Figure 1: Folding of Drawing Prints CHAPTER 7 ORTHOGRAPHIC DRAWING Orthographic projection is a means of representing a three-dimensional object in two dimensions. It uses multiple views of an object, from points of view rotated about the objects centre through increments of 90 degrees. Equivalently, the views may be considered to be obtained by rotating the object about its centre through increments of 90 degrees. The views are positioned relative to each other according to either of two schemes: first-angle or angle projection. In each- the appearances of views may be thought of as being “projected” onto planes that form a transparent around the object. “box” Primary or Principal Planes of Projection When drawing a number of views of an object, the object is viewed through a plane of projection from a point at infinity, thereby obtaining an accurate outline of the visible face of the object. However, the projection of one face will not provide an overall description of the object; other planes of projection must be used. Establishing an object’s true height, width and depth requires a front, top and side views, which are called the principal planes of projection. The three principal (or primary) planes of projection are known as the vertical, horizontal and profile planes. The angles formed between the horizontal and the vertical planes are called the first, second, third and fourth angles as shown in the diagram below. For practical purposes only the first and third is used. First Angle Projection In first angle projection, each view of the object is projected in the direction (sense) of sight of the object, onto the interior walls of the box.
Figure 1: Folding of Drawing Prints CHAPTER 7 ORTHOGRAPHIC DRAWING Orthographic projection is a means of representing a three-dimensional object in two dimensions. It uses multiple views of an object, from points of view rotated about the objects centre through increments of 90 degrees. Equivalently, the views may be considered to be obtained by rotating the object about its centre through increments of 90 degrees. The views are positioned relative to each other according to either of two schemes: first-angle or projection. In each- the appearances of views may be thought of as being “projected” onto planes that form a transparent “box” around the object. Primary or Principal Planes of Projection When drawing a number of views of an object, the object is viewed through a plane of projection from a point at infinity, thereby obtaining an accurate outline of the visible face of the object. However, the projection of one face will not provide an overall description of the object; other planes of projection must be used. Establishing an object’s true height, width and depth requires a front, top and side views, which are called the principal planes of projection. The three principal (or primary) planes of projection are known as the vertical, horizontal and profile planes. The angles formed between the horizontal and the vertical planes are called the first, second, third and fourth angles as shown in the diagram below. For practical purposes only the first and third angle is used. First Angle Projection In first angle projection, each view of the object is projected in the direction (sense) of sight of the object, onto the interior walls of the box.