First angle projection:
Assume that a small block is made 35 mm × 30 mm × 20 mm and that two of the corners are cut away as shown below in three stages. Figure 2 illustrates a pictorial view (3D view) of the block and this has been arranged in an arbitrary way because none of the faces are more important than the others. In order to describe the orthographic views (set of 2 or more view of an object obtained by different direction which resembles the original dimensions of the object), we need to select a principal view and in this case we have chosen the view in direction of arrow A to be the view from the front.
The five arrows point to different surfaces of the block and five views will result. The arrows themselves are positioned square to the surfaces, that is at 90° to the surfaces and they are also at 90°, or multiples of 90° to each other.
The views are designated as follows:
View in direction A is the view from the front,
View in direction B is the view from the left,
View in direction C is the view from the right,
View in direction D is the view from above,
View in direction E is the view from below.
In first angle projection the views in the directions of arrows B, C, D and E are arranged with reference to the front view as follows:
The view from B is placed on the right,
The view from C is placed on the left,
The view from D is placed underneath,
The view from E is placed above.
The experienced draughtsman will commit the above rules to memory. It is customary to state the projection used on orthographic drawings to remove all doubt, or use the distinguishing symbol which is shown on the arrangement in Fig. 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, 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.
Fig.3 First angle projection arrangement. Dotted lines indicate hidden edges and corners
Study the rearrangement shown below in Fig. 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.
Fig.4 Third angle projection arrangement
If a model is made of the block in Fig. 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. 5. Figure 5(a) shows the positioning for first angle and Fig. 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.
Fig.5 (a) First angle arrangement
The terms first and third angle correspond with the notation used in mathematics for the quadrants of a circle in Fig. 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.
Fig.5(b) Third angle arrangement
The intersection of the vertical and horizontal planes gives 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.
Fig.6 VP is the vertical plane. HP is the horizontal plane. AVP the auxiliary vertical plane. GL is the ground line
If the planes containing the three views are folded back into the plane of the drawing board, then the result is shown in Fig. 6 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.
Traditionally, front views are also known as front elevations, side views are often known as side or end elevations and the views from above or beneath are referred to as plans. All of these terms are freely used in industrial drawing offices.