Lesson Note on Orthographic Projections: First and Third Angle Views for SS 2

Class: Senior Secondary School 2 (SS2 / SSS2)

Term: 3rd Term

Week: 3

Age: 16 years

Duration: 45 minutes

Subject: Technical Drawing

Curriculum Theme: Technical Skills

Previous Lesson: Orthographic Projection: Meaning, Principles and Types.

Topic: ORTHOGRAPHIC PROJECTIONS

Subject Matter: Orthographic views (first angle projection, third angle projection)

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

• Define orthographic projection.
• Explain the principles of first angle projection.
• Explain the principles of third angle projection.

Affective Domain:

• Appreciate the importance of orthographic projection in technical communication.
• Show interest in drawing technical diagrams accurately.

Psychomotor Domain:

• Draw the orthographic views (front, plan, and end views) of simple objects using first angle projection.
• Draw the orthographic views (front, plan, and end views) of simple objects using third angle projection.
• Dimension orthographic views correctly according to standards.

Social Domain:

• Collaborate with peers to interpret and draw technical diagrams effectively.

Reference Materials

The following resources were used in planning this lesson:

• Senior Secondary Schools Education Curriculum
• State Unified Scheme of Work
• N.D. Bhatt, Engineering Drawing.
• https://www.technicaldrawing.org/orthographic-projection-explained
• https://www.youtube.com/watch?v=R9_T4fR2c9c

Instructional Materials

The teacher will teach this lesson with the aid of:

• Drawing board
• T-square
• Set squares (45° and 30°/60°)
• Pencils (HB, 2H)
• Eraser
• Compass
• Divider
• Protractor
• Drawing paper
• Pictorial drawings of various simple objects
• Examples of correctly drawn orthographic views (first and third angle)

Rationale for the Lesson

This lesson helps pupils understand how to represent three-dimensional objects accurately in two dimensions. This skill is important for communicating design and manufacturing information clearly in engineering and construction.

Prerequisite/Previous Knowledge

Pupils have basic knowledge of geometric constructions, different types of lines used in technical drawing, and sketching simple shapes.

Lesson Content/Board Summary

ORTHOGRAPHIC PROJECTIONS

Introduction to Orthographic Projection

Orthographic projection is a method of representing a three-dimensional object in two dimensions by projecting a series of views onto mutually perpendicular planes. It provides clear and unambiguous information about the object’s shape, size, and features, essential for manufacturing and construction.

Principal Views

There are six principal views, but typically three are sufficient to describe an object:

• Front View (Elevation): Shows the object as seen from the front.
• Top View (Plan): Shows the object as seen from above.
• End View (Side Elevation): Shows the object as seen from the side (left or right).

First Angle Projection

In first angle projection, the object is imagined to be in the first quadrant. The observer looks at the object, and the projection planes are placed behind the object. The views are then projected onto these planes.

Symbol: A truncated cone is typically used, with the smaller end to the right of the larger end.

Arrangement of Views:

• The Top View (Plan) is placed directly below the Front View.
• The Right End View is placed to the left of the Front View.
• The Left End View is placed to the right of the Front View.

Worked Example 1 (First Angle Projection):

Draw the orthographic views (Front, Top, and Right End Views) of a simple L-shaped block in first angle projection. Assume dimensions of 60mm (length), 40mm (width), and 50mm (height), with a step of 20mm from the top and 20mm from the right.

Step 1: Draw the Front View. Represent the main shape and overall height and length, including the visible step.
Step 2: Project lines downwards from the Front View to draw the Top View. Represent the length and width, showing the step’s outline.
Step 3: Project lines horizontally from the Front View and use a 45-degree line or compass to transfer width from the Top View to draw the Right End View. Represent the width and height, showing the step.
Step 4: Add appropriate dimensions to all views, ensuring no dimension is repeated.

Third Angle Projection

In third angle projection, the object is imagined to be in the third quadrant. The projection planes are placed between the observer and the object. The views are projected onto these planes.

Symbol: A truncated cone is typically used, with the larger end to the right of the smaller end.

Arrangement of Views:

• The Top View (Plan) is placed directly above the Front View.
• The Right End View is placed to the right of the Front View.
• The Left End View is placed to the left of the Front View.

Worked Example 2 (Third Angle Projection):

Draw the orthographic views (Front, Top, and Right End Views) of the same L-shaped block (from Example 1) in third angle projection.

Step 1: Draw the Front View. (This view will be identical to the Front View in first angle projection).
Step 2: Project lines upwards from the Front View to draw the Top View. (The shape will be the same as in first angle, but positioned above the Front View).
Step 3: Project lines horizontally from the Front View and use a 45-degree line or compass to transfer width from the Top View to draw the Right End View. (The shape will be the same as in first angle, but positioned to the right of the Front View).
Step 4: Add appropriate dimensions to all views, ensuring no dimension is repeated.

