ME: 121 Engineering Practices Lecture 2 Precision Measuring Instruments

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

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

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 Week 2

Different Parts of Vernier Calipers Vernier Caliper can be used to measure diameter, thickness very accurately. Week 2

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’. Week 2

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. Week 2

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. Week 2

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 Week 2

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 Week 2

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 Week 2

Principle of Vernier Calipers N divisions on the vernier scale is equal to (N-1) divisions on the main scale. Concept Structure Week 2

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 Week 2

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 Week 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 Week 2

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) Week 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 Week 2