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Geometry of a twist drill

Short answer

A twist drill has two helical flutes and two cutting lips. Typical angles: point angle 118° for general steel, helix angle about 30°, lip clearance 8–15°, chisel edge angle 120–135° (135° preferred).

Draw and label a twist drill and explain its geometry. State typical values of point angle, helix angle, lip clearance angle and chisel edge angle.

A twist drill is a rotary cutting tool with two helical flutes and two cutting lips, used to produce round holes.

  shank      body (flutes)         point
 |<---->|<------------------->|<-------->
 ====== |  ///////////////  |  /\
 tang   |  helical flutes   | /  \  lips
                              \/  chisel edge

Main parts

  • Shank: straight (small drills, held in a chuck) or Morse taper (large drills, with a tang that drives and helps ejection).
  • Body: carries two helical flutes that form the cutting edges, carry chips out and let coolant in.
  • Land: raised strip following the flute; the margin on it guides the drill and keeps diameter.
  • Web: central core thickness between flutes; it increases toward the shank for strength.
  • Point: cone formed by two lips and the chisel edge.

Geometry angles

FeatureTypical valueRole
Point (included) angle118° for general steel and iron; 135° to 140° for hard materials; 90° for plastics, 130° for brassControls cutting action and thrust
Helix angle24° to 32° (about 30° general)Rake angle at the lip; chip removal
Lip clearance angle8° to 15°Prevents the heel rubbing behind the lip
Chisel edge angle120° to 135°Depends on lip clearance; 135° preferred
  • The two lips must be equal in length and angle; otherwise the hole will be oversize and the drill may break.
  • The chisel edge does not cut but pushes metal sideways, producing high thrust; web thinning reduces it.

✓ Answer checked by Claude Opus 5.5 (high effort)

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