explain projection of point engineering drawing
Answers
Answer:
A POINT
The position of a point in engineering drawing is defined with respect to its distance from the three principle planes i.e., with respect to the VP, HP, & PP.
The point is assumed to be in the respective quadrant shown in figure 1(a). The point at which the line of sight (line of sight is normal to the respective plane of projection) intersects the three planes are obtained. The horizontal plane and the side planes are rotated so such that they lie on the plane containing the vertical plane. The direction of rotation of the horizontal plane is shown in figure 1 (b).
Explanation:
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:
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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
Object,
Projectors,
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
The above method represents the object by a pictorial view as an observer sees it. 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’.
Orthographic Projections
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. 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 below 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.
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