Page 67 - CITS - Fitter - Trade Theory

FITTER - CITS

Outside micrometer (metric)

Definition of outside micrometer
A micrometer is a precision instrument used to measure a job, generally within an accuracy of 0.01 mm.
Micrometers used to take the outside measurements are known as outside micrometers.
The parts of a micrometer are listed here.
Frame: The frame is made of drop-forged steel or malleable cast iron. All other parts of the micrometer are
attached to this.
Barrel/Sleeve: The barrel or sleeve is fixed to the frame. The datum line and graduations are marked on this.
Thimble On the bevelled surface of the thimble also, the graduation is marked. The spindle is attached to this.

Spindle: One end of the spindle is the measuring face. The other end is threaded and passes through a nut. The
threaded mechanism allows for the forward and backward movement of the spindle.
Anvil: The anvil is one of the measuring faces which is fitted on the micrometer frame. It is made of alloy steel
and finished to a perfectly flat surface.
Spindle lock nut: The spindle lock nut is used to lock the spindle at a desired position.
Ratchet stop: The ratchet stop ensures a uniform pressure between the measuring surfaces.

Working principle of outside micrometre
The micrometer works on the principle of screw and nut. The longitudinal movement of the spindle during one
rotation is equal to the pitch of the screw. The movement of the spindle to the distance of the pitch or its fractions
can be accurately measured on the barrel and thimble. On the barrel a 25 mm long datum line is marked. This
line is further graduated to millimetres and half millimetres (i.e. 1 mm & 0.5 mm). The graduations are numbered
as 0, 5, 10, 15, 20 & 25 mm. The circumference of the bevel edge of the thimble is graduated into 50 divisions
and marked 0-5-10-15..................... 45-50 in a clockwise direction.
In metric micrometers the pitch of the spindle thread is 0.5 mm. Thereby, in one rotation of the thimble, the spindle
advances by 0.5 mm
The distance moved by the spindle during one rotation of the thimble is 0.5 mm. Movement of one division of the
thimble = 0.5 x 1/50 = 0.01 mm
Least count of an outside micrometer
- Pitch of a spindle thread is 0.5 mm in one rotation of thimble spindle advances 0.5 mm.
- On barrel 25 mm datum line is further graduated to mm and half mm and numbered as 0,5,10,15,20,25 mm.
- Circumference of bevel edge of thimble is graduated into 50 equal divisions.
= Movement of one division of thimble.
0.5 ×1 ÷ 50 = 0.01 mm

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CITS : CG & M - Fitter - Lesson 10