TERM: FIRST TERM
SUBJECT: BASIC TECHNOLOGY
CLASS: JSS 3
WEEK NINE AND TEN
TOPIC: ORTHOGRAPHIC DRAWING
The draughtsman or engineer uses two basic methods to represent an object on paper, board, or similar surfaces. The two basic methods are:
Orthographic projection is used to show at least three views. The three views usually shown in orthographic projection are: front view (elevation), side view (either right side or left side), and plan (top view). Orthographic drawing is the method of producing the drawing of an object so it can be fabricated accurately.
CHARACTERISTICS OF ORTHOGRAPHIC DRAWING
IMPORTANCE OF ORTHOGRAPHIC DRAWING
Dimension of an orthographic drawing is usually measured in millimeters
METHODS OF PROJECTION
Orthographic views of objects are drawn on surfaces called planes. A projection plane can be likened to a screen on which a film is projected. If the screen is placed vertically, the plane is called a vertical plane. If it is placed horizontally, it is called a horizontal plane. It is equally called a side vertical plane, if it is placed by either the right hand side or the left hand side.
The method of projection varies depending on the rays of light the object is taken to the plane. When the projection rays are perpendicular to the plane, the projective method is called ORTHOGRAPHIC. If the rays are at right angle to the plane, the projection is called OBLIQUE. If the rays are taken to a particular station, the projective method will result PERSPECTIVE,thereby presenting the object as would appear to the eye.
ANGLES OF PROJECTION
The angle of projection in common use areof two types namely:
The first angle projection method is mainly used in European countries. In this projection, the front elevation drawing is followed by the plan elevation
The third angle projection method is mainly used in USA and Canada. In this projection, the plan elevation is followed by front elevation.
There are three types of views in orthographic drawing:
This is act of using lines to indicate the length and breadth of a particular object. Lines indicating extremes of dimension are drawn where required either to the right side or below it.
Arrowheads are drawn to point inwards and the figure is printed between the arrowheads in case of small dimension.
Hidden details are indicated with dotted lines. Hidden details and hatching line indicate a sectional view of objects. Rounded corners and small diameter holes should also be dimensioned
Read SCALES AND SCALE DRAWING
© Lesson Notes All Rights Reserved 2023