ME: 121Engineering Practices

Lecture 2

Precision Measuring Instruments

UNIT OF LINEAR MEASUREMENT

1) British System: mile

yard foot (‘) inch (“)

2) Metric System: kilometer

meterdecimetercentimetermillimeter

3) S.I. System : meter

CONVERSIONS

BRITISH SYSTEM:12 inch = 1 foot

36 inch = 1 yard 3 foot = 1 yard 1760 yard = 1mile

METRIC SYSTEM:1000micron = 1mm

10mm = 1cm10cm = 1decimeter10decm = 1meter1000meter = 1kilometer

4Week 2

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

5Week 2

Different Parts of Vernier Calipers

Vernier Caliper can be used to measure diameter, thickness very accurately.

6Week 2

Different Parts of Vernier Calipers

A Vernier calipers consists of mainly two partsPart 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’.

7Week 2

Different Parts of Vernier CalipersA 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.

8Week 2

Different Parts of Vernier Calipers

A Vernier calipers consists of mainly two partsPart 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.

9Week 2

Measuring Diameter

Use the outside jaws of the caliper as shown

The lower jaws 1,1(AB) are used to measure the thickness or external diameter of the tubes, cylinders or spheres.

10Week 2

Measuring Diameter

Use the inside jaws of the caliper as shown

The upper jaws 2,2 (CD) are used to measure the inner diameters of hallow bodies like tubes or holes.

11Week 2

Measuring Depth of a Hole

Use the depth gauge of the caliper as shown

The thin strip (3) is used to measure the depth of the objects like test tubes.

12Week 2

Principle of Vernier CalipersN divisions on the vernier scale is equal to (N-1) divisions on the main scale.

Concept Structure

13Week 2

Least Count of Metric Vernier Scale

Minimum length or thickness measurable with the vernier calipers is called its least count.

Calculating Least CountExample -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

Least Count is 1 - 9/10 = 1/10mm

The difference between one MSD and one VSD = L.C

L.C = 0.1mm

14Week 2

Least Count of Metric Vernier ScaleCalculating Least CountMethod – 1, Example -2

i.e. One Main Scale Division = 1mm = 1MSD

One Vernier Scale Division = 49/50mm = 1VSD

Least Count is 1 – 49/50 = 1/50mm = S/N

The difference between one MSD and one VSD = L.C

L.C = 0.02mm

In this vernier caliper main scale division (49mm) are divided in to 50 equal part in the vernier scale.

L.C = 1MSD – 1 VSDS = Value of One Main Scale Division N = Number of Equal Divisions on the Vernier Scale

15Week 2

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

16Week 2

How to Take the Reading

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)

17Week 2

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