# Lesson Notes By Weeks and Term - Junior Secondary School 3

ORTHOGRAPHIC DRAWING

TERM: FIRST TERM

SUBJECT: BASIC TECHNOLOGY

CLASS: JSS 3

REF MATERIALS

• EVANS BASIC TECH FOR JSS BOOK 3
• NERDC BASIC TECH FOR JSS BOOK THREE

WEEK NINE AND TEN

TOPIC: ORTHOGRAPHIC DRAWING

CONTENT

1. ORTHOGRAPHIC DRAWING
2. METHODS OF PROJECTIONS
3. ANGLES OF PROJECTION
4. DIMENSIONING

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
• Pictorial projection

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

1. Standardize lines are used
2. The plan, front and side views of the object are properly drawn
3. Objects are drawn to scale

IMPORTANCE OF ORTHOGRAPHIC DRAWING

1. It shows accurately the shape of the object
2. It shows clearly the size of each part of the object

Dimension of an orthographic drawing is usually measured in millimeters

EVALUATION

1. Define orthographic drawing
2. State characteristics orthographic drawing

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:

1. First angle projection
2. Third angle projection

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:

1. Horizontal plan views
2. Vertical plan views
3. Side plan views

DIMENSIONING

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

EVALUATION

1. Explain the two types of angle of projection .
2. Define dimensioning.
3. Differentiate between the angles of projection with the aid of a sketch.

GENERAL EVALUATION

1. Define orthographic projection.
2. Draw letter “F” and produce it in first angle projection.
3. Draw a rectangle of length 70mmand width 50mm and dimension it.

REFERENCE MATERIALS

1. NERDC, BASIC TECHNOLOGY,BOOK 1 pages 58-63

WEEKEND ASSIGNMENT

1. Which of the following lines can be used for hidden details A. hatching line  B. short dashes C. continuous thin lines D. Straight lines.
2. The method of projection in which the plan elevation comes before the front elevation is called A.  third angle B. first angle C. second angle projection D. fourth angle projection.
3. When the projection rays are perpendicular to the plane, the projective method is called  A. orthographic B. oblique C. perpendicular D. plane views .
4. If the rays are at right angle to the plane, the projection is called ______ A.  orthographic           B. oblique  C.  perpendicular  D. perspective .
5. If the rays are taken to a particular station, the projective method will result ____
1. orthographic  B.  oblique C. perspective (d) plane views.

THEORY

1. Define (a) Dimensioning  (b) orthographic drawing.
2. Briefly describe first angle and third angle projection with sketch.