The Anatomy of a Twist Drill Bit
A twist drill bit looks simple, but its geometry is the result of over a century of engineering refinement. Every dimension — from the helix angle of the flutes to the thickness of the web — affects how the bit cuts, how chips evacuate, and how long the tool lasts.
Understanding these features helps you select the right bit for your application and diagnose problems when drilling does not go as planned.
The Point (Tip)
The point is where cutting happens. It consists of two cutting lips, a chisel edge, and clearance faces.
Cutting lips — The two angled edges that do the actual cutting. They must be equal in length and angle for the bit to drill a round, on-size hole.
Chisel edge — The blunt center where the two clearance faces meet. On a conventional point, the chisel edge does not cut — it pushes material aside. A split point grinds this area into a cutting geometry.
Point angle — Typically 118° or 135°. Controls thrust force and self-centering ability.
Lip relief angle — The clearance behind each cutting lip, usually 8–12°. Too little relief causes rubbing; too much weakens the cutting edge.
The Flutes
Flutes are the helical grooves that spiral along the body of the drill. They serve two critical functions: they provide cutting edges and they evacuate chips from the hole.
Helix Angle
The helix angle is the angle of the flute spiral relative to the drill axis.
Standard helix (28–32°) — The default for general-purpose drilling in steel and most metals.
Slow helix (10–20°) — Better for brass, bronze, and plastics. The lower angle prevents the bit from grabbing and self-feeding in soft, gummy materials.
Fast helix (35–45°) — Aggressive chip evacuation for aluminum and deep-hole drilling. The steeper angle pulls chips out faster.
Flute Length vs Overall Length
The flute length determines the maximum drilling depth. Never drill deeper than the flute length — chips cannot evacuate past the flutes, causing packing, heat buildup, and breakage.
The Margins
Margins are the narrow raised strips along the leading edge of each flute. They are the only part of the drill body that contacts the hole wall.
Purpose — Guide the drill straight, burnish the hole wall for a smooth finish, and maintain hole diameter accuracy.
Width — Typically 0.2–0.5mm depending on drill diameter. Wider margins provide more stability but generate more friction.
Wear indicator — When margins wear down, the drill starts producing oversized or rough holes. This is often the first sign that resharpening is needed.
The Web
The web is the solid core of the drill between the two flutes. It provides structural rigidity.
Web thickness — Increases from the point toward the shank (web taper). A thicker web means a stronger bit but a larger chisel edge, which increases thrust force.
Thin web — Easier penetration, lower thrust force, but more prone to breakage. Common in smaller diameter bits.
Thick web — Maximum rigidity for large diameters and deep holes. Requires more thrust force.
Web thinning — A secondary grinding operation that reduces the chisel edge width without weakening the overall web. Common on premium drill bits.
The Body
The body connects the point to the shank and contains the flutes.
Body clearance (back taper) — The body diameter decreases slightly from point to shank (typically 0.02–0.08mm per 100mm). This prevents the body from binding in the hole.
Land — The full-diameter surface between flutes. Contains the margin and the body clearance.
The Shank
The non-cutting end that fits into the chuck or spindle. Common types include straight (cylindrical), Morse taper, and reduced (Silver & Deming).
Troubleshooting with Anatomy Knowledge
Oversized holes — Check margin wear, unequal lip lengths, or excessive runout
Rough hole finish — Worn margins, insufficient lip relief, or too-high feed rate
Bit walking off center — Unequal lip lengths, dull chisel edge, or no split point
Chip packing — Drilling deeper than flute length, wrong helix angle for material, or insufficient peck drilling
Premature breakage — Web too thin for the application, excessive feed, or chip packing in deep holes