Screw Gauge Method: Procedure, Precautions & Calculations Explained
A screw gauge is a tool for precisely measuring the diameter of a thin wire or the thickness of a metal sheet. You can usually measure length precisely up to 0.1 mm with Vernier Calipers. A screw gauge can be used to make more precise length measurements of up to 0.01 mm or 0.005 mm.
As a result, a Screw Gauge is a more precise instrument than Vernier Calipers. A screw has threads on it. Any two successive threads are separated by the same amount of space. By twisting the screw anticlockwise or clockwise in its nut, it can be moved backwards or forward.
We all know that Vernier calipers are one of the precision instruments that can actually measure with an accuracy of up to 0.1 mm. At the same time, higher accuracy can be attained using Screw Gauge. A Screw Gauge can take measurements up to 0.01 mm to even 0.005 mm. The sole idea behind this experiment is to find how to measure the thickness of a sheet of paper using a screw gauge.
Aim of the Practical
The aim of the experiment is to measure the thickness of a given sheet using a screw gauge.
What is the Apparatus Requirement in the Experiment?
Screw gauge
Sheet
Magnifying Lens
Apparatus Description
A screw gauge is made up of a U-shaped frame with a screwed spindle connected to a thimble. A scale graduated in mm is carved parallel to the thimble's axis. Pitch scale is the term for this. A sleeve is affixed to the screw's head.
A ratchet on the screw head prevents the screw from being overtightened. A circular scale known as the head scale is found on the thimble and is divided into 50 or 100 equal sections. The sleeve moves around the pitch scale when the screw is turned.
The anvil is a stud with a plane end surface that is positioned precisely opposite the screw tip on the 'U' frame. When the screw's tip makes contact with the anvil, the head scale's zero normally corresponds with the pitch scale's zero.
Theory
Screw gauge is one of the precision instruments that can be used to accurately measure the thickness of a paper, or even the diameter of a thin wire. The structure of a screw gauge consists of a U-shaped frame along with a screw spindle that is attached to the thimble. In a screw gauge, mm scales are engraved, running parallel to that of the thimble.
(Image will be uploaded soon)
The head section of the screw gauge consists of a ratchet that restricts the over-tightening of the screw. Within the thimble section, it consists of a circular scale that is divided into 50 or 100 equal parts. It is also termed as a head screw, which moves over the pitch scale while operating.
The anvil, which is the stud with a plane end surface, forms the U shaped frame. It can be found on the opposite side to the tip of the screw. You will find the zero of the head scale coinciding with that of the pitch scale. This occurs when the tip of the screw comes in contact with that of an anvil.
Pitch of the Screw Gauge
The pitch of the screw gauge can be defined as the distance traveled by the spindle per rotation. The pitch of the screw gauge can be determined by the distances traveled by the screw, divided by the total number of rotations.
The formula for pitch is given by:
Pitch of the Screw = \[\frac{\text{(Distance travelled by the screw)}}{\text{(Number of Full rotation taken)}}\]…………. (1)
Principle
The screw's linear distance traveled is proportional to the spin applied to it. The smallest distance that the instrument can reliably measure is the linear distance moved by the screw when rotated by one division of the circular scale. It's known as the instrument's least count.
Least Count of the Screw Gauge
When the tip of the screw gauge is turned by one division of the head scale, the least count (LC) is taken.
The formula for calculating least count is given by:
Least Count =\[\frac{\text{(Pitch)}}{\text{(Total number of divisions in the circular scale)}}\] …………. (2)
Zero Error and Zero Correction
By looking at the screw gauge image, one needs to consider the zero error in the calculation. The zero error can be calculated by completely rotating the screw until it touches the anvil. Make sure that the edge of the cap is located at the zero marking. The screw gauge needs to be kept vertical so that its zero is facing downward.
By attaining the same position, you can come across three circumstances:
The zero marks from the circular scale align with that of the reference scale. Here, no scope of zero error or zero correction can be found.
The zero marks from the circular scale actually remain above that of the reference scale. Here, the zero error is considered positive, while the zero correction is negative.
The zero marks from the circular scale are actually below the reference line. Here, the zero error is considered negative, while the zero correction is positive.
Procedure
In order to properly conduct the experiment, make sure to go through the following procedure carefully:
Determine the screw gauge's pitch (1) and least count (2) using the above equations.
To determine the zero error, make contact between the anvil and the screw. Perform it three times and keep track of the results. If no zero errors occur, the value 'zero error nil' is recorded.
Using the ratchet head, move the screw away from the anvil, insert the lead shot, and then return the screw to its original position. Stop then and there if the ratchet slips without moving the screw.
Count the number of pitch scale divisions visible and uncovered by the cap's edge. The pitch scale reading (PSR) is denoted by the letter N.
Count the number (n) of circular scale divisions that cross the reference line.
