1. 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.

2. Projection  Projection means an image or act of obtaining the image of an object. The image is referred to as a ‘view’.  The lines from the object to the plane are called the projectors.

3. Concept of Projection  Look at an object kept in front of an illuminating bulb as shown in the figure. Light rays from the bulb strike the object and create its shadow on the screen. The image thus obtained is represents a ‘view’ or the projection of the object. Fig. 1

4. Concept of Projection  The ‘view’ appears larger than the object as the light rays are divergent.  In such a case the projection is called oblique projection.

5. Concept of Projection  If the light-rays are parallel to each other and perpendicular to the plane, the ‘view’ or image formed would appear equal to the actual size of the object.  In such a case, the projection is called orthographic projection.  Ortho means perpendicular.

6. Orthographic Projection  When the projectors are parallel to each other and perpendicular to the plane of projection, the ‘view’ or projection is called orthographic projection.  Imagine that a person looks at the block shown in the figure from a theoretically infinite distance, so that the rays of light emerging from his eyes are parallel to one another and perpendicular to the front surface F

7. Concept of Projection  The projection system used in engineering drawing, similar to that shown in Fig.1, is depicted in Fig.2.  The light source is replaced by a person looking towards the object.  The lines of sight of the observer create a ‘view’ of the screen.  The screen is referred to as the Plane of Projection (POP).  The lines of sight are called projection lines or projectors. Thus observer, object and POP are the three basic elements of a projection system Fig. 2

8. Orthographic Projection  The view of this block will be the shaded figure, showing the front surface of the object in its true shape and proportion.  If these rays of sight are extended further to meet perpendicularly a plane (marked V.P.) set up behind the block, and the points at which they meet the plane are joined in proper sequence, the resulting figure (marked E) will also be exactly similar to the front face.

9. Orthographic Projection  This figure E is called the projection of the block on the plane V.P.  The lines from the block to the plane are also the projectors.  As the projectors are perpendicular to the plane on which the projection is obtained, this is called the orthographic projection of the front surface F of the block on the plane V.P.

10. Orthographic Projection  The projection is shown separately in Figure (b) at right.  It shows only two dimensions of the block, viz. height H and width W.  It does not show the thickness.  Thus we find that only one projection is insufficient for the complete description of the block or for that matter, any object.

11. Orthographic Projection  Let us further assume that another plane marked H.P. is hinged at right angle to the first plane, so that the block is in front of V.P. and above H.P.  The projection on H.P. (Figure P) shows the top surface of the block.

12. Orthographic Projection  If a person looks at the block from above, he will obtain the same view as the figure P.  It shows the width W and the thickness T of the block.  It however, does not show the height of the block W T

13. Orthographic Projection  One of the planes is now rotated or turned on the hinges so that it lies in the extension of the other plane.  This can be done in one of the two ways: (i) by turning the V.P. in the direction of the arrows A or (i) by turning the V.P. in the direction of the arrows A or (ii) by turning H.P. in the direction of the arrows B. W T

14. Orthographic Projection  The H.P. when turned and brought in line with the V.P. is shown by dashed lines.  The two projections can now be drawn on a flat (2-dimensional) sheet of paper in correct relation with each other.  When studied together, they supply all information about the shape and size of the block.

15. Orthographic Projection  Thus any solid may be represented by means of different orthographic views.  Different views of an object can be obtained by viewing the object from different directions.  Any orthographic view gives only two dimensions.  Any two orthographic views together give all the three dimensions.

16. Planes of Projection  Plane of Projection (POP) is a plane on which a particular view is projected.  We need different POPs to draw different views.  Two such planes perpendicular to each other are employed for the purpose of orthographic projections and are called the reference planes (RPs) or Principal Planes.

17. Principal Planes of Projection  Horizontal Plane (H.P.): A plane parallel to the ground (or horizon) is called Horizontal Plane (HP) or the Horizontal Reference Plane (HRP).  Vertical Plane (V.P.): A plane perpendicular to the ground and intersecting the horizontal plane is called Vertical Plane (VP) or Frontal Reference Plane (FRP).

