Screw Gauge Principles, Parts, and Accurate Measurement Steps
A screw gauge can be described as an object which is used to measure cylindrical and spherical objects. It gives precise measurements but can be a bit difficult to use. In technical terms, the screw gauge can be defined as a mechanical tool that facilitates the measurement of diameter, radius, or thickness of a thin metal sheet, or the thickness of a wire with maximum accuracy.
A screw gauge is an instrument that measures the diameter of thin objects like a wire. The name screw gauge is provided because it is most commonly used to measure the diameters of wire which in turn are governed by the standard numbers that are called the standard wire gauge.
A screw gauge also measures the thickness of small sheets such as glass and plastic.
Since a screw gauge works on the principle of a micrometer, that’s why we call it the principle of a micrometer screw.
Screw Gauge
When accurately cut, a single threaded screw is placed inside a closely fitted nut and rotated. The two types of motions occur, one is circular and the other is a linear motion of the screw along its axis.
The distance moved by the screw in one complete rotation of the screw equals the distance between the two consecutive threads of the screw gauge. This distance is called ‘Pitch’ and it is always a constant value.
Since the linear motion (the small distances) made by the screw gauge is hard to be measured, these linear distances are amplified into larger distances by the rotational motion of the screw. These rotations are easily measurable. The screw gauge is constructed in a way that follows the principle of ‘micrometer screw.’
Now, to measure the diameter of a given wire using a screw gauge, we need to know its structure.
Structure of a Screw Gauge
The screw gauge consists of a screwed spindle which is fitted with a U-shaped frame and is attached to the thimble. A graduated scale in mm is engraved parallelly over the axis of the thimble. To the head of the screw, a sleeve is attached.
There is a racket that is present at the head of the screw which avoids undue tightening of the screw. The circular scale which is present on the thimble is known as the head scale which is divided into 50 or 100 equal parts. The sleeves start moving over the pitch scale when the screw is worked. ‘Anvil’ is a stud that is fixed on the ‘U’ frame with a plane-ended surface. It is exactly opposite to the tip of the screw. The zero of the head scale coincides with the zero of the pitch scale when the top of the screw comes in contact with the anvil.
Determine the Diameter of Wire Using Screw Gauge
Pitch
Aim- To measure the diameter of a given wire by using a screw gauge
Apparatus- wire, screw-gauge, magnifying lens, half-meter scale
Procedure-
Observe the value of one linear scale division (LSD).
Find the pitch and the least count of the screw gauge step-wise.
The plane face B and the plane face A are brought into contact and see if there is any zero error. This is to be done thrice and if there is no zero error, then record the zero error nil.
Now, face B and face A is moved away and the wire is length-wise placed over face A and face B is moved towards face A by using the ratchet head R. Stop is R is turning without moving the screw.
After this, the number of visible divisions of the linear scale is noted and these divisions should be uncovered by the edge of the cap. This reading IV is called the LSR or the linear scale reading.
Now, the number of divisions of the circular scale n which are lying over the reference line is noted.
Now the steps 5 and 6 are repeated after the wire is rotated by 90° to measure the diameter in the perpendicular direction.
Now the steps 4,5,6, and 7 are repeated for five different positions which are separated equally on the length of wire. Each of the observations is recorded in a tabular form.
Now, the total reading is calculated and zero correction is applied in each case.
Mean of different values of diameter is taken.
After stretching the wire along a half-meter scale, the length of the wire is measured. Now, one end of the wire is kept at a known mark and the position of the other end is noted. The length of the wire is the difference between the two ends of the wire. This is done thrice and recorded.
To Determine the Diameter of A Wire By Screw Gauge
Firstly, we count the number of divisions on the linear scale on a place completely uncovered by the cap. Let’s suppose that we got the reading as 4.0 mm as a linear scale reading.
Now, we rotate the screw 3 times till the zero mark of the head scale reaches the reference line, it means one rotation is complete.
After four rotations, we note the reading and it comes out 6.0 mm. We got the linear distance mode as: 7.0 mm - 4.0 mm = 3.0 mm.
So, the pitch can be calculated as:
=\[\frac{Linear distance moved by a screw}{one rotation of the screw}\]=\[\frac{3mm}{3}\]
= 1 mm
So, the pitch of the screw is 1 mm or 0.1 cm.
So, the distance moved by the screw in one complete rotation of the circular cap is 1 mm.
Least Count of the Screw Gauge
A circular cap has 100 divisions, if the cap moves one division, then the distance moved is 1/100 of the pitch, which is the least count of the screw gauge.
So, the formula for the least count is:
L.C.=\[\frac{Pitch}{No of Division in a circular/head scale}\]=\[\frac{1}{100}\]
= 0.01 mm or 0.001 cm.
Zero error reading:
…… mm
……..mm
……..mm
Mean zero error…….mm.
The table used in the calculation
Formulas Used
Length of the wire, l= (i) ……cm, (ii)......cm, (iii)......cm.
