Micrometer / Screw gauge
Screw Gauge
Screw
gauge is a device which can measure very small lengths (or thickness) up to one
– hundredth part of a millimeter. It can be used to measure diameter of a wire,
thickness of a thin metal sheet, etc.
Description:
A Screw Gauge is shown
here and its main parts are described below.
C-Frame:
It’s a c shaped frame as
identified in the picture, is a rigid part that has both holding points for a
job or object to be measured. Its size depends on micrometer measuring range so
size of c frame increases as range expands to bigger.
Its main function is to
provide basic structure of a micrometer in which stationary anvil located at
one end and moveable spindle slides inward or outward trough other end of c
frame.
Zero
adjust screw:
It’s a screw located back
end of anvil shown in figure. As name shows it is to correct or adjust the zero
error of micrometer if there is some error found before the measurement during
test.
Anvil:
As discussed above it is a
small stationary cylindrical part of micrometer located in far end of c-frame
and acts as one holding point for measuring objects. So we can say it’s a one
of rigid measuring and holding point of micrometer.
Spindle:
A cylindrical long part
which is mounted through all other parts sleeve, lock nut and thimble. It is moveable
part and has a connection with ratchet as we rotate the ratchet clockwise or
counter clockwise the spindle slides out or inward to adjust it with compare to
measuring object size.
Anvil
Face and Spindle Face:
Faces of both anvil and
spindle which are opposite to each other are the measuring points of micrometer
and hold the measuring object collectively.
Lock
nut:
As
we know the mechanism of micrometer based on precision ground
threads of spindle so the lock nut works as stationary nut for this mechanism,
so rotation of this mechanism into lock nut controls the spindle movement.
Sleeve:
It’s a barrel type
cylindrical part which mounted on spindle and is main scale of micrometer
because main scale is engraved on the sleeve . Thimble rotates around that
sleeve and spindle. Its main function is indication of reading in millimeter in
case of imperial micrometer.
Thimble:
Thimble is also mounted on
spindle and a scale is engraved around it perimeter of thimble. Scale of
thimble is to show the measurement value in fraction.
Ratchet:
It’s a knurled thumb gripe
to rotate the spindle into desired direction for measuring process, provided
with ratchet action to avoid over tightening of micrometer across the measuring
object and also ensures equal pressure force of each measurement.
Principle of the screw guage:
The
screw gauge works on the principle of screw. When a screw is rotated in a nut,
it exhibits both linear and rotational motions. When a screw is moved in a
fixed nut , the linear distance travelled by the screw on the main scale when
the circular is given one complete rotation is called Pitch of the screw.
Least count of the screw guage:
Least count (L.C) = Pitch of the screw / Total
number of divisions on the circular scale generally, the pitch of the screw
guage is 0.5 mm and it has 50 divisions on its circular scale. Hence, L.C. =
0.5 mm / 50 = 0.01 mm or 0.001 cm
Determination of the diameter’d’ of a wire:
Place
the wire between the stud and spindle end as indicated in the diagram. Rotate
the thimble until the wire is firmly held between the stud and the spindle.
The
ratchet is provided to avoid excessive pressure on the wire. It prevents the
spindle from further movement - squashing the wire!.
To
take a reading first look at the main scale and note the main scale reading
(M.S.R).
Note
the division of the circular scale which coincides with the base line of the
main scale.
This
circular scale division (C.S.D) when multiplied by the least count (L.C) ,gives
the circular scale reading(C.S.R) Add the Main scale reading (M.S.R) and the
circular scale reading (C.S.R).This gives the observed reading.
Hence, Observed Reading
= Main scale reading + Circular scale reading
Observed
Reading = Main scale reading +( Circular scale Division*Least Count)