---
title: "Thread cutting on lathe and change gears"
subject: "Workshop Technology"
chapter: 6
key: "semester_2/workshop_technology:lathe-thread-cutting-change-gears"
source: https://www.bctengineer.com/q/semester_2/workshop_technology/lathe-thread-cutting-change-gears
site: BCT Engineering
---

# Thread cutting on lathe and change gears

Workshop Technology · Chapter 6: General Machining Processes “Chip-forming”

- Asked in: year not given, 3+5 marks
- Video: [The quest for metric threads   first change gear part 1 of 2](https://www.youtube.com/watch?v=6sKFjxNmTds) — ARW
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/workshop_technology/lathe-thread-cutting-change-gears
- Whole chapter: https://www.bctengineer.com/md/semester_2/workshop_technology/chapter-6

## Question

Explain how a screw thread is cut on a centre lathe. A lathe has a lead screw of 6 mm pitch. Determine the change gears (simple or compound, from a set of gears from 20 to 120 teeth in steps of 5) to cut a metric thread of 1.75 mm pitch.

## Short answer

The spindle and lead screw are connected via change gears so carriage advances one thread pitch per revolution. Use low speed, 60° V tool at centre height, engage half-nut, cut multiple passes feeding compound slide at 29°, reversing spindle to return. Gear ratio = 1.75/6 = 7/24. Simple: driver 35T, driven 120T (idler). Compound: 35 & 20 driving 40 & 60.

## Answer

#### Thread cutting on a lathe
The spindle and the lead screw are connected through a train of change gears so that, for each revolution of the job, the carriage moves through one pitch of the thread.
1. Set a low spindle speed and set the change gears (or the quick-change gear box) for the required pitch.
2. Fit a 60° V thread tool ground to the thread profile, and set it at centre height and square to the job using the thread (centre) gauge.
3. Engage the half-nut on the lead screw. For later passes, re-engage at the correct point using the thread-chasing dial; where the dial cannot be used (as here, since 6/1.75 is not a whole number), keep the half-nut engaged and reverse the spindle to bring the tool back.
4. Cut several passes, increasing the depth with the compound slide set at 29° (so the cutting is mainly on the leading flank). Withdraw the tool with the cross slide at the end of each pass and return the carriage to the start.
5. Check the thread with a gauge or nut.

#### Change gear calculation
The train ratio is the ratio of job rotation to lead-screw rotation:

$$
\frac{N_{spindle}}{N_{leadscrew}} = \frac{p_{lead\ screw}}{p_{thread}} \quad\Rightarrow\quad \frac{\text{Driver}}{\text{Driven}} = \frac{p_{thread}}{p_{lead\ screw}}
$$

$$
\frac{\text{Driver}}{\text{Driven}} = \frac{1.75}{6} = \frac{7}{24} = 0.2917
$$

**Simple train:** multiply the numerator and denominator by 5:

$$
\frac{7}{24} = \frac{35}{120}
$$

Both gears are in the set (20 to 120 teeth). So a 35-tooth driver on the spindle (stud) drives a 120-tooth gear on the lead screw, through an idler gear of any size, which only fills the gap and keeps the direction correct.

```text
 spindle gear 35 --> idler --> 120 lead screw gear
       (driver)                  (driven)
```

**Compound train (alternative):**

$$
\frac{7}{24} = \frac{7}{8}\times\frac{1}{3} = \frac{35}{40}\times\frac{20}{60}
$$

Check: $\frac{35}{40}\times\frac{20}{60} = 0.875 \times 0.3333 = 0.2917 = \frac{1.75}{6}$.

Here the 35 driver meshes with a 40, and a 20 on the same shaft as the 40 drives the 60 on the lead screw.

**Answer: simple train - driver 35 teeth, driven 120 teeth (with an idler); or compound train - drivers 35 and 20, driven 40 and 60.**

## Check yourself

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

1. How is the pitch of a thread cut on a lathe obtained?
   A. By the cross slide feed
   B. By a gear train linking spindle and lead screw so the carriage moves one pitch per job revolution
   C. By the tailstock
   D. By hand feeding
   Answer: B. Change gears connect the spindle and lead screw so the carriage advances one thread pitch per revolution. (Key point: Principle of thread cutting)

2. Why is the compound slide set at 29° while cutting a 60° thread?
   A. To cut on both flanks equally
   B. So cutting is mainly on the leading flank
   C. To raise the tool
   D. To lock the carriage
   Answer: B. Feeding at 29° makes the leading flank do most of the cutting, giving better chip flow. (Key point: Compound slide angle for threading)

3. Driver/Driven for a 1.75 mm thread with a 6 mm lead screw is
   A. 6/1.75
   B. 1.75/6 = 7/24
   C. 1.75 x 6
   D. 24/7
   Answer: B. Driver/Driven = p_thread/p_lead screw = 1.75/6 = 7/24. (Key point: Gear ratio formula)

4. Which compound train gives 7/24?
   A. (35/40) x (20/60)
   B. (35/40) x (60/20)
   C. (40/35) x (20/60)
   D. (20/40) x (20/60)
   Answer: A. 0.875 x 0.3333 = 0.2917 = 7/24. (Key point: Compound train)

## Related in Workshop Technology

- [Taper turning: set-over and compound angle](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Ataper-turning-setover-numerical) — Workshop Technology, chapter 6
- [Classification and main parts of centre lathe](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Alathe-classification-main-parts) — Workshop Technology, chapter 6
- [Gear manufacturing and simple indexing](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Agear-milling-indexing-numerical) — Workshop Technology, chapter 11
- [Work holding and tool mounting on lathe](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Alathe-work-tool-holding) — Workshop Technology, chapter 6

## Related in other subjects

- [Screw thread production and change gears](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Ascrew-thread-manufacturing-change-gears) — Manufacturing and Production Processes, chapter 11
- [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
- [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
- [Measuring thread pitch, angle and diameters](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Atoolmaker-microscope-thread-measurement) — Metrology, chapter 12