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Understanding The Definition, Working Principle, And Formula Of Screw Gauge

Getting the correct measurement is very crucial during an experiment. And at certain instances, you would be required to measure a minimal distance, which can only be measured using a specially designed measuring tool. One such measuring tool is a Screw Gauge. 

However, before you start using a screw gauge to measure any distance, you need to understand what is a Screw Gauge, its working principle and steps to operate this device.

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What Is a Screw Gauge?

A screw gauge is a measuring tool used to measure the diameter of circular objects or the thickness of very thin surfaces, such as sheets of metal. This instrument consists of various parts. These parts are as follows – 

  • A U shaped frame on which a spindle is attached. 
  • This spindle has grooves like screws on it, which allows it to move to and from. 
  • An anvil is attached on the other end of the U shaped frame. This anvil is used to hold the object to be measured in a fixed position.
  • The lock nut is placed above the screw to lock the device and help inaccurate measurements. 
  • The barrel of the screw gauge has measurements engraved on it. These measurements are used while measuring the dimension. It has two different kinds of scale, one is called pitch scale, which measures in terms of millimetres, and the other is called circular scale, whose revolution pushes the screw or spindle half millimetres. 
  • The thimble is the 2nd last part of the screw gauge and is used to screw the spindle towards the anvil to hold the object to measure. 
  • A ratchet is attached to the end of the gauge to prevent the thimble from slipping.Visit This Site: eblogz

Working Principle

Screw gauge primarily works on the principle of the screw. This instrument measures the distance travelled by the screw using its two scales, and these two measurements are then added in a specific manner to obtain the final measurement.

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As it works on the principle of the screw, it successfully magnifies small distances because the circular distance moved by the screw also contributes to the linear movement of the spindle. 

To understand the working of a screw gauge and its formula, you need to understand the two key concepts that govern a screw gauge. 

Pitch

Pitch is the ratio of the distance travelled by the screw when the spindle covers one full rotation. The exact value of pitch can only be calculated when the spindle reaches its maximum number of rotations. Now that the screw has reached the spindle measure the distance covered on the scale. Now apply this formula to calculate pitch.  

Pitch = Distance travelled by the screwNumber of rotations  

Here, the number of rotations is the total number of rotations the screw made to move from one end to another. The Pitch of a screw gauge is measured in millimetres.

Least Count

The least count is another crucial measurement that is essential for measuring using a screw gauge. The least count of a measuring gauge can be obtained by using the following expression. 

Least Count of Screw Gauge = Pitch Total number of divisions of the circular scale.  

What we obtained after solving this expression is the distance travelled by the screw when we turn it through one division on the circular scale.

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Formula Of Screw Gauge 

Now that you have understood the concept of pitch and least count, you can easily apply them in the formula of screw gauge to obtain the final measurement. 

Final Reading = Reading on pitch scale reading on circular scale X Least count

However, this simple calculation can become complex if there is an error in your screw gauge. Don’t worry because this problem can easily be rectified by making a small change in the formula. But first, let’s understand this problem. 

Zero Error

In some instances, there is a probability that the zero on the circular scale of your screw gauge does not fall in line with the zero on the pitch scale. This problem may cause serious errors in your calculations. There are two types of zero errors, which we will discuss, and you will also get to know how you can resolve those errors. 

Positive Zero Error

After tightening both the ends of a screw gauge, if you find that the circular scale’s zero falls below the pitch scale’s zero, this error is called positive zero error. However, this type of error cannot be rectified mechanically, but you can undo the damage done by this error in your calculation by doing the following changes. 

Final Reading = [Pitch Scale Reading Circular Scale Reading X Least count of screw gauge] – [ Positive zero error X Least count]

Here the positive zero error stands for the number of divisions below the zero of the pitch scale. 

Negative Zero Error

If the circular scale of the gauge ends up above the pitch scale, then the mechanical error will be called negative zero error. And this error can also be easily rectified by doing some modifications to the formula. So, the final reading will be calculated as follows. 

Final Reading = [Pitch Scale Reading Circular Scale Reading X Least count of screw gauge] [ Negative zero error X Least count]

The value of negative error is the number of divisions that lie above the zero pitch scale. 

Solved Example

Now that you have understood the concept let’s apply that through a simple question. 

  1. Calculate the final dimension of an object using a screw gauge. Suppose the pitch of a screw gauge is .5. And the 30th division of the circular scale matches with the zero of the pitch scale, and the pitch scale reading is 5mm.

Answer – 

Least Count of Screw Gauge = Pitch Total number of divisions of the circular scale  

= 0.550= 0.01mm

Now the final reading can be obtained as follows:

Final Reading = Reading on pitch scale reading on circular scale X Least count

 = 5 mm 30 x 0.01 = 5.30 mm

Things To Retain

A screw is a device used to measure small dimensions and is based on the principle of the screw. And to obtain a reading on the screw gauge, you need to calculate the pitch and the least count. However, there can be discrepancies in the calculation due to zero errors- positive or negative, which you would be required to rectify before finalising the measurements.

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