Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Design a mechanical system using CAD software.
• Plan and manage a mechanical project.
• Fabricate and assemble components using machining and welding.
• Troubleshoot and diagnose common mechanical problems.
• Produce a technical report documenting a project.

Lesson summary

This week, we delve into integrated mechanical applications and projects, focusing on the practical application of the theoretical knowledge you've gained throughout the year. This isn't just about passing an exam; it's about building the skills to design, create, and maintain mechanical systems that impact our daily lives in South Africa. From the water pumps that bring water to rural communities to the agricultural machinery that feeds our nation, mechanical technology is vital. This week will focus specifically on integrating various skills, such as CAD, machining, welding, and problem-solving, in the context of project-based learning.

Lesson notes

This week's focus is INTEGRATION. This means bringing together different skills and knowledge areas we've covered.

Let's break down the key concepts: Project Planning: This is the roadmap for your project.

It involves: Defining the Scope: Clearly stating what the project will achieve and its limitations.

Task Breakdown: Dividing the project into smaller, manageable tasks.

Resource Allocation: Determining the materials, tools, and personnel needed for each task.

Timeline Creation: Estimating the time required for each task and creating a schedule. Consider potential delays (like load shedding) and build in contingency.

Risk Assessment: Identifying potential problems and developing mitigation strategies (e.g., having backup materials).

CAD/CAM Integration: Using Computer-Aided Design (CAD) software to create detailed drawings and then using Computer-Aided Manufacturing (CAM) software to generate instructions for machines to manufacture the designed parts. This integration streamlines the manufacturing process and improves accuracy.

Material Selection: Choosing the right material for the job is critical.

Consider: Strength: The ability to withstand forces without breaking. Consider tensile strength, yield strength, and shear strength.

Hardness: Resistance to scratching and indentation.

Ductility: The ability to be drawn into wires.

Malleability: The ability to be hammered into thin sheets.

Corrosion Resistance: Ability to withstand environmental degradation (especially important in South Africa's varied climate).

Cost and Availability: Consider the cost of the material and its availability from local suppliers.

Machining Techniques: Applying various machining processes such as turning, milling, drilling, and grinding to shape and finish metal parts. Understanding cutting speeds, feed rates, and depth of cut is crucial for efficient and accurate machining.

Welding Techniques: Joining metal parts using heat and pressure. Common welding processes include arc welding, MIG welding, and TIG welding. Choosing the correct welding process and parameters is essential for a strong and durable weld. Safety precautions are paramount.

Fitting and Assembly: Accurately fitting and assembling manufactured parts to create a functioning mechanical system. This requires precise measurements, alignment, and fastening techniques.

Quality Control: Implementing procedures to ensure that manufactured parts and assembled systems meet the required specifications. This includes visual inspection, dimensional measurement, and functional testing.

Worked example

Example 1: Project Planning

You are tasked with designing and building a simple manual water pump for a small rural community that relies on borehole water.

Scope: Design and build a hand-operated water pump capable of lifting water from a borehole 10 meters deep and delivering it at a rate of 20 liters per minute.

Task Breakdown:

Task 1: Design the pump using CAD (2 days)

Task 2: Select materials (1 day)

Task 3: Machine pump components (5 days)

Task 4: Weld the pump frame (2 days)

Task 5: Assemble the pump (2 days)

Task 6: Test the pump (1 day)

Task 7: Paint and finish the pump (1 day)

Resource Allocation: Steel pipes, valves, piston, pump housing, welding machine, lathe, milling machine, hand tools, paint.

Timeline: Total project duration: 14 days (adjust for weekend and school hours). Gantt chart creation is recommended here.

Risk Assessment: Load shedding interrupting machining/welding (mitigation: generator backup or rescheduling), Material shortages (mitigation: sourcing from multiple suppliers).

Example 2: Material Selection and Calculation

You need to select a material for a lever in a mechanical system that will experience a maximum tensile force of 5000 N. The lever is 200mm long and 20mm wide, and you want the thickness to be sufficient to withstand the force with a safety factor of

3. You are considering mild steel and aluminum.

Calculate the required cross-sectional area:

Safety Factor = 3

Design Force = Maximum Force Safety Factor = 5000 N * 3 = 15000 N

Determine the allowable stress for mild steel and aluminum:

Mild Steel Yield Strength (σy) ≈ 250 MPa (250 N/mm²)

Aluminum Yield Strength (σy) ≈ 150 MPa (150 N/mm²)

Calculate the required area based on the allowable stress:

Mild Steel: Area = Design Force / Allowable Stress = 15000 N / 250 N/mm² = 60 mm²

Aluminum: Area = Design Force / Allowable Stress = 15000 N / 150 N/mm² = 100 mm²

Calculate the required thickness:

Mild Steel: Thickness = Area / Width = 60 mm² / 20 mm = 3 mm

Aluminum: Thickness = Area / Width = 100 mm² / 20 mm = 5 mm

Conclusion: Both materials are suitable, but mild steel requires a smaller thickness (3mm) compared to aluminum (5mm). Consider cost, weight, and corrosion resistance when making the final decision.