ENGINEERING DRAWING

LECTURE 3 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. Projection Methods

Parallel Perspective

Pictorial Orthographic 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) 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 Plane of projection is an imaginary flat plane which the image is created. Disadvantage of Perspective Projection

1. It is difficult to create. 2. It does not reveal exact shape and size. 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. Orthographic Projection

• Orthographic projection is a parallel projection technique in which the parallel lines of sight are perpendicular to the projection plane. Orthographic Projection The Glass Box Approach Opening the Box Final Views Six Orthographic Views: Laying out all six views Three Primary Views Construction of Views First and Third Angle Projections

• First Angle – International • Three Angle – USA Comparison of First and Third Angle Projections FIRST ANGLE PROJECTION THIRD ANGLE PROJECTION The object is assumed to be situated in the first The object is assumed to be situated in the third quadrant. quadrant. The object lies in between the observer and the The plane of projection lies in between the plane of projection. observer and the object. View from above (top view) is drawn below of View from above (top view) is drawn above of front view. front view. View from below (bottom view) is drawn above of View from below (bottom view) is drawn below of front view. front view. View from left (left side view) is drawn on the right View from left (left side view) is drawn on the left of front view. of front view. View from right (right side view) is drawn on left of View from right (right side view) is drawn on right front view. of front view. View from the rear (rear view) is drawn on left or View from the rear (rear view) is drawn on left or right of front view. right of front view. First Angle Projection Third Angle Projection Symbol of first and third angle projection Dimensioning Placing of Dimensions Basic Line Types 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. Good Practice Exercise

• Complete three orthographic views of the object shown on the next slide (Third Angle Projection). • Include visible, hidden, and center lines where appropriate. Object for exercise