Sectional Views of Machines for SS 3

Class: SS 3
Term: First Term
Week: 11
Age: 17 years
Duration: 60 minutes
Subject: Technical Drawing
Curriculum Theme: Orthographic Projection and Machine Drawing
Previous Lesson: Sectional Views of Buildings
Topic: SECTION AND SECTIONAL VIEW components
Subject Matter: Sectional view of machine

Specific Objectives

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

Cognitive Domain

• Define sectional view in technical drawing.
• Explain the purpose and importance of sectional views.
• Identify different types of sectional views.
• State the conventions for drawing section lines and cutting planes.

Psychomotor Domain

• Draw a full sectional view of a simple machine component.
• Draw a half sectional view of a symmetrical machine component.

Affective Domain

• Appreciate the clarity and detail provided by sectional views in technical communication.
• Demonstrate precision and neatness in drawing sectional views.

Reference Materials

The following resources were used in planning this lesson:

• 2014 Senior Secondary Education Curriculum (SSEC)
• Relevant State Unified Scheme of Work
• Technical Drawing textbooks for Senior Secondary Schools
• FCT ERC/NAPPS Scheme of work

Instructional Materials

The teacher will teach this lesson with the aid of:

• Machine parts and models (e.g., engine block, gear housing, pulley)
• Orthographic projections of machine components (with and without sectional views)
• Drawing board, T-square, set squares, pencils (HB, 2H), eraser
• Drawing paper
• Chalkboard/Whiteboard and markers

Rationale for the Lesson

This lesson is important because sectional views are fundamental to understanding the internal construction and features of machine parts, which cannot be clearly shown by external views alone. Mastering sectional views equips students with a critical skill for interpreting and creating technical drawings, essential for careers in engineering, manufacturing, and related technical fields. It enhances their ability to communicate design intent effectively and accurately.

Prerequisite/Previous Knowledge

Students should have prior knowledge of orthographic projection, basic drawing instruments and their uses, and the ability to interpret basic engineering drawings.

Lesson Content/Board Summary

SECTION AND SECTIONAL VIEW components

Meaning of Sectional View

A sectional view (or section) is a view of an object as if it were cut by an imaginary plane, revealing its internal features. It is used in technical drawing to show the interior construction of an object that would otherwise be hidden by external surfaces, making the drawing clearer and easier to understand.

Purpose and Importance of Sectional Views

Sectional views are crucial in technical drawing for several reasons:

1. They reveal internal details that are not visible in standard orthographic views, reducing the need for many hidden lines.
2. They simplify complex drawings, making them easier to interpret and understand.
3. They provide a clear representation of the object’s internal construction, material thickness, and assembly details.
4. They are essential for manufacturing and assembly processes, ensuring components are produced and fitted correctly.
5. They aid in design analysis and problem-solving by exposing hidden features.

Cutting Planes and Section Lines

1. Cutting Plane Line: This is an imaginary plane that “cuts” through the object to reveal its interior. It is represented by a thick chain line, made up of long dashes alternating with two short dashes, and terminated by thick short dashes or arrowheads indicating the direction of view.
2. Section Lines (Hatching): These are thin, parallel lines drawn on the cut surfaces of the object in a sectional view. They indicate the material that has been “cut” by the cutting plane. Standard section lines are typically drawn at 45 degrees to the main outlines of the view and are evenly spaced. Different materials can be represented by specific hatching patterns, though a general pattern is often used for simplicity.

Types of Sectional Views

Various types of sectional views are used depending on the complexity and symmetry of the object:

1. Full Section: This is obtained when the cutting plane passes entirely through the object, dividing it into two equal or unequal parts. One part is then imagined to be removed, and the remaining part is viewed.
2. Half Section: Used primarily for symmetrical objects, a half section is created when the cutting plane passes through only one-quarter of the object. This view shows both the interior and exterior of the object in a single view, separated by a centre line.
3. Removed Section: A removed section is a sectional view that is taken from its normal position and placed elsewhere on the drawing sheet. It is often used to show the cross-sectional shape of a feature, such as a rib or spoke, at a specific location.
4. Revolved Section: Similar to a removed section, a revolved section is drawn directly on the view from which it was taken, usually superimposed on the main view. It shows the cross-sectional shape of a feature by rotating the section 90 degrees about an axis perpendicular to the cutting plane.

Steps for Drawing a Sectional View of a Machine Component

To draw a sectional view, follow these general steps:

1. Draw the Orthographic Views: Begin by drawing the necessary orthographic views (e.g., front view, top view, side view) of the machine component.
2. Position the Cutting Plane: Decide where the cutting plane will pass through the object to best reveal the desired internal features. Draw the cutting plane line on the appropriate orthographic view, indicating the direction of sight with arrowheads.
3. Project the Sectional View: Project the lines and features from the orthographic views onto the plane where the sectional view will be drawn. Imagine the part of the object between the viewer and the cutting plane has been removed.
4. Draw Visible Outlines: Draw all visible outlines of the object as they would appear after the cut, including the newly exposed internal features.
5. Apply Section Lines: Hatch the surfaces that have been cut by the imaginary cutting plane with appropriate section lines (usually 45-degree parallel lines). Ensure that adjacent components in an assembly have different hatching angles or patterns.
6. Add Dimensions and Annotations: Add necessary dimensions and annotations to the sectional view to provide complete information about the component.

