ENS 207 engineering graphics Lecture 4: Orthographic projections, Multiview drawing

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: 1) Line of sight 2) Plane of projection (image plane or picture plane)

MEANING Orthographic projection is a parallel projection technique in which the parallel lines of sight are perpendicular to the projection 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 Parallel projection Perspective projection Line of sight Line of sight

Which Views to Present? General Guidelines Pick a Front View that is most descriptive of object Normally the longest dimension is chosen as the width (or depth) Most common combination of views is to use: Front, Top, and Side View Views other than the Principal Views are called Auxiliary Views Pick the views which will help in describing the object with highest clarity. Explain what is an auxiliary view. Explain that they are drawn to show specific features that are not clear in the Principal views.

AU 2005 The Idea is to have them take an object from the table. Declare front. FRONT View is the MOST DESCRIPTIVE VIEW OF THE OBJECT. The view that gives MORE INFORMATION ABOUT THE OBJECT. Rotate 90 degrees “up” to get top view. Rotate Back. Rotate 90 degrees clockwise to get right side. This give three principal views commonly used. AU 2005

Methods of Drawing Orthographic Projections First Angle Projections Method Here views are drawn by placing object in 1st Quadrant ( Fv above X-y, Tv below X-y ) Third Angle Projections Method Here views are drawn by placing object in 3rd Quadrant. ( Tv above X-y, Fv below X-y ) FV TV X Y X Y G L TV FV

Conventional Orthographic Views Height Depth Width Front View Top View Right Side View Note that the views are placed and aligned in the manner shown in the diagram. Remind the students that they have to follow the above convention for all their home work problems and exam problems. It is very important to maintain the alignment and correct placement relative to each other. Means line for top (and bottom) is straight across for both front view and right side view for example. Same thing between front and top for sides. Note : The following can be seen from the slide: Top View and front view have the same width Front View and Right / Left side view have the same height. The depth of Top view is same as the width of right/ left side view. AU 2005

Step 1 – Lightly Block Three Views Use very light lines for drawing in the construction lines Instructor: Construction lines are to be drawn lightly, gray vs. black, so that they will not reproduce on a Xerox machine and therefore do not have to be erased. AU 2005

S.V. F.V. ORTHOGRAPHIC PROJECTIONS x y PICTORIAL PRESENTATION IS GIVEN FOR T.V. S.V. F.V. ORTHOGRAPHIC PROJECTIONS FRONT VIEW L.H.SIDE VIEW FOR F.V. FOR S.V. x y PICTORIAL PRESENTATION IS GIVEN DRAW THREE VIEWS OF THIS OBJECT BY FIRST ANGLE PROJECTION METHOD TOP VIEW

S.V. F.V. FOR T.V. ORTHOGRAPHIC PROJECTIONS X Y FOR F.V. FOR S.V. FRONT VIEW L.H.SIDE VIEW X Y FOR F.V. FOR S.V. TOP VIEW PICTORIAL PRESENTATION IS GIVEN DRAW THREE VIEWS OF THIS OBJECT BY FIRST ANGLE PROJECTION METHOD

S.V. F.V. FOR T.V. ORTHOGRAPHIC PROJECTIONS X Y FOR S.V. FOR F.V. FRONT VIEW L.H.SIDE VIEW X Y FOR S.V. FOR F.V. TOP VIEW PICTORIAL PRESENTATION IS GIVEN DRAW THREE VIEWS OF THIS OBJECT BY FIRST ANGLE PROJECTION METHOD

ORTHOGRAPHIC PROJECTIONS FOR T.V. ORTHOGRAPHIC PROJECTIONS FRONT VIEW L.H.SIDE VIEW X Y FOR F.V. FOR S.V. TOP VIEW PICTORIAL PRESENTATION IS GIVEN DRAW THREE VIEWS OF THIS OBJECT BY FIRST ANGLE PROJECTION METHOD

ORTHOGRAPHIC PROJECTIONS FOR T.V. ORTHOGRAPHIC PROJECTIONS O 450 X FV Y 30 40 TV 30 D 40 15 FOR F.V. PICTORIAL PRESENTATION IS GIVEN DRAW FV AND TV OF THIS OBJECT BY FIRST ANGLE PROJECTION METHOD

PICTORIAL PRESENTATION IS GIVEN DRAW FV AND TV OF THIS OBJECT BY FIRST ANGLE PROJECTION METHOD ORTHOGRAPHIC PROJECTIONS 10 30 50 35 FV X Y RECT. SLOT FOR T.V. O 20 D 30 D 60 D TV FOR F.V. TOP VIEW

HIDDEN LINES Hidden lines are used to represent surfaces that are not directly visible in an orthographic view.

SLANTED SURFACES Slanted surfaces are surfaces that are not parallel to either the horizontal or vertical axis.

PROJECTION BETWEEN VIEWS

COMPOUND LINES A compound line is formed when two slanted surfaces intersect. The true length of a compound line is not shown in the front, top, or side views.

OBLIQUE SURFACES Oblique surfaces are surfaces that do not appear correctly shaped in the front, top, or side views

ROUNDED SURFACES Rounded surfaces are surfaces that have constant radii, such as arcs or circles. Surfaces that do not have constant radii are classified as irregular surfaces