## Pictorial Drawings

 Oblique Projection Perspective Drawing circles

Pictorial views (3D views)

An object can be shown three different views.

1. Isometric projection is a method of portraying, on a single view, a three-dimensional picture. It can be use for making rough sketches on site to clarify an issue. The angle between the principal axes is 120°. The scale ratio for isometric drawings is the same for each principal axis (Y:Z:X = 1:1:1) as can be seen in Figure 2 (a).

Selection of axises

The selection of the principal axis of an isometric view is very important. Most objects that you'll draw are put in a rectangular box. A rectangular box has eight corners which could be used as the principal axis for your isometric sketch as shown below. View (a) is the preferred view as it reveals more detail than all the others.

Figure 1

2. Consider the item or detail that you need to draw carefully and then select the best option for the view.

3. Oblique projection. - There are two types of oblique projection:
1) Cavalier (angle between projectors and projection plane is 45°. Perpendicular faces are projected at full scale)
This projection will not be used.
2) Cabinet (angle between projectors and projection plane is 45°. Perpendicular faces are projected at 50% scale)
In both projection, the main view is drawn as an orthographic projection and the side views are represented on inclined planes. The angle between the orthogonal and inclined plane is 45°). The scale ratio for oblique drawings is the same for two principal axis and half for the inclined plane (Y:Z:X = 1:1:0.5) as can be seen in Figure 2 (b).
An alternative of the oblique projection is to change the principal axis, refer to Figure 2 (c). This method gives an more appropriate view of the object to be drawn.
Great care should be taken when you select a viewing direction in oblique view. The best viewing direction is when the important details are shown in the face with the principal axis Y:Z:X = 1:1:0.67. Use the view that shows all details clearly.

4.             (a) isometric                       (b) oblique                             (c) oblique alternative

Figure 2

In oblique projections the main view is drawn as an orthographic projection and the two other side views are shown on an inclined plane. The angle of inclination is 45° to the horizontal and all measurements on the two oblique planes are halved (see Figure 3 (a). Oblique drawings can result in distortion. Sometimes a better option is to use the oblique alternative view as shown in Figure 3 (b).

(a) Oblique view                                             (b) Oblique view alternative

Figure 3

5. Perspective Drawings
A perspective drawing is similar to a photograph: as the lines of an object recede toward the back, they appear to converge or become closer together. The advantage of a perspective drawing over scale and full-size drawings is that the object looks more like what our eyes perceive if we viewed the actual object. That is, a three-dimensional perspective is provided. These drawings are very difficult to construct and not part of this course.

Figure 4

However, nowadays the Computer Aided Drafting packages (CAD) have 3-D facilities build in and the skill to draw in perspective mode is of no great importance in building and engineering drawing anymore.

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Circles in isometric projections

A circle drawn in isometric view is represented as an ellipse. Use the ordinates constructed on an orthogonal view and transfer them to the isometric view. Having established some points of the curve use the best fitted curve to connect the points either freehand or with a French curve. This principle is shown in Figure 2 below.

Figure 5

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