ENGINEERING DRAWING (CHAPTER-4)


๐Ÿ“˜ ENGINEERING DRAWING

Chapter 4 – Scales (Plain, Diagonal, Vernier)

4.1 Introduction

In engineering drawing, objects are often too large (like buildings, bridges, ships) or too small (like watch parts, IC chips) to be drawn in their actual size on a sheet.
๐Ÿ‘‰ Therefore, we use Scales to represent objects proportionally.

  • Definition:
    A scale is a measuring device used to reproduce the dimensions of an object proportionally smaller or larger than its actual size.

  • Example:

    • A building 30 m long is represented as 300 mm on paper using a 1:100 scale.

    • A small screw 5 mm long may be drawn as 50 mm on paper using a 10:1 scale.

4.2 Types of Scales

(A) Full Scale

  • When the drawing size = actual size.

  • Ratio = 1:1.

  • Used when object fits comfortably on sheet.

(B) Reducing Scale

  • When the drawing is smaller than actual size.

  • Ratio less than 1 (e.g., 1:2, 1:5, 1:10, 1:100).

  • Used in maps, buildings, large machines.

(C) Enlarging Scale

  • When the drawing is larger than actual size.

  • Ratio greater than 1 (e.g., 2:1, 5:1, 10:1).

  • Used for small machine parts, electronics components.

4.3 Representative Fraction (R.F.)

  • R.F. = Ratio of drawing size to actual size, expressed in the same unit.

R.F.=Length of drawingActual length of objectR.F. = \frac{\text{Length of drawing}}{\text{Actual length of object}}

Examples:

  1. If a line 50 mm on paper represents 5 m in reality:

    • Convert both to same unit → 5 m = 5000 mm.

    • R.F. = 50 / 5000 = 1/100.

  2. If R.F. = 1/20 and actual length = 200 cm, drawing length =

    • 200 ÷ 20 = 10 cm.

4.4 Types of Scales in Engineering Drawing

(1) Plain Scale

  • Can read two units (e.g., cm and mm, m and dm).

  • Constructed by dividing a line into equal parts.

Steps for Construction (Example):

  • R.F. = 1/50, Maximum length to measure = 6 m.

  • Length of scale = (R.F.) × (Max length) = (1/50) × 6000 mm = 120 mm.

  • Draw a line 120 mm long, divide into 6 equal parts (each = 1 m).

  • Subdivide the first division into 10 parts (each = 0.1 m = 10 cm).

  • Mark “0” at first division; left side shows decimeters, right side shows meters.

๐Ÿ‘‰ With this scale, you can read measurements up to 6 m correct to 10 cm.

(2) Diagonal Scale

  • Can read three units (e.g., m, dm, cm OR km, hm, m).

  • Works on principle of similar triangles.

Steps for Construction (Example):

  • R.F. = 1/50, Maximum length = 6 m.

  • Length of scale = 120 mm (same as plain scale).

  • Draw a rectangle of height 50 mm and base 120 mm.

  • Divide base into 6 parts (1 m each).

  • Subdivide the first part into 10 (for decimeters).

  • Draw vertical lines, then divide the leftmost vertical into 10 equal parts.

  • Draw diagonals → gives centimeters.

๐Ÿ‘‰ With this, you can read up to 6 m correct to 1 cm.

(3) Vernier Scale

  • Used for very accurate measurements.

  • Can read small fractions of a unit (e.g., mm, minutes, seconds).

  • Based on difference between main scale divisions (MSD) and vernier scale divisions (VSD).

Types:

  1. Forward Vernier – 1 VSD < 1 MSD.

  2. Backward Vernier – 1 VSD > 1 MSD.

Example (Construction):

  • Suppose R.F. = 1/25, Maximum length = 4 m.

  • Length of scale = (1/25) × 4000 = 160 mm.

  • Construct main scale = 4 m.

  • Suppose 10 VSD = 9 MSD →

    • 1 VSD = 0.9 MSD.

    • Least Count = 1 MSD – 1 VSD = 0.1 MSD.

๐Ÿ‘‰ This Vernier scale can measure lengths up to 4 m accurate to 1 cm.

4.5 Comparative (Special) Scales

  • Used to compare measurements in two different units (e.g., kilometers and miles, nautical miles and kilometers).

  • Useful in maps and navigation.

4.6 Practical Applications of Scales

  • Civil Engineering: Building plans, road maps, railway layouts.

  • Mechanical Engineering: Machine parts, gears, bolts.

  • Electrical Engineering: Circuit diagrams (scaled layouts).

  • Geography/Maps: Distances between cities, maps of countries.

4.7 Summary of Chapter

  • Scale = tool to represent large/small objects on paper.

  • Types: Full, Reducing, Enlarging.

  • R.F. expresses the ratio of drawing length to actual length.

  • Plain scale → two units; Diagonal scale → three units; Vernier scale → fractional accuracy.

  • Scales are widely used in civil, mechanical, electrical, and map-making fields.


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