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3.6 Pictorial Sketching

A pictorial sketch represents a 3D object on a 2D sheet of paper by orienting the object so you can see its width, height, and depth in a single view.

Pictorial sketches are used frequently during the ideation phase of engineering design to record ideas quickly and communicate them to others. Their similarity to how the object is viewed in the real world makes them useful for communicating engineering designs to nonengineers. Later in the design process, pictorial drawings are often used to show how parts fit together in an assembly and, in part catalogs and manuals, to make it easy to identify the objects.

This chapter examines three common methods used to sketch pictorials: isometric sketching, which is a subtype of the general category of axonometric projection, oblique sketching, and perspective sketching. Figure 3.30 shows perspective, isometric, and oblique sketches of a stapler. Figure 3.31 shows pictorial sketches for backpack concepts.

3.30

3.30 Three Types of Pictorial Sketches

3.31

3.31 Pictorial sketching is used frequently to convey preliminary design ideas, as in these backpack concept sketches. (Courtesy of André Cotan.)

Each of the pictorial methods differs in the way points on the object are located on the 2D viewing plane (the piece of paper).

A perspective sketch presents the most realistic looking view. It shows the object much as it would appear in a photograph—portions of the object that are farther from the viewer appear smaller, and lines recede into the distance.

An axonometric sketch is drawn so that lines do not recede into the distance but remain parallel. This makes isometric views easy to sketch but takes away somewhat from the realistic appearance.

An oblique sketch shows the front surface of the object looking straight on and is easy to create, but it presents the least realistic representation because the depth of the object appears to be out of proportion.

Various types of pictorial drawings are used extensively in catalogs, sales literature, and technical work. They are often used in patent drawings; in piping diagrams; in machine, structural, architectural design, and in furniture design; and for ideation sketching. The sketches for a wooden shelf in Figure 3.32 are examples of axonometric, orthographic, and perspective sketches.

3.32

3.32 Sketches for a Wooden Shelf Using Axonometric, Orthographic, and Perspective Drawing Techniques. The axonometric projections in this sketch are drawn in isometric. (Courtesy of Douglas Wintin.)

In axonometric and oblique drawings, distant features are not shown proportionately smaller, the way they appear in a photograph or to our vision. Edges that are parallel on the object always appear parallel in an axonometric view. The most common axonometric projection is isometric, which means “equal measure.” When a cube is drawn in isometric, the axes are equally spaced (120° apart). Though not as realistic as perspective drawings, isometric drawings are much easier to draw. CAD software often displays the results of 3D models on the screen as isometric projections. Some CAD software allows you to choose between isometric, axonometric views (see Figure 3.35 for examples), or perspective representation of your 3D models on the 2D computer screen. In sketching, dimetric and trimetric sometimes produce a better view than isometric but take longer to draw and are therefore used less frequently.

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