Orthographic Projection

A tutorial on the basics and techniques of Orthographic Projection in technical drawing

Introduction

Orthographic projection is a method of representing three-dimensional objects usually by three two-dimensional drawings in which each parallel line is drawn parallel to the plane of view.

Expected Learning Outcomes

  1. Identify and differentiate between the various views in orthographic projection.
  2. Draw accurate orthographic projections from 3D objects, ensuring precision in dimensions and details.
  3. Read and interpret orthographic drawings, displaying the relationship between different views and the 3D object.
  4. Apply orthographic projection techniques to solve real-world problems, creating detailed technical drawings for engineering and architecture.

Note to the Learner

The figures used in this lesson are colored for the sake of visualization to better elaborate the concepts. Do not apply color to your drawings if you are using CAD software, and do not shade your drawings if you are drawing by hand manually.

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Types of Orthographic Projection

There are two separate methods of drawing in orthographic projection: the first angle projection and the third angle projection. The latter method is becoming more popular throughout the world, and may soon become the only accepted system in drawing.

At present, in Kenya, both methods are used. The building industry mostly uses the first-angle projection, while other engineering trades prefer the third-angle projection. Consequently, the learner should be conversant with both these methods and their differences.

The names first angle and the third-angle are derived from mathematics.
A derived angle is divided into four right angles thus forming four equal quadrants.
The first angle is from 0 to 90 degrees in the first quadrant and the third angle is from 180 degrees to 270 degrees in the third quadrant.

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Views in Orthographic Projection

The views used in orthographic projection are:

  1. Top View (Plan)
  2. Front View (Elevation)
  3. Side (End) View
  4. Sectional Views
  5. Auxiliary Views (in various situations)

The front elevation displays height (H) and length (L), the end elevation displays width (W) and height (H), and the plan elevation displays length (L) and width (W).

Forms Of Orthographic Projection

Orthographic projection overcomes the drawbacks of difficulty in drawing circles and curves on 3-sided objects. The key difference between the first and third angle projections is the relative positioning of views.

  1. The first angle projection: this form places the object in the first quadrant,with the plan below the front elevation and the end elevation on the left or right side.
  2. The third angle projection: this places the object in the third quadrant, with the plan above the front elevation and the end elevation on the left or right side depending on the view.

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Differences between 1st angle orthographic projection and 3rd angle orthographic projection

Comparison Table

1st angle orthographic projection 3rd angle orthographic projection
1. The object is kept in the first quadrant. 1. The object is assumed to be kept in the third quadrant.
2. The object lies between the observer and the plane or projection. 2. The plane of projection lies between the observer and the object.
3. The plane of projection is assumed to be non-transparent. 3. The plane of projection is assumed to be transparent.
4. In this method, when the views are drawn in their relative positions, the plan comes below the front elevation, the view of the object as observed from the left-side is drawn to the right of elevation. 4. In this method, when the views are drawn in their relative positions, the plan comes above the front elevation, left hand side view is drawn to the left-hand side of the front elevation.

Projection Symbols

It is good practice to indicate the method of projection used in your drawings to distinguish between plan and front views front drawings. The symbols are derived from the views of a conical frustum and are used to specify the method of projection (first angle or third angle).
Below are the symbols used to represent the methods of projection:

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Differences Between the first angle orthographic projection and the 3rd angle orthographic projection.
Derivation of the symbols(3rd angle projection)
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Derivation of the symbols(1st angle projection)

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Hidden Lines in Orthographic Projection

Hidden details are represented by short dashed lines,indicating features that cannot be seen from a particular view but are necessary to define the part or assembly.Hidden lines are omitted when not needed to avoid clutter in the drawing.

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Important Lines Used in Orthographic Projection

The following lines are essential in orthographic projections:

  1. Center Lines: Represented by a long dash and a shorter dash,used to indicate the center of circular or symmetrical features.
  2. Hidden Lines: Short dashed lines representing hidden details inside the object.

Frequently Asked Questions