Class: Senior Secondary School 2 (SS2 / SSS2)
Term: First Term
Week: 6
Age: 16 years
Duration: 45 minutes
Subject: Metal Work
Curriculum Theme: Pre-Vocational Studies
Previous Lesson: .
Topic: Gas Welding
Subject Matter: Meaning of gas welding (joining metals using gas flame), oxy-acetylene welding principle, low pressure system, high pressure system, safety precautions

Specific Objectives

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

Cognitive Domain:

Define gas welding.
Explain the oxy-acetylene welding principle.
Differentiate between low pressure and high pressure gas welding systems.
List at least five safety precautions for gas welding.

Affective Domain:

Appreciate the importance of safety in gas welding operations.
Develop an interest in learning more about different welding techniques.

Psychomotor Domain:

Identify the main components of a gas welding setup.
Observe a gas welding operation safely.

Participate in discussions about safe workshop practices.
Collaborate with peers to list safety rules.

Reference Materials

The following resources were used in planning this lesson:

Senior Secondary Schools Education Curriculum
State Unified Scheme of Work
Metalwork Technology for Senior Secondary Schools by E.O. Akintunde

Instructional Materials

The teacher will teach this lesson with the aid of:

Gas cylinders (oxygen and acetylene)
Regulators
Hoses
Welding torch
Goggles
Gloves
Welding apron

Rationale for the Lesson

This lesson helps pupils understand gas welding as an important metal joining process. It enables them to recognize the equipment involved and the critical safety measures needed to perform welding operations without harm.

Prerequisite/Previous Knowledge

Pupils should have basic knowledge of metals and their properties, as well as general workshop safety rules.

Lesson Content/Board Summary

Gas Welding

Meaning of Gas Welding

Gas welding is a fusion welding process that uses a flame produced by the combustion of a fuel gas (commonly acetylene) and oxygen to melt and join metals. The intense heat from the flame melts the edges of the metals to be joined, and a filler rod may be used to add material to the joint.

Oxy-Acetylene Welding Principle

The oxy-acetylene welding principle involves mixing oxygen and acetylene gases in a welding torch. When ignited, this mixture produces a high-temperature flame (up to 3,200°C or 5,800°F) that is hot enough to melt most common metals. The flame is directed at the joint, causing the metal edges to melt and fuse together. A filler rod, made of a similar material to the base metals, is often added to strengthen the joint.

Types of Gas Welding Systems

Gas welding systems are primarily categorized by the pressure at which acetylene is supplied to the torch.

Low Pressure System

In a low-pressure system, acetylene is generated at low pressure (typically 0.1 to 1.0 psi) using a carbide-to-water generator. The acetylene then flows to the torch where it is mixed with oxygen. These systems are less common now due to safety concerns and the availability of cylinder gases.

High Pressure System

In a high-pressure system, both oxygen and acetylene are supplied from cylinders at high pressure. Regulators are used to reduce these high cylinder pressures to a safe working pressure suitable for the welding torch. This is the most common and widely used system today due to its convenience and safety features.

Safety Precautions in Gas Welding

The following are important safety precautions to observe during gas welding:

Always wear appropriate personal protective equipment (PPE) such as welding goggles or a face shield, leather gloves, and a welding apron.
Ensure the working area is well-ventilated to prevent the accumulation of fumes and gases.
Check all hoses and connections for leaks before starting work, using soapy water.
Keep cylinders upright and secured to prevent them from falling.
Never use oil or grease on oxygen cylinder valves or regulators, as it can cause an explosion.
Light the torch with a spark lighter, not a match or cigarette lighter.
Store oxygen and acetylene cylinders separately or at a safe distance from each other.
Turn off cylinder valves when work is completed or during breaks.
Avoid directing the flame towards flammable materials or persons.

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 greets the pupils, reviews the previous lesson on metal joining methods, and then introduces gas welding as another important method, showing pictures or actual equipment if available.
Pupils’ Activity: Pupils respond to greetings, recall previous knowledge, and observe the introductory materials.
Learning Point: Pupils are prepared for the new topic and understand its relevance to previous learning.

