While a pictorial sketch may be adequate at the earlier stages of design development, the 2D orthographic projection – a plan and two elevations – is the conventional way of communicating the form and geometry of the product from design to production. It must be a complete and unambiguous representation of the object, and various conventions have grown up over the years to represent a 3D object on paper.
The method of orthographic projection is to float the designed object inside an imaginary box comprising planes at right angles to one another. If the sides of this box are thought of as being windows and are peered through in turn, then the silhouette or projection of the designed object onto the top horizontal window is called the plan; the projection of the same object onto the front vertical window is the front elevation and onto an adjacent side vertical window an end or side elevation. Now imagine the box to be hinged.
The horizontal plan is swung up through 90 degrees and the end elevation is also swung round through 90 degrees, so that all three projections now lie on the same plane. In practice, the elevation of the front face is drawn immediately below the plan and the end elevation of the right-hand side of the object (when viewed from the front) to the right-hand side of the front elevation – all lined up and in scale so that common dimensions can be taken off. This arrangement is known as third-angle projection. In what is known as first-angle projection, the views are arranged such that the front elevation is above the plan, and the end elevation of the right-hand side of the object is to the left of the front elevation.
The advantage of the third-angle over first angle projection is that the features in adjacent views are placed in juxtaposition, making it easier to project one view from another when drawing, and to associate these features when dimension or reading the drawing. UK and European practice favors first-angle (although third-angle projection is commonplace in the UK, and BS308 gives it equal status with first-angle projection): US practice favors third-angle projection. Third-angle projection is considered by many to be a more logical system.
For example, when drawing a long thin object, such as a train, a third-angle projection would be easier to read: for a train facing to the left in side elevation, the view of the front (the front elevation) would be to the left of the side view. A section is a slice through the object, projected orthogonality, to show internal details or changes in profile for complex shapes. The cut solid may be cross-hatched if this aids clarity, and the point at which the section is taken is indicated on one of the views according to the national or international standard being conformed to.
Unlike a real cut cut section, sections on an orthographic drawing will often show, by convention, details it would not be possible to see with the naked eye. An auxiliary projection is a view taken from any angle, not necessarily at right angles to the object, and not orthogonal to the other views, which may be added to the set of orthogonal projections to make an understanding of the form clearer.