Page 126 - CITS - Fitter Trade Pactical - Volume - 2

FITTER - CITS

MODULE 3

EXERCISE 36: Measuring the thread Dimension by various
methods by thread Micrometer

Objectives

At the end of this exercise you shall be able to
• Identify elements of thread
• Describe the feature of thread micrometer
• Measuring effective diameter of thread using thread micrometer
• Measuring effective diameter of thread by three wire method.

Following thread elements considered in thread measurement.

1 Pitch of Screw Thread
2 Angle of thread
3 Effective (Pitch) diameter of thread
Screw thread Micrometer: This micrometer is used to measure the effective diameter (pitch diameter) of
screw threads . This dimension is important because the area of the thread flanks in the vicinity of the pitch
line is where the greatest transmission of force occurs between mating parts.

Fig 1

This is very similar to the ordinary micrometer in construction but has facilities to change the anvils.
The anvils are replaceable and are changed according to the profile and pitch of the different systems of
threads. (Figs 2 & 3)
The three-wire method: This method uses three wires of the same diameter for checking the effective diameter
and the flank form. The wires are finished with a high degree of accuracy.
The size of the wires used depends on the pitch of the thread to be measured.
For measuring the effective diameter, three wires are suitable placed between the threads. (Fig 4)
e measuring wires are fitted in wire-holders which are supplied in pairs. One holder has provisions to fix one
wire and the other for two wires. (Fig 5)

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CITS : CG & M - Fitter - Exercise 35