1. MICROMETER
The micrometer is divided from two greek words. Micro means small or little and meter
means a measurement. It is easily understood that the word micrometer means, it is an
instrument made to measure the length or diameter of a piece of job within an accuracy of 0.01
mm.
PARTS OF A MICROMETER
THIMBLE
BARREL/SLEEVE
LOCK NUT
FRAME
ANVIL SPINDLE
RATCHET STOP
FRAME:- It is a semicircular in shape made of drop-forged steel or malleable cast iron.
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.
2. SPINDLE:-
BARREL/SLEEVE:-
THIMBLE:-
RATCHET STOP:-
LOCK NUT:-
One end of the spindle is the measuring face. The other end is threaded and
passes through a nut. The threaded mechanism allows for forward and backward
movement of the spindle.
The barrel or sleeve is fixed to the frame. It is a hollow piece of steel heaving
internal threads on some length. The datum line and graduations are marked on
this.
This is a hollow cover rigidly fixed to the spindle when the thimble is rotated the
spindle rotates. The bevelled edge of the thimble is divided into 50 equal circular
divisions.
The ratchet stop ensures a uniform pressure between the measuring surfaces
The lock nut is used to lock the spindle at a desired position.
3. WORKING PRINCIPLE:-
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.
GRADUATIONS:-
In metric micrometers the pitch of the spindle thread is 0.5 mm.
There by, in one rotation of the thimble, the spindle advances by 0.5 mm.
On the barrel a 25 mm long datum line is marked. This line is further graduated in to
millimeters and half millimeters ( 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, 20, 25, 30, 35, 40, 45, & 0 in clockwise direction.
LEAST COUNT OF METRIC MICROMETER:
In one complete rotation of the thimble, the spindle is forward or backward 0.5 mm.
The circumference of the bevel edge of the thimble is graduated into 50 divisions.
So value of one division of thimble is 0.5/50 = 0.01 mm.
RANGES OF OUTSIDE MICROMETER
Outside micrometers are available in ranges of 0 to25 mm,25 to 50 mm, 50 to 75 mm, 75 to 100
mm, 100 to 125 mm & 125 to 150 mm. It is specified by the minimum and maximum dimension
which can be measured by it.
For all ranges of micrometers, the graduation marked on the barrel is only 0 - 25 mm
A test piece is given for testing of minimum capacity of micrometer
4. READING OF MICROMETER MEASUREMENTS
Show the reading 33.12 mm
At first note the minimum range = 25.00 mm
No of division above datum line = 8 =8x1 mm = 8.00 mm.
No of thimble division in line with the barrel datum line= 12x 0.01 = 0.12 mm.
TOTAL READING = 33.12 mm
SHOW THE READING 5.78 mm
At first note the minimum range (0-25 mm) = 00.00 mm
No of division above datum line = 5 =5x1 mm = 5. 00mm.
No of thimble division in line with the barrel datum line= 27x 0.01 = 0.28 mm.
No of division below the datum say 1 , 1x0.5 mm = 0.50 mm
TOTAL READING = 5.78 mm
5. A) POSSITIVE ERROR:-
B) NEGATIVE ERROR:-
ERRORS IN MICROMETER -
When the faces of the anvil and the spindle contact closely, the
datum line and the zero of the thimble must coincided. If the thimble
does not, it is sure that the micrometer has got some error.
There are two kinds of errors such as -
While the faces of the anvil and spindle are in contact, the zero on
the thimble does not reach the datum line. The number of divisions
requires to line up, that number of divisions is called positive (+) error
The zero line on the thimble passes over the datum line at the
time of measuring faces are in contact. The number of divisions
passes over the datum line is called negative (-) error.
VERNIER MICROMETER:-
In ordinary micrometers we can measure to an accuracy of 0.01 mm. But in vernier micrometers
we can measure to an accuracy of 0.001 mm.
Vernier micrometers are similar to ordinary micrometers in construction but additional vernier
graduations given above the datum line.
There are 10(ten) such vernier graduation lines marked parallel above the datum line.
The sapce between these 10lines is equal to 9 divisions in the thimble.
The value of 10 vernier division is = (0.01x9) = 0.09 mm
The value of one vernier division =0.09/10 mm =0.009mm
The least count = 1 thimble division -1 vernier division = (0.01 -0.009) mm = 0.001 mm.
6. Show the reading in vernier micrometer 5.783mm
Range of micrometer is 0 t0 25 mm
Full mm division visible on the barrel =5x1mm=5.00mm
Half mm division visible after the full mm division
=1x0.5mm=0.50mm
Thimble division below the datum line =28x0.01mm=0.28mm
Vernier division coinciding with thimble =3x0.001mm =0.003mm
Total reading=5.783mm
