Orthographic Projections

Orthographic Projections are the types mainly used in the production of aircraft engineering drawings of components and structures. They are drawn as if the viewer is infinitely remote from the object and rays lead out from the object so that the projection lines of opposite sides appear to be parallel. This method of projection provides a two-dimensional view of only one surface of the object. This means it must have multiple views (usually three, but there can be as many as six) of the relevant surfaces (drawn on three mutually perpendicular planes) to provide an accurate depiction of the whole object.

The two conventions that are used for orthographic projections are stated below.

• The First Angle Projection (older): The internationally recognised symbol of the truncated cone (frustum) is the key as to whether the first or third angle projection is being portrayed on a drawing. The first angle projection is being used when the truncated end of the cone is viewed and the two concentric circles are drawn at the remote end of the cone. In the same way, the surface of interest is drawn remote from that surface in first angle projections.

FIGURE

• The Third Angle Projection (more recent): These projections show the surface of interest drawn adjacent to that surface, in the same manner that the two concentric circles are drawn adjacent to the truncated end of the cone.

FIGURE

Note: It is possible to find the cone reversed (end for end), but the location of the two concentric circles relative to the truncated end will always provide the information as to how the drawing is to be read.