Subject : Basic Technology
Class : Basic 9 / JSS 3
Term : 1st Term / First Term
Previous Knowledge : The pupils have been taught
during their previous lesson or class
Behavioural Objectives : By the end of the lesson, learners will be able to
- define oblique drawing
- say the steps that are involved in oblique drawing
Topic: Orthographic Projection
Orthographic projection is a method of drawing an object be means of plane views. While isometric drawing looks at all the three views (or faces) of an object together, orthographic looks at the same three views separately but linked together. In orthographic projections, we can look at each view and see its “correct or true shape”.
Orthographic projection is the only drawing that shows the shape, angles and size of each part of the article to be constructed as they really are. Because of this peculiar characteristic, it is one of the universally accepted drawings in the construction industry, therefore the drawing is usually used for production purposes. Ordinary pictorial drawing is not sufficient to build a house or fabricate parts of a machine. What is most needed is orthographic projection of the article. Therefore, technical drawing students must be able to read and interpret this drawing correctly. It requires a lot of practice, patience, time and mental alertness to be able to visualize readily the given pictorial drawings and draw the item in orthographic projection. The students should be able to visualize direct projection i.e. to see clearly from the ‘minds’ eyes’.
The block suspended in a glass box
Principal Planes of Orthographic Projection
Orthographic projection uses two main planes called the principal planes of projection, they are the vertical plane planes and the horizontal planes. These two planes intersect, producing four quadrants. The object to be drawn is imagined to be drawn is imagined to be placed in one of these quadrants, and orthographic views of it are projected on to the planes, the object is normally positioned so that its main faces are parallel to them. This ensures that views of the faces are true sizes and shapes.
Principal planes of projection
- Vertical Plane: The views on the vertical plane are front elevation and end elevation (side elevation).
- Horizontal plane: This shows the plan in the quadrants in the figure above. The quadrant 0-90ocontains the first angle and quadrant 180o-270o contains the third angle. Since the second and fourth quadrants would combine the two systems, we do not use them.
Angles of Projection
There are two main methods or orthographic projection as earlier said, they are:
- First angle projection
- Third angle projection
The principal views are the front elevation side (end) elevation and the plan.
- Front elevation: This is the main view, it is obtained by looking at the given front of the object to be drawn. The front elevation is seen in the vertical plane.
- End (side) elevation: This is obtained by looking by looking at the given side of the object. The end elevation is also seen in the vertical plane [i.e. auxiliary/side vertical plane (S.V.P)].
- Plan: This view is obtained by looking vertically down on the given object from the above. The plan is seen on the horizontal plane (H.P).
First Angle Projection
This method in developed in the United Kingdom. In first angle projection, the elevations at the front are the horizontal plane placed below the x and y above the plan. The hidden detail is shown by short dashes. It should be noted that the left side elevation is usually placed at the right hand side.
The Topic is presented step by step for easy assimilation by the learners.
Step 1 : The teacher revises the previous topics that had been taught during the precious lessons
Step 2 : The subject teacher introduces the new topic of the learners
Step 3 : He or she allows the students to give their own contributions and he corrects them when the needs arise
Briefly define the following terms :
- What do you understand by Oblique drawing
- Briefly explain the principal views of orthographic projection