A single orthographic projection almost never provides enough information to fully describe a component. This is because all of the information about dimensions parallel to the viewing direction are lost completely. A multiview drawing combines several orthographic projections into a single document, often including a pictorial representation like the photo shown below in the top right. By comparing multiple views, one can reconstruct the original 3D object.

Adjacent views are related by a 90-degree rotation around the fold line separating the two views. A multiview drawing usually consists of three views: a Front view, a Top or Bottom view, and a Left or Right view. Although in theory the part could be placed in any orientation, the views are usually chosen to coincide with a natural symmetry of the part—for example, the front, right, and top of the car shown above. These are called the principal views.

Placement and Alignment of Views

The views in a multiview drawing are placed with respect to one another according to a specific convention. In the United States, Canada, Japan, and Australia, the third-angle projection convention is used. Proper alignment and placement may be obtained by the following procedure: imagine that the part is enclosed inside a glass box. Project the part onto each of the panels of the glass box using orthographic projection. Finally, unfold the panels of the glass box. Using this rule, the right view is placed to the right of the front view, and the top view is placed above the front view. This might seem like the only intuitive placement, but the rest of the world actually uses the first-angle projection convention, where the right view is placed to the left of the front view, etc.

The glass box method also ensures correct alignment of views. The same feature shown in two adjacent views must lie on a line perpendicular to the fold line. Notice that the side-view mirror on the car above appears at the same vertical position in both the front and right views, while the wheels appear at the same horizontal position in the front and top views.

The right and top views also share a common axis along the length of the car. The same feature in these two views will appear the same distance away from the fold line. This fact is useful when constructing your own multiview projections by hand and correlating features between views, but fold lines are not shown in usual engineering practice, so there is no specific convention dictating the spacing between adjacent views.

Choosing Appropriate Views

You don’t always need three unique views to completely describe an object. Sometimes you need more, and those additional views might be of a special kind like a section view, auxiliary view, or detail view. Sometimes you only need two, particularly if the part is symmetric about a plane or axis.

You should start by selecting the most descriptive view of the object. You will generally choose this as your front view, and then construct additional views from it as necessary to fully define the object. Each view will give different information and will be best-suited to answering specific questions about the object.

We will demonstrate this with a child’s tiger toy and a porcelain crane.

The details of the tiger’s face are most visible in the front view, but the right view gives a better perspective on its legs. The approximate surface area of the crane’s wings are best seen in the top view, which shows their width and length, but the curvature is only visible in the front view.

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