---
title: "Screw thread production and change gears"
subject: "Manufacturing and Production Processes"
chapter: 11
key: "semester_4/manufacturing_and_production_processes:screw-thread-manufacturing-change-gears"
source: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/screw-thread-manufacturing-change-gears
site: BCT Engineering
---

# Screw thread production and change gears

Manufacturing and Production Processes · Chapter 11: Screw Threads and Gear manufacturing Methods

- Asked in: year not given, 4+2 marks
- Video: [Part 1- Simple & compound gear train, calculation for thread cutting](https://www.youtube.com/watch?v=-Oo8LWh51ms) — ITI Punjab Skills
- 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/screw-thread-manufacturing-change-gears
- Whole chapter: https://www.bctengineer.com/md/semester_4/manufacturing_and_production_processes/chapter-11

## Question

(a) Describe the methods of producing screw threads by machining, rolling and grinding, with their relative merits. (b) A lathe has a lead screw of 6 mm pitch. Find the change gears (driver and driven) to cut a thread of 2.5 mm pitch, and explain the basic rule.

## Short answer

Threads are made by single-point cutting, taps and dies, milling, rolling and grinding; rolling gives highest strength and rate, grinding highest accuracy. For thread cutting, job pitch over lead screw pitch equals driver over driven. With 6 mm lead screw and 2.5 mm pitch, use driver 25 teeth and driven 60 teeth.

## Answer

#### (a) Production of screw threads

**Machining (cutting):**
- *Single-point cutting on a lathe:* a tool ground to the thread form is moved along the work by the lead screw, taking several passes. It gives any pitch and size and is accurate, but slow.
- *Taps and dies:* internal threads are cut by a tap and external threads by a die, in one or a few passes; fast for small sizes.
- *Thread milling:* a rotating multi-tooth cutter is used for large threads, worms and lead screws; on CNC machines thread milling is done by helical interpolation.
- *Thread chasing, whirling.*

**Thread rolling:** the blank is pressed between hardened dies (flat reciprocating or round rolling dies), and the thread is formed by cold plastic deformation. There are no chips; the grain follows the thread contour, and the surface is work hardened and smooth. It gives higher strength, much better fatigue life and a very high production rate, but it suits only ductile materials of limited hardness, and the dies are costly.

**Thread grinding:** a wheel dressed to the thread form grinds the thread, either by traverse (single-rib wheel) or by plunge (multi-rib wheel). It gives the highest accuracy and finish and can cut hardened material (gauges, lead screws, ball screws), but it is costly and slow.

| Method | Accuracy | Rate | Strength |
|---|---|---|---|
| Single-point cutting | Good | Low | Normal |
| Tap and die | Fair | Medium | Normal |
| Rolling | Good | Very high | Highest |
| Grinding | Highest | Low | Normal |

#### (b) Change gears

For thread cutting, the work (spindle) and the lead screw are linked by a gear train so that the carriage moves one pitch of the job per revolution of the work:

$$
\frac{\text{Pitch of job}}{\text{Pitch of lead screw}} = \frac{\text{Driver (on spindle)}}{\text{Driven (on lead screw)}}
$$

$$
\frac{2.5}{6} = \frac{5}{12} = \frac{25}{60}
$$

The driver on the spindle stud has **25 teeth** and the driven gear on the lead screw has **60 teeth** (a simple train, with gears available in steps of 5 teeth from 20 to 120). If these exact gears are not available, a compound train such as $\frac{20}{40}\times\frac{50}{60}$ gives the same ratio $5/12$.

**Answer:** driver 25 teeth, driven 60 teeth (ratio 5/12).

## Check yourself

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

1. What advantage does thread rolling give?
   A. Highest accuracy on hardened material
   B. Works on brittle materials
   C. Grain follows thread contour, work hardening, higher strength and fatigue life
   D. No die cost
   Answer: C. Rolling is chipless but only for ductile materials. (Key point: Thread rolling)

2. When is thread grinding used?
   A. For highest production rate
   B. For soft ductile materials
   C. For cheap bolts
   D. For highest accuracy and hardened material, e.g. gauges and ball screws
   Answer: D. It is costlier and slow. (Key point: Thread grinding)

3. What is the change gear rule?
   A. Pitch of job / pitch of lead screw = driver (spindle) / driven (lead screw)
   B. Driven / driver = pitch of job / pitch of lead screw
   C. Pitch of job x pitch of lead screw = ratio
   D. Driver teeth equal lead screw pitch
   Answer: A. The carriage must move one job pitch per spindle revolution. (Key point: Change gear rule)

4. What gears cut 2.5 mm pitch with a 6 mm lead screw?
   A. Driver 60, driven 25
   B. Driver 25, driven 60 (ratio 5/12)
   C. Driver 24, driven 60
   D. Driver 30, driven 50
   Answer: B. 2.5/6 = 5/12 = 25/60. (Key point: Gear selection)

## Related in Manufacturing and Production Processes

- [Lathe operations, taper turning and turning calculations](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Alathe-operations-taper-turning-numerical) — Manufacturing and Production Processes, chapter 7
- [Grinding wheel specification and selection](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Agrinding-wheel-designation-selection) — Manufacturing and Production Processes, chapter 8
- [CNC lathe part program with G and M codes](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Acnc-lathe-part-program-gcode) — Manufacturing and Production Processes, chapter 9
- [Chvorinov's rule and riser design](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Achvorinov-rule-riser-design) — Manufacturing and Production Processes, chapter 4
- [Flat rolling draft, roll force and power](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aflat-rolling-force-power-numerical) — Manufacturing and Production Processes, chapter 5
- [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

## Related in other subjects

- [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
- [Production of screw threads](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Athread-production-methods) — Workshop Technology, chapter 11
- [Screw jack with square thread](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Ascrew-jack-square-thread-numerical) — Machine Design II, chapter 8
- [Thread errors and effect of pitch error](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Ascrew-thread-errors-and-pitch-effect) — Metrology, chapter 12