ORTHOGRAPHIC DRAWING

ENGINEERING DRAWING - LECTURE 6: ORTHOGRAPHIC DRAWING

Principle of Projection

• Engineering drawing is based on the fundamental principle of projection.
• A projection is a drawing or representation of an entity on an imaginary plane or planes.
• Projections should convey all three dimensions, along with other details, on a sheet of paper.
• A projection is obtained by viewing an object from a point of sight (at eye level) and tracing its features in a correct sequence.
• A projection involves four components:
  1. The actual object that the drawing or projection represents.
  2. The eye of the viewer looking at the object.
  3. The imaginary projection plane (which can be thought of as the viewer's drawing paper).
  4. The imaginary lines of sight, called projectors, which are perpendicular to the projection plane in orthographic projection.

Types of Projection

The two broad types of projection are:

• Parallel Projection:
  • Orthographic projection (e.g., Top view, Front view, Left-side view, Right-side view, Rear view, Bottom view)
  • Oblique projection
  • Axonometric projections / Pictorial drawings
• Perspective Projection: Represents objects as they appear to the eye, with receding parallel lines converging.

Orthographic Projections

• There are six principal views for every object: Front, Rear, Right side, Left side, Top, Bottom.
• Orthographic projection can be visualized by suspending an object inside a hinged "glass box." Points of the object are projected onto the planes (walls) of the box. The box is then unfolded to lay flat, displaying the different views.
• The three regular views in orthographic drawings are the top (or plan) view, front view, and a side (or end) view.
• Necessary views are the minimum number of views required to completely describe an object.

Orientation of the Front View Principles:

1. The front view should show a large surface of the part parallel to the viewing plane.
2. The front view should show the shape of the object clearly.
3. The front view should show the object in a usual, stable, or operating position, especially for familiar objects.
4. Usually, screws, shafts, tubes, and other elongated parts are drawn in a horizontal position.

Orthographic Projection Systems

The two main systems for projecting views are:

• First Angle Projection
• Third Angle Projection

First Angle Projection

• The observer looks through the object to the planes of projection to produce the view being observed.
• The object is imagined to be in the first quadrant.
• The object lies between the observer and the plane of projection.
• The plane of projection is assumed to be non-transparent.
• View Placement: When views are drawn in their relative position:
  • Top View comes below Front View.
  • Right Side View is drawn to the left side of the elevation (Front View).
  • Left Side View is drawn to the right side of the elevation (Front View).

Third Angle Projection

• The drawing of the view of the object is produced on a plane placed between the observer and the object.
• The object is imagined to be in the third quadrant.
• The plane of projection lies between the observer and the object.
• The plane of projection is assumed to be transparent.
• View Placement: When views are drawn in their relative position:
  • Top View comes above the Front View.
  • Right Side View is drawn to the right side of the elevation (Front View).
  • Left Side View is drawn to the left side of the elevation (Front View).

Orthographic Projection Symbols

Standard symbols are used to indicate whether First Angle or Third Angle projection is utilized in a drawing. These symbols represent the two views of a truncated cone drawn in the respective projection systems.

• The symbols should be included with the drawing to show the projection used.
• The symbols MUST be drawn and NOT sketched!
• Unless otherwise stated, the first angle projection is to be used for this course.

Hidden Lines, Centre Lines, Dimensions

• Hidden Lines:
  • Represent features on an object that are not visible in the particular view being drawn (implying they are covered by other features).
  • Hidden details are represented by thin short dashes.
• Centre Lines:
  • Used primarily to indicate locations of circular, cylindrical, or spherical features.
  • Can also specify locations of other principal symmetries.
• Dimensions:
  • Drawings must be dimensioned correctly.
  • Dimensions must not be repeated on different views.

Examples

The lecture provides numerous examples illustrating the application of orthographic projection principles. These examples demonstrate how to derive the various 2D orthographic views (Front, Top, Side) from a 3D isometric representation of an object, including the correct use of hidden lines and centerlines for features like holes, slots, and cutouts.