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Micrometer, also called a micrometer screw gauge, is made up of a calibrated screw. Micrometers are used for precise measurement of components in mechanical engineering and machining, along with metrological instruments like dials, verniers, and digital callipers. A micrometer is an instrument which is usually utilised to make accurate linear measurements of dimensions, like diameter, thickness, and lengths of solid bodies. A micrometer is composed of a C-shaped frame with a movable jaw functioned by an integral screw.
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Key Terms: Micrometer, Screw gauge, Caliper, Frame, Anvil, Barrel, Digital Micrometers, Verniers
What is Micrometer?
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Micrometers are used to precisely measure small linear quantities such as length and thickness. It can measure quantities up to the range of 0.01mm to 0.005 mm. It is normally used in applications where accurate measurements are essential such as the hardware industry.
Micrometer
A micrometer is an instrument typically utilised to make accurate linear measurements of dimensions, like diameter, thickness, and lengths of solid bodies. A micrometer is composed of a C-shaped frame with a movable jaw functioned by an integral screw.
Frequently Asked Questions Related to MicrometerQues: What is the pitch of a micrometer? (1 Mark) Ans: The pitch of a micrometer is 1 mm and it shows 100 divisions on a circular scale. Ques: How to read a micrometer? (2 Marks) Ans: To read the micrometer in thousandths, the number of vertical divisions visible on the sleeve is multiplied by 0.025". To this output, the number of thousandths indicated by the line on the thimble is added, which best coincides with the central long line on the sleeve. Ques: What is the range of a micrometer? (2 Marks) Ans: Micrometers have measuring ranges of between 25mm and 1 inch depending upon their types. This deciphers the incremental metric measurements of 0-25mm, 25-50mm, 50-75mm, and so on. The imperial versions are equal to 0-1 inches, 1-2 inches, 2-3 inches, etc. Ques: What is the least count of a micrometer? (1 Mark) Ans: The least count of a micrometer is 0.01 mm. The least count can be estimated by: \(\frac{Pitch}{No. \ of \ Divisions \ on \ Circular \ Scale}\). Ques: True or False: Screw is located inside the barrel. (1 Mark) Ans: True. The screw found inside the barrel is usually considered the heart of micrometer. |
Parts of Micrometer
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Different Parts of a Micrometer are elaborated below:
- Frame: The significant part of the micrometer is the C-shaped frame that holds together the anvil and the sleeve. It is a heavy metal frame with a high thermal mass. To prevent any substantial heating of the frame, it is normally covered with insulated plastic.
- Anvil: It is that part of the instrument that is attached to the frame. The sample to be measured is rested against the anvil.
- Sleeve/Barrel: The circular component consists of a linear scale also called a pitch scale.
- Screw: It is considered to be the heart of the micrometer. It is placed inside the barrel which is used to determine the measurement.
- Locknut: A locknut is used to hold the spindle stationary and not let the sample move or get loose.
- Spindle: It is that part of the micrometer that faces against the anvil and helps hold the sample tightly. The thimble is used to move the spindle towards or away from the anvil.
- Thimble: The part that engulfs the barrel is the thimble. It contains circular measurements to determine the rotations.
- Ratchet: Located at the end of the instrument, a ratchet is used to limit the applied pressure by sliding at a predetermined torque.
Parts Of Micrometer
Heart of a Micrometer
The heart of a micrometer is the screw located within the barrel.
- The precision of the thread form of the screw determines the accuracy of the measurement.
- A screw’s thread can be identified as the ridges felt when touching a screw.
- The thread is the helical structure that converts torque to linear force.
Types of Micrometers
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There are two common types of micrometers:
- Outside Micrometer
- Inside Micrometer
Outside Micrometers
An external micrometer (or outsider micrometer) is used to measure the outer surfaces of any component. It provides accurate measurements of the components and is a very common type of micrometers used in manufacturing industries.