18. Principal Planes of Projection Apart from these two, there is one more Principal Plane:  Profile Plane (P.P.): A plane perpendicular to the HP and the VP and intersecting both of them, is called Profile Plane (PP) or Profile Reference Plane (PRP).

19. Principal Planes of Projection  It is important to note that thee RPs are imaginary only.  They are assumed to be transparent so that the observer can look through them.

20. Auxiliary Plane of Projection  A plane inclined to the HP and perpendicular to the VP is called Auxiliary Inclined Plane (AIP).  A plane inclined to the VP and perpendicular to the HP is called Auxiliary Vertical Plane (AVP). AVP

21. Orthographic Views  Each of the RPs act as the POP for the corresponding view., e.g.: 1. The H.P., for the view from above or the top view (TV). Top view is also called plan. 2. The V.P., for the view obtained from the front, i.e. Front View (FV) or elevation.

22. Orthographic Views 3. Side Views: When the observer looks at the object from side, i.e., from his left- hand side or right-hand side, the view obtained is called side view (SV). SV is seen on the PP. I. Left-Hand Side View: When the observer views the object from his left-hand side, the view obtained is called left- hand side view (LHSV). II. Right Hand Side View: When the observer views the object from his right-hand side, the view obtained is called as right-hand side view (RHSV).

23. Orthographic Views 4. Bottom View: When the observer looks to the object from below, the view obtained is called bottom view (BV) or bottom plan. 5. Rear View: When the observer looks to the object from back, the view obtained is called rear view (RV) or back view or rear elevation.

24. The Four Quadrants  When the planes projection are extended beyond the line of intersection, they form four quadrants or dihedral angles which may be numbered as in the figure.  The object may be situated in any one of the quadrants, its position relative to the planes being described as ‘ above or below HP’, and in front of or behind VP.  The planes are assumed to be transparent.  The projections are obtained by drawing perpendiculars from the object to the plane, i.e., by looking from the front and from above.

25. The Four Quadrants  They are shown on the flat surface by rotating one of the planes as already explained.  It should be remembered that the first and the third quadrants are always opened out while rotating the planes.

26. Methods of Projection- First Angle Projection Method  In this method, the object is assumed to be situated in front of the V.P. and above the H.P., i.e., in the first quadrant and then projected on these planes.  The object lies between the observer and the POP.  In this method when views are drawn in their relative positions, TV comes below the FV view.  Each projection shows the view of that surface of the object which is remote from the plane on which it is projected and which is nearest to the observer.

27. Methods of Projection- Third Angle Projection Method  In this method, the object is assumed to be situated behind the V.P. and below the H.P., i.e., in the third quadrant.  The planes of projection are assumed to be transparent.  They lie between the object and the observer.

28. Methods of Projection- Third Angle Projection Method  When the observer views the object from the front, the rays of sight intersect the VP.  The figure formed by joining the points of intersection in correct sequence gives the front view of the object.  The top view is obtained in a similar manner by looking from above.

29. Methods of Projection- Third Angle Projection Method  Each projection shows the view of that surface of the object which is nearest to the plane on which it is projected.

33. Reference Line  Considering the front view which is the view as seen from the front, independently, the HP coincides with the line xy. In other words xy represents H.P.  Similarly considering the top-view which is obtained by looking from above, the same xy represents the V.P.

34. Reference Line  Hence, when these two projections are drawn in correct relationship with each other, xy represents both the HP as well as the VP.  This line xy is called the reference line.

35. Orthographic Projections  Further, in the first-angle projection method, the HP is always assumed to be so placed as to coincide with the ground on or above which the object is situated. Hence, in this method, the line xy is also the line for the ground.  In the third-angle projection method, the HP is assumed to be placed above the object. The object may be situated on or above the ground. Hence, in this method, the line xy does not represent the ground. The line for ground, denoted by letters GL, may be drawn parallel to xy and below the front view.