Orthographic Projection JSS3 Basic Technology Lesson Note

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 the front view (elevation), side view (either the 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

The 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 a right angle to the plane, the projection is called OBLIQUE. If the rays are taken to a particular station, the projective method will result in PERSPECTIVE, thereby presenting the object as would appear to the eye.              

ANGLES OF PROJECTION 

The angle of projection in common uses is of 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 the USA and Canada. In this projection, the plan elevation is followed by the 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 an 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 lines 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 the letter “F” and produce it in first angle projection.
3.         Draw a rectangle of length 70 mm and width 50mm and dimension it.

READING ASSIGNMENT 

Read SCALES AND SCALE DRAWING

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 a 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 in ____.     A.  orthographic  B.  oblique C. perspective (d) plane views.

THEORY 

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