Mechanical(Analogue) outside Micrometer and Digital outside Micrometer The outside micrometer is the most commonly used type of micrometer It is used to measure external dimensions such as the outside diameter of an object.
Types of Outside Micrometers:
- Rotating Disc Micrometer
- Point Micrometer
- Tube Micrometer
- Non-Rotating Disc Micrometer
- Spline Micrometer
- Blade Micrometer
Inside Micrometers
Inside micrometers are used to measure inside dimensions like the inside diameter of a hole or tube.
Types of Inside Micrometers:
- Caliper-type inside micrometers
- Tubular and rod inside micrometers
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Working Principle Of Micrometer
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A normal screw consists of threads that are separated by a definite distance. When the screw rotates, the distance it advances is equal to the distance of separation between the two threads. This distance is called the pitch of the screw. The pitch is usually 1mm or 0.5mm.
Working Principle Of Micrometer
The core working principle of the micrometer is that it converts the rotations data into a linear parameter. Therefore the number of rotations can be easily correlated with the amount of axial movement.
Pitch of the screw: The pitch of the screw is the linear distance covered by the spindle for every single rotation. It is given by the expression:
| \(Pitch (p) = \frac{Distance \ moved \ by \ screw \ spindle}{No. \ of \ full \ rotation \ given} \) |
Least Count of the screw: The least count of the screw is the distance the spindle travelled when the circular scale in the thimble has gone through one division. It is given by:
| \(Least \ Count (LC) = \frac{Pitch}{Total \ number \ of \ division \ on \ the \ circular \ scale} \) |
Zero Error and Zero Correction: When the anvil and the spindle are brought in contact, the linear scale’s reference line should coincide with the zero of the circular scale. In that case, the micrometer is said to have no zero error.
No Zero Error
But if the zero of the circular scale lies below the reference line, then it is said to have a positive zero error. The resultant measurement of the sample should be finally subtracted by this value.
Positive Zero Error
If the zero of the circular scale lies above the reference line, then it is said to have a negative zero error. The resultant measurement of the sample should be finally added by this value.
Negative zero error
After these measurements are noted down, the sample to be measured is kept between the anvil and the spindle firmly. The main scale reading (MSR) and circular reading are measured and noted down. The value is then adjusted according to the acquired zero error. Multiple measurements of the sample are taken by laterally rotating it. The mean value is taken as the final value.
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Uses of Micrometer
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There are many uses of micrometer, including:
- Micrometers are most commonly used in engineering and mechanics.
- It is used in the industrial manufacturing of moving parts such as pistons where a slight mistake in the dimension could lead to unfitting components. For optimal function of hardware components such as pipes and bearing, accurate dimensions are measured using a micrometer.
- Digital micrometers are now available which reduce the discrepancies that were dealt with in the manual ones. The major advantage is that these could be fitted with other hardware tools and precise measurements could be easily taken.
Uses of Micrometer
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Things To Remember
- Micrometer is a measuring instrument used to measure extremely small quantities.
- It can measure quantities up to the range of 0.01 mm to 0.005 mm.
- The screw is called the heart of the micrometer as the total accuracy of the measurement is dependent on the thread of the screw.
- The core working principle of the micrometer is that it corresponds the number of rotations to the axial movement of the screw.
