Outside Micrometer – Parts, Principle, Graduation, Reading and Construction

In the ITI Turner trade, precise measurement is essential for producing accurate components. One of the most important precision measuring instruments used for external measurements is the outside micrometer. It is used to measure the outside diameter and thickness of objects with very high accuracy. This chapter explains the construction, parts, working principle, graduation, and reading of the outside micrometer in detail.

Introduction to Outside Micrometer

An outside micrometer, also known as a micrometer screw gauge, is a precision instrument used to measure external dimensions such as diameter, thickness, and length with high accuracy, usually up to 0.01 mm or 0.001 mm.

It is widely used in engineering workshops and manufacturing industries.

Construction of Outside Micrometer

The outside micrometer is made of a rigid frame and a precision screw mechanism. The frame is usually made of steel or cast iron to provide strength and stability.

The main components are mounted on the frame to ensure accurate measurement.

Main Parts of Outside Micrometer

• Frame: U-shaped body that holds all parts
• Anvil: Fixed measuring surface
• Spindle: Movable measuring surface
• Sleeve (Barrel): Contains main scale markings
• Thimble: Rotating part with circular scale
• Ratchet Stop: Ensures uniform pressure
• Lock Nut: Locks the spindle in position

Principle of Outside Micrometer

The outside micrometer works on the principle of a screw and nut mechanism. When the thimble is rotated, the spindle moves forward or backward along the axis.

The linear movement of the spindle is proportional to the rotation of the screw. This allows very small measurements to be taken accurately.

Pitch of Micrometer Screw

The pitch is the distance moved by the spindle in one complete rotation of the thimble.

Example:

• Pitch = 0.5 mm (commonly used)

Graduation of Outside Micrometer

The micrometer has two scales:

1. Main Scale (Sleeve Scale)

• Marked in millimeters
• Each division represents 1 mm or 0.5 mm

2. Thimble Scale

• Divided into equal parts (usually 50 or 100 divisions)
• Each division represents a fraction of a millimeter

Least Count of Outside Micrometer

The least count is the smallest measurement that can be read using the instrument.

Least Count = Pitch / Number of divisions on thimble

Example:

• Pitch = 0.5 mm
• Number of divisions = 50

Least count = 0.5 / 50 = 0.01 mm

Reading of Outside Micrometer

The reading is taken in two parts:

1. Main Scale Reading (MSR)

Read the value on the sleeve scale visible just before the thimble edge.

2. Thimble Scale Reading (TSR)

Note the division on the thimble scale that coincides with the reference line.

Total Reading

Total Reading = MSR + (TSR × Least Count)

Example

MSR = 5 mm
TSR = 25 divisions
Least count = 0.01 mm

Total reading = 5 + (25 × 0.01) = 5.25 mm

Uses of Outside Micrometer

• Measuring external diameter
• Measuring thickness of materials
• Precision measurement in machining

Advantages

• Very high accuracy
• Reliable and precise
• Easy to handle

Limitations

• Limited measuring range
• Requires careful handling

Precautions While Using Micrometer

• Clean measuring surfaces
• Use ratchet for uniform pressure
• Check zero error
• Avoid excessive force

Maintenance of Micrometer

• Keep clean and dry
• Store in case
• Lubricate lightly
• Avoid dropping

Application in Turner Workshop

In the Turner trade, outside micrometers are used for accurate measurement of diameters and thickness of workpieces. They ensure precision and quality in machining operations.

Proper use of micrometers helps in maintaining tight tolerances.

Conclusion

The outside micrometer is an essential precision measuring tool in the ITI Turner trade. Understanding its construction, principle, graduation, and reading method is important for accurate measurement.

With proper use and maintenance, it provides reliable and precise results.

In conclusion, mastering the use of outside micrometer is crucial for producing high-quality components in engineering workshops.