Engineers are confronted with the task of communicating the design, development and structures of machines to manufacturers and builders. The shape and size of various parts of a machine and its structure must be recorded on plane sheets in a systematic way for communication. The pictorial view of the object does not carry all the details, especially the inner details and correct shape of complicated parts. Different methods, therefore, are implied for describing the exact shape based on the ‘projectors’ drawn by engineers.
Principle of Projection
If straight lines are drawn from various points on the contour of an object to meet a plane, the object is said to be projected on that plane. The figure formed by joining, in correct sequence, the points at which these lines meet the plane, is called the projection of the object. The lines from the object to the plane are called projectors.
Projection on a Single Plane
If straight lines are drawn from the various points on the contour of an object to meet a plane, the figure obtained on the plane is called the Projection of the object. The object is said to be projected on the plane. In other words, we can say that the projection of an object on a plane is the shadow of the object on the plane showing each and every edge line of the object. The imaginary lines drawn from the object to the plane are called projectors or projection lines. The plane on which the projection of the object is taken is called plane of projection. Suppose an object is placed in front of a screen and light thrown on the object (assuming the light rays to be parallel to each other and perpendicular to the screen) then a true shadow of the object is obtained on the screen. This shadow is the projection on the object on the plane of screen showing the contour line of the object.
Types of Projections
The projections are classified according to the method of taking the projection on the plane. A classification of projection is shown below:
Factors on Which Type of Projection Depends
Different views of an object can be drawn by projections. Thus, every drawing of an object will have four things on which projection depends
- Plane of projection, and
- Observer’s eye or station point.
Methods of Projection
In engineering drawing following four methods of projection are commonly used, these are:
- Isometric projection
- Oblique projection
- Perspective projection
- Orthographic projection
In the above methods (the first three methods represent the object by a pictorial view as an observer sees it. These projections, which come under the category of ‘Pictorial Drawing’ have already been discussed in detail here. In these methods of projection, a three-dimensional object is represented on a projection plane by one view only. While in the orthographic projection an object is represented by two or three views on the mutual perpendicular projection planes. Each projection view represents two dimensions of an object. For the complete description of the three-dimensional object, at least two or three views are required. Orthographic projection comes under the category of ‘Non-Pictorial Drawing’.
The word orthographic means straight description. The straight description here stands for the parallel projectors from the object to infinity. If a perpendicular picture plane is inserted between the projectors, a picture is formed having the same shape and size as that of the object. If an observer at position ‘A’ moves to infinity, the projectors to his eyes becomes parallel to the object and he observes the same shape and size as that of the object. (Refer Fig 2). The view so formed is known as the orthographic projection. Similarly, the parallel projectors shall form the pictures on the respective picture planes from the positions B and C. Usually two views are sufficient for simple objects, but the help of three or more views is necessary for complicated objects. These picture planes are mutually perpendicular to each other and are known as ‘Principal Planes’ of projectors, named Horizontal Plane (HP), Vertical Plane (VP) and Profile Plane (PP).
Methods of Orthographic Projection
The two methods of projections are:
- First angle projections
- Third angle projections
Figure 3 shows four quadrants formed by the intersection of horizontal and vertical planes. The intersecting line of the planes is called the co-ordinate axis. The revolving direction of the horizontal plane shows that quadrants I and III are “open” but II and IV quadrants become “closed” when the horizontal plane coincides with the vertical plane. It is obvious that the closed quadrant has no use for the purpose of projectors as the views taken on these will overlap.
First Angle Projections
This method of projection is popular in Europe, especially in Britain. Bureau of Indian Standard has also recommended it now. Figure 4 shows an object placed in the first quadrant. Parallel projectors in the direction ‘A’, from the object, forms a picture on the vertical plane (VP) which is known as Front View or Front Elevation. Similarly, parallel projectors from the direction of ‘B’ forms the picture on the horizontal plane (HP), known as Top View or Plan. A mutually perpendicular plane to both HP and VP, known as profile plane (PP) also receives projectors from the object from the direction C. The view on the profile plane is known as Side View or Side Elevation. The three planes containing the views are then opened on a plane as shown in Fig. 5. The symbol of first angle is shown in Fig 6.
Although two or three views are enough to reveal an object, the projectors from six directions of the object in the first angle are shown, if necessary. (Refer Fig 7). The views are to be shown symmetrically as shown in Fig 7 (b). The view from the top (Direction B) placed underneath. The view from the front (Direction A) is placed in the centre. The view from the left side (Direction C) is placed on the right side of view A. The view from the right side (Direction D) is placed on the left side. The view from the bottom (direction E) is placed on the top as “E” view. The view from the rear (Direction F) may be placed on the right or left side of C or D views.
Third Angle Projections
This system of projection is known as the American system. The object is placed in the 3rd quadrant. The planes are imagined to be made of transparent material, say a glass box. (Refer Fig. 8). The front wall of the box is assumed to be hinged to the other walls as shown in the figure. The parallel projectors in all the six directions form respective views on the walls of the box serving as picture planes. The hinged walls of the box are opened and laid down on a plane. (Refer Fig. 9). The placement of various views is in a systematic way. The view from the top is placed above the Front View (FV). The view from the right-hand side is placed on the right side of FV. The view from left hand side is placed on the left side of FV. The view from bottom is placed underneath the FV. The view from the rear may be placed on the right or the left of the side views. The symbol for the third angle is given in Fig 10.
Comparison of First Angle Projection and Third Angle Projection method
|Sl No.||First Angle Projection Method||Third Angle Projection Method|
|1||The object is kept in the first quadrant.||The object is assumed to be kept in the third quadrant.|
|2||The object lies between the observer and the plane of projection.||The plane of projection lies between the observer and the object.|
|3||The plane of projection is assumed to be non- transparent.||The plane of projection is assumed to be transparent|
|4||In this method, when the views are drawn in their relative positions, Plan (Top view) comes below the elevation (Front view), the view of the object as observed from the left- side is drawn to the right of elevation.||In this method, when the views are drawn in their relative positions, Plan comes above the elevation, left hand side view is drawn to the left-hand side of the elevation.|
|5||This method of projection is now recommended by the “Bureau of Indian Standards” from 1991.||This method of projection is used in U.S.A and also in other countries.|