In engineering drawing, angle projection refers to the standard methods used to represent a three-dimensional (3D) object on a two-dimensional (2D) plane. These methods are crucial for creating orthographic views that accurately convey the object's shape and size. The two primary systems of angle projection are first angle projection and third angle projection.
These systems determine the placement of the different views (such as front, top, and side views) relative to each other on the drawing sheet. The choice of method affects how the object is conceptually positioned between the observer and the projection plane.
First Angle Projection
Based on the reference provided:
- The first angle projection is one of the ways to represent three-dimensional objects in two dimensions.
- It is widely used in Europe and most of the world, but not typically in the United States.
- For obtaining the orthographic views from 3D objects using this method, we divide the plane into four quadrants.
In essence, with first angle projection, the object is conceptually placed between the observer and the projection plane. When you project the views onto these planes, they are then laid out on the 2D drawing sheet.
Key Characteristics of First Angle Projection:
- View Placement: The view is placed as if projected through the object onto the plane behind it. For example:
- The top view is placed below the front view.
- The right side view is placed to the left of the front view.
- Quadrant: It's based on placing the object in the first quadrant of a conceptual 3D space.
- ISO Standard: Primarily used in countries following ISO standards for technical drawing.
Third Angle Projection
Third angle projection is the other main system, commonly used in the United States and other regions. In this system, the projection plane is conceptually placed between the observer and the object.
Key Differences (First Angle vs. Third Angle):
- View Placement:
- First Angle: Top view below front, Right side view left of front.
- Third Angle: Top view above front, Right side view right of front.
- Conceptual Position:
- First Angle: Object is between observer and plane.
- Third Angle: Plane is between observer and object.
- Quadrant:
- First Angle: Object in the first quadrant.
- Third Angle: Object in the third quadrant.
Engineers and designers must indicate which projection method is used on a technical drawing, usually with a symbol, to ensure correct interpretation of the views.
