Orthographic and Multiview Projections

Orthographic and Multiview Projections

To accurately document the three-dimensional geometry of an object onto two-dimensional paper, engineers use a system called orthographic projection. This forms the foundation of all standard engineering drawings.

Principles of Orthographic Projection

Orthographic projection is a method of representing a 3D object in 2D. 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 that projection plane.

Because the line of sight is perfectly perpendicular to the face of the object being viewed, the true shape and true length of features are preserved on that viewing plane. This eliminates the perspective distortion found in artistic drawings or photographs.

Multiview Drawings

A single orthographic projection can only show two of an object's three dimensions (e.g., width and height, but not depth). Therefore, to completely define a 3D object, a systematic arrangement of multiple projections is required. This is called a multiview drawing.

The Six Principal Views

Imagine the object suspended inside a glass box. Its features are projected onto the six sides of the box. Unfolding this box creates the six principal views:

  1. Front View: Usually selected as the view showing 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 almost never necessary to use all six views. The rule is to use the minimum number of views required to completely 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 like casting a shadow).
    • Arrangement: The top view is placed below the front view; the right side view is placed to 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 it like looking through a window at the object).
    • Arrangement: The top view is placed above the front view; the right side view is placed to the right of the front view.
    • Usage: Standard in the United States and Canada (ASME standard).

Engineering drawings must always include a standard symbol in the title block to indicate whether first-angle or third-angle projection is used, because misinterpreting the views can lead to manufacturing a mirrored or backward part.

Projection Aids and Alignment

To ensure the views correspond correctly, 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.

In drafting, projection lines or a 45-degree miter line are often used to accurately transfer depth dimensions from the top view to the side view.