SUBJECT: MECHANICAL MEASUREMENT &

METROLOGY

TOPIC: MICROMETER

(PRECISION MEASURING INSTRUMENT)

Semester:IInd sem.of Ist Year DME

MICROMETER

• INTRODUCTION:

• MAIN PARTS OF MICROMETER

• PRINCIPLE

• LEAST COUNT

• READING OF MICROMETER

• USE

INTRODUCTION:

• A micrometer allows a measurement of the size of a body.

• It is one of the most accurate mechanical devices in common use.

MAIN PARTS OF MICROMETER• U-shaped steel frame : The outside micrometer has U or C shaped frame. It holds all the

parts of micrometer together .The frame is generally made of steel, cast steel, malleable C. I. of light alloy. It is desirable that the frame of Micrometer be provided with conveniently placed finger grips of heat insulating material.

• Anvil and Spindle : The Micrometer has fixed anvil protruding 3 mm fro the left hand side of the frame. The diameter of the anvil is the same as that of the spindle. Another movable anvil is provided on the front of the spindle. The anvil are accurately ground and lapped. The spindle engages with the nut.

MAIN PARTS OF MICROMETER cont.

• Lock nut : A lock nut is provided on the micrometer spindle. It is used to lock the spindle when the Micrometer is at its

correct reading.

• Sleeve or Barrel: The sleeve is accurately divided and clearly marks in 0.5 mm

division along its length, which serves as main scale. It is chrome platted and adjustable for zero setting.

MAIN PARTS OF MICROMETER cont.

• Thimble : Thimble can be moved over the barrel. It has 50 equal division around its circumference. Each division having a value of 0.01 mm.

• Ratchet :

The ratchet is provided at the end of thimble. It is used for accurate measurement.

PRINCIPLE OF MICROMETER

Micrometer works on the principle of screw and nut.

We know that when a screw is turned through a nut through one revolution, it advances by one pitch distance i.e. one revolution of the screw corresponds to linear movement of a distance equal to pitch of the thread.

LEAST COUNT OF MICROMETER

• Least count is the minimum distance which can be measured accurately by the instrument.

L. C. of Micrometer = ___Pitch of spindle screw___

No. of divisions on the Thimble

Suppose the pitch of the spindle screw is 0.5 mm and Number of divisions on the thimble are 50

Therefore, Least Count of Micrometer = 0.5 / 50 =0.01 mm