Mean Diameter of the wire,
D=\[\frac{D_1(a)+D_1(b)+...+D_3(a)+D_3(b)}{6}=....mm+....cm\]
Mean length of the wire,
l=\[\frac{l_1+l_2+l_3}{3}=....cm\]
Volume of the wire,
V=\[\pi (\frac{D}{2})^2l=....cm^3\]
Conclusion
This is all about how a screw gauge can be used for measuring the diameter of a given wire. There is a list of precautions that need to be taken care of to get the right values. Focus on the process and learn how to use it to measure the diameter of small objects.
FAQs on How to Measure the Diameter of a Wire Using a Screw Gauge
1. What is a screw gauge and what is its primary use in a physics lab?
A screw gauge is a precision measuring instrument used to determine the dimensions of very small objects, typically with an accuracy of 0.01 mm. Its primary use in a physics lab is to measure the diameter of a thin wire or the thickness of a thin sheet of metal or glass, which cannot be measured accurately with a standard ruler or even a Vernier caliper.
2. What are the main steps to measure the diameter of a wire using a screw gauge?
To measure the diameter of a wire, you should follow these steps:
First, determine the least count (LC) and check for any zero error of the instrument.
Place the wire between the anvil and the spindle.
Rotate the thimble using the ratchet until the wire is gently held. The ratchet will make a clicking sound to prevent over-tightening.
Record the Main Scale Reading (MSR), which is the last visible division on the main scale.
Record the Circular Scale Division (CSD) that coincides with the reference line.
Calculate the observed diameter using the formula: Diameter = MSR + (CSD × LC).
Finally, apply the zero correction to get the accurate diameter.
3. How do you determine the 'least count' of a screw gauge?
The least count (LC) of a screw gauge is the smallest value it can measure accurately. It is calculated by dividing the pitch of the screw by the total number of divisions on the circular scale. The formula is:
Least Count (LC) = Pitch / Total number of divisions on the circular scale.
For a typical school laboratory screw gauge, the pitch is 1 mm and there are 100 divisions on the circular scale, making the least count 1 mm / 100 = 0.01 mm.
4. What is the difference between the 'pitch' and the 'least count' of a screw gauge?
The pitch is the linear distance the screw moves along the main scale when the circular scale is given one complete rotation. It represents the distance between two consecutive screw threads. In contrast, the least count is the distance the screw moves when the circular scale is rotated by just one division. The least count determines the ultimate precision of the instrument, while the pitch is a fundamental parameter used to calculate it.
5. How are positive and negative zero errors identified in a screw gauge?
Zero error is identified by bringing the flat end of the screw in contact with the anvil without placing any object between them.
Positive Zero Error: This occurs if the zero mark of the circular scale is below the reference line. The error is positive, and the correction will be negative.
Negative Zero Error: This occurs if the zero mark of the circular scale is above the reference line. The error is negative, and the correction will be positive.
No Zero Error: If the zero mark of the circular scale perfectly coincides with the reference line, there is no zero error.
6. Why is it important to take multiple readings at different points along the wire when measuring its diameter?
It is crucial to take multiple readings because a real-world wire is often not perfectly uniform; its thickness can vary slightly along its length and in different orientations. By measuring the diameter at several different points and calculating the average (mean diameter), you can minimize the impact of these random variations and any minor observational errors. This leads to a more accurate and representative value for the wire's true diameter.
7. How does the precision of a screw gauge compare to that of a Vernier caliper?
A screw gauge is significantly more precise than a Vernier caliper. A standard screw gauge has a least count of 0.01 mm. In comparison, a typical Vernier caliper has a least count of 0.1 mm or sometimes 0.02 mm. This means the screw gauge can measure dimensions up to 10 times smaller than a Vernier caliper, making it the preferred instrument for measuring the diameter of a thin wire or the thickness of a paper sheet.
8. What is the difference between zero error and backlash error in a screw gauge?
Zero error is a static error that exists due to improper calibration or manufacturing defects, where the zero of the circular scale doesn't align with the reference line when the studs are in contact. It is a constant error that needs correction. On the other hand, backlash error is a dynamic error caused by wear and tear in the screw threads. It occurs when the direction of rotation is reversed, causing a lag in the screw's linear motion. To avoid it, one should always approach the final reading by moving the thimble in the same direction.
9. After finding the diameter, how is the radius of the wire calculated?
Once the corrected mean diameter (d) of the wire has been accurately determined, the radius (r) is simply calculated by dividing the diameter by two. The formula is r = d / 2. Using the mean diameter from multiple readings ensures that the calculated radius is also as accurate as possible.
10. What are some real-world applications where precise measurements using a screw gauge are important?
Precise measurements from a screw gauge are vital in many fields:
In electronics and electrical engineering, for measuring the gauge of wires used in winding coils, transformers, and motors.
In mechanical engineering and manufacturing, for quality control of small components like ball bearings, nuts, and bolts to ensure they meet strict design tolerances.
In materials science, for determining the thickness of thin films, foils, and coatings to study their physical properties.
