Micrometer Screw gauge
Micrometer Screw gauge
It is the instrument used for precise measurement of
quantities such as length, diameter and thickness. It consist of two scales a
main scale & circular scale.
Pitch: - It is the distance between two consecutive threads of
a screw
or It is the distance moved in one
rotation by screw.
Zero error - When the screw of the micrometer completely touches
the fixed stud; the zero of the circular scale does not coincide with the reference
line then instrument have zero error. It is of two types – (i) positive error (ii) negative error
Correction of error
Steps
1. Count the number of division beyond the reference line.
2. multiply step 1 with L.C
3. For positive error subtract the data of step 2 with total reading & for negative error add the data of step 2 with total reading.
E.g.
for fig (b)
number of division beyond the reference line = 2
2 x L.C = 2 x 0.01 mm = 0.02 mm
Let total reading is 10 mm
then correct reading = 10 - 0.02 = 9.98 mm
similarly for fig (c)
number of division beyond the reference line = 4
4 x L.C = 4 x 0.01 mm = 0.04 mm
Let total reading is 10 mm
then correct reading = 10 + 0.04 = 10.04 mm
Steps
1. Count the number of division beyond the reference line.
2. multiply step 1 with L.C
3. For positive error subtract the data of step 2 with total reading & for negative error add the data of step 2 with total reading.
E.g.
for fig (b)
number of division beyond the reference line = 2
2 x L.C = 2 x 0.01 mm = 0.02 mm
Let total reading is 10 mm
then correct reading = 10 - 0.02 = 9.98 mm
similarly for fig (c)
number of division beyond the reference line = 4
4 x L.C = 4 x 0.01 mm = 0.04 mm
Let total reading is 10 mm
then correct reading = 10 + 0.04 = 10.04 mm
Taking a measurement
- First find whether there is an error on the micrometer.
- Then find its least count using the pitch.
- Adjust the object between stud & screw by moving the circular scale; rotate the ratchet for confirmation if it produce sound.
- Find the main scale reading. If main scale shows an additional reading; we have to add that as well.
- Then find the value of the circular scale that coincides with the main scale.
- Multiply the circular scale reading with the L.C.
- Finally add the step 6 to the main scale reading.
Total reading = M.S.R + (C.S.D x L.C)
Correct reading = total reading + Error
Correct reading = total reading + Error
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