engineering drawing...theory of projection and orthographic projection •projection: any kind of...
TRANSCRIPT
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ENGINEERING
DRAWING
LECTURE 3
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Theory of Projection and Orthographic
Projection
• Projection:
Any kind of representation of an object on a paper, screen or similar
surface by drawing or by photography is called the projection of that
object.
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Projection Methods
Parallel
Pictorial Orthographic
Perspective
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Projection Theory
• The projection theory is used to graphically represent 3-D objects on 2-D
media (paper, computer screen).
• The projection theory is based on two variables:
i. Line of sight
ii. Plane of projection (image plane or picture plane)
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Line of sight is an imaginary ray of light between an
observer’s eye and an object.
• There are 2 types of LOS : parallel and converge
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Plane of projection is an imaginary flat plane which the
image is created.
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Disadvantage of Perspective Projection
1. It is difficult to create.
2. It does not reveal exact shape and size.
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Pictorial Projection
• The Projection in which the length, breadth (width) and height of an object
is shown in one view is known as pictorial projection.
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Orthographic Projection
• Orthographic projection is a parallel projection technique in which the
parallel lines of sight are perpendicular to the projection plane.
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Orthographic Projection
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The Glass Box Approach
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Opening the Box
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Final Views
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Six Orthographic Views:
Laying out all six views
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Three Primary Views
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Construction of Views
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First and Third Angle Projections
• First Angle – International
• Three Angle – USA
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Comparison of First and Third Angle Projections
FIRST ANGLE PROJECTION THIRD ANGLE PROJECTION
The object is assumed to be situated in the first
quadrant.
The object is assumed to be situated in the third
quadrant.
The object lies in between the observer and the
plane of projection.
The plane of projection lies in between the
observer and the object.
View from above (top view) is drawn below of
front view.
View from above (top view) is drawn above of
front view.
View from below (bottom view) is drawn above of
front view.
View from below (bottom view) is drawn below of
front view.
View from left (left side view) is drawn on the right
of front view.
View from left (left side view) is drawn on the left
of front view.
View from right (right side view) is drawn on left of
front view.
View from right (right side view) is drawn on right
of front view.
View from the rear (rear view) is drawn on left or
right of front view.
View from the rear (rear view) is drawn on left or
right of front view.
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First Angle Projection
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Third Angle Projection
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Symbol of first and third angle projection
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Dimensioning
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Placing of Dimensions
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Basic Line Types
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Meaning of Lines
• Visible lines represent features that can be seen in the current view.
• Hidden lines represent features that can not be seen in the current view.
• Center line represents symmetry, path of motion, centers of circles, axis of
axisymmetric parts.
• Dimension and Extension lines indicate the sizes and location of features
on a drawing.
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Good Practice
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Exercise
• Complete three orthographic views of the object shown on the next slide
(Third Angle Projection).
• Include visible, hidden, and center lines where appropriate.
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Object for exercise