Drilling, boring and reaming

Why it matters

Holes are the most common machined feature — for bolts, dowels, bearings, oil passages. A drill makes the hole quickly but not accurately; boring corrects its position and straightness; reaming gives the final size and finish. Knowing which to use, and how to estimate their time and power, is everyday process-planning work.

Key ideas

Twist drill geometry

• Point angle (2p) — commonly 118° for general work on steel; larger (130°–140°) for hard materials, smaller (90°–100°) for soft or brittle ones.
• Helix angle — the angle of the flutes; it acts as the rake angle at the outer corner of each lip (typically 20°–30°). The effective rake falls towards the centre and becomes strongly negative at the chisel edge.
• Lips (cutting edges) — two, each removing half the feed per revolution.
• Chisel edge — at the web, it does not cut cleanly but extrudes metal; it produces a large part of the thrust force. Web thinning or a pilot hole reduces thrust.
• Lip clearance (about 8°–12°), margin, body clearance and flutes (carry chips out and fluid in).

Drilling machines. Bench and pillar (upright) drills, radial drills (arm swings and head traverses — for large, heavy work), gang and multi-spindle drills (production), deep-hole and gun drills (L/D > 10), and machining/turning centres.

Related hole-making operations

• Boring — enlarging an existing hole with a single-point tool. It corrects hole location, straightness and roundness because the tool follows the spindle axis, not the old hole. Long boring bars (large overhang L/D) are prone to chatter; carbide or damped bars help.
• Reaming — a multi-flute tool removes a small allowance (typically about 0.1–0.5 mm on diameter depending on size) to give accurate size (tolerance around IT7) and good finish. A reamer follows the existing hole, so it cannot correct position or straightness. Run at a lower speed (roughly half to two-thirds of the drilling speed) and a higher feed than drilling.
• Counterboring, countersinking, spot facing — seats for bolt heads and nuts.
• Tapping — cutting internal threads with a tap.
• Trepanning — cutting an annular groove to leave a solid core; for large holes in plates.
• Centre drilling — a short stiff drill to start holes accurately.

Typical sequence for an accurate hole: centre drill → drill (undersize) → bore (to correct position) → ream (to size and finish).

Formulas

• N = 1000·V / (π·D) (rev/min; V in m/min at the drill periphery; D drill diameter in mm)
• MRR = (π·D²/4)·f·N (drilling from solid, mm³/min; f feed in mm/rev)
• MRR = (π/4)·(D² − d²)·f·N (enlarging, boring or reaming from d to D)
• A = (D/2)·cot p (length of the drill point, mm; p = half the point angle) — add it to the hole depth for a through hole, plus a small over-travel.
• tm = (L + A + O) / (f·N) (min)
• Uncut chip thickness per lip: t = (f/2)·sin p; width of cut per lip: w = (D/2) / sin p.
• Power: P = u·MRR (u specific energy from data) and torque Mt = P / ω, with ω = 2π·N/60 (rad/s). Thrust and torque can also be estimated from empirical data-book formulas.

Worked examples

Example 1 (standard). A 20 mm hole is drilled through a 40 mm plate with an HSS drill (point angle 118°) at V = 25 m/min and f = 0.25 mm/rev. Allow 2 mm over-travel. Find N, the drilling time and the MRR.

1. N = 1000·V/(π·D) = 25 000/(π × 20) = 398 rev/min.
2. Point length A = (D/2)·cot p = 10 × cot 59° = 10 × 0.6009 = 6.01 mm.
3. tm = (L + A + O)/(f·N) = (40 + 6.01 + 2)/(0.25 × 397.9) = 48.01/99.47 = 0.483 min ≈ 29 s.
4. MRR = (π·D²/4)·f·N = 314.16 × 0.25 × 397.9 = 31 250 mm³/min.

Example 2 (GATE level). A 20 mm drill (point angle 118°) cuts steel at f = 0.2 mm/rev and N = 400 rev/min. The specific cutting energy is 3 J/mm³ (given). (a) Find the uncut chip thickness per lip, the power and the torque for drilling from solid. (b) Find the power if the hole is instead enlarged from a 10 mm pilot hole at the same feed and speed.

1. t = (f/2)·sin p = 0.1 × sin 59° = 0.0857 mm.
2. MRR = (π × 20²/4) × 0.2 × 400 = 314.16 × 80 = 25 133 mm³/min = 418.9 mm³/s.
3. P = u·MRR = 3 × 418.9 = 1257 W.
4. ω = 2π × 400/60 = 41.89 rad/s → Mt = P/ω = 1257/41.89 = 30.0 N·m.
5. Enlarging: MRR = (π/4)(20² − 10²) × 0.2 × 400 = 235.6 × 80 = 18 850 mm³/min = 314.2 mm³/s → P = 3 × 314.2 = 943 W. The pilot hole removes the chisel-edge region, so the thrust force drops far more than the 25 % fall in power suggests.

Common mistakes

• Forgetting the drill-point length when timing a through hole.
• Using the full feed per lip; each of the two lips takes f/2.
• Using D² − d² with radii instead of diameters (or forgetting the π/4).
• Expecting a reamer to straighten a hole — only boring corrects location and straightness.
• Reaming at drilling speed; reamers need lower speed and adequate allowance (too little allowance makes them rub and glaze).
• Converting N to ω without the 2π/60 factor when finding torque.

For GATE PI

• NAT on drilling/boring/reaming time including point length, MRR from solid and when enlarging, power and torque from specific energy.
• Chip thickness per lip and the role of point angle.
• MCQs on drill geometry (helix angle as rake, chisel edge and thrust), the boring vs reaming distinction, and hole-making sequences.

Quick check

1. Drill 10 mm, V = 25 m/min. Spindle speed?
2. Which feature of a twist drill produces most of the thrust?
3. Can reaming correct the position of a hole?
4. Feed 0.3 mm/rev, point angle 118°: uncut chip thickness per lip?
5. MRR when enlarging a 15 mm hole to 20 mm at f = 0.1 mm/rev, N = 600 rev/min?

Answers: 1. 25 000/(π × 10) ≈ 796 rev/min. 2. The chisel edge. 3. No — boring does that; reaming only sizes and finishes. 4. 0.15 × sin 59° = 0.129 mm. 5. (π/4)(400 − 225) × 0.1 × 600 = 8247 mm³/min.