MICROMETER(SCREW-GAUGE)
A screw gauge is a measuring instrument used for measuring very small distances of the order of 1/100th part of a mm.It was first invented by William Gascoigne (England) in the 17th Century.
It is used for measuring diameter of circular objects .
Example:
1)thickness of a needle
2)thickness of a wire
PRINCIPLE:
It works on the principle of a Screw.A Screw moves forward or backward linearly when its head is rotated forward or backward.
PITCH:
It is defined as the horizontal distance moved forward by the screw when it's head is given a complete rotation.The distance moved by the screw for every complete rotation is constant.The constant distance moved by the screw for one complete rotation of head is called pitch of the screw.
Pitch=Distance moved by the screw/No of complete rotations
The pitch of the screw gauge is usually 1mm or 0.5mm
CONSTRUCTION:
A Screw gauge consist of the following main parts:
1)Main Scale
2)Circular Scale
3)Studs
4)U shape metallic frame
MAIN SCALE:
A millimeter scale is engraved along the (line of graduation) reference line. This scale is called the main scale.
CIRCULAR SCALE:
It is the head scale or circular scale.It has 100 equal divisions .
STUDS:
There are two studs which serves as the two jaws.
LEAST COUNT/ACCURACY:
It is the smallest distance that the instrument can measure (correctly).
The accuracy of screw gauge is 0.001cm.
RATCHET:
The scale can be moved forward or backward with a ratchet arrangement.
When the screw moves forward and the studs come in contact the ratchet becomes free.It indicates that the contact has been established.
ZERO ERROR:
When the two studs of a screw gauge are brought in contact with each other, the zero of the circular scale should coincide with the reference line on main scale.Then there is no zero error.
When the zero of the circular scale does not coincide with the reference line, the screw gauge is said to have zero error.
POSITIVE ZERO ERROR:
The zero error is positive if on bringing studs in contact,the zero of the circular scale is below the reference line on the main scale.
NEGATIVE ZERO ERROR:
The zero error is negative if on bringing studs in contact, the zero of the circular scale is above the reference line on the main scale
Calculation of Least Count:
Least Count
=Pitch/No of divisions on circular scale
=1mm/100
=0.1cm/100
=0.001cm
READINGS:
=Linear scale reading+ Circular scale reading
=Linear scale reading+n×Least Count
STEPS TO TAKE THE READINGS OF SCREW GAUGE:
1)Read and record the reading of main scale .
2)Read and record the readings of Circular scale at the point where the main scale and circular scale coincide.
3)Multiply the reading of circular scale by the least count of screw gauge (0.001cm)
4)Add the result of step 1 and step 3 to get the final readings.
Every thing is clear 😍👍
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