---
title: "Geometry of a twist drill"
subject: "Workshop Technology"
chapter: 5
key: "semester_2/workshop_technology:twist-drill-geometry"
source: https://www.bctengineer.com/q/semester_2/workshop_technology/twist-drill-geometry
site: BCT Engineering
---

# Geometry of a twist drill

Workshop Technology · Chapter 5: Drills and Drilling Processes

- Asked in: year not given, 6 marks
- Video: [Drill Tool Nomenclature | Parts of Drill Tool | Drill bit terminology](https://www.youtube.com/watch?v=K39bnxmIz7Q) — ADTW Study
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/workshop_technology/twist-drill-geometry
- Whole chapter: https://www.bctengineer.com/md/semester_2/workshop_technology/chapter-5

## Question

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.

## 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).

## Answer

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

```text
  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
| Feature | Typical value | Role |
|---|---|---|
| Point (included) angle | 118° for general steel and iron; 135° to 140° for hard materials; 90° for plastics, 130° for brass | Controls cutting action and thrust |
| Helix angle | 24° to 32° (about 30° general) | Rake angle at the lip; chip removal |
| Lip clearance angle | 8° to 15° | Prevents the heel rubbing behind the lip |
| Chisel edge angle | 120° 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the typical included point angle of a drill for general steel and iron?
   A. 90°
   B. 118°
   C. 135° to 140°
   D. 60°
   Answer: B. 118° is standard for general steel and iron; harder materials use 135° to 140°. (Key point: Point angle)

2. The helix angle of the flutes (about 30°) mainly provides
   A. Clearance behind the lip
   B. Rake angle at the lip and chip removal
   C. Hole position accuracy
   D. The chisel edge
   Answer: B. The helix gives the rake at the lip and lets chips travel out of the hole. (Key point: Role of helix angle)

3. What does the chisel edge do?
   A. Cuts cleanly with low force
   B. Does not cut but pushes metal sideways, causing high thrust
   C. Guides the drill in the hole
   D. Carries coolant
   Answer: B. The chisel edge extrudes metal rather than cutting it; web thinning reduces the thrust. (Key point: Chisel edge)

4. If the two lips of a drill are unequal in length or angle, the result is
   A. A smaller hole
   B. An oversize hole and possible drill breakage
   C. A faster cut
   D. No effect
   Answer: B. Unequal lips make one lip do more work, so the hole is oversize and the drill may break. (Key point: Unequal lips)

## Related in Workshop Technology

- [Drilling time and metal removal rate](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Adrilling-time-numerical) — Workshop Technology, chapter 5

## Related in other subjects

- [Stepped shaft: angle of twist at free end](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Ashaft-angle-of-twist-free-end) — Strength of Materials, chapter 6
- [Typical ABCD values of long lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Along-line-abcd-typical-values) — Power System Analysis I, chapter 5
- [Solid shaft 300 kW, 120 rpm with twist limit](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Asolid-shaft-300kw-120rpm-twist-limit) — Strength of Materials, chapter 6
- [Steel shaft 200 hp: torque, shear, twist](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Asteel-shaft-200hp-150rpm) — Strength of Materials, chapter 6