---
title: "Hand tapping and tap drill size"
subject: "Workshop Technology"
chapter: 3
key: "semester_2/workshop_technology:hand-tapping-tap-drill"
source: https://www.bctengineer.com/q/semester_2/workshop_technology/hand-tapping-tap-drill
site: BCT Engineering
---

# Hand tapping and tap drill size

Workshop Technology · Chapter 3: General Hand Working Operations

- Asked in: year not given, 6 marks
- Video: [Tech Basics - Drilling Tapping Project](https://www.youtube.com/watch?v=y1wczTswkTQ) — IMAT 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/workshop_technology/hand-tapping-tap-drill
- Whole chapter: https://www.bctengineer.com/md/semester_2/workshop_technology/chapter-3

## Question

Explain the procedure of cutting an internal thread by hand using a set of taps. Calculate the tap drill size for an M12 x 1.75 thread and state the purpose of a die.

## Short answer

Drill the hole to the tap drill size 10.25 mm (nearest drill 10.2 mm) for M12×1.75. Chamfer, start the taper tap square, turn forward 1/2 turn then back 1/4 to break chips with cutting oil, then use intermediate and plug taps. A die cuts external threads on a rod.

## Answer

A tap is a hardened threaded tool with flutes used to cut an internal thread in a drilled hole. It is turned by a tap wrench. A die cuts an external thread on a rod, held in a die stock.

#### Set of taps
| Tap | Chamfer | Use |
|---|---|---|
| Taper (first) | 8 to 10 threads tapered | Starts the thread; used first |
| Intermediate (second) | 3 to 5 threads | Deepens the thread |
| Plug/bottoming (third) | 1 to 2 threads | Finishes the thread to the bottom of a blind hole |

For through holes, taper or plug tap alone is often enough.

#### Procedure
1. Drill the hole to the tap drill size and chamfer its mouth.
2. Hold the work firmly in a vice with the hole axis vertical.
3. Fit the taper tap in the wrench and start it square to the work by light downward pressure.
4. Turn the wrench 1/2 to 1 turn forward, then 1/4 turn back to break and clear the chips; use cutting oil (not for cast iron).
5. Remove the tap, clear the chips and repeat with the intermediate and plug taps in order.
6. Check the thread with a bolt or a go/no-go gauge.

#### Tap drill size
Tap drill size = major diameter - pitch (for ISO metric threads of about 75% thread depth).

$$
\begin{aligned} d_{tap} &= D - p \\ &= 12 - 1.75 = 10.25\ \text{mm} \end{aligned}
$$

The nearest standard drill is 10.2 mm.

**Answer: tap drill size = 10.25 mm (use 10.2 mm drill).**

#### Die
A die cuts an external thread on a bar. The bar end is chamfered, the die is started square in the die stock and turned forward with periodic reverse to break chips, with cutting oil.

## Check yourself

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

1. What is the tap drill size for an M12 x 1.75 thread?
   A. 13.75 mm
   B. 10.25 mm
   C. 12 mm
   D. 9.5 mm
   Answer: B. Tap drill = major diameter - pitch = 12 - 1.75 = 10.25 mm, so a 10.2 mm drill is used. (Key point: Tap drill size)

2. In which order is a set of taps used?
   A. Plug, intermediate, taper
   B. Taper, intermediate, plug
   C. Intermediate, taper, plug
   D. Plug only
   Answer: B. The taper tap starts the thread, the intermediate deepens it, and the plug tap finishes to the bottom. (Key point: Order of taps)

3. Why is the tap wrench turned back a quarter turn after each half to one turn forward?
   A. To loosen the job
   B. To break and clear the chips
   C. To cool the vice
   D. To measure the thread
   Answer: B. Reversing breaks the chip and clears it from the flutes, preventing jamming and tap breakage. (Key point: Turning the tap)

4. What does a die do?
   A. Cuts an internal thread in a hole
   B. Cuts an external thread on a rod or bar
   C. Drills the tap hole
   D. Checks the thread
   Answer: B. A die, held in a die stock, cuts an external thread on a bar. (Key point: Purpose of die)

## 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
- [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

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