Orthographic and Multiview Projections

Orthographic and Multiview Projections

To accurately capture the three-dimensional geometry of an object on a two-dimensional sheet of paper, engineers use a system called orthographic projection. This is the foundation of all standard engineering drawings.

Principles of Orthographic Projection

Orthographic projection is a method of representing a 3D object in two dimensions. It relies on the concept of projecting the features of the object onto an imaginary viewing plane (the projection plane) using parallel projectors that are perpendicular (orthogonal) to the projection plane.

Because the line of sight is perfectly perpendicular to the face of the object being viewed, true shapes and true lengths are preserved on that specific viewing plane, eliminating the perspective distortion seen in artistic drawings or photographs.

Multiview Drawings

A single orthographic view can only show two of the three dimensions of an object (e.g., width and height, but not depth). Therefore, to completely define a 3D object, multiple orthographic views must be systematically arranged. This is known as a multiview drawing.

The Six Principal Views

An object can be imagined as being suspended inside a glass box. The object's features are projected onto the six sides of the box. Unfolding this box creates the six principal views:

  1. Front View: Usually chosen to show the most characteristic shape of the object.
  2. Top View: Projected onto the top of the box.
  3. Right Side View: Projected onto the right side of the box.
  4. Left Side View: Projected onto the left side of the box.
  5. Bottom View: Projected onto the bottom of the box.
  6. Rear View: Projected onto the back of the box.

In standard engineering practice, it is rarely necessary to use all six views. The rule is to use only the minimum number of views required to fully define the object without ambiguity. The most common arrangement is a three-view drawing: Front, Top, and Right Side.

Projection Angles (First vs. Third Angle)

When unfolding the "glass box," there are two internationally recognized standards for arranging the views on the drawing sheet:

  1. First-Angle Projection:

    • Concept: The object is placed between the observer and the projection plane. (Think of it as casting a shadow).
    • Arrangement: The Top view is placed below the Front view; the Right Side view is placed on the left of the Front view.
    • Usage: Standard in Europe, Asia, and most of the world (ISO standard).

  2. Third-Angle Projection:

    • Concept: The projection plane is placed between the observer and the object. (Think of looking through a window at the object).
    • Arrangement: The Top view is placed above the Front view; the Right Side view is placed on the right of the Front view.
    • Usage: Standard in the United States and Canada (ASME standard).

Drawings must always include a standard symbol in the title block indicating whether First-Angle or Third-Angle projection is being used, as misinterpreting the views can lead to a part being manufactured backward or inside out.

Projection Aids and Alignment

To ensure views correspond correctly to one another, they must be perfectly aligned:

  • The Top view must align vertically with the Front view.
  • The Side view must align horizontally with the Front view.

Drafting often uses projection lines (or a 45-degree miter line) to transfer depth dimensions accurately from the Top view to the Side view.