1. ME: 121 Engineering Practices
Lecture 2
Precision Measuring Instruments

2. UNIT OF LINEAR MEASUREMENT
British System:
mile
yard
foot (‘)
inch (“)
2) Metric System:
kilometer
meter
decimeter
centimeter
millimeter
3) S.I. System :

3. CONVERSIONS BRITISH SYSTEM: 12 inch = 1 foot 36 inch = 1 yard
3 foot = 1 yard
1760 yard = 1mile
METRIC SYSTEM:
1000micron = 1mm
10mm = 1cm
10cm = 1decimeter
10decm = 1meter
1000meter = 1kilometer

4. Vernier Calipers
Vernier Calipers are precision tools used to make accurate measurements to within .001 in (1/1000 in.) or 0.02 mm, depending on whether they are inch or metric vernier calipers
MATIRIAL: It is made from nickel chromium steel
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5. Different Parts of Vernier Calipers
Vernier Caliper can be used to measure diameter, thickness very accurately.
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6. Different Parts of Vernier Calipers
A Vernier calipers consists of mainly two parts
Part 1:
A 2cm wide 15cm long rectangular metal strip
.The left end bottom side of this strip consists of a fixed jaw 1 (A) and at the same end jaw 2(C) at the top of this strip.
On the strip a scale (5) is graduated in Inches along the upper edge and another scale (4) is graduated in Centimeters along the lower edge.
This is called Main Scale ‘S’.
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7. Different Parts of Vernier Calipers
A Vernier calipers consists of mainly two parts
Part 2:
A metal frame V called vernier slides over the Main Scale ‘S’.
At the bottom of this frame V a button 8(P) is attached, which helps to fix this vernier at any desired place on the main scale.
This vernier frame consists of jaw1 (B) at the bottom and a jaw 2(D) at the top .
Two scales are graduated on this frame corresponding to two scales on the Main Scale ‘S’. The two scales 6 and 7 on the vernier are called Vernier scale.
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8. Different Parts of Vernier Calipers
A Vernier calipers consists of mainly two parts
Part 2:
Vernier scale consists of equal number of divisions.
When we move vernier frame over the main scale, a thin strip (3) will be projected out.
The projection will be exactly equal to the distance between Jaws 1(AB) i.e the thickness of the object between jaws.
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9. Use the outside jaws of the caliper as shown
Measuring Diameter
The lower jaws 1,1(AB) are used to measure the thickness or external diameter of the tubes, cylinders or spheres.
Use the outside jaws of the caliper as shown
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10. Use the inside jaws of the caliper as shown
Measuring Diameter
The upper jaws 2,2 (CD) are used to measure the inner diameters of hallow bodies like tubes or holes.
Use the inside jaws of the caliper as shown
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11. Measuring Depth of a Hole
The thin strip (3) is used to measure the depth of the objects like test tubes.
Use the depth gauge of the caliper as shown
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12. Principle of Vernier Calipers
N divisions on the vernier scale is equal to (N-1) divisions on the main scale.
Concept Structure
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13. Least Count of Metric Vernier Scale
Minimum length or thickness measurable with the vernier calipers is called its least count.
Calculating Least Count
Example -1
In the vernier caliper shown in fig the main scale division (9mm) are divided in to 10 equal parts in the vernier scale.
i.e. One Main Scale Division = 1mm
One Vernier Scale Division = 9/10mm
The difference between one MSD and one VSD = L.C
Least Count is 1 - 9/10 = 1/10mm
L.C = 0.1mm
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14. Least Count of Metric Vernier Scale
Calculating Least Count
Method – 1, Example -2
In this vernier caliper main scale division (49mm) are divided in to 50 equal part in the vernier scale.
i.e. One Main Scale Division = 1mm = 1MSD
One Vernier Scale Division = 49/50mm = 1VSD
The difference between one MSD and one VSD = L.C
L.C = 1MSD – 1 VSD
S = Value of One Main Scale Division
N = Number of Equal Divisions on the Vernier Scale
Least Count is 1 – 49/50 = 1/50mm = S/N
L.C = 0.02mm
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15. Least Count of Metric Vernier Scale
Calculating Least Count
Direct Approach:
If the smallest division on main scale is 1mm (S) and the no. of divisions on vernier scale is 20 (N) than the least count of vernier caliper will be,
L.C = S/N = 1/20 mm = 0.05mm
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16. How to Take the Reading Total Reading = M.S.R + (V.C x L.C)
The reading on the main scale just before the zero of the vernier is noted.
This is called Main scale reading (M.S.R)
The number of division (n) on the vernier which coincides perfectly with any one of the main scale divisions is noted. This is called vernier coincidence (V.C)
The vernier coincidence (V.C=n) is multiplied by least count to get the fraction of a main scale division i.e., V.C x L.C
This is added to the main scale reading (M.S.R) to total reading,
Total Reading = M.S.R + (V.C x L.C)
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17. How to Take the Reading
EXAMPLE OF READING
Main scale reading = 35mm = M.S.R
The vernier division coinciding with the main scale is the 20th division
Fraction Value = V.C x L.C = 20 x 0.02 = 0.20mm
Total reading = M.S.R+(V.C x L.C) = 35mm+0.20= 35.20mm
35.20 mm
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