Page 130 - CITS - Fitter Trade Pactical - Volume - 2

FITTER - CITS

EXERCISE 37 : Demo on Thread Micrometer and its

applications

Objectives
At the end of this exercise you shall be able to
• use screw thread micrometer.
• measurement pitch diameter of V thread
• measuring effective diameter of thread using three wire method.

A screw thread micrometer, unlike a regular micrometer, is specifically designed to measure the key dimensions
of screw threads. These dimensions include the:
• Pitch diameter: This is the diameter measured between the flanks of the thread, essentially the average
diameter of the screw.
• Major diameter: This is the diameter measured at the peaks of the threads.
• Minor diameter: This is the diameter measured at the roots of the threads.
Here’s a demonstration of how to use a screw thread micrometer:
1 Setting the anvils:
• Most screw thread micrometers have two sets of anvils: a fixed anvil and a movable spindle with a measuring
anvil.
• Some micrometers may have interchangeable anvil tips for different thread sizes.

• Consult the micrometer’s manual for specific instructions on setting the anvils for the desired thread type
(e.g., metric, Unified Coarse Thread - UNC, etc.).
• There might be setting standards or gage blocks included to ensure accurate anvil positioning.
2 Measuring the thread:
• Place the screw thread between the anvils, with the flanks of the thread fitting into the V-shaped grooves
on the anvils.
• Gently rotate the ratchet thimble until the anvils make contact with the screw thread, but avoid overtightening.
A ratchet stop mechanism is usually present to provide a consistent measuring force.
• Read the measurement on the micrometer’s sleeve and thimble scales. The reading will typically be the
pitch diameter of the thread

115

CITS : CG & M - Fitter - Exercise 36