Introduction for Screw Gauge Measurement:
Screw gauge: The screw gauge is used to measure the lengths of the given object accurately. Thickness of wire, plate and sheet of paper can be accurately determined using the screw gauge. The lengths are measured by finding the distance advanced by the tip of a fine screw.
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The screw gauge is in the shape of a U which is in metal frame.A hollow cylinder with fixed thread cut inside is attached to one end of the metal frame. A fine screw can work inside the cylinder. The screw gauge also has a pitch scale. A pitch scale is a millimeter scale, marked on the cylinder which is parallel to the axis of the screw.
A sleeve with a beveled edge is attached to the head of the screw and the edge is divided into 100 equal divisions which is called as head scale. The tip of the screw has a plane surface. A stud with a plane parallel surface is fixed to the other end of the U-shaped frame. A ratchet attached to the head of the screw prevents excessive tightening of the screw.
The principle of screw is used for working the screw gauge. When a screw working inside a fixed nut is rotated over an axis its tip advances through a distance proportional to the number of rotations of the screw.
The linear distance through which the tip moves for one rotation is equal to the distance among any two consecutive threads which is called pitch of the screw.
Pitch and least count of a screw gauge:
Put the zero of the head scale coincide with any division of the pitch scale and now write down the reading of pitch scale. Now produce 10 full rotations to the head scale and now write the change in the pitch scale reading.
Therefore the pitch is written as
Pitch = Distance moved on the pitch scale / Number of rotations
Distance moved by the screw gauge for 10 rotations = 10 mm
Distance moved by the screw gauge for one rotation = `(10)/(10)` =1 mm
Then the pitch = 1 mm
Least count: When the screw is turned through 1 division of the head scale the distance moved by the tip of the screw is called least count of the screw.
That is, the least count= Pitch of the screw / Number of head scale decisions.
= `1/(100)` =0.01 mm
The measurement of head scale reading is = corrected head scale coincidence `xx` Least count
The measurement of thickness of the wire = pitch scale reading + head scale reading
All the measured values should be tabulated for finding the thickness. The same measurement procedure should be repeated for various positions of the wire. Finally average value should be calculated. It the thickness of given wire.