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Sample Questions
Ques. A micrometer with a pitch of 0.5 mm and a circular scale with 50 divisions is used to measure the thickness of a thin sheet of Aluminium. Before starting the measurement, it is found that when the two jaws of the micrometer are brought in contact, the 45th division coincides with the main scale line and that the zero of the main scale is barely visible. What is the thickness of the sheet if the main scale reading is 0.5 mm and the 25th division coincides with the main scale line? (3 marks)
Ans. Least count= pitch / no. of division on a circular scale
→ 0.5 mm / 50
→ 0.01 mm
Negative zero error = – 5 × LC = -5 × 0.01 = -0.05 mm
Measured value = main scale reading + screw gauge reading – zero error
→ 0.5mm + 25 × 0.01mm – (-0.05mm)
→ 0.75mm + 0.05mm
→ 0.80mm
Ques. The least count of a micrometer is 0.001cm. Using it the diameter of a wire is measured as 0.084cm. The relative error in this measurement is? (3 marks)
Ans. We know that the relative error in a screw gauge or micrometer is given by the ratio of least count to the measured value. We have,
L.c = 0.001cm
and the measured value of the diameter is given as,
\(R.E= \frac{l.c}{d}\)
Hence, relative error in the measurement will be,
\(R.E= \frac{0.001}{0.084}\)
⇒\(R.E = \frac{1}{84}\)
Ques. The screw of the micrometer screw gauge moves through a distance of 2mm when it is turned through 4 rotations. Find the pitch of the screw. If the circular scale is divided into 100 equal parts. Find the least count of the micrometer screw gauge. (3 marks)
Ans. Distance travelled by screw = 2mm
No. of rotations given = 4mm
Pitch of screw = No.of rotations travelled / No.of rotations given
→ 2mm/4 = 0.5 mm
Number of divisions of circular scale = 100
Least count of micrometer screw gauge = pitch of screw/ no.of circular divisions
→ 0.5 / 100 = 0.005mm
Hence the pitch of the screw is 0.5mm and the least count is 0.005mm
Ques. If the pitch of the micrometer screw gauge screw is 0.1mm and its circular scale is divided into 100 equal parts. Find the least count of the micrometer screw gauge. (2 marks)
Ans. Pitch of the screw = 0.1mm
No.of divisions on circular scale = 100
Least count = pitch of screw / no. of divisions on the circular scale
L.c = 0.1/100
L.c = 0.001 mm
Ques. When a micrometer with a least count of 0.01mm is used to measure the diameter of the wire, the reading on the sleeve is found to be 0.5mm and the reading on the thimble is found to be 27 divisions. What is the correct diameter of the wire if the zero error of the gauge is +0.005cm? (2 marks)
Ans. Least count of the micrometer = 0.001 mm
Main scale reading = 0.5 mm
Circular scale reading = 27
Shown reading = MSR + CSR x LC
→ 0.5 + 27 X 0.001 = 0.77mm
Ques. When a micrometer of least count 0.01mm is used to measure the diameter of a rod, the reading on the sleeve is found to be 1.6 cm and the reading on the thimble is found to be 48 divisions. What is the correct diameter of the rod if the zero error is -0.003 cm? (2 marks)
Ans. Least count of micrometer = 0.01mm
Main scale reading = 1.6cm = 16mm
Circular scale reading = 48
Shown reading = MSR + CSR x LC
→ 16 + 48 x 0.01 = 1.648 cm
Zero error = -0.003 cm
Corrected reading = shown reading - zero error with proper sign
1.648 + 0.003 =1.651 cm
Thus the correct reading is 1.651 cm
Ques. The pitch of a micrometer is 1 mm and there are 100 divisions on the circular scale. While measuring the diameter of a wire, the linear scale reads 1 mm and the 47th division on the circular scale coincides with the reference line. The length of the wire is 5.6 cm. Find the curved surface area (in cm) of the wire in the appropriate number of significant figures. (3 marks)
Ans. The distance moved on the linear scale when circular scale makes one complete rotation is p = 1mm(pitch). The number of divisions on the circular scale is N=100. Thus, one division on the circular scale is LC=p/N = 0.01mm. The linear scale reading (LSR) is 1 mm and the circular scale reading (CSR) is 47. Thus, the diameter of the wire is
d= LSR + CSR × LC
= 1 + 47 × 0.01 = 1.47mm
= 0.147 cm
The curved surface area of the cylinder is
A = π d l= 3.14 (0.147) (5.6)
= 2.5848 cm2
Rounding off to two significant digits gives A=2.6cm2
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