Typical AutoCAD drawings. Third angle projection

2.Typical AutoCAD drawings

The following examples are meant to convey to the reader the extensive range of draughting facilities available from software associated with basic programs. Obviously there is a certain amount of overlap in the scope of programs and often alternative ways of performing similar operations. Figures 3.6 and 3.7 show pictorial drawings of an engine development. A pictorial view can easily be generated after drawing orthographic views, which give the dimensions in three planes at right angles to each other. Figure 3.8 shows a drawing of a cycle. Figure 3.9 demonstrates the realistic effect of rendering. The viewing point and orientation is adjustable. Architectural drawings for the design of a shopping mall are reproduced in Fig. 3.10 and Fig. 3.11. They show the outline of the development and how the completed construction could appear.

3.Third angle projection

The difference between first and third angle projection is in the arrangement of views and, with reference to the illustration in Fig. 4.4, views are now positioned as follows: View B from the left is placed on the left, View C from the right is placed on the right, View D from above is placed above, View E from below is placed underneath. Study the rearrangement shown below in Fig. 4.4 and remember the above rules because it is vital that the principles of first and third angle projection are understood. The distinguishing symbol for this method is also shown. If a model is made of the block in Fig. 4.1, and this can easily be cut from polystyrene foam used in packing, then a simple demonstration of first and third angle projection can be arranged by placing the block on the drawing board and moving it in the direction of the four chain dotted lines terminating in arrows in Fig. 4.5. Figure 4.5(a) shows the positioning for first angle and Fig. 4.5(b) for third angle projection. The view in each case in the direction of the large arrow will give the five views already explained. The terms first and third angle correspond with the notation used in mathematics for the quadrants of a circle in Fig. 4.6 the block is shown pictorially in the first quadrant with three of the surfaces on which views are projected. The surfaces are known as planes and the principal view in direction of arrow A is projected on to the principal vertical plane. The view from D is projected on to a horizontal plane. View B is also projected on to a vertical plane at 90° to the principal vertical plane and the horizontal plane and this is known as an auxiliary vertical plane. Another horizontal plane can be positioned above for the projection from arrow E, also a second auxiliary vertical plane on the left for the projection of view C. Notice that the projections to each of the planes are all parallel, meeting the planes at right angles and this a feature of orthographic projection. The intersection of the vertical and horizontal planes give a line which is the ground line GL. This line is often referred to as the XY line; this is useful in projection problems since it represents the position of the horizontal plane with reference to a front view and also the position of the vertical plane with reference to a plan view. Many examples follow in the text. If the planes containing the three views are folded back into the plane of the drawing board, then the result is shown in Fig. 4.7 where dimensions have also been added. The draughtsman adjusts the distances between views to provide adequate spaces for the dimensions and notes. To describe a simple object, a draughtsman does not need to draw all five views and it is customary to draw only the minimum number which completely illustrate the component. You will note in this particular case that we have omitted views which contain dotted lines in preference to those where corners and edges face the observer. Many parts do not have a definite ‘front’, ‘top’ or ‘side’ and the orientation is decided by the draughtsman, who selects views to give the maximum visual information.

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