Orthographic Projection

To manufacture a three-dimensional part accurately, an engineer must communicate its exact geometry on two-dimensional paper. Orthographic projection is the standard technique used to achieve this. It allows complex 3D objects to be represented completely and unambiguously through multiple 2D views.

The Purpose of Orthographic Projection

Imagine a 3D object suspended inside a glass box. If you project straight lines from the object (perpendicularly) onto the sides of the glass box, and then unfold the box, you create flat, 2D representations of the object from multiple angles.

This mathematical projection method ensures that features are shown in their true size and shape when viewed directly, free from the perspective distortion that occurs in artistic sketches or photography.

Front, Top, and Side Views

In a standard multiview drawing, an object is typically represented by three primary views:

1. Front View: Usually chosen as the view that shows the most characteristic shape of the object or its natural operating position. This is the primary reference view.
2. Top View (Plan View): The view looking directly down onto the object from above. It is aligned directly above or below the front view (depending on the projection standard).
3. Side View (Profile View): The view looking at the object from the right or left side. It is aligned horizontally level with the front view.

Working together, these three views provide enough spatial information to completely define the dimensions (length, width, height) of most standard parts.

Alignment Between Views

The core of orthographic projection is strict alignment between views:

• The Front and Top views share the same width.
• The Front and Side views share the same height.
• The Top and Side views share the same depth.

Because of this alignment, features correspond exactly across the drawing. If there is a hole in the top view, its depth can be traced straight down into the front view using hidden (dashed) lines.

Basics of Reading Multiview Drawings

Reading a multi-view drawing involves mentally reconstructing the 3D object from its 2D projections.

• Start with the front view to grasp the basic profile.
• Look to the top and side views to determine the depth and thickness of the features shown in the front view.
• Pay close attention to solid lines (visible object edges) and dashed lines (hidden features). A solid circle in the top view could be a protruding cylinder or a drilled hole; checking the front or side view will immediately reveal which one it is by whether solid or dashed lines are used.

Projection Standards (First vs. Third Angle)

There are two primary conventions for unfolding the "glass box":

• First-Angle Projection: Primarily used in Europe and Asia (object is between the observer and the projection plane).
• Third-Angle Projection: Primarily used in the US and Canada (projection plane is between the observer and the object). Both methods are mathematically valid. The choice depends on regional standards, and the specific standard used is always indicated by a symbol in the drawing's title block.