Examples of Machine Components for Sectional Views

Machine components that frequently require sectional views to show internal details include:

1. Engine blocks and cylinder heads
2. Gearboxes and transmission housings
3. Valves and pumps
4. Bearings and bushings
5. Pulleys with internal webs
6. Flanged couplings

Teaching Methods/Instructional Techniques

Discussion, Demonstration, Guided Practice, Question and Answer, Explanation, Observation, Practical Activity.

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Questioning/Recall

Teacher’s Activity: The teacher greets the students and asks them to recall what they know about orthographic projection and how it helps in representing objects. The teacher then introduces the concept of internal features and how they are often hidden in standard orthographic views, leading to the need for sectional views.

Pupils’ Activity: Pupils respond to questions about orthographic projection and listen attentively to the introduction of sectional views.

Learning Point: Need for sectional views

Step 2: Defining Sectional View

Time: 10 minutes

Teaching Skill: Explanation/Demonstration

Teacher’s Activity: The teacher defines a sectional view, explaining it as a view of an object as if cut by an imaginary plane. Using a physical machine component model (e.g., a hollow block or a simple housing), the teacher demonstrates how an imaginary cut would reveal internal features. The teacher also explains the purpose and importance of sectional views in technical drawing.

Pupils’ Activity: Pupils listen, observe the demonstration, and ask questions for clarification. They note down the definition and purpose.

Learning Point: Meaning and purpose

Step 3: Understanding Cutting Planes and Section Lines

Time: 10 minutes

Teaching Skill: Explanation/Illustration

Teacher’s Activity: The teacher explains the concept of a cutting plane line, illustrating its representation on the board (thick chain line with arrowheads). The teacher also explains section lines (hatching), demonstrating how they are drawn at 45 degrees on the cut surfaces to indicate material. Examples of different hatching patterns for various materials can be briefly mentioned.

Pupils’ Activity: Pupils observe the illustrations on the board, practice drawing cutting plane lines and section lines in their sketch pads, and ask questions.

Learning Point: Cutting planes, section lines

Step 4: Exploring Types of Sectional Views

Time: 10 minutes

Teaching Skill: Explanation/Comparison

Teacher’s Activity: The teacher explains and illustrates the different types of sectional views: full section, half section, removed section, and revolved section. For each type, the teacher uses simple sketches or pre-drawn examples to show how they are formed and when they are typically used, emphasizing the distinction between them.

Pupils’ Activity: Pupils observe the illustrations, differentiate between the types of sectional views, and make notes.

Learning Point: Types of sectional views

Step 5: Guided Practice – Drawing a Full Section

Time: 10 minutes

Teaching Skill: Guided Practice/Demonstration

Teacher’s Activity: The teacher guides students through the steps of drawing a full sectional view of a simple machine component (e.g., a cylindrical block with a central hole). The teacher demonstrates on the board, showing how to draw the orthographic views, position the cutting plane, project the section, and apply section lines. The teacher ensures students follow along in their drawing pads.

Pupils’ Activity: Pupils follow the teacher’s demonstration, drawing the full sectional view of the given component in their drawing pads.

Learning Point: Drawing full sections

Step 6: Guided Practice – Drawing a Half Section

Time: 5 minutes

Teaching Skill: Guided Practice/Demonstration

Teacher’s Activity: The teacher then guides students to draw a half sectional view of a symmetrical machine component (e.g., a flanged coupling). The teacher emphasizes how one-quarter of the object is imagined to be removed, showing both internal and external features in one view.

Pupils’ Activity: Pupils follow the teacher’s guidance, attempting to draw a half sectional view in their drawing pads.

Learning Point: Drawing half sections

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. What is a sectional view and why is it important in technical drawing?
2. Describe the difference between a full section and a half section.
3. How are cutting plane lines and section lines represented in a drawing?
4. Mention two machine components where sectional views are commonly used.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Understanding sectional views

Step 8: Note-Taking

Time: 10 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on sectional views, their types, and conventions into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Recording lesson notes

Step 9: Conclusion

Time: 5 minutes

Teaching Skill: Consolidation

Teacher’s Activity: The teacher summarises the key points of the lesson, reiterating the importance of sectional views for clarity in technical communication and encouraging students to practice drawing various sectional views. The teacher ensures any final misconceptions are addressed.

Pupils’ Activity: Pupils listen to the summary and ask any remaining questions.

Learning Point: Sectional views consolidated

Continuous Assessment/Further Study

Type: Homework/Practice Exercise

Instruction: Using your drawing instruments, complete the following tasks on a drawing sheet:

1. Draw the orthographic views (front, top, and side) of a simple machine component (e.g., a stepped shaft with a keyway).
2. Draw a full sectional front view of the same component, showing all internal details clearly.
3. Draw a half sectional front view of a given symmetrical object (e.g., a simple pulley).

Lesson Keywords

• Sectional View – A view of an object as if cut by an imaginary plane to reveal internal features.
• Cutting Plane Line – An imaginary line indicating where an object is cut to create a sectional view.
• Section Lines – Parallel lines drawn on cut surfaces in a sectional view to indicate material.
• Full Section – A sectional view where the cutting plane passes entirely through the object.
• Half Section – A sectional view of a symmetrical object, showing both internal and external features.
• Removed Section – A sectional view placed outside the main view.
• Revolved Section – A sectional view drawn directly on the main view, rotated 90 degrees.

Differentiation

For students who grasp the concepts quickly, the teacher can provide more complex machine components for drawing sectional views or challenge them to identify errors in pre-drawn sectional views. For students who are struggling, the teacher can provide simpler components, offer more one-on-one guidance during drawing practice, and use physical models more extensively to explain how the cut reveals internal features.

Suggested Lesson Videos

Search YouTube for: sectional views technical drawing SS3