2. SCREW GAUGE
Aim: To measure the i)Thickness of the Glass Plate ii) Diameter of the metal wire
iii) Volume of the given Glass Plate.
Apparatus : Screw Gauge , Glass Plate and Metal wire .
Description : Screw Gauge consists of U shaped metallic frame. To one side of this U
frame a long hallow cylindrical tube with a nut inside it, the inner side of cylindrical
nut contains a uniform thread cut in it. On the other side of U frame a fixed stud S1
with a plane face is attached.
A screw S2 is fitted in the cylindrical nut. One side of the screw S2 has a plane
face similar to that of stud S1. The faces of S1 and S2 are plane and parallel to one
another. The other end of the screw carries a milled head ‘H’ attached to a cap ‘C’
with a sloping edge. When the head H is rotated, the screw moves ”to and fro” in
the nut.The milled head H is provided with a safety device ‘D’ to rotate the head H.
When the object is held between the stud S1 and screw S2 and the head H is
rotated using the safety device (D), it produces crackling sound when optimum
pressure is applied on the object.
4. SCREW GAUGE
The outer surface of long cylindrical nut consists of a thick horizontal line ‘P’
parallel to the axis of cylindrical tube .This line ‘P’ is called Index line. Along
the index line a scale is graduated in millimeters. This scale is called Pitch
Scale. On the sloping edge of the cap ‘C’ a circular scale is graduated, which
consists of 100 equal divisions, this scale is called Head scale.
Theory : The screw gauge works on the principle of screw.
When we rotate the head ‘H’ by means of safety device ‘D’ through one
complete rotation, the distance moved by the screw for every complete rotation
is constant. This constant distance moved by the screw for one complete
rotation of head ‘ H ‘ is called Pitch of the screw. If the head scale has 100
equal divisions, then the distance moved by the screw for even 1/100 of a
complete rotation can be measured accurately, this is called Least count of
screw gauge.
5. SCREW GAUGE
Pitch of the screw
No. Of divisions on H scale
Therefore Least count (L.C) =
Procedure :First we have to determine the least count of the given Screw
gauge.
To determine the least count of the screw gauge, the head ‘H’ is rotated
through certain (say 5) number of complete rotations .The distance
moved by the sloped edge over the pitch scale is measured.
Now substitute these values in the formula of pitch of the screw =
Pitch of the screw
No. Of divisions on H scale
Therefore Least count (L.C) =
Distance moved by sloped edge over the pitch scale
No. Of rotations of the screw
Pitch of the screw=
6. SCREW GAUGE
Now check whether the given screw gauge has any ZERO ERROR or
not. To determine the ZERO ERROR, the head H is rotated until the flat
end of the screw S2touches the plane surface of the stud s1 (do not
apply excess pressure) i.e we have to rotate the head only by means
of safety device ‘D’ only.
7. SCREW GAUGE
When S1 and S2 are in contact ,the zero of the head scale perfectly
coincides with the index line as in Fig-(a). In such case there will be
no ZERO ERROR and no correction is required.
When S1 and S2 are in contact ,the zero of the head scale is below the
index line as in Fig(b), such ZERO ERROR is called positive ZERO
ERROR, and the correction is negative.
When S1 and S2 are in contact ,the zero of the head scale is above
the index line as in Fig(c) , such ZERO ERROR is called negative
ZERO ERROR, and the correction is positive.
a) Determine the thickness of glass plate : The given object glass plate is
held between the two parallel surfaces of fix stud S1 and screw tip S2.
8. SCREW GAUGE
Note the completed number of divisions on pitch scale, which is called PITCH SCALE
READING (P.S.R). The number of the head scale division coinciding with the index
line is noted, which is called OBSERVED HEAD SCALE READING n’. If the given
screw gauge has ZERO ERROR (x) the correction is made by adding or subtracting
the ZERO ERROR (x) from the OBSERVED HEAD SCALE READING n’. The corrected
value (n’-x) or (n’+x) is called the HEAD SCALE READING (H.S.R) n.
To calculate the fraction the H.S.R (n) is multiplied by the least count (L.C).