Dimensioning Orthographic Views

Correct dimensioning is essential for manufacturing. Key rules include:

• Dimension lines should be thin, continuous lines, placed parallel to the feature being dimensioned.
• Extension lines extend from the object outline but do not touch it.
• Arrowheads should be neat, sharp, and touch the extension lines.
• Dimension figures should be placed above the dimension line, centrally, and readable from the bottom or right side of the drawing.
• Avoid duplicating dimensions or placing them on hidden lines.
• Overall dimensions, feature dimensions, and location dimensions should be included clearly.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Set Induction

Teacher’s Activity: The teacher asks pupils how engineers and designers communicate the exact shape and size of objects for construction or manufacturing.

Pupils’ Activity: Pupils offer ideas such as sketches, models, or technical drawings.

Learning Point: The lesson will introduce orthographic projection as a precise method for representing objects.

Step 2: Explanation of Orthographic Projection and Principal Views

Time: 7 minutes

Teaching Skill: Explanation/Illustration

Teacher’s Activity: The teacher defines orthographic projection and explains the concept of principal views (Front, Top, End) using a simple physical object or a pictorial drawing.

Pupils’ Activity: Pupils listen attentively, observe the demonstrations, and ask clarifying questions.

Learning Point: Pupils understand the definition and basic views of orthographic projection.

Step 3: First Angle Projection

Time: 10 minutes

Teaching Skill: Demonstration/Explanation

Teacher’s Activity: The teacher explains the principle of first angle projection, its symbol, and the standard arrangement of views. The teacher then demonstrates drawing the orthographic views of a simple object on the board or chart using first angle projection.

Pupils’ Activity: Pupils observe the demonstration carefully and take notes on the principles and view arrangement.

Learning Point: Pupils learn how to represent objects using first angle projection.

Step 4: Practice First Angle Projection

Time: 7 minutes

Teaching Skill: Guided Practice

Teacher’s Activity: The teacher provides a different pictorial drawing of a simple object and guides pupils to draw its orthographic views in first angle projection on their drawing paper, emphasizing correct use of instruments and dimensioning.

Pupils’ Activity: Pupils attempt to draw the orthographic views, applying the rules of first angle projection and dimensioning.

Learning Point: Pupils gain practical experience in drawing first angle projections.

Step 5: Third Angle Projection

Time: 7 minutes

Teaching Skill: Demonstration/Explanation

Teacher’s Activity: The teacher explains the principle of third angle projection, its symbol, and the standard arrangement of views. The teacher demonstrates drawing the orthographic views of a simple object (possibly the same as in Step 3 for comparison) using third angle projection.

Pupils’ Activity: Pupils observe the demonstration, noting the differences in symbol and view arrangement compared to first angle projection.

Learning Point: Pupils learn how to represent objects using third angle projection.

Step 6: Practice Third Angle Projection

Time: 5 minutes

Teaching Skill: Guided Practice

Teacher’s Activity: The teacher provides another pictorial drawing of a simple object and guides pupils to draw its orthographic views in third angle projection, ensuring correct view placement and dimensioning.

Pupils’ Activity: Pupils draw the orthographic views, applying the rules of third angle projection and dimensioning.

Learning Point: Pupils gain practical experience in drawing third angle projections.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. Define orthographic projection.
2. State two principal views used in orthographic projection.
3. Describe the arrangement of views in first angle projection.
4. Describe the arrangement of views in third angle projection.
5. Draw the standard symbol for first angle projection.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 1 minute

Teaching Skill: Summarizing

Teacher’s Activity: The teacher summarizes the key points of the lesson, reiterating the importance of orthographic projection in technical communication and the differences between first and third angle projection.

Pupils’ Activity: Pupils listen and note down the summary points.

Learning Point: Reinforcement of the lesson’s main concepts.

Lesson Keywords

• Orthographic – A type of projection in technical drawing where views of an object are projected onto planes that are perpendicular to each other.
• Projection – The process of representing a three-dimensional object on a two-dimensional surface.
• First Angle Projection – A method of orthographic projection where the object is placed in the first quadrant, and the projection planes are behind the object.
• Third Angle Projection – A method of orthographic projection where the object is placed in the third quadrant, and the projection planes are between the observer and the object.
• Front View – The view of an object as seen from the front, also known as Elevation.
• Plan – The view of an object as seen from above, also known as Top View.
• End View – The view of an object as seen from the side (left or right side elevation).
• Dimensioning – The process of adding measurements and notes to a drawing to specify the size and features of an object.

Differentiation

For pupils who are struggling, the teacher will provide simpler pictorial drawings and offer more one-on-one guidance during drawing practice. For advanced pupils, more complex objects or tasks requiring multiple views can be assigned, encouraging them to explore more intricate dimensioning techniques.

Note for teachers using this lesson plan

Ensure that pupils have access to proper drawing instruments and understand their correct usage. Emphasize the importance of neatness and accuracy in technical drawing. Use physical models or 3D software to help pupils visualize the objects and their projected views before drawing. Pay close attention to the correct placement of views and dimensioning rules for both first and third angle projections.