--- title: "Gear manufacturing methods and indexing" subject: "Manufacturing and Production Processes" chapter: 11 key: "semester_4/manufacturing_and_production_processes:gear-manufacturing-indexing-hobbing" source: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/gear-manufacturing-indexing-hobbing site: BCT Engineering --- # Gear manufacturing methods and indexing Manufacturing and Production Processes · Chapter 11: Screw Threads and Gear manufacturing Methods - Asked in: year not given, 3+5 marks - Video: [Gear Manufacturing Methods | ENGINEERING STUDY MATERIALS](https://www.youtube.com/watch?v=YDCEzVpe94k) — Engineering Study Materials - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/gear-manufacturing-indexing-hobbing - Whole chapter: https://www.bctengineer.com/md/semester_4/manufacturing_and_production_processes/chapter-11 ## Question (a) Classify the methods of manufacturing gears and explain gear hobbing with its advantages. (b) A spur gear with 24 teeth and module 3 mm is to be cut on a milling machine by form milling using a dividing head (worm ratio 40:1). Calculate the indexing, the pitch circle diameter, outside diameter, whole depth and circular pitch. Also state the indexing for 37 teeth. ## Short answer Manufacturing methods: casting, form milling, generating (hobbing, shaping), finishing; hobbing is accurate and fast. Indexing: 40/24 = 1 + 10/15; 40/37 = 1 + 3/37. Module 3, 24 teeth: PCD 72 mm, OD 78 mm, depth 6.75 mm, circular pitch 9.42 mm. ## Answer #### (a) Methods of gear manufacture 1. **Forming/shaping from material:** casting (sand, die, investment), powder metallurgy, forging, cold rolling, extrusion and stamping (thin gears) - low cost for large quantity but lower accuracy. 2. **Machining (cutting):** - *Form milling:* a form cutter (one of a set of 8 cutters for each module, numbered by tooth number range) cuts one tooth space at a time; the work is indexed in a dividing head. Slow and less accurate. - *Generating:* a tool and work move relative to each other as if a pair of gears (hobbing, gear shaping by pinion or rack cutter) - accurate, with one cutter for all tooth counts of one module. 3. **Finishing:** shaving, grinding, lapping, burnishing to improve accuracy and finish. **Gear hobbing:** the **hob** is a worm-like cutter with gashes forming cutting teeth. Hob and gear blank rotate in a fixed speed ratio (as worm and wheel), and the hob is fed across the face of the blank. For a single-start hob and $z$ teeth, the blank turns $1/z$ revolution for each revolution of the hob. Tooth form is generated by the successive cuts. Advantages: continuous cutting and high production rate; one hob cuts all gears of the same module and pressure angle; good accuracy; can cut spur, helical, worm wheels (not internal gears or gears near shoulders). #### (b) Indexing and gear dimensions **Simple indexing** with 40:1 dividing head: the crank turns per division $$ n = \frac{40}{z} $$ **For $z = 24$:** $$ n = \frac{40}{24} = 1\frac{2}{3} = 1 + \frac{10}{15} $$ One full turn and **10 holes in a 15-hole circle** (or 20 holes in a 30-hole circle). **For $z = 37$:** $$ n = \frac{40}{37} = 1\frac{3}{37} $$ One full turn and **3 holes in a 37-hole circle**. **Gear dimensions** (module $m = 3$ mm, $z = 24$): | Item | Formula | Value | |---|---|---| | Pitch circle diameter | $mz$ | 72 mm | | Outside diameter | $m(z+2)$ | 78 mm | | Whole depth | $2.25m$ | 6.75 mm | | Circular pitch | $\pi m$ | 9.42 mm | **Answer:** 1 turn + 10 holes on the 15-hole circle (24 teeth); 1 turn + 3 holes on the 37-hole circle (37 teeth); PCD 72 mm, OD 78 mm, depth 6.75 mm, circular pitch 9.42 mm. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main difference between form milling and generating (hobbing) of gears? A. Form milling uses one cutter for all tooth counts of a module; generating needs a set of 8 cutters B. Form milling cuts one tooth space at a time with a form cutter; generating moves tool and work like a pair of meshing gears, with one cutter for all tooth counts C. Generating is slower and less accurate than form milling D. Form milling is a finishing process and generating is a casting process Answer: B. A form cutter (one of a set of 8 per module) cuts one tooth space at a time, whereas generating methods such as hobbing use one cutter per module for any tooth count and are more accurate. (Key point: Generating vs form milling) 2. In hobbing a gear with z teeth using a single-start hob, how are the hob and blank related? A. The blank turns z revolutions per hob revolution B. The blank turns 1/z revolution for each revolution of the hob C. The blank stays fixed while the hob indexes D. The blank and hob turn at equal speeds Answer: B. Hob and blank rotate in a fixed ratio like worm and wheel, so a single-start hob turns the blank 1/z revolution per hob revolution. (Key point: Hobbing principle) 3. With a 40:1 dividing head, how many crank turns per division are needed for 24 teeth? A. 1 full turn plus 10 holes on a 15-hole circle B. 1 full turn plus 3 holes on a 37-hole circle C. 2 full turns plus 10 holes on a 15-hole circle D. 24 full turns Answer: A. n = 40/24 = 1 2/3 = 1 + 10/15, so one full turn and 10 holes in a 15-hole circle. (Key point: Indexing calculation) 4. For module 3 mm and 24 teeth, what are the pitch circle diameter, outside diameter and whole depth? A. 72 mm, 75 mm, 6.75 mm B. 72 mm, 78 mm, 6.75 mm C. 78 mm, 72 mm, 6.00 mm D. 72 mm, 78 mm, 9.42 mm Answer: B. PCD = mz = 72 mm, OD = m(z+2) = 78 mm, whole depth = 2.25m = 6.75 mm; 9.42 mm is the circular pitch. 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