ENGINEERING DRAWING (CHAPTER 12)

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📘 ENGINEERING DRAWING

Chapter 12 – Isometric Projection

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12.1 Introduction

• In orthographic projection, we need multiple views (FV, TV, SV) to represent an object.

• But sometimes we need a single pictorial view that shows the 3D appearance.

• This is achieved by Isometric Projection.

👉 “Isometric” = equal measure (from Greek iso = equal, metron = measure).

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12.2 Principle of Isometric Projection

• Object is imagined inside a transparent cube.

• Cube is tilted so that one of its body diagonals is perpendicular to the plane of projection.

• As a result:

  • All three principal axes (X, Y, Z) are equally inclined (120° apart).

  • Scale along each axis is equally reduced (isometric scale).

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12.3 Isometric Axes and Planes

• Isometric Axes → Three lines meeting at a point at 120° angles.

  • One vertical, two inclined at 30° to horizontal.

• Isometric Planes → Planes formed by any two isometric axes.

  • They appear as parallelograms instead of squares.

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12.4 Isometric Scale

• In isometric projection, lengths are not true lengths but are reduced.

• A special scale called Isometric Scale is used.

Construction of Isometric Scale:

1. Draw a horizontal line (true length line).

2. Draw a line at 45° and mark true lengths on it.

3. Draw another line at 30° from starting point.

4. Project lengths from 45° line to 30° line.

5. The divisions on 30° line give isometric lengths.

👉 Isometric Projection = uses isometric scale.
👉 Isometric Drawing = uses true scale (slightly larger).

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12.5 Isometric Projection of Simple Shapes

(A) Cube

• Draw three axes (one vertical, two at 30°).

• Mark isometric length on each.

• Complete six faces as parallelograms → cube appears in 3D.

(B) Prism & Pyramid

• Prism → Base drawn on isometric plane, project height vertically.

• Pyramid → Base as parallelogram, apex projected above center.

(C) Cylinder

• Draw ellipse for base (since circle appears as ellipse).

• Project axis upwards, draw another ellipse for top.

• Join with tangents.

(D) Cone

• Base as ellipse.

• Apex above center.

• Join apex with base ellipse.

(E) Sphere

• Always appears as circle in isometric projection (diameter = isometric length).

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12.6 Isometric Projection of Planes and Circles

• A square plane appears as a parallelogram.

• A circle appears as an ellipse.

Four-Center Method for Ellipse in Isometric:

1. Draw isometric square equal to circle diameter.

2. Using four centers, draw approximate ellipse inscribed in square.

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12.7 Isometric Views of Composite Solids

• Many engineering objects are combinations of solids (cube + cylinder, prism + cone, etc.).

• Draw each part separately in isometric and combine.

👉 Example: A lathe spindle head may combine cylinder + frustum + holes.

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12.8 Difference Between Isometric Projection and Isometric Drawing

Aspect	Isometric Projection	Isometric Drawing
Scale	Isometric scale (reduced)	True scale
Appearance	Slightly smaller	Slightly larger
Usage	Exact projection (theoretical)	Common in practice (for clarity)

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12.9 Applications

• Mechanical Engineering → Machine parts, tools, assemblies.

• Civil Engineering → Buildings, layouts, furniture drawings.

• Electrical Engineering → Layout of machines, equipment.

• Architecture → 3D visualization of designs.

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12.10 Summary of Chapter

• Isometric = equal measure (all axes at 120°).

• Isometric projection uses isometric scale (reduced).

• Shapes → squares → parallelogram, circles → ellipse, sphere → circle.

• Complex objects = combination of simple solids.

• Widely used for pictorial representation of engineering designs.

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