An inside micrometer is used to measure large internal dimensions, such as the internal diameter of holes, bores, and registers. It is especially useful where a vernier caliper cannot provide sufficient accuracy.

Types of Inside Micrometer

There are two main types of inside micrometers used in industry:

1. Analog Inside Micrometer

2. Digital Inside Micrometer

• Least count (Analog): 0.01 mm

• Least count (Digital): 0.001 mm

Measurements can be taken in millimetres or inches, depending on the instrument. Inside micrometers are available in various designs and ranges to suit different internal measurement requirements.

Construction of Inside Micrometer

A typical inside micrometer consists of the following parts:

• Measuring head (micrometer unit)

• Extension rods

• Spacer (spacing collar)

• Handle

Measuring Head

The measuring head includes:

• Thimble – has circular markings and provides readings up to 50 divisions, known as thimble division (TD)

• Sleeve (barrel) – carries the main scale and sub-scale, known as main scale division (MSD), usually up to 25 mm

Anvil and Rod Attachment

At the front end of the measuring head (anvil):

• A hole is provided to insert extension rods

• A locking screw is used to secure the extension rod firmly

• Another hole is provided for fitting the handle

At the rear end, a screw is provided for thimble adjustment.

Handle

The handle is used to insert and support the inside micrometer when measuring deep holes or bores.

Range of Inside Micrometer

The measuring head and spindle are made of high-quality steel, while the measuring faces are made of high-grade tool steel for wear resistance.

Extension Rod Ranges

Extension rods are used to measure large internal diameters and are available in the following ranges:

• 50–75 mm

• 75–100 mm

• 100–125 mm

• 125–150 mm

• 150–175 mm

• 175–200 mm

For smaller diameters, a spacer (typically 12 mm) is used along with the measuring head.

How to Measure Internal Diameter Using an Inside Micrometer

Some inside micrometers are also designed with one fixed and one movable contact end, similar to jaws.

The working of an inside micrometer is similar to an external micrometer and involves three basic steps:

1. Fixing the extension rod and spacer as per the required range

2. Adjusting the thimble using the screw to expand the measuring faces

3. Taking the measurement by gently rocking the micrometer to find the maximum reading

Accurate and careful measurement is essential to avoid measurement errors, which may lead to leakages, poor fits, or assembly issues.