| 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. |
| metadata | {"priority":7,"promptSignals":{"phrases":["drilling operation","twist drill","drill bit","drilling speed","drilling feed","reaming","drill thrust force"],"minScore":3}} |
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.
