = π D N / 1000 [m/min; D in mm, N in rpm]

Feed per revolution: f [mm/rev] Feed rate: v_f = f × N [mm/min]

Recommended cutting conditions (HSS drill):

Material	V_c [m/min]	f [mm/rev per 10mm D]
Carbon steel (1045)	20–30	0.15–0.30
Stainless 304	8–15	0.10–0.20
Cast iron (gray)	20–30	0.15–0.25
Aluminum	60–100	0.15–0.35
Ti-6Al-4V	5–10	0.05–0.15
Inconel	3–6	0.05–0.10

Carbide drills: 3–5× higher V_c than HSS for same material

Thrust Force and Torque

Thrust force (empirical, from Merchant): F_thrust = K_t × d × f × B_w [N]

K_t = specific thrust (material constant); approximate by: F_thrust ≈ 2.5 × (π D²/4) × P_z / (V_c / 60) (rough estimate)

Practical Machinability model: F_thrust = C_t × D^a × f^b [N] C_t, a, b from manufacturer tables

Torque: M = C_m × D^c × f^d [N·m] Power: P = 2π N M / 60 [W]

Drill Life and Taylor Equation

V_c T^n = C (same form as turning) n ≈ 0.12–0.20 (HSS); n ≈ 0.25–0.35 (carbide)

Tool life indicators (drilling): Flank wear V_B ≥ 0.3 mm; drill diameter change > 0.05 mm; breakage; surface finish degradation Reconditioning: regrind point; HSS drill regrind 5–10× before discard

Reaming

Finishing operation after drilling; removes 0.1–0.3 mm material Accuracy: H7 tolerance routinely (±0.012 mm for 10mm hole) Surface finish: Ra 0.8–1.6 μm (vs. 3.2–6.3 μm drilled)

Reaming speeds: V_ream ≈ V_drill / 3 to V_drill / 2 (much slower for dimensional control) Feed: 2–3× drilling feed rate (more teeth, lower chip load per tooth)

Reamer types: Machine reamer: multi-flute (6–8); cylindrical; solid or shell Spiral flute: better chip evacuation; used for blind holes Adjustable (expansion) reamer: adjusts for wear; expensive but adjustable

Boring

Enlarges and corrects an existing hole; single-point tool on boring bar Accuracy: IT5–IT7 (0.006–0.025 mm tolerance) Perpendicularity: better than drilling; corrects drill wander Surface finish: Ra 0.8 μm achievable

Boring bar deflection: δ = F_radial × L³ / (3EI) [limit to < 0.05 mm for accuracy] Carbide boring bar: E = 620 GPa → 3× stiffer than HSS for same diameter

Deep Hole Drilling (L/D > 10)

Challenges: chip evacuation, coolant delivery, drill wander Methods: BTA (Boring and Trepanning Association): internal chip evacuation; single lip drill; to L/D = 200+ Gundrilling: external flute; coolant through drill; L/D > 100; Ra 0.4–0.8 μm; straightness 0.05–0.1 mm per 100 mm Ejector drilling: double tube; L/D = 100–200; high productivity

Coolant pressure: 70–200 bar for gundrilling (must overcome chip back-pressure) Pecking cycles: for drilling without deep-hole equipment; periodic retraction for chip clearing

GD&T Hole Tolerances

Standard hole tolerances (ISO 286): H6: ±0 to +0.011 mm (10mm hole) — precision fit H7: ±0 to +0.021 mm — standard reamed hole H8: ±0 to +0.035 mm — general purpose H9: ±0 to +0.052 mm — loose tolerance

Positional tolerance: True position ⌀0.1 at MMC → well controlled for mating parts For through holes with fasteners: positional = 0.5 × (fastener clearance per hole)

Chip Types and Coolant

Drilling chips: spiral chips for ductile; segmented for brittle Chip breakage: important for deep holes; use chipbreakers on drill point for stainless/Ti Coolant: flood for standard; high-pressure through-spindle for hardened/Ti/Inconel; MQL for aluminum

Output

Provide: V_c [m/min], N [rpm], f [mm/rev], v_f [mm/min], F_thrust [N], M [N·m], P [kW], hole tolerance (achievable without/with reaming), surface finish Ra [μm], drill type (HSS/carbide, standard/deep hole), coolant method.