Screw gauge is an instrument which works based on the principle of micrometer screw. When the width of the material to be measured is close to 0.01mm, screw gauge is used. Screw gauges are typically used to measure diameter of wires/rods, also used to measure thickness of metal sheet and so on. In a standard screw, each thread is separated by a predetermined distance and the thread separation equals the distance traveled by a screw in forward direction in one complete rotation. In other words, when a screw is rotated in a nut, the distance moved by the tip of the screw is directly proportional to the number of rotations.
Parts of screw gauge
The Screw Gauge consists of a ‘U’ shaped metal frame
A hollow cylinder is attached to one end of the frame
Pitch Scale-Sleeve
Thimble -Head/Circular scale
Ratchet
S1-spindle
S2-Anvil
Pitch of the Screw gauge
Pitch refers to the linear distance between a screw's threads (1mm or 0.5mm).
Pitch scale
It is a scale running parallel to the axis of the screw. It is marked in mm. The spindle of the screw passes through the sleeve cylinder
Thimble/Head scale
The screw is connected to a hollow cylinder which rotates along with nut on turning. i.e., thimble. It is a circular scale marked in mm, attached to the screw. The thimble of a screw gauge has 50 divisions for one rotation. The spindle advances 1mm when the screw is turned through two rotations.
Least Count of a Screw Gauge
The least count of screw gauge is 0.01mm which indicates that one can measure the dimension in steps of 0.01mm.
Least Count can be defined as the minimum value measured by a screw gauge.
In screw gauge with 0-50 division on head scale
Least count =0.5/50=0.01mm
Errors in a screw gauge
To get accurate readings, these unmercenary errors should be avoided.
Zero Error of a Screw Gauge
If the zero of the head scale coincides with the pitch scale axis, there is no zero error.
n-number of divisions
Zero Error Z.E = (n x L.C) = (0 x 0.01)
Zero Correction, Z.C = 0 mm
Positive Zero Error
If the zero of the head scale lies below the pitch scale axis, the zero error is positive.
Eg., 7 th division of the head scale coincides with the pitch scale axis. then the zero error is positive) and is given by,
Z.E = + (n x L.C)= + (7 x 0.01)
Zero Correction Z.C = – (7 x 0.01) =-0.07mm
Negative Zero Error
If the Zero of the head scale lies above the pitch scale axis, the zero error is negative. E.g., 48 th division coincides with the pitch scale axis, then the zero error is negative and is given by,
Z.E = – (50 – 48) x 0.01
Zero Correction Z.C = + (50 – 48) x 0.01=+0.02mm