SCREW GAUGE
What is Screw Gauge?
A screw gauge is an instrument that accurately measures small lengths and diameters. The balance is comprised of a calibrated screw and a scale. Fine threads, one turn, and a short distance. Turning the screw causes movement of the measuring spindle against or away from the reference surface that stays put. The screw gauge has a scale from which one can take readings. This is an exact measure of an object being studied.
It consists of two scales: The pitch Scale and Circle Scale.
Pitch scales
Pitch scale determines how far does the screw move in one turn. It assists in making sure that actual determination is done by tracking the screw movement and its turns.
Circle scale
A numerical scale that runs along the periphery of the screw is known as the circular scale. It’s just like a ruler but in a circular shape.
Total Reading = Pitch Scale Reading + Circular Scale Reading × least count of the gauge
Screw Gauge Formula
pitch
This is the amount of travel of the screw for every turn which it completes. The formula for pitch of the Screw is:
Screw Gauge Pitch = Distance Moved By Screw/Number of Full Rotation Given
Least Count
The screw gauge measures it precisely, the smallest measure. The formula for least count (LC) is:
Least Count = Pitch/Total Number of Division on Circular Scale
Least count of micrometer screw gauge = (1 mm)/(100) = 0.01 mm
Micrometer screw gauge is defined as an instrument that is used for measuring the diameter of thin wires, the thickness of small sheets such as glass or plastics.
Least count of micrometer =
Least count of
Parts of Screw Gauge
Screw :
The measurement element of this screw gauge is the main feature.
Anvil :
In this case, the anvil has an object placed on it, and the spindle moves against the object.
Spindle :
As the thimble is turned, the spindle will move towards the anvil.
Thimble :
The moving part of the screw gauge is called this.
Thimble lock :
In order to fix the spindle when making a measurement this part is put under tension. It is sometimes called lock-nut or lock-ring.
Ratchet stop :
This keeps the thimble from revolving any more than is necessary for it.
Barrel/Main Scale :
This is referred to as a sleeve or stock, which is one of the non-movable components marked either linearly or Vernier-wise, as displayed in the diagram of screw gauge.
Working Principle of a Screw Gauge
A screw gauge, or micrometer screw gauge, measures small dimensions with high precision. It operates on the principle of a finely threaded screw converting rotational motion into linear displacement.
Key Components:
Frame: Holds all parts together.
Anvil: Fixed end where the object is placed.
Spindle: Movable screw that advances towards the anvil.
Thimble: Rotates and is marked with a scale.
Sleeve: Stationary part with a linear scale.
Ratchet Stop: Ensures uniform pressure.
Lock: Secures the spindle in place during measurement.
Measurement Process:
Pitch: Distance the spindle moves per thimble rotation (e.g., 0.5 mm per rotation).
Least Count: Smallest measurable value, calculated as ( \text{Pitch} / \text{Thimble Divisions} ). For example, if the pitch is 0.5 mm and there are 50 thimble divisions, the least count is ( 0.5 , \text{mm} / 50 = 0.01 , \text{mm} ).
Main Scale Reading (MSR): Reading on the sleeve.
Thimble Scale Reading (TSR): Aligned division on the thimble.
Calculation: [ \text{Total Reading} = \text{MSR} + (\text{TSR} \times \text{Least Count}) ]
Errors in Screw Gauge
There are many reasons why errors may occur during the screw gauge measurement. Here are some common errors associated with screw gauges:
Zero Error
Positive Zero Error: Thimble scale zero mark moves ahead of main scale zero mark when anvil and spindles touch leading to positive zero error (+Z).
Negative Zero Error: Thimble scale zero marking lags behind main scale zero mark whenever contact occurs between the anvil and spindles thus resulting into negative zero error (-Z)
Parallax Error: It happens where the eye does not run parallel with the scale of measurements causing misreading at the eye level. To do this successfully, make sure your eye is upright with scale line – if you want to get clear information.
Frictional Errors: These may result in mistakes, particularly in accurate readings. This error can be minimized with regular cleaning and lubrication.
Wear and Tear: Continuous usage may lead to wear and tear of the screw gauge components, affecting its accuracy over time. Regular maintenance and calibration are necessary to minimize errors due to wear.
Temperature Variation: Changes in temperature can cause the metal components of the screw gauge to expand or contract, leading to inaccuracies in measurements.
Human Errors: Incorrect handling, misalignment of the object being measured, or misinterpretation of readings can result in human-induced errors.
Incorrect Calibration: If the screw gauge is not calibrated correctly or if the least count is not accurately determined, it can lead to measurement errors.
