ENGINEERING DRAWING (CHAPTER -7)


๐Ÿ“˜ ENGINEERING DRAWING

Chapter 7 – Orthographic Projections (Principles)

7.1 Introduction

  • An engineer must represent a 3D object (machines, buildings, structures) on a 2D drawing sheet.

  • This is done using Projection.

Definition:
Projection is the method of representing the shape and size of an object on a 2D plane by drawing straight lines from the object to the plane.

7.2 Types of Projection

  1. Perspective Projection

    • Rays converge to a point (eye).

    • Objects look realistic (like human vision).

    • Not used in engineering (distorted scale).

  2. Parallel Projection

    • Rays are parallel (imaginary projectors).

    • True shape and size preserved.

    • Subtypes:

      • Orthographic Projection (perpendicular projectors).

      • Oblique Projection (inclined projectors).

      • Axonometric Projection (isometric, dimetric, trimetric).

๐Ÿ‘‰ Engineering drawing mainly uses Orthographic Projection.

7.3 Principle of Orthographic Projection

  • Object is imagined inside a Glass Box (transparent cube).

  • Rays are drawn perpendicular to projection plane.

  • The views (projections) are obtained on different sides of the box.

  • Then the glass box is unfolded onto a 2D sheet.

Main Projection Planes:

  1. Vertical Plane (VP) – Front view

  2. Horizontal Plane (HP) – Top view

  3. Profile Plane (PP) – Side views

7.4 Methods of Projection

(A) First Angle Projection (used in India & Europe)

  • Object is placed between observer and plane.

  • Views are arranged as:

    • Front View (FV) → Center

    • Top View (TV) → Below FV

    • Right Side View (RSV) → Left of FV

    • Left Side View (LSV) → Right of FV

๐Ÿ‘‰ Symbol: Truncated cone (ISO standard).

(B) Third Angle Projection (used in USA, Canada)

  • Plane is placed between observer and object.

  • Views are arranged as:

    • Front View (FV) → Center

    • Top View (TV) → Above FV

    • Right Side View (RSV) → Right of FV

    • Left Side View (LSV) → Left of FV

๐Ÿ‘‰ Symbol: Truncated cone (different arrangement).

7.5 Difference Between First Angle & Third Angle Projection

Aspect First Angle Third Angle
Object Position Between observer & plane Plane between observer & object
View Arrangement Top view below FV, side views opposite Top view above FV, side views same side
Used in India, Europe USA, Canada
Symbol Cone with base left, apex right Cone with base right, apex left

7.6 Types of Views in Orthographic Projection

  1. Front View (Elevation) – Seen from front.

  2. Top View (Plan) – Seen from above.

  3. Side View (Profile) – Seen from sides.

  4. Auxiliary Views – For inclined surfaces (true shape).

  5. Sectional Views – When object is cut to show inside details.

7.7 Visualization of Orthographic Projections

  • To understand projections, practice 3D → 2D visualization.

  • Example: Cube

    • FV = Square

    • TV = Square

    • Side view = Square

  • Example: Cylinder (standing vertically)

    • FV = Rectangle

    • TV = Circle

  • Example: Cone (standing vertically)

    • FV = Triangle

    • TV = Circle

7.8 Rules for Orthographic Projections (BIS Standards)

  1. Use First Angle Projection unless specified.

  2. Minimum two views must be drawn (usually FV + TV).

  3. Hidden lines (dashed) show invisible edges.

  4. Center lines show symmetry (axes, holes).

  5. Views should be aligned properly.

  6. Choose the minimum number of views that clearly describe the object.

7.9 Practical Applications

  • Mechanical Engineering – Machine parts, gears, bolts, engines.

  • Civil Engineering – Building plans, elevations, sections.

  • Electrical Engineering – Layouts of panels, wiring diagrams.

  • Architecture – Floor plans, elevations.

7.10 Summary of Chapter

  • Projection → method of representing 3D object on 2D sheet.

  • Orthographic Projection uses perpendicular projectors.

  • Two main methods: First Angle (India, Europe) and Third Angle (USA).

  • Views include Front, Top, Side, Auxiliary, Sectional.

  • Correct arrangement, line conventions, and alignment are essential.


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