Introduction

In Chapter 1 it was stated that there are two sets of rules that apply to engineering drawing. Firstly, there are the rules that apply to the layout of a drawing and secondly the rules pertaining to the manufacture of the artefact. This chapter is concerned with the former set of rules, called the 'drawing layout rules'. These define the projection method used to describe the artefact and how the 3D views of it can be represented on 2D paper. These will be presented in terms of first and third angle orthographic projections, sections and cutting planes, auxiliary projections as well as trimetric, dimetric, isometric and oblique projections.

The chart in Figure 2.1 shows the types of drawing projections. All engineering drawings can be divided into either pictorial projections or orthographic projections. The pictorial projections are non-specific but provide visualisation. They can be subdivided further into perspective, axonometric and oblique projections. In pictorial projections, an artefact is represented as it is seen in 3D but on 2D paper. In orthographic projections, an artefact is drawn in 2D on 2D paper. This 2D representation, rather than a 3D representation, makes life very much simpler and reduces confusion. In this 2D case, the representation will lead to a specification that can be defined by laws. The word oriho means correct and the word graphic means drawing. Thus, orthographic means a correct drawing which prevents confusion and therefore can be a true specification which, because orthographic projections are clearly defined by ISO standards, are legal specifications. Orthographic projections can be subdivided into first and third angle projections. The two projection

PROJECTIONS

I

┬ź

Perspective

Oblique

\ Axonometrie / First Angle | Third Angle

[Visualization ¡Specification

Figure 2.1 The different types of engineering drawing projections

Figure 2.1 The different types of engineering drawing projections methods only differ in the manner in which the views are presented. The third angle projection method is preferred.

Whichever projection method is used, the representation is achieved by projectors which are effectively rays of light whose sources are on one side of an artefact passing over the artefact and projecting its image onto a 2D drawing sheet. This is similar to the image or shadow an artefact would produce when a single light source projects the shadow of an artefact onto, say, a wall. In this case, the wall is the picture plane. The various types of pictorial and orthographic projections are explained in the following sections.

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