Understanding Projection Systems in Technical Drawing

Projecting Points and Shapes

Projecting points onto a plane involves rays passing through the points and intersecting the plane. These intersections are called projections. Projecting a shape involves projecting its vertices.

Projection Systems

There are two main systems of projection:

Perspective Projection

When the rays originate from a single point, the system is called perspective projection. The resulting shape is proportionally smaller or larger.

Parallel Projection

When the rays are parallel, the system is called parallel projection. The resulting shape is congruent (same shape and size). There are two variations:

1. Oblique Projection: The rays are oblique to the projection plane.

2. Orthographic Projection: The rays are perpendicular to the projection plane. This is also known as multiview projection.

Multiview Projection

The multiview system uses two perpendicular projection planes. Their intersection is called the XY line.

The elevation (front view) is projected onto the vertical plane (V), and the plan (top view) is projected onto the horizontal plane (H).

In multiview projection:

• Depth is the distance from a point to the vertical plane.

• Height is the distance from a point to the horizontal plane.

Points are labeled with capital letters, and lines with lowercase letters. Horizontal projections use a subscript 1 (A₁) or a prime superscript (A'). Vertical projections use a subscript 2 (A₂) or a double prime superscript (A").

Representing Solids in Multiview

Representing solids in multiview may require a third projection: the side elevation.

Axonometric Projection

In technical drawing, axonometric projection is a type of parallel projection used to create pictorial views of three-dimensional objects in two dimensions. The dimensions of the represented objects will appear slightly reduced.

This system uses three perpendicular planes that intersect to form three axes. There are three types of axonometric projection:

• Isometric Projection: The most common type. The axes form three 120° angles, resulting in equally foreshortened objects.

• Dimetric Projection: Two angles are equal, and the third is different. Two object dimensions are equally reduced, and the third is reduced differently.

• Trimetric Projection: Three different angles result in three different reductions.

Oblique Axonometric Projection: When one projection plane is parallel to the viewer, two axes are at 90°, and the third is oblique.