To measure the diameter perpendicularly, repeat steps 4 and 5 after rotating the lead shot by 900 degrees. Fill in the blanks with your observations in the tabular column.
Calculate the total reading by using the equation gives as:
t = PSR + corrected HSR = N+(n x L.C) and apply zero correction in each case.
Find the mean of the various values.
Make sure to insert the sheets in between the studs of the screw gauge. Take the calculation of thickness from five different positions.
Make sure to calculate the average thickness, while determining the correct thickness by zero error.
Experiment Observation Table
Calculation
Least count _____ mm
Zero Error _____ mm
Mean thickness of the paper _____ mm
Mean corrected thickness of the paper
Thickness observed through screw gauge – Zero error _____ mm
Final Result
As a result, the thickness of the given sheet of paper comes out is _____ mm
Sources of Error
The thickness of the sheet may not be consistent.
Although backlash-related errors can be reduced, they cannot be entirely eliminated.
FAQs on How to Measure the Thickness of a Sheet Using a Screw Gauge
1. What is a screw gauge and what is its main principle?
A screw gauge, also known as a micrometer, is a precision instrument used for accurately measuring the dimensions of small objects like the thickness of a sheet. Its working principle is based on the magnification of linear motion using a screw. For every complete rotation of the thimble, the screw moves forward by a fixed linear distance called the pitch. This mechanism allows for extremely precise measurements, often up to 0.01 mm.
2. How do you measure the thickness of a given sheet using a screw gauge?
To measure the thickness of a sheet, you should follow these steps:
- First, determine the least count of the instrument and check for any zero error.
- Place the given sheet between the anvil and the spindle of the screw gauge.
- Rotate the thimble using the ratchet until the sheet is gently but firmly held. The ratchet makes a clicking sound to prevent over-tightening.
- Record the Main Scale Reading (MSR) and the Circular Scale Reading (CSR) that aligns with the main line.
- Calculate the final thickness using the formula: Total Reading = MSR + (CSR × Least Count) ± Zero Error Correction.
3. Why is a screw gauge considered more precise than a Vernier caliper for this experiment?
A screw gauge is more precise than a Vernier caliper primarily because it has a smaller least count. The least count of a standard screw gauge is 0.01 mm, whereas for a typical Vernier caliper, it is 0.1 mm. This tenfold increase in precision is due to its screw mechanism, which converts a small linear advancement into a large, easily readable rotation on the circular scale, making it ideal for accurately measuring the thickness of very thin objects like a metal sheet.
4. What is the importance of checking for zero error before measuring the sheet's thickness?
Checking for zero error is a critical step for obtaining an accurate measurement. A zero error is a systematic error that occurs if the zero mark on the circular scale does not coincide with the main line when the jaws of the screw gauge are fully closed.
- A positive zero error occurs if the circular scale's zero is below the main line and must be subtracted from the final reading.
- A negative zero error occurs if the zero is above the main line and must be added to the final reading.
5. How would you record observations in a table when measuring a sheet's thickness?
For the CBSE Class 11 experiment, an observation table for measuring sheet thickness should include the following columns:
- Serial Number (to record multiple readings taken at different points on the sheet).
- Main Scale Reading (MSR) in mm.
- Coinciding Circular Scale Division (CSD).
- Circular Scale Reading (CSR), calculated as CSD × Least Count.
- Observed Thickness (MSR + CSR).
- The final step is to calculate the mean of all observed thickness values and apply the zero error correction to get the Corrected Thickness.
6. What are the common sources of error when using a screw gauge, apart from zero error?
Besides zero error, other common sources of error in a screw gauge experiment are:
- Backlash Error: This is a mechanical error due to wear and tear in the screw threads. To avoid it, you should always rotate the thimble in a single direction when approaching a measurement.
- Parallax Error: This happens if your eye is not positioned directly perpendicular to the scale while taking the reading.
- Excessive Pressure: Applying too much force can compress the sheet, leading to an underestimated thickness. It is important to always use the ratchet for the final tightening.
7. What happens if you apply too much pressure with the ratchet when measuring a soft metal sheet?
If you apply excessive pressure when measuring a soft or malleable sheet (like one made of aluminium or copper), you risk deforming or compressing the sheet. This would cause the spindle and anvil to come closer than the actual thickness of the material, resulting in a measurement that is inaccurately low. The ratchet mechanism is designed specifically to prevent this by applying a uniform, calibrated pressure for every measurement.
8. How does a change in temperature affect the accuracy of a screw gauge?
A screw gauge is typically made of metal and is therefore subject to thermal expansion and contraction. If the ambient temperature increases, the instrument can expand slightly. This expansion can alter the pitch of the screw and the calibration of the scales, leading to inaccurate readings. For highly precise applications in science and engineering, measurements are often performed in temperature-controlled environments to minimise this effect.
