Lecture 14

Lecture 14

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  PROJECTIONS INTRODUCTION TO PROJECTIONS Introduction4. Perspective projection-when the projectors converge to a point, the projection thus obtained is known as perspective projection I think you all know how to choose scale and how to take dimensions so let us start with projections you might have some idea of projection as you might have studied in physics but here concept is somewhat different so let us start with first of all is what is it but before this we will discuss some objec- tives of this chapter. Now we will discuss the most important and widely used method by engineers. Yes, orthographic projection is the most important method out of these so will discuss this in detail. Before starting orthographic projection we must discuss planes of references in which we have to draw projections.Chapter Learning Objectives Planes of ReferencesAfter reading this chapter, you will be able to the following questions In study of orthographic projection, two principle planes are used. They are known as coordinate planes. One plane is horizontal and the other vertical and they intersect each other at right angles. This can be drawn as You have seen the planes divide themselves into four angular spaces or dihedral angles. The horizontal plane is known as H.P and the vertical plane is known as V.P.Their line of intersection is known as the ground line denoted by the line XY.The projection of an object on H.P is known as the plan and the projection on the V.P is known as elevation. • Discuss projection and its methods (in brief). • Explain multiview projection in detail • Discuss projection of points in detail. • Discuss projection of straight lines in detail. • Discuss various types of projection of planes. What is Projection? A projection is defined as a view imagined on to a plane known as projection plane. In graphic language shape of an object is described by projection, which is image of the object, formed by the rays of sight taken in some particular direction from the object into a picture plane. Depending upon the orientation of the object, location of the point of sight, and the direction of lines of sight relative to the picture plane, different types of projections such as ortho- graphic, isometric, perspective, oblique etc. can be obtained. The point, from which the observer is assumed to view the object, is called station point or the center of projection and the lines of rays drawn from the object to the plane are called projectors. Methods of Projection There are four methods of projections commonly used 1. Orthographic projection 2. Isometric projection 3. Oblique projection 4. Perspective projection. Let us discuss small introduction of each one of these so that we can understand examples 1. Orthographic projection-when the projectors are Orthographic Projection While projecting any object on a plane if the rays of sight are taken perpendicular to the plane of projection, the projection method is then called orthographic projection. In this method the observer is assumed to be at infinite distance from the plane of projection, such that the projectors will be parallel to each other. perpendicular to the plane of projection the projection is known as orthographic projection. 2. Isometric projection-when the projectors are parallel but inclined to one plane of projection, the projection thus obtained is known as isometric projection 3. Oblique projection-when the projectors are parallel to each other and oblique to the plane of projection, the projection drawn is known as oblique projection.
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  To draw sideview rotate the AVP in the direction of arrow shown, so as to make it coincide with the V.P. looking the object from the left, the left side view is obtained and drawn on the right of the front view. There are two systems by which an object can be represented. a. First angle projection b. Third angle projection Both these are named according to the quadrant in which the object is imagined to be placed for purposes of projection of projection. As far as the shape and sizes of views are concerned there is no difference between these two systems. The only difference lies in the relative positions of the various views. Orthographic Projection (front view) Principle of First Angle Projection When the object is situated in first quadrant, that is in front of V.P and H.P, the object lies in between the observer and the plane of projection. In this case the object is to be transparent and the projectors are imagined to be extended from various points of the object to meet the projection plane. The meeting points when join in order form an image. This is the principle of first angle projection. Initial view Final arrangement of views Principle of Third Angle Projection When the object is situated in third quadrant that is in front of V.P and below H.P, the projection plane lies between the observer and the object. This means that the plane of projec- tion is said to be transparent. The intersection of this plane with the projectors from all the points of object would form an image on the transparent plane. This is the principle of third angle projection. Orthographic Projections (front & top views) Here the figure shows the two principal planes H.P and V.P and another auxiliary vertical plane (AVP). AVP is perpendicular to both V.P and H.P. Front view is drawn by projecting the object on the V.P and top view is drawn by projecting on the H.P the projection on the AVP as seen from the left of the object and drawn on the right hand of the front view, is called left side view. Initial view Final arrangement of views
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  Comparison of FirstAngle and Third Angle Projection. If one projects lines from the corners of the object (with each line at 90 degrees to a surface of the glass), until these lines intersect the glass, one lay out 6 views each of which represents the object as it is seen by the various planes of glass. S.no. First Angle Projection Third Angle Projection 1. The object to be assumed The object to be assumed in first quadrant. in third quadrant. 2. The object lies between the The plane of projection observer and the plane of lies between the observer projection. and the object. 3. The plane of projection is The plane of projection is assumed to be assumed to be transparent non- transparent 4. Object is situated on or Object is situated on or above the H.P above the ground 5. Symbol Symbol 6. Front view comes above Top view comes above Having projected the respective views of each face of the object onto the glass surfaces we can unfold the glass. top view. This method of front view. This method projection is recommended of projection is used in by the B.I.S from 1991. U.S.A and also in other countries. Making Orthographic Views Representation drawing and sketching is concerned with what you see or drawing which you imagine as if you actually saw it. For making orthographic views we can consider an object enclosed in an imaginary glass box, positioned such that the planes of glass are parallel to the major surfaces of the object and 90 degrees to each other. Until all the views are laid out in the same plane as the front view giving the figure. This process uses principles of orthographic projection the advantage of this process is that it’s possible to apply a uniform a system of coordinates to the pictures created in each view. If we correctly draw the individual views onto the pieces of glass, fold them up into the middle of box, the resulting intersec- tions will define the three dimensional object. From above example we can designate and find out the relative positions of views as follows: • View from the front- front view- View from the top-top view • View from the left- left side view- View from the right- right side view • View from the below-bottom view-View from the rear- rear view