---
title: "Gear manufacturing and simple indexing"
subject: "Workshop Technology"
chapter: 11
key: "semester_2/workshop_technology:gear-milling-indexing-numerical"
source: https://www.bctengineer.com/q/semester_2/workshop_technology/gear-milling-indexing-numerical
site: BCT Engineering
---

# Gear manufacturing and simple indexing

Workshop Technology · Chapter 11: Screw Threads and Gear Manufacturing Methods

- Asked in: year not given, 3+3 marks
- Video: [Milling Machine  Spur Gear](https://www.youtube.com/watch?v=qmNRxBA_R2k) — SJB Institute of Technology
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/workshop_technology/gear-milling-indexing-numerical
- Whole chapter: https://www.bctengineer.com/md/semester_2/workshop_technology/chapter-11

## Question

Name the methods of manufacturing gears and compare gear hobbing with form milling. A spur gear of module 3 mm and 34 teeth is to be cut on a milling machine using the dividing head (ratio 40:1). Find (a) the outside diameter and tooth depth, and (b) the indexing for each tooth by simple indexing, using the Brown and Sharpe plate holes 15, 16, 17, 18, 19 and 20.

## Short answer

Gear manufacturing methods include casting, forming, machining, and finishing. Hobbing generates teeth continuously with higher accuracy and speed, while form milling cuts one tooth at a time with a form cutter and requires indexing. For module 3 mm, 34 teeth: outside diameter = 108 mm, tooth depth = 6.75 mm. Simple indexing: 1 full turn plus 3 holes on the 17-hole circle.

## Answer

#### Gear manufacturing methods
- Casting (sand casting, die casting) for rough gears.
- Forming: powder metallurgy, extrusion, cold rolling of gears.
- Machining: milling with a form cutter, gear shaping, gear hobbing, gear planing, broaching.
- Finishing: shaving, grinding, lapping, honing.

#### Hobbing versus form milling
| Form milling | Hobbing |
|---|---|
| A form (disc) cutter cuts one tooth space at a time | A hob (worm-like cutter) cuts continuously; the teeth are generated by rolling |
| Needs an indexing head; each cutter profile serves a range of tooth numbers, so it is slightly inexact | Higher profile accuracy; one hob cuts all gears of the same module |
| Slow; for single parts or small lots | Fast; for large production |
| Only spur and helical gears | Spur, helical gears and worm wheels |

#### (a) Gear dimensions
$$
\begin{aligned} D_o &= m(z+2) = 3\times 36 = 108\ \text{mm} \\ \text{Tooth depth} &= 2.25m = 6.75\ \text{mm} \\ \text{Pitch circle diameter} &= mz = 102\ \text{mm} \end{aligned}
$$

A module 3 involute cutter for the 26 to 34 teeth range is used: No. 4 in the Brown and Sharpe 8-cutter numbering (No. 5 in the metric module cutter numbering, which runs the other way).

#### (b) Indexing
Simple indexing with a 40:1 worm gear:

$$
\text{Index crank turns} = \frac{40}{z} = \frac{40}{34} = 1\frac{6}{34} = 1\frac{3}{17}
$$

So after cutting each tooth, turn the crank by 1 full turn plus 3 holes in the 17-hole circle (set the sector arms to 3 spaces).

**Answer: outside diameter = 108 mm, tooth depth = 6.75 mm; indexing = 1 full turn + 3 holes on the 17-hole circle.**

## Check yourself

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

1. How does hobbing differ from form milling?
   A. Hobbing cuts one tooth space at a time
   B. Hobbing cuts continuously by generation and is fast; form milling cuts one space at a time with indexing
   C. Form milling is for mass production
   D. Form milling can cut worm wheels
   Answer: B. The hob generates teeth by continuous rolling, suiting large production; form milling needs indexing. (Key point: Hobbing vs form milling)

2. What is the outside diameter of a module 3, 34-tooth spur gear?
   A. 102 mm
   B. 108 mm
   C. 105 mm
   D. 111 mm
   Answer: B. Do = m(z + 2) = 3 x 36 = 108 mm; 102 is the pitch circle diameter. (Key point: Outside diameter)

3. What is the tooth depth?
   A. 6.75 mm
   B. 3 mm
   C. 6 mm
   D. 7.5 mm
   Answer: A. Depth = 2.25 m = 6.75 mm. (Key point: Tooth depth)

4. What is the crank movement per tooth for 34 teeth with a 40:1 head?
   A. 34/40 turn
   B. 1 turn and 3 holes in the 17-hole circle
   C. 1 turn and 6 holes in the 15-hole circle
   D. 2 turns exactly
   Answer: B. 40/34 = 1 6/34 = 1 3/17, so 1 full turn plus 3 holes on the 17-hole circle. (Key point: Simple indexing)

## Related in Workshop Technology

- [Thread cutting on lathe and change gears](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Alathe-thread-cutting-change-gears) — Workshop Technology, chapter 6
- [Files: cut, grade and common types](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Afile-grades-cuts-types) — Workshop Technology, chapter 2
- [Upsetting diameter and force calculation](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Aupsetting-diameter-force-numerical) — Workshop Technology, chapter 9

## Related in other subjects

- [Gear manufacturing methods and indexing](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Agear-manufacturing-indexing-hobbing) — Manufacturing and Production Processes, chapter 11
- [Lewis method module for spur gears](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Alewis-spur-gear-module-design) — Machine Design I, chapter 9
- [Tooth forces on a spur gear pinion](https://www.bctengineer.com/question.md?key=semester_6%2Ftheory_of_mechanism_and_machine_i%3Aspur-gear-tooth-forces-numerical) — Theory of Mechanism and Machine I, chapter 8
- [Spur gear pair dimensions numerical](https://www.bctengineer.com/question.md?key=semester_6%2Ftheory_of_mechanism_and_machine_i%3Aspur-gear-pair-geometry-numerical) — Theory of Mechanism and Machine I, chapter 4