Orthographic Projection

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

The Purpose of Orthographic Projection

Imagine a 3D object suspended inside a glass box. If you project lines straight out from the object (perpendicularly) onto the sides of the glass box, and then unfold the box, you create a flat, 2D representation 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 sketching or photography.

Front, Top, and Side Views

In standard multi-view drawings, an object is typically represented by three primary views:

Front View: Usually chosen as the view that shows the most characteristic shape of the object or its natural operating position. It is the primary reference view.
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).
Side View (Profile View): The view looking at the object from the right or left side. It is aligned horizontally with the front view.

These three views working together provide enough spatial information to fully define the dimensions (length, width, height) of most standard parts.

Relationship Between Views

The key to orthographic projection is the strict alignment between the 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 line up exactly across the drawing. If there is a hole in the top view, its depth can be tracked perfectly straight down into the front view using hidden lines.

Reading Simple Multi-View Drawings

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

Start with the Front view to understand the basic profile.
Look at the Top and Side views to determine the depth and thickness of the features shown in the Front view.
Pay close attention to solid (visible) lines and dashed (hidden) lines. A solid circle in the Top view might be a protruding cylinder or a drilled hole; checking the Front or Side view will immediately reveal which it is based on whether it uses visible or hidden lines.

Projection Standards (First vs. Third Angle)

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

First-Angle Projection: Used primarily in Europe and Asia. (The object is between the observer and the projection plane).
Third-Angle Projection: Used primarily in the United States and Canada. (The projection plane is between the observer and the object). Both are mathematically sound; the choice depends on regional standards, and the specific standard used is always indicated by a symbol in the title block.