Page 8 - 9-phy-1 Measurements
P. 8
Positive Zero Error
If the zero of circular scale is below the datum line, there is positive zero error.
In Fig 1.3.2(b), zero error is positive where 2nd division of the circular scale
coincides with the datum line. Multiply this number with the least count to
obtain value of zero error.
Positive zero error = 2 × LC = 2 × 0.01 mm = 0.02 mm
DO YOU KNOW? Negative Zero Error
If the zero of the circular scale is above the datum line, there is negative zero
Screw gauge works on the error. In Fig 1.3.2(c) zero error is negative where 46th division of the circular
principle of screw and nut. scale is in front of the datum line. Subtract this number from total number of
When the screw is turned divisions on circular scale (50 – 46 = 4). Multiply resultant number with the
through one revolution, it least count to obtain value of negative zero error.
advances by one pitch Negative zero error = 4 × LC = 4 × 0.01 mm = 0.04 mm
length.
Zero Correction
When the zero Error is negative, the zero correction is positive.
Positive zero correction = + 0.04 mm
When zero error is positive, the zero correction is negative.
Negative zero correction = -0.02 mm
Screw gauge, flat metal strip or kink free wire.
ΠFind the Least Count.
Close the anvil & spindle by rotating the ratchet
till it clicks freely.
Ž Check the zero error and hence calculate the What is the purpose of
ratchet?
zero correction.
! Hold the metal strip between the anvil &
‘
• Add or subtract the ’ spindle.
zero correction as “
condition demands. Tighten it gently by the ratchet till it clicks.
”
• Take the diameter of • Note the main scale reading (M) which is just under or before the sleeve
the wire from
different places. of the circular scale.
• Do not press the jaws Note the division (n) on the circular scale which coincides with the
of the screw gauge
very hard. datum line.
• Do not rotate thimble Multiply this number with the least count and
unnecessarily in
backward or forward calculate the fractional value as (X)
direction.
X = n × LC
• Rotate the screw
gently using the Add the values ‘M’ and ‘X’ to obtain the
ratchet.
thickness of the strip (or diameter of wire). Why the metal strip should
Apply zero correction to determine the correct be flat or wire be kink free?
value of thickness t = Y ± ZC
Take two more readings at different positions and find the average
thickness.
8
If the zero of circular scale is below the datum line, there is positive zero error.
In Fig 1.3.2(b), zero error is positive where 2nd division of the circular scale
coincides with the datum line. Multiply this number with the least count to
obtain value of zero error.
Positive zero error = 2 × LC = 2 × 0.01 mm = 0.02 mm
DO YOU KNOW? Negative Zero Error
If the zero of the circular scale is above the datum line, there is negative zero
Screw gauge works on the error. In Fig 1.3.2(c) zero error is negative where 46th division of the circular
principle of screw and nut. scale is in front of the datum line. Subtract this number from total number of
When the screw is turned divisions on circular scale (50 – 46 = 4). Multiply resultant number with the
through one revolution, it least count to obtain value of negative zero error.
advances by one pitch Negative zero error = 4 × LC = 4 × 0.01 mm = 0.04 mm
length.
Zero Correction
When the zero Error is negative, the zero correction is positive.
Positive zero correction = + 0.04 mm
When zero error is positive, the zero correction is negative.
Negative zero correction = -0.02 mm
Screw gauge, flat metal strip or kink free wire.
ΠFind the Least Count.
Close the anvil & spindle by rotating the ratchet
till it clicks freely.
Ž Check the zero error and hence calculate the What is the purpose of
ratchet?
zero correction.
! Hold the metal strip between the anvil &
‘
• Add or subtract the ’ spindle.
zero correction as “
condition demands. Tighten it gently by the ratchet till it clicks.
”
• Take the diameter of • Note the main scale reading (M) which is just under or before the sleeve
the wire from
different places. of the circular scale.
• Do not press the jaws Note the division (n) on the circular scale which coincides with the
of the screw gauge
very hard. datum line.
• Do not rotate thimble Multiply this number with the least count and
unnecessarily in
backward or forward calculate the fractional value as (X)
direction.
X = n × LC
• Rotate the screw
gently using the Add the values ‘M’ and ‘X’ to obtain the
ratchet.
thickness of the strip (or diameter of wire). Why the metal strip should
Apply zero correction to determine the correct be flat or wire be kink free?
value of thickness t = Y ± ZC
Take two more readings at different positions and find the average
thickness.
8
