How do you differentiate between first-angle and third-angle projection in a drawing?

Short Answer

The difference between first-angle and third-angle projection lies in the arrangement of views in an orthographic drawing. In first-angle projection, the object is placed between the observer and the projection plane, and the views appear opposite to their actual positions. In third-angle projection, the projection plane is between the observer and the object, and the views appear in their natural positions.

To differentiate between these projections in a drawing, first-angle projection places the top view below the front view and the right-side view to the left, while third-angle projection places the top view above the front view and the right-side view to the right. Each method follows standard symbols to indicate the projection type.

Detailed Explanation

Differences Between First-Angle and Third-Angle Projection

  1. View Placement
    • First-Angle Projection
      • The top view is placed below the front view.
      • The right-side view is placed on the left and vice versa.
    • Third-Angle Projection
      • The top view is placed above the front view.
      • The right-side view is placed on the right and the left-side view on the left.
  2. Object and Projection Plane Position
    • First-angle projection: The object is placed between the observer and the projection plane.
    • Third-angle projection: The projection plane is placed between the observer and the object.
  3. Standard Symbols Used
    • First-angle projection symbol: A truncated cone with the base on the left.
    • Third-angle projection symbol: A truncated cone with the base on the right.
  4. Common Usage in Different Regions
    • First-angle projection: Used in Europe, India, and Asia (ISO Standard).
    • Third-angle projection: Used in the USA and Canada (ANSI Standard).

Identifying Projection Type in a Drawing

  1. Look at the View Arrangement
    • If the top view is below the front view, it is first-angle projection.
    • If the top view is above the front view, it is third-angle projection.
  2. Check the Projection Symbol
    • A first-angle projection drawing has a cone symbol with the base on the left.
    • A third-angle projection drawing has a cone symbol with the base on the right.
  3. Observe the Side Views
    • In first-angle projection, the right-side view appears on the left.
    • In third-angle projection, the right-side view appears on the right.
Conclusion

The difference between first-angle and third-angle projection is mainly in view placement and object positioning. First-angle projection places views in opposite locations, while third-angle projection follows a natural arrangement. To identify the projection type in a drawing, check the position of views and the standard projection symbol used.