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FITTER - CITS

LESSON 7 : Introduction to taps and die

Objectives

At the end of this lesson you shall be able to
• state features and uses of threading hand taps
• state the different types of die
• state the method of thread cutting and safety

Introduction to tap and it’s specification

They are made from high carbon steel or high speed steel, hardened and ground.
The threads are cut on the periphery and are accurately finished.

To form the cutting edges, flutes are cut across the thread.
The end of the shank of the tap is made of square shape for the purpose of holding and turning the taps.
The end of the taps are chamfered (taper lead) for assisting, aligning and starting of the thread.
The size of the taps, the thread standard, the pitch of the thread, the dia. of the tapping hole are usually marked
on the shank.
Marking on the shank are also made to indicate the type of tap i.e. first, second and plug.
Type of tap in a set
Hand taps for a particular thread are available as a set consisting of three pieces.

A hand tap set is used to manually cut an internal thread. Normally, a hand tap set consists of three taps: taper
tap, center tap and finishing tap. The hand taps have a square shaft to be clamped securely in thread holding
tools. The hand taps can be clamped in an adjustable tap wrench and holding tools with ratchet.
These are:
– first tap or taper tap
– second tap or intermediate tap

– plug or bottoming tap
These taps are identical in all features except in the taper lead.
First tap or taper tap: The taper tap is to start the thread. It is possible to form full threads by the taper tap in
through holes which are not deep. The taper lead distributes the cutting force over a large area, and the taper
shape helps the thread to start. They can therefore be used to start a thread prior to use of second or bottom
leads, or for through holes.
Second tap or intermediate tap: Second taps have a lead of 3-5 threads at 8 degrees per side. They are the
most popular and can be used for through holes, or blind holes where the thread does not need to go right to the
bottom.
Plug or bottoming tap: The bottoming tap (plug) is used to finish the threads of a blind hole to the correct depth.
Bottom taps have a chamfer (lead) of 1–2 threads, the angle of the lead being around 18 degrees per side. They
are used to produce threads close to the bottom of blind holes
For identifying the type of taps quickly - the taps are either numbered 1,2 and 3 or rings are marked on the shank.

The taper tap has one ring, the intermediate tap has two and the bottoming tap has three rings.

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CITS : CG & M - Fitter - Lesson 6