Thickness of the Glass plate = Total reading = P.S.R + n× L.C – – – – – (1)
Changing the position of glass plate , 5 readings should be taken, and recorded in
the table-1. Every time calculate the total thickness of the glass plate using equation
(1).
9. SCREW GAUGE
Average of the 5 readings of the glass plate should be calculated, to get the
average thickness(t) of the given glass plate.
Table -1 ( Thickness of glass plate ) :
Average thickness of the glass plate (t) = mm
S.NO.
M.S.R
a cm
Vernier Coincidence
(n)
Fraction
b = n×LC
Total Reading
(a+b) cm
1
2
3
4
5
10. SCREW GAUGE
b) Determine the radius(r) of the given metal wire :The given object metal wire is held
between the two parallel surfaces of fix stud S1 and screw tip S2 .
Note the completed number of divisions on pitch scale, which is called PITCH
SCALE READING (P.S.R). The number of the head scale division coinciding with
the index line is noted, which is called OBSERVED HEAD SCALE READING n’.
If the given screw gauge has ZERO ERROR (x) the correction is made by
adding or subtracting the ZERO ERROR (x) from the OBSERVED HEAD SCALE
READING n’. The corrected value (n’-x) or (n’+x) is called the HEAD SCALE
READING (H.S.R) n.
To calculate the fraction the H.S.R (n) is multiplied by the least count (L.C).
Diameter of the given wire = Total reading = P.S.R + n × L.C
11. SCREW GAUGE
Changing the position of metal wire, 5 readings should be taken, and recorded in
the table-2. Every time calculate the total diameter (d) of the metal wire using
equation (1).
Average of the 5 diameter of the metal wire should be calculated, to get the
average diameter(d) of the given metal wire.
Radius (r) of the metal wire = d/2 mm
Table – 2 (Diameter of the metal wire):
S.NO.
M.S.R
a cm
Vernier Coincidence
(n)
Fraction
b = n×LC
Total Reading
(a+b) cm
1
2
3
4
5
12. SCREW GAUGE
Precautions : i ) Pitch scale reading (P.S.R) should be taken carefully without
parallax error
ii ) Head scale reading (H.S.R) should be taken carefully without parallax
error
iii )Screw must be rotated by holding the safety device ‘D’.
v ) The screw is rotated in only one direction either clock wise or anti-clock
wise to avoid the back lash error.
Observations : i ) Zero error =
ii) Zero correction = mm
iii ) Distance moved by the head for 5 complete revolutions = mm
iv ) Number of head scale divisions =
iv ) Do not apply excess pressure on the object held between the surfaces
S1 and S2.
13. SCREW GAUGE
Pitch of the screw
No. Of divisions on H scale
Vi) Least count (L.C) =
c ) Volume of Glass plate (v) : The length ( l ) , breadth ( b) are determined
using vernier calipers and thickness ( t ) of the glass plate is determined
using screw gauge. The values of l ,b and t are substituted in the equation of
volume.
V = l × b × t
Distance moved by sloped edge over the pitch scale
No. Of rotations of the screw
Pitch of the screw=
V )
14. SCREW GAUGE
What is Pitch of Screw ?
It is the distance traversed by the screw for one complete rotation of head ‘ H ‘
is called Pitch of the screw.
What is the formula to calculate Pitch of Screw
What is Least count of screw gauge ?
What is the formula to calculate Least count of a screw gauge?
Distance moved by sloped edge over the pitch scale
No. Of rotations of the screw
Pitch of the screw=
Minimum amount of thickness which can be measured by the screw
gauge is called it's Least count.
Pitch of the screw
No. Of divisions on H scale
Least count (L.C) =
15. SCREW GAUGE
If Pitch scale is in of.50mm and number of head scale divisions are 50 calculate L.C of the screw gauge.
L.C = Pitch of screw / No of head scale divisions = 0.5/50 =0.01mm
What are the differences between Vernier calipers & Screw gauge.
I) screw gauge is more accurate instrument when compared to vernier calipers. L.C of Screw
gauge is 0.01mm and L.C of Vernier calipers is 0.1mm . Smaller the L.C higher will be the accuracy.
Ii) We cannot measure inner diameters & depths with Screw gauge, But, we can measure them
with Vernier calipers.
