2.

Screw Gauge

Aim:
To measure
(a) diameter of a given wire
(b) thickness of a given sheet using screw gauge.

Apparatus Required:
Screw Gauge, wire,metal sheet

Principle:
A Screw Gauge is an instrument of higher precision than a Vernier Callipers. In any
ordinary screw, there are threads and the separation between any two consecutive
threads is the same. The distance advanced by the screw when it makes its one
complete rotation is the separation between two consecutive threads. This distance is
called the Pitch (p) of the screw. It is usually 1 mm or 0.5 mm.

Pitch of the Screw Gauge

The linear distance covered by the tip of the screw in every rotation of the circular scale
is called the pitch of a screw gauge. This movement of the spindle is shown on an
engraved linear millimeter scale on the sleeve. To find the pitch, give full rotation to the
screw and note the distance advanced by the circular scale over the pitch scale.

The pitch can be represented as:

Least Count of the Screw Gauge

On the thimble there is a circular scale which is divided into 50 or 100 equal parts. We
are using a screw gauge which has 50 circular divisions. The Least count (LC) is the
distance moved by the tip of the screw, when the screw is turned through 1 division of
the circular. The least count can be calculated using the formula;

Determination of Zero Error:

When the stud and spindle are brought in contact with each other, the zero of circular
scale should coincide with reference line of main scale. In that case the screw gauge
have no zero error. However, when the zero of circular scale does not coincide with
reference line of main scale, the screw gauge is said to have zero error.

The zero error is said to be positive zero error if on bringing the spindle in contact with
stud, if the zero of the circular scale lies to the bottom of the reference line Owing to
this error, the measured readings will be systematically bigger than the actual value by
the same amount. Hence the error is to be subtracted from the observed readings. If on
the other hand, the zero of the circular scale lies to the top of the reference line it is said
to be negative

zero error. Owing to this error, the measured readings will be systematically smaller
than the actual value by the same amount. Hence the error is to be added from the
observed readings.

To determine the error, bring the spindle in contact with the stud and note the reading on
the linear as well as circular scale. If the linear scale reading is x and circular scale
reading is n’ then zero error is given by ± (x + n’ × LC ). Zero correction (e) is always
negative of zero error. In our case, as shown in the fig. 2.3, the linear scale reading is
zero and the circular scale zerois 2 divisions bellow the reference. Therefore, the zero
error is: -[0 + 2 × 0.02] = – 0.04 mm.

So, the Zero correction (e) is = -[-0.04] = 0.04 mm.

= Measured reading – (-0.04) for positive error

Procedure:

1. Find the value of one linear scale division (L.S.D.).

Measurement of diameter of the wire

1. Bring the spindle B in contact with the stud A and calculate the zero error. If there
is no zero error, then note down zero error nil.
2. Move the face B away from face A. Place the wire lengthwise (as shown in the
fig.2.5) over face A and move the face B towards face A using the ratchet head
R. Stop when R turns (slips) without moving the screw with click sound.
3. Note the number of divisions of the main scale reading (M.S.R) visible before the
edge of circular scale.
4. Note the number (n) of the division of the circular scale lying over reference line.
5. Repeat steps 5 and 6 after rotating the wire by 90° for measuring diameter in a
perpendicular direction.
6. Repeat steps 4, 5, 6 and 7 for five different positions separated equally
throughout the length of the wire. Record the observations in table.
7. Find observed diameter and apply zero correction in each case.
8. Take mean of different values of actual diameter.
9.
Measurement of thickness of a given sheet
1. Repeat steps 1, 2, 3, 4, 5 and 6. Instead of wire place the rigid sheer between
face A and B.
2. Find the thickness of the sheet as shown in fig. 2.5 at five different position of the
sheet, spread over the surface of the sheet.
 3. Record the observations in the table 2.2.
4. Find the observed thickness and apply zero correction in each case.
5. Take mean of different values of actual thickness.

RESULT
The diameter of the given wire as measured by screw gauge is ... mm

PRECAUTIONS
1. Rachet arrangement in screw gauge must be utilised to avoid undue
pressure on the wire as this may change the diameter.
2. Move the screw in one direction else the screw may develop “play”.
3. Screw should move freely without friction.
4. Reading should be taken atleast at four different points along the
length of the wire.
5. View all the reading keeping the eye perpendicular to the scale to
avoid error due to parallax.

SOURCES OF ERROR
1. The wire may not be of uniform cross-section.
2. Error due to backlash though can be minimised but cannot be
completely eliminated.

Diagram:
OBSERVATIONS AND CALCULATION
The length of the smallest division on the linear scale = ... mm
Distance moved by the screw when it is rotated
through x complete rotations, y = ... mm
Pitch of the screw = y
x
= ... mm
Number of divisions on the circular scale n = ...
Least Count (L.C.) of screw guage
=
pitch
No. of divisions on the circular scale
= ... mm
Zero error with sign (No. of div. × L. C.) = ... mm
Measurement of the diameter of the wire

Mean diameter = ... mm