Step 2: Meaning of Gas Welding

Time: 7 minutes
Teaching Skill: Explanation
Teacher’s Activity: The teacher defines gas welding, emphasizing the use of a gas flame to join metals by melting them.
Pupils’ Activity: Pupils listen attentively and take notes on the definition of gas welding.
Learning Point: Pupils understand the basic concept of gas welding.

Step 3: Oxy-Acetylene Welding Principle

Time: 7 minutes
Teaching Skill: Explanation/Illustration
Teacher’s Activity: The teacher explains the principle of oxy-acetylene welding, detailing how oxygen and acetylene gases are mixed and ignited to produce a high-temperature flame.
Pupils’ Activity: Pupils listen, ask questions for clarification, and note down the key aspects of the welding principle.
Learning Point: Pupils grasp how the oxy-acetylene flame is generated and used for welding.

Step 4: Low and High Pressure Systems

Time: 8 minutes
Teaching Skill: Comparison/Explanation
Teacher’s Activity: The teacher explains and differentiates between low pressure and high pressure gas welding systems, highlighting their operational differences and safety aspects.
Pupils’ Activity: Pupils listen, compare the systems, and identify the advantages of the high-pressure system.
Learning Point: Pupils can distinguish between the two main types of gas welding systems.

Step 5: Demonstration of Gas Welding Operation

Time: 7 minutes
Teaching Skill: Demonstration
Teacher’s Activity: The teacher briefly demonstrates the setup of gas welding equipment (regulators, hoses, torch) and, if possible, a short, safe ignition and flame adjustment (without actual welding) to illustrate the process.
Pupils’ Activity: Pupils observe the teacher’s demonstration carefully, identifying the different components.
Learning Point: Pupils visualize the practical aspects of gas welding and recognize the equipment.

Step 6: Safety Precautions

Time: 4 minutes
Teaching Skill: Listing/Discussion
Teacher’s Activity: The teacher lists and briefly explains the essential safety precautions that must be followed during gas welding operations.
Pupils’ Activity: Pupils listen, contribute additional safety ideas if any, and note down the precautions.
Learning Point: Pupils understand the critical importance of safety and learn specific precautions.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

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

What is gas welding?
Briefly explain the principle of oxy-acetylene welding.
State two differences between a low-pressure and a high-pressure gas welding system.
List at least three safety precautions for gas welding.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 2 minutes
Teaching Skill: Summarization
Teacher’s Activity: The teacher summarizes the main points of the lesson, reiterating the definition, principle, types of systems, and safety aspects of gas welding. The teacher assigns homework to research other types of welding processes.
Pupils’ Activity: Pupils listen to the summary and copy down the homework.
Learning Point: Pupils consolidate their understanding of gas welding.

Lesson Keywords

Gas welding – A process that uses a fuel gas and oxygen flame to melt and join metals.
Oxy-acetylene – A mixture of oxygen and acetylene gases used to create a high-temperature flame for welding.
Regulator – A device used to control and reduce the high pressure of gases from cylinders to a safe working pressure.
Welding torch – A tool that mixes fuel gas and oxygen and provides a nozzle for the flame.
Acetylene – A fuel gas used in gas welding, known for its high flame temperature when combined with oxygen.
Oxygen – A gas used in gas welding to support combustion and increase flame temperature.

Differentiation

For pupils who grasp concepts quickly, the teacher can encourage them to research the different types of flames (neutral, carburizing, oxidizing) and their applications. For pupils needing more support, the teacher can provide simplified diagrams of the gas welding setup and review the safety precautions individually.

Note for teachers using this lesson plan

Ensure all safety measures are strictly adhered to during any demonstration involving gas welding equipment. If actual ignition is not feasible or safe, use detailed diagrams, videos, or virtual simulations to illustrate the process. Emphasize the practical application of safety precautions to prevent accidents in the workshop.