Screw Gauge
A micrometer, sometimes known as a micrometer screw gauge, is a device incorporating a
calibrated screw widely used for accurate measurement of components in mechanical
engineering and machining as well as most mechanical trades, along with other metrological
instruments such as dial, vernier, and digital calipers.
Construction
It consists of a U-shaped metal frame F with a fixed stud A.
1. The right moveable and R (called Ratchet) is a cylinder type whose other end is called
spindle.
2. A static linear scale or the main scale (usually in mm) is marked on a cylinder.
3. The second scale (moveable) is called a circular scale or a cap scale (with 50 or 100
divisions) which is drawn on a cap called thimble ‘T’ fitted over a cylinder.
4. The circular scale is designed in such a way that one complete rotation of the cap moves
it horizontally by 1 mm on the main scale if circular scale cap moves it horizontally by 1 mm
on the main scale if circular scale division is 100 and 0.5 mm in the main scale if the circular
division is 50.
Least Count
A screw gauge is also known as a micrometer is a precision instrument.
Least count=pitch/number of division of thimble.
5/50=0.01 mm.
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One division of thimble =0.01 mm with reference to straight line on sleeve. This is least
count of screw gauge or micrometer.
Least Count Formula
Pitch of screw gauge
least count=
Total no of division on circular scale
Pitch of Screw gauge
The pitch of the screw is the distance moved by the spindle per revolution. To find this, the
distance advanced by the head scale over the pitch scale for a definite number of complete
rotations of the screw is determined.
Zero Error
In case of vernier calipers it occurs when a zero on main scale does not coincide with
a zero on vernier scale.
Positive zero error
Negative zero error
Zeroerror =Noof line × least count of SG
How to use screw gauge?
Let us find the diameter of Wire:
Fix the given wire between the stud and spindle by turning the ratchet in the clockwise
direction.
Note the main scale reading (M.S.R) (up to complete division visible on the horizontal
line just beyond the circular scale.
Note the circular scale division says ‘n’ which is exactly in front of the index line or
reference line.
Multiply ‘n’ with the least count (L.C) to get a fraction of the circular scale reading
(C.S.R) to be added into the main scale reading (M.S.R).
The diameter of the wire will be the diameter of wire= (MSR+ C.S.R) + zero correction.
Repeat the experiment at least three times by gipping the wire at different places to obtain
its average diameter.
Practical Related to Vernier Caliper:
Calculate the area of ball bearing by measuring its diameter using Screw Gauge.
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Observation and Calculations
Least count =
Zero Error =
Table:
S.No Main scale No of line Circular Diameter Zero Correction Diameter of
reading Matching Scale (mm) Error (mm) BB
(mm) reading Y = X = X’ (mm) (mm)
X (mm) Y’ = Y+Z.E
X’
+ +¿
Mean Diameter = 3 ¿ mm
Diameter
radius= mm
2
Area=π r 2 mm2
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