JSS3 Basic Technology Lesson Note on Orthographic Projection

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TOPIC: Orthographic Projection

Sub-Topic:

• Meaning, Components (principal planes, principal views)
• Angles of projection

(b) Placing view in the quadrants

(c) Dimensioning techniques.

SUB-TOPIC 2   ANGLES OF PROJECTION

The angle of projection depends on which of the four quadrants the object is placed on. The two principal orthographic projections are

• The first angle projection (English projection)
• The third angle projection (American projection)

The difference between these two is the method of the arrangement of the views.

THE FIRST ANGLE PROJECTION

The first angle projection is the projection in which the plan is located directly below the front elevation (view). What is seen from the left –hand side is drawn on the right hand side. What is seen from the right-hand side is drawn on the left hand-side. Hence, the plan is located directly below the front elevation (views)

FRONT	SIDE
PLAN

FIRST ANGLE PROJECTION

PROPERTIES OF FIRST ANGLE PROJECTION

The following are the properties of the first angle projection.

• The plan is always projected below the front view (elevation)
• The right-hand side view is shown on left hand side of the front view
• The left-hand side is shown on the right hand side of the front views.

FIRST ANGLE PROJECTION

EVALUATION

• What is orthographic drawing?
• State the two angles of orthographic projection.
• State properties of first angle projection.
• What are the three views in orthographic projection

Content Development (NOTES FOR SECOND LESSON/PERIOD TWO)

THE THIRD ANGLE PROJECTION

This is the projection in which the plan is located directly above the front view (elevation). What is seen on the left–hand side is drawn on the left –hand side. What is seen on the right-hand side is drawn on the right side. Hence; the plan is located directly above the front elevation (views).

PLAN
FRONT	SIDE

PROPERTIES OF THIRD ANGLE PROJECTION

The following are the properties of third-angle projection.

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